Mechanically Strengthened Aerogels through Multiscale, Multicompositional, and Multidimensional Approaches: A Review

In recent decades, aerogels have attracted tremendous attention in academia and industry as a class of lightweight and porous multifunctional nanomaterial. Despite their wide application range, the low mechanical durability hinders their processing and handling, particularly in applications requiring complex physical structures. “Mechanically strengthened aerogels” have emerged as a potential solution to address this drawback. Since the first report on aerogels in 1931, various modified synthesis processes have been introduced in the last few decades to enhance the aerogel mechanical strength, further advancing their multifunctional scope. This review summarizes the state‐of‐the‐art developments of mechanically strengthened aerogels through multicompositional and multidimensional approaches. Furthermore, new trends and future directions for as prevailed commercialization of aerogels as plastic materials are discussed.


Introduction
Porous nanomaterials have received noteworthy attention in nanoscience and nanotechnology.Aerogels are unique meso or macrostructured 3D porous materials with an ultralow density and large specific surface area. [1]The term aerogel was first introduced by Kistler in 1931 refer to gels whose liquid content has been replaced by air, without the collapse of the gel solid network. [2]The skeletal and pore structure of aerogels can be regulated at the nanoscale level to attention to these stages during the synthesis of gels.To date, various strategies have been employed to advance the modulus of aerogels, including the reinforcement of organic precursors and the use of polymers, carbon nanotubes, and nanowires. [7,8]However, it is difficult to simultaneously focus on all properties, such as density, surface area, and thermal stability.It is well known that owing to their excellent mechanical strength, low density, fine elasticity, good electrical conductivity, and extremely high aspect ratio, carbon nanotubes (CNTs) and graphene are ideally considered as the best candidates to fabricate ultralight, elastic, and conductive aerogels. [7]Studies have demonstrated the multifunctionality of aerogels composed of carbon-based 1D or 2D nanomaterials. [4,7,9]The aerogels are intriguing materials for multiple applications due to their unusual mechanical properties and multifunctionality.Various building blocks, including as 1D CNTs, SiO 2 nanofibers (NFs), 2D graphene and MXene nanosheets, polymers, cellulose, and biomass, have been used in the last five years to fabricate advanced 3D multidimensional and multicompositional materials. [7,10]Moreover, the intrinsic properties of the building blocks are reflected in their 3D structures, which endow the resulting aerogels with unique features.
This review is structured as follows: first, in the introduction section, we introduce various multidimensional and multicompositional nanomaterials, particularly aerogels.Figure 1 presents information regarding papers published in the aerogel research field in the last decade; particularly, on the mechanical properties of aerogels as well as the dependence of the mechanical properties of different kinds of aerogels on their density.In the next section, aerogels are classified based on their multidimensional structures, multicompositions, and multiscalability of aerogels, including hierarchical structure, and carbonization of aerogels to improve their functionality.In addition, we explain methods and reasons for the synthesis of multidimensional, multicompositional, and multiscaled aerogels.The overview of the main experimental steps involved in the fabrication of multidimensional and multicompositional aerogels highlights the peculiarities of each step involved.The third section of the review is dedicated to colloidal dispersions; particularly, it focuses on the various interparticle forces and how they can be modified experimentally.Thereafter, gelation chemistry is discussed in the succeeding section, and this comprises diverse mechanistic aspects and it provides experimental guidelines on the effective gelation of stable dispersions.Next, we briefly compare aerogels prepared using various new methods to those prepared using the traditional molecular sol-gel chemistry.A brief discussion on silica, non-silicate, synthetic polymer, biopolymer, and carbon-based aerogels, and the parameters for improving their mechanical strength is elaborated in the main section of the review.Lastly, the review is concluded with some important future perspectives and scope for aerogel researchers.

Chemical Engineering for Enhancing the Mechanical Strength of Aerogels
Generally, porous nanomaterials are classified into three types based on pore size: microporous (pore size < 2 nm), mesoporous (2 nm < pore size < 50 nm), and macroporous (pore size > 50 nm).The most important and common synthesis technique for aerogels is the sol-gel process.Sol-gel processing technology has been known since Ebelman's initial synthesis of silica gels in 1845. [11]The new ceramics were created for more sophisticated applications by combining inorganic sols and gels in a technique known as "sol-gel".Sol is a colloidal particle dispersion in a liquid media.The size of solid particles dispersed in colloidal sol  [4i] Copyright 2018, Elsevier.18c] Copyright2019, Elsevier.18a] Copyright 2012, Wiley-VCH.18b] Copyright 2020, American Chemical Society.
are between 2 nm and 0.2 μm.Typically, water, alcohol, or other organic liquids (depending on the final product) are preferred as solvents for dispersing colloidal particles in sol.In contrast, gel is an interconnected semisolid network that expands stably through a liquid medium, and the size of gel can be limited to the container size.The sol-gel chemistry started in the 19th century with formation of gel using SiCl 4 precursor. [7]Further, the detailed chemistry of hydrolysis and condensation for the synthesis of sol was developed based on the presence of silicon alkoxide precursors.However, the rate of hydrolysis and condensation, as well as the hydrolysis of different kinds of alkoxide, can lead to an extensive phase separation and limit the formation of ternary or quaternary aerogels.Nevertheless, sol-gel processing is still one of the most reliable methods for chemical engineers and material scientists.The synthesis of different types of aerogels and their expected microstructures is presented in Figure 2.
The overall sol-gel process can be divided into different subparts [3] i. Oligomerization to form small clusters ii.Polymerization of the clusters into the sol iii.Gelation of sol yielding a 3D network that extends all over the reaction system iv.Aging of the gel to strengthen the network v. Drying of gel by adopting a suitable method, e.g., supercritical alcohol drying (SAD), [12] supercritical CO 2 drying (SCD), [1] ambient pressure drying (APD), [13] freeze-drying (FD), vacuum drying (VD), [14] etc.

Gelation Chemistry
In 1974, Flory grouped gels into four different types, comprising ordered, lamellar gels, covalent polymer networks, and networks of physically aggregated polymers, and disordered particulate gels. [11]However, in 1996, Kakihana provided a more helpful classification for distinct gel types produced by sol-gel chemistry, which is utilized to prepare inorganic solids.Sol-gel-based materials can be classified into different types owing to the great diversity of sols and gels.Traditionally, sol is formed through the hydrolysis and condensation of silicon alkoxide precursors, but in recent years, many modifications have been adopted in the sol-gel synthesis of nanomaterials. [15]In gelation chemistry, the structure of polymeric networks can be controlled at the molecular level, and this is promising for the synthesis of aerogels with tailored properties for interesting applications. [3,16]Gelation occurs when polymerization reactions reach a critical value, a polymer of infinite size is formed compared to the atomic or molecular scale.Flory-Stockymayer model of gelation provides a mechanism for the construction of polymerized gel network via the successive addition of molecular branches using monomers.Ceramists and inorganic chemists developed a systematic classification method for gels, which clearly distinguishes them from colloidal gels.According to Flory's classification, gels can be categorized as particulate or polymeric.The polymeric network contains a 3D solid backbone composed of chains of particles of <1.5 nm size, which cannot be easily observed by electron microscopy techniques.
The Flory-Huggins (FH) model is a well-known theoretical model for predicting the transition from solution to polymerization in presence of the appropriate solvents. [17]Depending on the temperature, this model can predict the phase diagram for solvent and polymer networks with one or two phases that are in equilibrium.Additionally, the degree of polymerization further widens the miscibility gap created by the two distinct phases in a single solution.Compared to other polymeric aerogels, RF aerogels are distinct owing to the irreversible phase diagram of their FH model.Consequently, equilibrium thermodynamics cannot determine the degree of polymerization using thermodynamic models.
Frey and Theil examined the interaction of cellulose in a saltwater mixture using an experimental phase diagram. [19]Another study reported that sol-to-gel transition in cellulose aerogels occurs via three different routes, including intrinsic reaction (using sodium hydroxide solution), neutralization reaction, and formation of a reversible and thermodynamically stable miscibility gap. [20]The thermally activated process for gelation can be explained by considering Arrhenius-type behavior, as expressed below: where  is a gelation constant.In addition, gelation can be induced in base solutions of cellulose by changing the pH or neutralization reaction.
For synthetic polymer aerogels (polyimide aerogel), in addition to diamines and dianhydrides, a trifunctional amine is used as a cross-linker. [21]The molecular weight of polymer chains increases until the polymers become insoluble in the solvent.Thus, polymerization processes convert soluble monomers into insoluble polymers in the aerogel skeletons.Some polymer systems require the addition of cross-linking agents during gel formation to promote strong interactions between the solid polymeric components for the development of porous structures.Numerous factors have been observed to influence the gelation time, including the polymer concentration, chemical composition, additives, temperature, and pH. [22]To promote gelation, some polymers (for example, phenolic resin and polyacrylamide) require the addition of curing agents (for example, silica nanoparticles and alumina nanoparticles), whereas other polymers (thermo-plastic polymers) achieve gelation immediately, resulting in the formation of a strengthened network. [23]umerous techniques have been employed for the synthesis of biopolymer-based aerogels. [24]These aerogels exhibit attractive and distinctive features because of the amazing flexibility and durability of the sol-gel synthesis process, which can be paired with APD to synthesize xerogel; FD to synthesize cryogel; or SD to synthesize aerogel.The aging and cross-linking processes are necessary during the synthesis of biopolymer-based aerogels.To date, various synthesis techniques have been investigated for the fabrication of 1D nanofiber and its assemblage into 3D fibrous aerogels. [25]Various methods may be utilized to fabricate 3D fibrous aerogels with various structural characteristics using either a single assembly of fibers or an aggregated assembly.The morphological network of fiber-based aerogels significantly impacts both their mechanical and functional characteristics.Furthermore, solgel synthesis is one of the conventional wet chemical processes for preparing 2D material-based aerogels. [26]Thus, the synthesis process can be precisely managed and the appropriate microstructure of the gels can be formed owing to the ease in regulating the reaction and the low processing temperature.Worsley et al. reported the synthesis of graphene aerogel for the first time using the resorcinol-formaldehyde (RF) assisted sol-gel chemical method. [27]In addition, Qu et al. developed a surfactant-foaming sol-gel method for the synthesis of graphene aerogels. [28]Using this method, enormous graphene hydrogel blocks were produced, and GO-liquid crystals were effectively reconstructed using the microbubbles as templates.Molecular precursors utilized in conventional solgel synthesis include organic alkoxides and chlorides. [7]Owing to the difficulties in determining suitable precursors for many 2D-materials-based aerogels, the use of the sol-gel process for preparing these materials is restricted due to the amorphous nature of the obtained material.b) polymerization of silica at different pHs, [53] c) schematic arrangements of two different cyclic arrangements of SiO 2 structural units, [53] d) ideal synthesis process of radical polymerization and hydrolysis-polycondensation for synthesis of aerogels, [38] e) ideal synthesis process of biopolymer (e.g., chitosan) aerogels [54] f) molecular dynamics (MD) simulation models for the synthesis of silica and graphene aerogels, [55] and g) typical temperature-pressure phase diagram for simple substance and drying routes.a-c) Adapted with permission. [53]Copyright 2016, Elsevier.38b] Copyright 2018, American Chemical Society.e) Adapted with permission. [54]Copyright 2020, Wiley-VCH.f) Adapted with permission. [55]Copyright 2021, Elsevier.

Hydrolytic Polycondensation and Addition Condensation
Functional aerogels are generally synthesized via hydrolysis polycondensation reactions, and the pH of the solution plays a major role in these reactions, and it plays a vital role for controlling the morphology of the final aerogel materials (Figure 3a,b).Silicon alkoxides are the most widely used precursors for synthesizing transparent, hydrophobic, thermally insulating, and monolithic silica-based aerogels.When silicon alkoxides are used, the alkoxide is converted to silanol species during hydrolysis, and these hydrolyzed silanol species further participate in the subsequent condensation reactions, which can occur in two pathways, as reported by Parale et al.: [29] i. condensation of two silanol species to form a siloxane linkage and one water equivalent, ii.Condensation of a silanol species with another alkoxide molecule to form a siloxane bond and one alcohol equivalent.
Figure 3c shows the primary cyclic arrangements of SiO 4 tetrahedra.The low polarity of the Si-O bond (the Si atom bears a small positive partial charge of ≈+0.32 in TEOS) in silicon alkoxides delays the gelation process. [7]To address this, acid or base catalysis can be employed depending on the functionality.The structure of aerogels can be altered based on the product of successive hydrolysis and polycondensation reactions.In addition, the textural, morphological, and mechanical properties of aero-gels can be controlled based on the type of catalysis reaction employed: Acid catalysis provides a nearly polymeric gels texture, whereas base catalysis favors denser colloidal silica particles (Figure 3b).The colloidal particles network obtained can be gradually changed to a polymeric network by altering some synthesis conditions.Thus, the structure of aerogels can be designed to achieve the desired functional properties via the chemical control of the mechanisms and kinetics of sol-gel reactions, as well as the selection of the sol-gel parameters, such as type of precursors, acid or base catalysts, solvent to precursor ratio, water to precursor ratio, surface modifications, and washing and drying conditions. [3,7]The schematic of the sol-gel synthesis of aerogels is systematically presented in Figure 3d.A precursor, solvent, water, and catalyst are required for the fabrication of aerogels, and the network of silica and graphene aerogels can be predicted using molecular dynamics (MD) simulation, as presented in Figure 3e.
Generally, previous studies on RF aerogels clearly have demonstrated that the R:F molar ratio 1:2 is stoichiometrically sufficient for forming a branched polymer network.Pekala et al. theoretically discussed a model for R/F concentration as a function of time, and found that the resorcinol concentration decreased rapidly during the two-step sol-gel process compared to the gradual decrease in the formaldehyde concentration. [30]n addition, R/C and R/W molar ratios are crucial to form homogeneous sol-to-gel transition, where the concentration of acid and base used in the particular sol-gel process should be considered.

Epoxide Assisted Gelation
A previous study employed an epoxide-initiated gelation method for the synthesis of non-silicate inorganic oxide aerogels, where organic epoxides are used as initiators for hydrolysis and condensation processes. [37]For sol-gel reaction, epoxides can be used as proton scavengers despite their use in organic synthesis (ringopening reactions, epoxide cleavage, epoxide reduction, asymmetric epoxidation, and epoxide ring closure) owing to their reactivity and ability to participate in various reactions.In this procedure, an acid protonates the epoxide oxygen before the conjugate base attacks the epoxide ring nucleophilically to open the ring.That is, epoxides consume the protons during the ring-opening reaction, and the protonated epoxide undergoes irreversible ringopening reaction in the presence of a suitable nucleophile (such as the counterion of the metal salt).The epoxide approach involves the combination of a solvated metal complex and epoxide to create a homogenous solution before a considerable pH increase.Controlled olation and oxolation reactions can be achieved via the relatively gradual and uniform pH increase during the reaction, which eventually results in the creation of the metal oxide gel network or a stable metal oxide sol.However, the immediate precipitation of the condensed metal oxide species from the solution owing to the fast polymerization of the metal oxide or hydroxide limits the use of this method for the synthesis of aerogels.Nevertheless, there are various techniques that can be used to circumvent the rapid polymerization in the epoxide ring opening, including the use of initiators, temperature control, employing catalysts, selection of suitable solvent, and use of inhibitors.

Radical Polymerization
Radical polymerization (RP) is one of the remarkable processes for chain polymerization, and it includes both free RP and controlled/living RP, and owing to its simplicity, it is widely used in polymer synthesis. [38]This process can be performed in aqueous media, as well as organic solvents, and the choice of the solvent depends on the monomers and desired properties of the synthesized polymer.In the 19th century, a repeating unit structure of known polymers, including polystyrene (PS), polyvinyl carbonate, and polyvinyl acetate, was fabricated using free radical polymerization. [39]Currently, free radical polymerization reactions are understood to proceed under kinetic than thermodynamic control.To date, various methods have been devel-oped for the synthesis of polymer aerogels using RP, including controlled/living RP, such as atom transfer RP (ATRP), [40] reversible addition-fragmentation chain transfer RP (RAFT), [41] and nitroxide-mediated stable free RP (SFRP). [42]eventis et al. successfully prepared PS aerogels using surfaceinitiated free RF, and the successful preparation can be attributed to the coexistence of vinyl monomers with sol-gel precursors, and the ability to rapidly switch between the two processes. [38]Cross-linked aerogels made of polystyrene and polymethylmethacrylate are examples of the strongest materials prepared using this approach.In a previous study, our group designed phenyl cross-linked TEOS precursor using RP, and further strengthened the silica aerogel network using the sol-gel process. [15]Kanamori et al. adopted the RP method to polymerize vinylsilanes and could obtain a double/triple cross-linked transparent, as well as mechanically enhanced, polyvinyl-polysiloxane aerogels by combining these precursors with other alkoxysilane precursors. [38,43]

Nucleation and Crystal Growth
Nucleation and crystal growth in biopolymer aerogels involve complex processes that are influenced by various factors including solution chemistry, temperature and pressure, aging, crosslinking, additives, and interactions. [44]When solute molecules start to form a stable cluster, nucleation and crystal growth are initiated.The biopolymer sol composition and concentration play an important role in the nucleation process.In addition, the molecular weight, polymer chain conformation, solvent, and the presence of functional groups in the biopolymer can affect the nucleation process.Therefore, it is necessary to use solvents with high boiling point during aerogel production.The crystal growth begins after nucleation and proceeds with the addition of solute molecules to the nucleus, resulting in continuous growth.Temperature and pressure control is another factor that plays a crucial role in gelation and drying, which can directly affect the crystal growth.Particularly, a slow aging and drying process can enable the formation of larger crystals.
The freezing profile of water provided by Zaragotas et al., who investigated the ice nucleation and growth, indicated that there also are two phases of crystallization: nucleation and crystal growth. [45]During the rapid nucleation period, a certain stimulus induces the crystallization of the super-cooled solvent, which results in the formation of the crystal core and crystal nucleus.As the size of the ice crystal is determined by the initial nucleation temperature and crystallization rate, as reported by a previous study, it is essential to control the formation of the ice nucleus. [46,47]n a typical synthesis, the biopolymer sol is converted into gel, which is then subjected to controlled cooling below its freezing point.The decrease in temperature induces the formation of ice crystals in the solution, and the cooling rate can influence the size and distribution of ice crystals, as well as the final network of aerogels.Generally, nucleation involves the initial formation of ice crystals, followed by the growth of these ice crystals via the addition of more water molecules for the continuous growth of ice crystals (endothermic process).Furthermore, FD is preferred for the removal of the solvent by phase transition directly from solid (ice) to vapor under low-pressure conditions.Lastly, a highly porous structure is obtained via the sublimation of the ice crystals in the biopolymer matrix.

Carbonization
Carbonization of aerogels involves the conversion of the organic components of aerogels, typically made from polymers or other organic materials, into carbon, while preserving the textural properties of aerogels, with a 3D network based on the polyaromatic structures. [9,48]Carbon aerogels (CAs), [49] organic aerogels, [50] and mixed metal oxide aerogels [51] often undergo calcination and activation stages in addition to the preparation steps mentioned for aerogels.Aerogel under atmospheric pressure is subjected to high temperatures, often exceeding 600 °C, in an inert atmosphere of N 2 or Ar during the calcination process.This step develops the carbonaceous structure in aerogels and is based on the breakdown and conversion of natural or synthetic organic materials to produce aerogels with a high carbon content.For this process, the temperature is often increased in a furnace to a point above the point at which the organic molecules form the gel undergo pyrolytic carbonization.

Necking Enhancement
The formation of the primary particles and secondary particles of aerogels depends on the pH of the reaction (Figure 3b).The secondary particles link together to initiate gelation, resulted in neck regions with mesoporous network.A solid gel network is made up of a number of dangling ends that are connected to the main network by only one end.These dangling branches not only contribute to the physical features of the aerogel network, such as density, porosity, and specific surface area, but, they also act as barriers to the migration of molecules or colloidal particles into gel.However, the elasticity or electric conduction of the gel network was not affected by dangling branches.The suppression of dangling branches leave a linking network at the two ends of other neighboring branches known as "active branches".Generally, the number of active branches (N a ) in the gel network can be used to determine the elastic modulus of a gel.According to the Flory-Stockymayer gelation model, N a can be considered as a function of the extent of the reaction (), as expressed in Equation (2): This formula expresses the relationship between the textural and theoretical properties of gel networks, such as the osmotic swelling behavior of polymeric gel.Using N a , the static shear modulus, G, can be calculated as expressed in Equation (3), [52] where V m is the volume per mole of polymer chains, R is the gas constant, and T is the temperature.
However, the shear modulus of oxide gels depends on the water content, as its affinity varies with the water content, which is proper for soluble boehmite or clay sols.The modulus for such materials depends on the concentration of the gel: [52] G ≈ C n (4)

Fibril Network Strengthening
Various biomacromolecules, such as chitin [56] and cellulose, [57] and synthetic polymers, such as polyimide (PI) [58] and polyurethane (PU), [59] were investigated for the production of aerogels.The possible mechanical strengths of these materials vary substantially by a few orders of magnitude (from 10 1 to 10 2 MPa), despite the fact that the majority of them can form 3D microfibrillar networks in similar levels of solid content.The chemical cross-linking between fibrils might affect the mechanical characteristics of the network, [60] whereas there are no notable quantitative correlations between the various microstructural factors and macroscopic mechanical responses.Furthermore, even when Maxwell rigidity theory or Gibson-Ashby models for cellular solids are utilized, it is still challenging to establish exact theoretical calculations owing to the unpredictability of fibrillar networks.
The preparation of networks in fibrillar aerogels are of great interest to the scientific community.The aggregation of polymer chains during the gelation process and the subsequent formation of entanglements that eventually result in a 3Dinterconnected network give materials their fibrous appearance, as explained in a previous study. [61]Despite the fact that this process can be accurately predicted using techniques, such as molecular dynamics, similar to the case in silica aerogels, this approach would be computationally intensive owing to the complexity and multiplicity of the process.In this situation, there are limitations caused by time-and length-scale effects, such as correlating data with already available experimental data from analytics.A model that can accurately describe the fibrillar morphology of aerogels has long been desired.To simulate the gelation in Ca-alginate hydrogels, Depta et al. proposed the use of a mesoscale model inside a discrete element method-based Langevin dynamics framework. [62]he Ca-alginate system, employing alginate molecules and Ca 2+ in aqueous conditions as the model system, could reasonably capture the aggregation of polymer fibers and the subsequent shape.

Drying
To advance the sol-gel process, aerogel researchers should focus on drying processes, as the drying condition of gels has a significant impact on the structural, as well as physical properties, of aerogel materials.During the drying process, the solvent in the gel pores can be removed without altering its porous structure.To date, several drying approaches have been developed to tune the properties of aerogels.In this subsection, the different drying methods, APD, SD, and FD were discussed and these are shown in the pressure-temperature phase diagram (Figure 3f).

Ambient Pressure Drying Process
The APD method is a conventional drying method for the longterm production of silica aerogels, and can significantly reduce the fabrication cost of aerogels, while ensuring safety. [4,13,63]However, the strong capillary pressure and irreversible shrinkage of the aerogel hinders the further application of APD.Additionally, the condensation reactions between silanol groups without surface treatment resulted in the formation of dense aerogels (xerogels) with a high-volume shrinkage.Therefore, it is essential to consider surface modification/hydrophobization of alco-/hydrogel before subjecting it to a APD. [64]This modification process not only prevents transient structural collapse during the initial stages of drying, but also induces a spring-back effect near the end of the drying process, allowing the gel to recover most of its original pore volume and the production of low-density, mesoporous aerogels.
However, the drying stress can be suppressed by adopting two strategies to produce aerogels with low-volume shrinkage and good mechanical stability under APD conditions.The first strategy involves the use of a low-surface tension solvent during the solvent exchange to reduce the capillary stress during APD.Several studies have reported the use of low-surface tension solvents, such as hexane and acetone, for reducing the capillary stress during APD. [65]The other strategy involves increasing the strength of the gel framework to withstand high capillary stress during APD.Many researchers have employed the APD method to synthesize aero-/xerogels, including silica-based aerogels, polymer-reinforced silica aerogels, polymer-modified silica aerogels, and RF aerogels.Recently, Saito et al. developed mechanically strong and scalable CNF aerogels with mesoporous network using the APD approach. [66]Furthermore, Wu et al. prepared transition metal carbide/nitride (MXene) aerogels utilizing ultrathin cellulose nanofiber and employed APD to obtain 2D MXene nanosheets. [67]Malfrait et al. prepared strong, machinable urea-modified chitosan aerogels using APD. [68]However, the APD method can result in the collapse or shrinkage of the aerogel structure, forming a denser material with reduced porosity.The extent of the overall structural damage depends on factors, such as the gel composition, the rate of solvent removal, and the choice of solvent.

Supercritical Drying Process
To obtain monolithic aerogels with lower density, preserved porous structure, and high porosity, SD is the ideal drying method.Particularly, SD can be used to obtain highly porous and uniform aerogel structures. [69]This process can minimize the capillary stress, thus easily removing the solvent in the pores of the gel network under supercritical conditions without affecting the porous structure of the aerogels.It is essential to focus on the gel-strengthening phenomena before performing SD.However, high operation cost and high-pressure operation are the two drawbacks affecting the further application of SD.
The SD process can be divided into two sub-processes: i) SAD, [8] also known as high-temperature SD, in which the alcogels are dried in alcohol solvent at a supercritical state (e.g., ethanol SAD conditions, 265 °C, 120 atm) used during synthesis and ii) SCD (low-temperature SD), [70] in which the synthesis solvent can be replaced by a low-temperature solvent, such as CO 2 , after which the aerogels are dried in CO 2 at a supercritical state (40 °C, 90 atm.).
Monolithic aerogels with low-volume shrinkage can be commonly synthesized using the SD process.To date, studies have reported the fabrication of various aerogels with remarkable physical, textural properties, and mechanical strengths, including the transparent and flexible silica, [71] organic (RF, [72] and phenolfurfural, [73] ) metal oxide (TiO 2 , [4] SnO 2 , [4] ) and fiber reinforced [74] aerogels using SAD.However, owing to high-temperature drying approach, this method has some limitations.To address these limitations, the SCD approach was developed to synthesize synthetic silica, [43] polymer, [75] biopolymer, [76] and organic [77] based aerogels.Furthermore, the organic or polymer-derived aerogels can be processed carbonization in an inert atmosphere to obtain carbon aerogels.Given the poor solubility of water in supercritical CO 2 , there are issues with using water as the synthesis solvent, so it must be substituted by an organic solvent.As the presence of water in the system is unavoidable owing to water condensation reactions and the addition of acidic or basic water as the catalysts, solvent cleaning is also advised before SCD when employing alkoxides as precursors.Consequently, the number of processing stages increases, thus increasing the high-pressure CO 2 and cost, and reducing the size of the aerogels owing to restrictions in the autoclave size compared to alternative drying processes, which is an additional drawback of this approach.

Freeze-Drying Process
Another drying method of aerogels is FD, and this is the most widely used drying method for the synthesis of 1D-or 2D-based aerogels. [78]To prevent the formation of a gas/liquid interface, at low pressure and temperature, the solvent in the hydrogel is frozen and eliminated via sublimation.The FD process is a simple, eco-friendly, and affordable technique compared to SCD.However, prior to drying hydrogels using FD, the freezing rate and precursor concentration should be optimized to control the porous structure.The volume shrinkage during FD can be prevented by fast freezing in the presence of liquid nitrogen, and subjecting the aerogel to sublimation under vacuum conditions before drying.This process is commonly used for hydrogels, as water is the more commonly used solvent compared to other organic solvents.The only exception is tert-butanol (t-BuOH), which is recognized as a good solvent for enhancing the surface area owing to the formation of smaller crystals compared to water.Generally, the lyophilization cycle involves three steps: i. Solvent exchange with the drying solvent, ii.Freezing, iii.Sublimation under vacuum conditions.
In most cases, the microstructure of gels formed using this process is disturbed owing to the crystallization of the solvent.Hence, the controllable pore size is in the micrometer scale or longer compared to samples prepared using SCD.This process has certain drawbacks, including the challenge of making monoliths and the need to exchange solvents while preventing the formation of micrometer-size pores (pores with diameters more than 10 μm).The final aerogels structural characteristics will depend on the drying procedure, and the choice of the techniques to be employed will depend on the chemical system and the eventual use of the dried material.In conclusion, all three drying processes have their advantages and limitations.
Among the aforementioned drying methods, APD is a low-cost approach, but it is limited to the synthesis of silica-based aerogels.The synthesis of transparent and flexible aerogels using APD and SCD has been reported.Many researchers have employed the SAD method to synthesize monolithic metal oxide-based aerogels and silica-based aerogels.However, SAD requires high temperature and pressure with alcohol solvents, which makes this method unsafe.To address these limitations, SCD and FD methods were developed as preferred drying methods owing to their low-temperature environment, and use for the synthesis of synthetic polymer, biopolymer, and polymer-reinforced silica aerogels.The tables in this review present a list of silica, non-silicate, synthetic polymer, natural/biopolymer, and carbon-based aerogels prepared using the aforementioned drying methods.Multicompositional as well as multidimensional/multiscale aerogels can be easily synthesized by following an appropriate drying condition.

Modern Synthesis Methods for Mechanically Strengthened Aerogels
In recent years, some new methods (other than the traditional sol-gel approach) were invented for the synthesis of 3D porous aerogels to enhance their mechanical properties.

Self-Assembly Process, Hydrothermal, and Click Chemistry
Self-assembly [79] is a technique based on the phenomenon of spontaneous structure formation at the atomic level, and the formation of a 3D network structure via the self-organization of anisotropically grown low-dimensional materials.This technique can implement a 3D structure by inducing self-organization and structuring processes through variable control in reaction and control of the growth/distribution/reactivity, which determines the growth of nanomaterials in several dimensions and the selforganization process of nanostructures.Recently, our group reported mercaptosuccinic acid-assisted assembly process to design 1D-fiber-based aerogels. [80]In addition, a common method for creating 2D material-based aerogels is self-assembly. [81]The limitations of the precursors will no longer be applied to the self-assembly approach, in contrast to the conventional sol-gel method.In addition to graphene, the other 2D nanomaterial aerogels have also been widely developed using self-assembly method. [81,82]The layers of 2D nanomaterials are joined together via electrostatic contact, hydrogen bonds, and p-p interaction with the help of a cross-linking agent.The synthesis process offers the benefits of moderate conditions, easy operation, and low cost, which enables large-scale production of highly effective adsorbents for water filtration.Furthermore, the most frequent wet chemical method for creating 2D-based aerogels is the hydrothermal process. [83]It refers to a technique for preparing materials at high pressure and temperature in an airtight pressure vessel using water as the solvent, which can increase the rate of gelation while simultaneously providing the consistency of the gel.This technique can be used to control the growth of 1D/2D materials to form a constituent material.
Furthermore, click chemistry [84] is one of the beneficial techniques for the short-term gelation of materials by controlling the surface chemistry of nanoparticles.This technology employs reactions, such as Diels-Alder [85] and Glaser-Hay coupling, [86] on the surface to accelerate reactions by inducing instantaneous reactions using thermal, UV, or ionic stimulations.To achieve aerogel formation, surface chemistry control technology must be supported for nanomaterials, and in the case of inorganic materials, reactions, such as addition, nucleophilic substitution, and electrophilic elimination, should be employed.To achieve the desired aerogel pore structure, detailed conditions, such as surface functional group, nanomaterial:solvent ratio control, porosity control, and reaction activation method, should be optimized to realize properties suitable for the application. [87]

Design of Aerogels using Double or Multi-Cross-Linking
In 2002, Leventis et al. developed cross-linking strategies with polymeric materials on silica using poly(hexamethylene diisocyanate) cross-linking. [88]Thereafter, polymeric cross-linking has been further investigated with various polymeric materials.Additionally, Leventis and Meador groups in NASA Glenn Research Center have jointly invented "X-Aerogels," which demonstrate 300 times higher Young's modulus compared to native silica aerogel by polyurethane cross-linking. [89]In addition, surface functional groups containing nucleo-and electrophilic groups, such as epoxy or amino groups, can be controlled during the aerogel synthesis. [90]Various types of polymers have been used to cross-link aerogel, to enable the control of the strength and flexibility of silica aerogels. [91]This method has been further developed for other metal oxide aerogels. [92]n recent years, researchers have focused on improving the mechanical properties of aerogels using double or triple crosslinking methods.Cross-linkers can initiate gelation, enhance mechanical properties, and draw into most of the 3D aerogels/hydrogels.In addition, an aerogel structure known as double or multi-cross-linked aerogel can be formed using two or more different kinds of cross-links. [38,93]Double cross-linked network aerogels with high mechanical strength and toughness are typically synthesized using free-radical polymerization, by adopting both chemical and physical cross-linking, one followed by the other, or both in one pot. [94]Many studies have confirmed the improved mechanical properties of double cross-linked 3D aerogels/hydrogels compared to chemical or physical cross-linked aerogels/hydrogels.Most hydrogel networks are formed using a double cross-linking approach, which enables the formation of a more stable internal structure and improves the mechanical properties.In double cross-linked networks, one cross-linking can enhance the mechanical strength of the aerogel (covalent cross-linking), while the other improves its functionality (noncovalent cross-linking). [94]In the case of hydrogels, double networks can be constructed via covalent and/or non-covalent crosslinking between different polymer chains.Here, non-covalent interactions can be further simplified as ionic bonding, hydrogen bonding, and hydrophobic interactions, which are important physical interactions in supramolecular chemistry.

Tailoring Pore Structure and Engineering of Mechanically Enhanced Aerogels
Various studies have investigated the synthesis of aerogels using inorganic and inorganic-organic hybrid precursors.The gelation of sodium silicate (water glass) and metal alkoxides (organosilanes) can dominate the sol-gel process, in which one can empirically control the textural properties at the microscopic level.Therefore, the need to optimize experimental conditions during the sol-gel process has been emphasized to predict the final properties of aerogels, including porosity, density, and mechanical properties.However, these properties cannot the predicted when cross-linkers or biopolymers are added into a primary precursor solution.The use of additives or the templating approach, which adds an extra degree of freedom during synthesis, can be very well-organized for pore structure engineering.Additionally, low-molecular-weight block copolymer surfactants, emulsions, and solid particles can be used as templating agents for the synthesis of aerogels.In general, the hydrolysis and condensation of the organometallic precursor are subjected to the presence of a polymeric template, such as poly(ethylene oxide), poly(acrylic acid), and amphiphilic block copolymers, in a sol-gel-based templating technique. [95]Hierarchically organized aerogels with a macropore size of 0.1-80 μm and uniform mesopores with a size of 2-60 nm have been synthesized using the templating approach. [96]Target gels can be synthesized within a template gel using low-molecular-weight gelators, and these template gels can be extracted during the solvent exchange process, forming an organogel with a 70 nm pore-size fiber-like structure. [97]During oil-in-water emulsion process, the pore size of aerogels can be tuned based on the water droplet size, which is used in the synthesis of metal oxide and biopolymer aerogels. [98]The main advantage of the oil-in-water emulsion approach is the easy extraction of the oil phase using SCD technology.
To achieve the formation of a 3D gel with the desired pore network, the drying process must be considered as it significantly affects the resultant pore structure of aerogels.The most commonly used drying technique for aerogel is FD owing to its low-risk factors and the ability to achieve the pore engineering of aerogels during this process.Furthermore, the meso-tomacro pore ratio can be adjusted by altering the freezing rate, temperature gradient, and liquid phase composition.There are distinct differences in the pore structure engineering of FD and SCD methods.For example, cellulose hydrogels dried using the SCD method exhibit a hierarchical structure with mesopores and macropores between fibrils and hairy beads, respectively. [99]owever, for the same cellulose cryogels synthesized using FD, a sheet-like macroporous interconnected morphology can be observed. [100]o date, various forms of biopolymer aerogels have been reported, including flexible, marshmallow-like, foamy forms, and more transparent silica-aerogel-like forms. [24,44]The microstructure of aerogels can be adjusted over a wide range by employing various material sources, extraction and processing tech-niques, as well as synthesis and drying procedures.One of the main justifications for synthesized biopolymer composite aerogels is the strengthening effect by biopolymers.However, aerogels made of soy protein, whey protein, silica, and organosilane (PMSQ) have all been strengthened using nanocellulose. [101]hao et al. designed pectin-silica hybrid aerogels with neck-free nanoscale network with thicker struts, and achieved improved mechanical strength compared to the classical neck-limited inorganic aerogels. [102]In addition, the chitosan aerogels are nonbrittle family member of the majority of polymer and biopolymer aerogels. [54]olymer aerogels are a prospective, non-brittle substitute for silica aerogel, but their extremely poor high-temperature stability limits their extensive application.Although polyimide is well recognized for its outstanding heat tolerance, polyimide aerogels frequently experience significant volume shrinkage when exposed to temperatures above 200 °C. [103]To address the drawbacks of fragile GO aerogels, a small quantity of easily accessible, thermally cross-linkable poly(acrylic acid) (PAA) can be incorporated. [104]This method can significantly improve the mechanical properties while preserving all other desirable characteristics of GO aerogels, such as their high porosity, electronic conductivity, low density, and high surface area.Takeshita et al. synthesized translucent, non-brittle, and waterproof aerogels using trimethylsilylated chitosan nanofibers and investigated their hydrophobization mechanism. [105]The morphological diversity in polymer aerogel can be attributed to parameters, such as the molecular structure of the monomers (e.g., aromatic versus aliphatic, rigid versus flexible, difunctional versus polyfunctional, the functional group density at the monomer molecular level), the solubility properties of the medium (e.g., polarity, ability to develop dispersion forces, and hydrogen bonding), the concentrations of the monomers and the catalyst, and the gelation temperature.

Brief Introduction of Evaluation Methods for the Mechanical Properties of Mechanically Strengthened Aerogels
According to Gibson and Ashby, pore architecture has a significant impact on the mechanical response of porous materials. [106]dentifying the causes of fracture failure in aerogels can enable the development of more efficient structures and improved performance.Using only the pore geometry, solid phase stiffness, and zero-porosity strength, a study revealed the possibility of theoretically calculating solutions for the stiffness, tensile strength, and pore strength (damaging pore pressure, frost, and fire) of some ideal porous materials. [107]Pore geometry is a critical common denominator that influences both the stiffness and strength of aerogels, and the mechanical properties of aerogels are the most important properties for the widespread applications of aerogels, as discussed in the introduction section.Processes, such as percolation theory, cluster-cluster, or monomer-cluster growth, can affect the gelation mechanism [108] The power-law increase of physical parameters such as elastic modulus in experiments underpins the link between gel formation and percolation theory.Gibson and Ashby devised a structural model based on the bending of cubic cells to establish the mechanical properties of porous open cell framework to characterize the mechanical behavior of porous open cell networks.It was found that Young's modulus (E) of porous materials is directly proportional to the square of the bulk density,  b .

Nanoindentation
Nanoindentation is an excellent method for investigating the mechanical characteristics of small volumes of materials at submicron sizes.It is important to understand the characteristic elastic properties of porous materials to characterize their mechanical properties, and the microstructural characteristics of the specimens play a big role in these qualities.Nanoindentation is a well-known method for determining the mechanical properties of films as it requires a small amount of space (at submicron scales) and does not involve the removal of films from substrates. [109]Various modelling methodologies have been developed to correlate porosity with attributes of porous materials, with most of these methods treating porous solids as two-phase microstructures with pores as the second phase in terms of porosity.This method is useful for determining the mechanical properties of brittle or soft aerogels, aerogel films, and membranes. [109,110]

Compression/Tensile Modulus
The mechanical strength of aerogels is typically measured using various testing methods.Inorganic aerogel and 2D assemblies, as well as organic materials, such as polyimides (PI), polyurethanes (PU), and polysaccharides, exhibit unique characteristics and require specific techniques for mechanical property measurement.The mechanical properties of aerogel, similar to other conventional materials, may be evaluated using the compression/tensile testing technique.Nevertheless, the observed behavior varies depending on the specific material composition and structural characteristics.
Young's modulus, often denoted as "E" is a fundamental mechanical property that describes the resistance of a material against deformation.It indicates how much a material will be elongated or compressed under a given load and provides information about its ability to withstand deformation. [111]Additionally, it quantifies how a material responds to an applied force or stress, which results in deformation or strain.Young's modulus is defined as the ratio of stress to strain within the elastic deformation range of a material.In mathematical terms, it can be expressed as: where E is the Young's modulus (in pascals, Pa or N m −2 ), "stress" is the force applied to the material (in newtons, N) per unit area (in square meters, m 2 ), and "strain" is the proportional change in length (dimensionless) of the material in response to the applied stress.
To calculate Young's modulus experimentally, tensile test or compression test is required.Higher Young's modulus values indicate that greater force must be applied to cause deformations, and vice versa. [112]The mechanical properties of various types of aerogels can be determined using modulus and bulk density power law relation, indicating that the Young's modulus increases strongly with bulk density. [113]∝  b  (6)   where  b is the bulk density, and  is ≈3.2-3.8.
When plotting the relative compressive modulus, E/E s , of different produced microlattices against their relative density, / s , the modulus exhibits a scaling behavior proportional to (/ s ) 2 . [114]Aerogels with densities below 10 mg cm −3 have a steeper scaling of the ratio of elastic modulus (E) to the elastic modulus of the solid material (E s ) because of the inefficient load transfer between ligaments [115] Most of the loading curves of polymer demonstrate a linear elastic zone for minor strains (<10%), which is then followed by a yield behavior accompanied by a stress plateau. [116]Studies have evaluated the mechanical strength of 1D-or 2D-based aerogels using tensile test, atomic force microscopy, and peel test, whereas compressive Young's modulus test has been mostly employed for all other types of aerogels.The key difference when measuring mechanical strength in multidimensional materials, such as 2D materials, is the need for techniques that can handle the atomically thin nature of these materials and their unique mechanical responses.In contrast, aerogel compositions require methods tailored to their porous and low-density structures.Therefore, we can adopt any convenient method to determine Young's modulus of aerogels.

MD Simulation
Computational developments from the nanoscale to the continuum scale have facilitated the comprehension of the relationship between the structure and properties of aerogels.MD simulations with model sizes of up to a few million atoms have been used in research to investigate the structural, thermal, mechanical, and fracture properties of aerogels. [55,117]MD simulations, as compared to other approaches, provide a higher resolution of mechanical processes in space and time.As a result, over the last two decades, MD simulations have significantly helped to understand the behavior of composite materials and their atomiclevel properties.Currently, owing to advances in computer science and technology, MD simulations may be performed on systems with millions of atoms with simulation timeframes as short as 1 ms.In addition, through atomic modeling, the variation in the properties of aerogels based on the material, formation process, and dimension can be predicted.For example, the finite element method using continuum modelling can be used to predict the characteristics and functional aspects of an aerogel based on its aerogel-forming structure.43a] Copyright 2018, American Chemical Society.SEM and visible photo of silica-based composite aerogel: Adapted with permission. [12]Copyright 2022, Elsevier.121c] Copyright 2020, American Chemical Society.121a] Copyright 2020, American Chemical Society.121b] Copyright 2021, American Chemical Society.121d] Copyright 2022, Elsevier.

Development of Mechanically Strengthened Aerogels: Traditional to Multicompositional/Multidimensional and Multiscaling Approaches
As one of the lightest materials in the world, "aerogel" can be used in diverse fields as a thermal insulating material in harsh temperature environments.In addition to other physical properties, mechanical property is the most important factor that needs to be considered for cost-effective and long-term aerogel applications.Silica aerogel is a well-known material that exhibits brittle behavior, but its mechanical properties, as well as other physical properties, can be improved using various insitu and ex-situ approaches. [4,118]For the past decades, efforts have been devoted to enhance the mechanical strength of aerogels, such as reinforcing with mechanically strong natural or synthetic polymers, double cross-linking polymerized precursor, triple-cross-linking polymerized precursor, and compositing with nanofibers/nanotubes/nanosheets. [38,43,63,119]In the last few years, researchers developed a new type of aerogels with enhanced mechanical strength. [43,120]Figure 4 shows the types of aerogels and their corresponding range of modulus values, which can be useful to tailor particular aerogels for diverse applications.Currently, various types of aerogels have been synthesized including silica, such as carbon aerogels, non-silicate aerogels, polymer-based aerogels, and fiber-based aerogels.
To design advanced aerogels with enhanced mechanical properties, their multidimensional and multiscale strengthening should be considered.This process involves strengthening aerogel materials at multiple length scales and in multiple dimensions to improve their overall performance.The main goal of this study is to present additional properties required by aerogels reinforced using multidimensional or multiscaling approaches, such as toughness, ductility, and lightweight characteristics.To understand the concept, we divided this section into five subsections based on the parameters for designing mechanically enhanced aerogels with multidimensional/multiscaling networks.The section starts with the enhancement of the mechanical strengths of colloidal silica aerogels (0D), and non-silicate aerogels by tuning porous networks (multiscaling, multicompositional), reinforcing with polymer/carbon fibers (multidimensional, multicompositional, and multiscaling).Furthermore, non-silicate aerogels reinforced via compositization (0D, 1D, or 2D), reinforcement of fibers, fibril network, and pore network design are discussed.Synthetic polymer-based aerogels exhibit colloidal, as well as fibril-like networks.The mechanical properties of this aerogel can be enhanced by cross-linking, surface modification or coating, and reinforcing with 1D, and 2D nanomaterials.In addition, the fibril network natural/biopolymer aerogels prepared using compositization, surface modification, reinforcement of fibers or 2D materials, and the effect of the mechanical strength are comprehensively discussed.The carbon-based aerogel synthesized without and with reinforcing 1D, 2D carbon, or other inorganic nanomaterials is also discussed.Overall, aerogels reinforced in multicompositional/dimensional/scalable approaches can be synthesized by compositing, reinforcing dimensional networks, and pore structure tuning (hierarchical pores).Therefore, we divided this section based on the type of aerogels, to highlight the potential of all three categories in each type of aerogels, as well as the various strategies for mechanically enhanced aerogels.This section is briefly summarized in Table 2, which briefly explains the strategies for enhancing the mechanical strength of various aerogels and their relationship with the topic of the review.

Silica-Based Aerogels
Aerogels were first invented by Kistler in 1931 at Stanford University, USA, using a sodium silicate precursor. [2]Owing to the timeconsuming and tedious procedures, Teichner et al. re-discovered aerogels in 1968, and introduced the use of alkoxysilanes for the synthesis of aerogels.These are the most representative aerogels composed of 0D colloidal particles and have been prepared using different synthesis precursors, mainly sodium silicate and silicon alkoxides (tetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS), and methyltrimethoxysilane (MTMS)) and processes (APD, SCD, and FD). [16,65,122]As mentioned previously, silica aerogels are fragile in nature and cannot be used for longterm applications owing to their weak "necklace" like 0D particle network Figure 5a.
The physical as well as mechanical properties of aerogels mostly depend on the synthesis conditions, as aerogels synthesized under mildly acidic conditions are stiffer than those prepared under basic conditions. [125]The mechanical strength of aerogels can be enhanced by subjecting them to suitable aging, drying, and heat treatments (Figure 5b,c). [16,113,126]To maximize the modulus and strength of aerogels, a longer aging time (24 h in the case of Figure 5b,c) is preferable to a shorter aging time (2 h).Nevertheless, it is difficult to precisely determine the mechanical strength of pristine silica aerogels owing to: 1) the highly fragile nature of aerogels, 2) difficulty in preparing the specimen in the required form, and 3) the small load forces that need to be applied.The modulus of silica aerogels generally depends on density. [127]At higher densities, aerogel breaks after a very small strain, exhibiting glass-like behavior.However, at smaller densities (80 to 150 kg m -3 ), the silica aerogels can withstand up to 80% compression strain without breaking (Figure 5b-e).Additionally, the mechanical strength of aerogels is also affected by the type of aerogel precursors, as aerogels synthesized using TEOS or TMOS precursors are more fragile compared to those synthesized using MTMS or MTES precursors. [128]The thermal conductivity of various aerogels as a function of their mechanical strength is presented in Figure 5f, and the image indicates that silica aerogels with thermal insulation properties exhibit low mechanical strength, which hinders their long-term thermal insulating applications.
One of the most effective ways to improve the mechanical properties of silica aerogels is to increase the network's connectivity at the nanoscale, which is highly dependent on the preparation circumstances.In addition, the elastic properties of silica d) compressive stress vs compressive strain profiles for different types of aerogels, [123] e) variation in the elastic modulus of silica aerogels compared to those obtained using MD simulation and in available literature, [124] and f) comparison of the mechanical strength and thermal conductivity ranges of different aerogels. [16,102]16b] Copyright 2021, Elsevier.d) Adapted with permission. [123]Copyright 2013, American Chemical Society.124b] Copyright 2019, Elsevier.f) Adapted with permission. [102]Copyright 2015, Wiley-VCH.aerogels can be improved by employing organo-substituted precursors, as used in organically modified aerogels. [129]The mechanical strength of aerogels can be enhanced not only by the presence of alkyl or aryl surface groups but also by the Ostwald ripening mechanism for inter-particle neck growth. [129,130]he mechanical properties of silica aerogels can be further enhanced via functionalization using different organo-substituted alkoxysilanes and this also provides the required environment suitable for applications (for example, adsorption, catalysis, and sensing) owing to the altered chemical properties compared to native silica aerogels.The final properties or structural changes depend on the conditions of the sol-gel process (e.g., two-step sol-gel process) rather than the type of functional groups: during the two-step sol-gel process, hydrolysis, and polycondensation can be promoted under different catalytic conditions, resulting in a network formation with decreased cross-linking density.However, the hydrophobic network causes macroscopic phase separation and leads to an increase in the particle size, which strengthens the aerogel network.Given the difficulty in tailoring the desired porous morphology directly from organo-substituted alkoxysilanes, surface functionalization using alkoxysilanes bearing desired functional groups is often employed. [131]However, Loy et al. employed (RO) 3 Si(CH 2 ) n Si(OR) 3 (n = 2, 6, 8, 10, 14) type precursors without the addition of silicon alkoxides, [132] and confirmed that the length of the spacer "n" influences the final network structure of aerogels.
Iswar et al. revealed the construction of dense and strong silica aerogels by systematically evaluating the effect of aging times on the mechanical strength of various aerogel samples. [16]They observed that the density of silica aerogels increased with a decrease in the aging time, and this density increase is accompanied by an increase in the stiffness of the silica aerogels because of larger shrinkage.Additionally, they confirmed that highly dense aerogels exhibited a brittle behavior compared to the low-density silica aerogels, which exhibited a more flexible nature.It is difficult to directly compare the mechanical properties of aerogels in previous studies because of the variations in the synthesis and measurement protocol.However, many studies provided a piece of unique information regarding the effect of morphology and density of silica aerogels on the fracture properties; however, it is difficult to compare the mechanical strength to density as it depends on the type of aerogels and their composites.
Silica aerogel reinforcement with fibers provide a balance between mechanical properties and thermal insulation. [117]Therefore, many aerogel industries have employed a similar approach and fabricated aerogel blankets (Figure 6a).This is a versatile route for facilitating aerogel manufacturing, in which the fiber concentration and homogeneous dispersion with silica sol can support the enhanced mechanical and insulation properties.Recently, many researchers focused on the use of recycled or waste fibers to enhance the mechanical properties of silica aerogel composites. [133]Among the several developed techniques, the reinforcement of silica aerogels with fibers and covalent crosslinking with reactive molecules or polymers are considered the best methods for achieving the desired mechanical properties for silica aerogels.Using these techniques, silica aerogels with extraordinary mechanical strength can be achieved; however, there is a tradeoff with the density of the silica aerogels (Figure 6b,c).To address this tradeoff, He et al. pioneered an investigation into ceramic fiber-reinforced silica aerogels to achieve extra strength and thermal insulation. [134]These types of composites can be prepared by dispersing fibers in the sol prior to gelation, followed by Figure 6.a) Schematic presentation of the synthesis of fiber-reinforced silica aerogels, [138] b) stress-strain profile for mullite fiber-reinforced silica aerogels, [140] c) effect of fiber density on the mechanical properties of silica aerogels, [138] d) process for the synthesis of polymer reinforced silica aerogels and representative set of monomers used, [141] and e) effect of PVP wt% on the Young's modulus of bending of silica aerogels. [142]-c) Adapted with permission.[138] Copyright 2021, Elsevier. b) Adpted under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[140] Copyright 2021, The Authors, published by MDPI.d) Adapted with permission.[141] Copyright 2011, American Chemical Society.e) Adapted with permission. [142]Copyright 2008, American Chemical Society.
SCD or APD, which is a reasonable approach for preparing mechanically strengthened silica aerogel composites.Generally, the fiber reinforcement directly increases the bulk density compared to the native silica aerogels, but also results in the stiffness of the aerogel.However, only a moderate improvement in the mechan-ical properties a observed if silica-based materials are used as the reinforcing fibers.Shao et al. reported a twofold improvement in the compressive strength of composite silica aerogels. [135]They observed that silica aerogels showed a compressive strength of 1.2 MPa when reinforced with 2 wt% attapulgite, and this is Figure 7. a) Chemical cross-linking [145] and b) bending stress vs strain curves of ethylene-or ethenylene-bridged polysiloxane aerogels; [145] c) optical image of ethylene-or ethenylene-based polysiloxane aerogels, [145] d) polyethylsilsesquioxane-and polyvinylsilsesquioxane-based aerogels; [146] e) optical images [146] and f) compression behavior of the aerogels. [146]a-c) Adapted with permission. [145]Copyright 2017, American Chemical Society.d-f) Adapted with permission. [146]38b] and triple-cross-linked [43b] silica aerogels using polymerized alkoxysilane precursors, c) morphology of double-cross-linked [38b] and triple-cross-linked [43b] aerogels, d) compression and bending behavior of triple-cross-linked aerogels, [43a,e] ) schematic of molecular-scaled structure variations, and high-mechanical-strength PI-PVPMS aerogels (synthesis and mechanical properties). [181]38b] Copyright 2018, American Chemical Society.43b] Copyright 2020, American Chemical Society.43b] Copyright 2020, American Chemical Society.43a] Copyright 2018, American Chemical Society.e) Adapted with permission. [181]Copyright 2019, Elsevier.
12 times higher than the compressive strength that can be endured by silica aerogel composites reinforced with 10.5 wt% silica fibers. [136]Li et al. prepared inorganic fiber-reinforced aerogels, and observed their improved mechanical and insulation properties. [137]eng et al. confirmed the relatively high modulus of aluminasilica composite aerogels (ASA). [138]Furthermore, mullite fiberreinforced ASA (MFAS) with improved comprehensive strength, which was attributed to the synergistic effect between ASA and mullite fibers, has been developed.The stress-strain curve behavior of MFAS was divided into the linear stage (at 0-3% strain) and the nonlinear stage (at 3-20% strain), respectively.During the linear stage, the compressive stress increased almost linearly for low-strain values due to ASA compression behavior.A less steep increase in stress at 3-12% strain and high strain (>12%) was observed and the density of MFSA increased, thus enhancing its mechanical strength.The ceramic framework-reinforced aerogel composites have been potentially used in thermal insulation due to their high compressive strength and low thermal conductivity.Hong et al. prepared porous ZrO 2 ceramics impregnated with silica aerogels using a camphene-based freeze-casting method. [139]The porous ZrO 2 ceramics exhibited low compressive strength (9.2-25.5 MPa) compared to dense ZrO 2 ceramics.Although the combination of these ceramics with porous silica aerogels improves the compressive strength to 15.4-36.8MPa, there is a tradeoff between the comprehensive strength the porosity and density of the aerogels.
As it addresses the hydrophobicity and thermal stability of the silica backbone, polymer reinforcement of hybridized aerogels is one of the cutting-edge methods for increasing the mechanical strength of silica-based aerogels.Figure 6d shows the process for the synthesis of polymer-reinforced silica aerogels with enhanced mechanical strength using different monomers.A study reported polymer-reinforced silica aerogels with a Young's modulus value of 700 MPa, which is the highest value reported for the same type of composite materials.However, the fabrication of these aerogels requires the use of toxic reagents, such as acetonitrile and carcinogenic chemicals.In addition, these aerogels were mostly translucent, and no reports were available on their high-temperature thermal conductivities.Another approach for achieving improved mechanical properties using polymers is by introducing a polymer into the silica network through co-hydrolysis with surface-functionalized coreshell polymer particles with silica precursor.Fidalgo et al. created nanohybrid silica/polymer aerogels and examined the effect of polymer nanoparticle size and composition on aerogel characteristics. [143]They observed that the core-and shell-type combination contributed to an improvement in the mechanical properties of the composite aerogels.Wei et al. incorporated PVP polymer into native silica aerogels via APD to improve their mechanical strength (Figure 8e). [142]They determined the flexural modulus of bending, E, using Equation ( 7) where P is the maximum loading force,  is the deflection of bending, L (20 mm) is the support span, b (20.4 mm) is the width of the sample, and h (5.4 mm) is the height of the sample.It is typically difficult to determine the modulus of native silica aerogel owing to its very small value and fragile nature.In contrast, the PVP-silica composite aerogels exhibit a modulus of 24.3-39.3MPa at a PVP content of 0.5-1.0wt% (Figure 6e).Owing to this high modulus, the PVP-silica composite aerogel can be used in long-term, large-scale thermal insulation applications for energy-saving practices.One of the best methods to synthesize these hybrid aerogels is via surface modification by 3-aminopropyltriethoxysilane (APTES) using the coprecursor method followed by the conformal coating of polymers, such as polyurethane, polyurea, and polystyrene, as well as epoxies. [90,144]Studies have reported that the further cross-linking of amine-modified silica aerogels (bulk density of 190 kg m −3 ) with isocyanate (density of 478 kg m −3 ) enhanced the compressive strength of the aerogel from 4.1 MPa with a maximum strain of 5.7% to 186 MPa at 77% strain. [90]he mechanical flexibility of a material is one of the most desirable properties for many applications, and polymer-crosslinked aerogels are an effective solution for designing flexible silica aerogels.However, cross-linking negatively affects the density and light transmittance of the resultant aerogels.Such negative influences can be avoided by altering the silica backbone to organically modify the polysiloxane networks.Rao et al. first demonstrated the fabrication of flexible polymethylsilsesquioxane (PMSQ)-based aerogel using MTMS as the sole precursor and a two-step sol-gel process. [176]Generally, MTMS-based polysiloxane aerogels are more flexible owing to their lower crosslinking density (three alkoxy groups).Kanamori et al. fabricated more compressible, transparent, and robust MTMS-based flexible aerogels by incorporating urea and surfactant in the synthesis solution to control the pore size and phase separation. [177]ramer et al. modified TEOS-derived aerogels using flexible polydimethylsiloxanes (PDMS), and achieved 30% compression at a PDMS addition content of 20 wt%. [178]Several efforts have been devoted to enhance the mechanical properties of aerogels, such as aging, incorporating fibers, and reinforcing reagents.However, these materials have limited Young's moduli (1.6-23 MPa) and exhibit hydrophilicity.Kanamori et al. developed a new approach for synthesizing hydrophobic aerogels and xerogels using MTMS as the only precursor in the presence of a surfactant, followed by SCD or APD. [177]Further, organic-inorganic hybridization is one of the most advanced methods for decreasing the fragility of native silica aerogels; however, such hybrid aerogels are mostly opaque because of the macroscopic phase separation induced during gelation.Therefore, researchers have developed many promising strategies for designing transparent, as well as flexible, polysiloxane-based aerogels, from MTMS and other organo-substituted alkoxysilane precursors.Kanamori et al. reported the fabrication of transparent monolithic aerogels using trifunctional MTMS precursor via a one-step solgel process, [177] and observed that the aerogel demonstrated reversible deformation upon uniaxial compression.In addition, they prevented macroscopic phase separation and promoted gelation (to avoid macroscopic phase separation) using surfactant and urea.The stress-strain curves shown in Figure 7 confirm the high modulus (7.41 MPa) and compression (80%) of these mono-lithic aerogels, and their ability to return to more than 90% of their original size after compression.Furthermore, Shimizu et al. used ethyltrimethoxysilane (ETMS) and VTMS as a single precursor for the fabrication of transparent and flexible aerogels. [146]articularly, the polyvinylsiloxane (PVSQ, VTMS-based) aerogels treated with a radical initiator (azobisisobutyronitrile AIBN) show strong bridging networks with superior compressive stress compared to polymethylsiloxane (PMSQ, MTMS-derived) aerogels.These drastic enhancements in the mechanical properties are ascribed to the radical polymerization of vinyl groups, improving the molecular-level connectivity between Si atoms and influences the bulk properties of PVSQ aerogels.These results are similar to those for the molecular-level connectivity of Si atoms (Si-O-Si or Si-CH 2 -CH 2 -Si), which has advanced effects on the bulk moduli of hybrid glass systems. [179]enerally, the modulus of aerogel increases with increasing the silicon concentration owing to an increase in density.91b] However, an increase in the VTMS and BTMSH concentrations decreased the modulus values of the cross-linked aerogels because of less silyl bonding by VTMS (less stiff backbone) and very high hexylene loadings from BTMSH (introduces more flexible links).Isocyanate-cross-linked silica aerogels have a stronger power law dependency between density and modulus (n ≈ 3.99 in E ∝  n for base-catalyzed, cross-linked aerogels) than that of pristine silica aerogels.It was due to the conformal coating with the polymer, tunes the microstructure of aerogels more efficiently at a lower density by increasing the neck regions between particles.For pristine silica aerogels, modulus and density are reported as a n exponent of 3-3.7, depending on the synthesis conditions.The flexural strength of 0.25 mol% aminated silica aerogels modified by chemical vapor deposition (CVD) at 0.230 g cm −3 density is 650 kPa, which is three times greater than the strength of CVD-modified silica aerogels with the same density and 31 times greater than the strength of original silica aerogels before CVD treatment (0.075 g cm −3 ). [8]The heterogeneous crust, which produced a tempering effect, and the capillary filling of micropores during the CVD process may be responsible for the increase in mechanical strength.
To reflect the effect of molecular structure on the mechanical properties, Shimizu et al. developed ethylene-bridged polymethylsiloxane (Ethy-BPMS) and ethenylene-bridged polymethylsiloxane (Ethe-BPMS) aerogels (Figure 7). [145,180]The ethylene (─CH 2 ─CH 2 ─) and ethenylene (─CH═CH─) bridges in the transparent, hydrophobic aerogels endowed the Si-O-Si network with higher strength.The network of these aerogels is similar to that of PMSQ, except for the partial substitution of oxygen in PMSQ by ethylene or ethenylene bridges.These organic parts in the cross-linked network affect the macroscopic mechanical properties of these aerogels, as the bridged aerogels are more flexible and viscoelastic than PMSQ aerogels.To synthesize bridged aerogels, researchers have employed strong acid and a strong base with a liquid surfactant as the main component of the solvent system, and the fabricated transparent aerogel displayed more homogeneous and finer pores (pore size ≈ 20 nm) and skeletons (≈10 nm), suppressing light as per the Mie scattering phenomena.Irrespective of their microstructures, PMSQ, Ethy-BPMS, and Ethe-BPMS aerogels exhibit similar compression behavior.Figure 7a,b shows the schematics of the networks of PMSQ, Ethy-BPMS, and Ethe-BPMS aerogels and their bending mechanical properties.A clear difference can be observed in the mechanical properties of these three different aerogels.For example, ethenylene demonstrated higher resilience properties against compression compared to ethylene bridges owing to its higher rigidity.Figure 7c shows the visible images of Ethy-BPMS aerogels, confirming their highly transparent nature.In addition, the synthesis, optical photographs, and mechanical properties of PESQ and PVSQ aerogels synthesized using VTMS and ETMS precursors are presented in Figure 7d-f.It is confirmed that the concentration of precursor, type of precursor, and amount of solvent could affect the transparency as well as mechanical flexibility of bridged silica aerogels.
Furthermore, Zu et al. developed transparent, machinable, super compressible, and highly bendable polyvinyl-polysiloxanebased aerogels using a versatile double cross-linking method (Figure 8). [38]First, a single alkenylalkoxysilane took part in radical polymerization to obtain polyalkenylalkoxysilane, followed by the subsequent hydrolytic polycondensation to obtain doubly cross-linked aerogels.The schematics of the synthesis method of these aerogels are presented in Figure 8a.Further, the incorporation of CNTs in VMDMS-derived aerogels has been reported, and this is schematically shown in Figure 8b and the corresponding morphology is presented in Figure 8c.Direct hydrolysis and polycondensation of the trifunctional precursors, VTMS and ATMS (allyltrimethoxysilane) produce cyclic and cage-like closed species with macroscopic phase separation between the polar solvent and hydrophobic condensates.Therefore, the current double cross-linking approach includes the radical polymerization of various silane precursors and base-catalyzed accelerated gelation before macroscopic phase separation.The high compression and bending flexibility of polyvinylpolysilsesquioxane (PVPSQ), polyallylpolysilsesquioxane (PAPSQ), polyvinylpolymethylsiloxane (PVPMS), and polyallylpolymethylsiloxane (PAPMS) aerogels were ascribed to the combination of polysiloxanes and hydrocarbon chains constituents (Figure 8d,e). [43,120]The increased elasticity of PAPSQ aerogels compared to PVPSQ aerogels may be due to the positioning of the branched hydrocarbon portion in the PAPSQ network between two neighboring silicon atoms.Zu et al. developed more flexible polyorganosilane hybrid aerogels using a triple cross-linking approach, as presented in Figure 8f.This unique triple-network structure provides excellent mechanical properties (Young's modulus ≈ 2.19 MPa) with complete recovery after 80% compression.Furthermore, high-mechanicalstrength and fire-retardant PI-PVPMS composite aerogels have been fabricated by incorporating amine-modified PI into PVPMS backbone.The physical and mechanical properties of siloxanebased aerogels and their composites are summarized in Table 3.

Metal-Oxide-Based Aerogel
Silica and polysiloxane aerogels are generally synthesized using alkoxysilane precursors, such as TEOS and MTMS, as discussed in the previous section.In addition, many researchers have attempted to develop precursors for non-silicate-based aerogels; however, it is difficult to control the reaction kinetics of sol-gel reactions. [182]Itoh et al. [183] and Tillotson et al. [184] reported the use of epoxide-based sol-gel systems as precursors for synthesizing metal halide salts.A highly strained ring in the epoxide acts as the proton scavenger to accelerate the reaction kinetics by forming oxo-and hydroxo-complexes.The main benefits of the epoxide-initiated sol-gel chemistry are: 1) ready availability of metal salts compared to metal-alkoxide-based precursors, 2) possibility of synthesizing various types of metal-oxide-based aerogels, and 3) the combinations of mixed metal oxide aerogels.Table 4 provides the information regarding different strategies used in the synthesis of mechanically enhanced non-silicate aerogels.
Various strategies have been reported for the enhancement of the properties of metal oxide aerogels.Hamza et al. [207] demonstrated the enhancement of Young's modulus of metal oxide aerogels by controlling their morphology.Generally, aerogels synthesized using this approach consist of colloidal-type pearl-like secondary particles.However, they recognized that the morphology, as well as crystal structure, of alumina aerogels can be altered based on the types of metal salts used.For example, boehmite with a nanoleaflet-type 2D microstructure was formed when aluminum chloride was used as the precursor, whereas colloidal 0D-type microstructure was formed when aluminum nitride is used as the precursor (Figure 9).Although there is no significant enhancement in their morphologies, there were significant differences when both aerogels were annealed at 800 °C.After annealing, the Young's modulus of the nanoleaflet type aerogels increased from 5.70 to 80.20 MPa with a change in its density from 130 to 240 mg cm −3 , whereas there was no such enhancement in Young's modulus of the colloidal-type alumina aerogel.
In addition, Eychmüller et al. [113] demonstrated that the mechanical properties of metal oxide-based aerogels can be enhanced by varying the metal oxide composition.To demonstrate the effect of the composition of the properties, a previous study prepared aerogels in molded bodies using a facile epoxy method, after which the aerogels were annealed at 300 °C.Thereafter, the research team compared the single compound alumina, gallia, iron oxide, and zirconia aerogels.They observed that after annealing at 300 °C, the particle sintering tends to shrink the aerogels, with 45% and 42%, for iron oxide and zirconia, respectively, and these shrinkage values are more than the alumina (29%) or gallia (23%).The alumina aerogels possess 1 order higher Young's modulus of ≈2.9 MPa than the brittle Gallia (≈0.36 MPa), which is 1 order of magnitude lesser.On the other hand, iron oxide and zirconia have high Young's moduli of 8.1 and 10.7 MPa, respectively, but further fractures at strains less than 6%.The gels appear to create a co-network of sheet-like alumina and sponge-like zirconium (Zr) when doped alumina aerogels with up to 20% Zr (Figure 9a-d).Additionally, doping alumina aerogel with Zr reduced its specific surface area from 709 m 2 g −1 (for pure Al 2 O 3 ) to 509 m 2 g −1 (for 20 at% Zr-Al 2 O 3 sample).In contrast, when Al 2 O 3 aerogel was doped with Zr, Young's modulus increased from 2.9 to 10.82 MPa, whereas the density increased from 49.8 to 86.6 mg cm −3 .
Leventis et al. synthesized vanadia (VO x ) aerogel using two different precursors of alkoxide and metal salt (Figure 9e-g). [208] They created a robust lightweight polymer-cross-linked vanadium oxide aerogel (X-VO x ) by nanocasting a 4 nm thin layer of an isocyanate-derived polymer on the normal vanadia aerogel's entangled worm-like skeletal framework.Additionally, vanadia aerogels prepared using epoxide-assisted gelation of VOCl 3 and epichlorohydrin show better properties than the silica gels synthesized from alkyl orthosilicates. [209]nadia aerogels were cross-linked with isocyanate-derived polyurethane/polyurea, epoxides, and styrene on the surface of the aerogel structures.These cross-linking enhanced the mechanical properties by more than 3 times under ambient conditions, with the aerogel achieving a Young's modulus of 623.3 MPa.In addition, the modulus increased to 1083.4 MPa after exposure to extreme condition of −180 °C. ) to a constant value of ≈480 m 2 g −1 , [113] c) photos of semi-transparent molded bodies before and after the compression test, [113] d) a significantly increased slope of Young's modulus and compression strength by enhancing the alumina scaffold through zirconia addition, [113] e) a photograph of X-VO x , f) compression test under different temperature conditions, [208] and g) native vanadia (VO x ) aerogel (density 0.078 g cm −3 ) and a cross-linked vanadia (X-VO x ) aerogel (density 0.428 g cm −3 ). [208]a-d) Adapted with permission. [113]Copyright 2018, American Chemical Society.f,g) Adapted with permission. [208]Copyright 2008, Springer Nature.

Non-Metal-Oxide-Based Aerogel
Recently, Xu et al. reported boron nitride-based aerogel (BNAG) with double-negative indices. [210]They created hyperbolic architectured ceramic aerogels with nanolayered double-pane walls that have a negative Poisson's ratio (0.25) and a negative linear thermal expansion coefficient (1.8 × 10 −6 K −1 ).These aerogels exhibit robust mechanical as well as thermal stability with ultralow densities (≈0.1 mg cm −3 ), superelasticity of up to 95%, and near-zero strength loss after intense thermal shocks (275 °C s −1 ) or intense thermal stress at 1400 °C, as well as ultralow thermal conductivity in vacuum (≈2.4 mW m −1 K −1 ) and in air ) uniaxial compression of hBNAGs with repeatable strain up to 95% with inset showing the experimental snapshots of one compression cycles, [210] c) experimental snapshots of the cross-sectional views and the corresponding scanning electron microscopy (SEM) images of the negative Poisson's ratio behavior of the hBNAGs under uniaxial compression, [210] d) compression stress-strain curve of the SiC-SiO 2 core-shell nanowire aerogel in the axial direction showing four deformation regions, [211] e) comparison of the specific modulus of the AH-SSCSNWA with those of other aerogels with random structure, [211] f) SEM image and optical photograph of ZrO 2 -Al 2 O 3 nanofibrous aerogel membranes showing excellent flexibility, [121] and g) compressive stress vs strain curves during loading-unloading cycles and experimental snapshots of a cycle. [121]a-c) Adapted with permission. [210]Copyright 2019, The Authors, published by The American Association for the Advancement of Science.d,e) Adapted with permission. [211]Copyright 2020, American Chemical Society.121c] Copyright 2020, American Chemical Society.

Synthetic Polymer-Based Aerogel
Sixty years after organic aerogels were firstly composited with their inorganic counterparts, Pekala et al. developed a bottom-up synthesis approach for RF aerogels. [212]Consequently, numerous polymer-based aerogels have been reported, and can be synthesized by varying the phenolic resin chemistry (phenol-furfural (PF), [34] cresol-formaldehyde, [32] melamine-formaldehyde, [31] ) based on the polymer type, such as polyurethane (PU), [35] polyurea, [91] polybenzoxazine, [35] poly(vinyl alcohol), [213] and PI's, [16] and based on soluble polymers (polystyrene (PS), [214] polyacrylonitrile, [215] and PEDOT:PSS).Most of the polymeric aerogels contain polymer chains cross-linked by covalent bonds, such as RF, [216] PF, [33] and PI aerogels. [217]RF aerogels are synthesized by addition-polycondensation, which can be performed under acidic and basic pH conditions.The molecular structure of the most promising monomers used during the synthesis of polymer aerogels (specifically polyimide aerogels) are presented in Figure 11.The properties of polymer aerogels are largely similar to those of silica-based aerogels, and their enhanced mechanical strength enables their use in a wide range of applications, particularly in aerospace applications.The morphology of polymer aerogel is determined by the dynamic nature of polymer chains during the entire preparation process.The representative solid phases in synthetic polymerbased aerogels mostly include PI, polyamide, PU, and PS (Figure 12a,b).
Mechanically strong materials with strong thermal insulating properties in ultra-low temperature regions (aerospace and polar regions) are in demand.Therefore, it is essential to control the microscopic structure of stiff porous materials to achieve superior elasticity at harsh environmental conditions.This can be achieved using polymer aerogels if they can maintain mechanical flexibility at low temperatures.Wang et al. designed hierarchical and honeycomb-like MF aerogels using chitosan and MF resin, and confirmed the great mechanical resilience with excellent thermal insulation of the aerogel. [219]They observed the increased rigidity of the aerogels along the radial direction with a maximum Young's modulus of 18.51 MPa, which can be mainly attributed to the combination of chemical cross-linking and directional microstructure.The rigid MF resin, which contains a large number of rigid triazine rings, may cross-link with highstrength (24.1 MPa) alginate to form stiff and robust 3D networks with high mechanical strength. [220]The honeycomb structure of MF can be useful in harsh environmental conditions between −20 and 100 °C. [219]Park et al. prepared RF aerogels with different catalyst concentration, which provides a high maximum compressive elastic modulus of 1.3 MPa, and the morphology and mechanical properties for this aerogel are provided in Figure 12d,e. [221]For space applications, the RF/silica aerogels achieved thermal conductivities of 0.0173 and 0.0196 W m −1 K −1 at 8 and 12 mbar vacuum, respectively, and a low temperature. [222]mong synthetic organic polymer aerogels, polyimide (PI) Figure 11.Monomers are used for the synthesis of polymer aerogels. [218]rogels are the most investigated owing to the combination of durability and thermal stability (Figure 12f,g). [78,218,223]he typical freeze-casting process is illustrated in Figure 13a.Both chemical cross-linking and physically templated processes can be used to construct 3D porous aerogel network.The optical image and SEM images of freeze dried polystyrene aerogels are shown in Figure 13b.For example, PI is a polymer with an imide ring (-CO-N-CO-) structure in its main chain.Aromatic and aliphatic PIs are separated into two classes based on various structural units of their molecular chains.Because of the large number of benzene and imide ring structures, aromatic PI exhibits improved thermal and mechanical properties than aliphatic PI.Various efforts have been devoted to improve the intrinsic properties of PI aerogels in the last decade. [36]Generally, PI aerogels can be synthesized by a reaction between poly(amic acid) (PAA) synthesized from dianhydrides and diamines through a thermal imidization process. [245]However, if there are only physical interactions between polymer chains, their resultant mechanical properties would not be stable and would limit their long-term industrial applications.A previous study found that the introduction of a suitable crosslinker and its percentage play an important role in maintaining the intrinsic properties of aerogels, while providing a high mechanical strength. [245,246]A research group affiliated with NASA Aerogel Research Laboratory first developed mechanically strong, robust, and durable polymer aerogels. [16,223,247]Meador et al. successfully developed and validated processes for synthesizing PI aerogels. [223]218d] a-c) Adapted with permission. [244]Copyright 2017, Wiley-VCH.d,e) Adapted with permission. [221]Copyright 2017, Elsevier.218d] Copyright 2012, American Chemical Society.
different types of cross-linkers, such as 1,3,5-triaminophenoxy benzene (TAB), octa(aminophenyl)silsesquioxane (OAPS), and 1,3,5-benzenetricarbonyl trichloride (BTC).They observed that TAB can be used to improve the mechanical strength of aerogels with a high Young's modulus of 30 MPa; however, this results in a density of as high as 0.33 g cm −3 .Synthesizing PI aerogels with an appropriate crosslinker and uniform distribution throughout the polymer matrix is a challenging task.Lin et al. reported 4,4'-oxidianiline (ODA) cross-linked PI aerogels with a maximum modulus value of 161.2 MPa.A previous study investigated PI aerogels using in situ SEM observation combined with stress-strain curves (Figure 13c).The study revealed that PI aerogels are composed of two linearly elastic regions: in the first region, elastic deformation was observed as the spaces between layers were gradually compressed as the layer structure was subjected to compression load.Hence, the stress increases gradually while the strain is higher at this stage.Furthermore, the density of the structure increased as the strain reached 41%.The calculated elastic modulus was 1.09 MPa, which is 15-fold higher than that at the first stage.
Cashman et al. [121] examined the substitution of 2,2'dimethylbenzidine (DMBZ) with shorter branched neopentyl spacer (1,3-bis(4-aminophenoxy)-2,2-dimethylpropane (BAPN).They observed that the performance of PI aerogels depends on the type of spacers used in the reaction.The 25 mol% BAPN resulted in the formation of an oligomeric network, where BAPN was isolated along the chain (Figure 13c).However, 75 mol% BAPN resulted in higher shrinkage, which is related with improved flexibility of the aliphatic linkages, and more collapsed networks were observed (Figure 13c).Additionally, the number of cross-linking points is important for controlling the properties.The maximum compression modulus reported was 0.139 GPa, and the curves of the aerogels prepared using 25 and 50 mol% BAPN were similar, whereas that of the aerogel prepared using 75 mol% BAPN reached higher stress when tested with the same strain level.
The comparison between Young's modulus versus density of PI aerogels prepared using different cross-linkers is presented in Figure 13d.The graph (Figure 13) indicates that the Young's modulus of the N3300A cross-linked aerogels was slightly higher than other aerogels with same density, which was attributed to the contribution of the hydrogen bonding derived from the urea linkages.The aromatic diamine and acid anhydride constituents raise the stiffness of PI aerogels owing to their inherent molecular rigidity and high intermolecular forces.Antenna applications require flexible PI aerogels.Therefore, Pantoja et al. improved the flexibility of PI aerogels by incorporating aliphatic spacers into the PI backbone. [250]They designed tough and flexible PI aerogels using different-length aliphatic spacers consisting of 3 and 12 methylenes with aromatic diamine.The PI aerogels exhibited a uniform fibrous network morphology.The amount of polymer and spacer can affect the surface area of the prepared aerogels.The stress-strain curves of the prepared PI aerogels followed the compressive behavior of cellular solids with three different regions (Figure 13e).In the first region, the initial rise was observed, which was attributed to the elastic bending of the cell walls, after which a plateau was observed in the second region, confirming the wall buckling and yielding, and this was followed by a sharp rise in the final region owing to the densification of the interface where cell walls meet.Typically, the modulus of all aerogels takes a path with the density when one could compare the same backbone chemistry shown in Figure 13e.The maximum compression modulus of the flexible PI aerogels was reported to be 51.4MPa.Wang et al. developed poly(vinyl alcohol) (PVA)based aerogels combined with PAA using the FD method.The combined PVA-PAA aerogel achieved a maximum compressive strength of 0.94 MPa (Figure 13f).
It is necessary to simultaneously achieve elasticity and hydrophobicity in some polymer aerogels; hence, new strategies via specific molecular design have been developed.For example, the ring-opening polymerization of benzoxazine not only tunes the surface chemistry of materials but also constructs a 3D network, and this has inspired researchers to develop polybenzoxazine aerogels. [229,251]Most polybenzoxazine-based aerogels are brittle in nature; thus, cross-linking with another monomer should be considered to achieve improved mechanical strength.Ma et al. [248] developed a dual-cross-linking approach to design poly(siloxane-benzoxazine) via the FD process.Many chemical cross-linkers used in PI aerogel synthesis are expensive, and the complex synthesis approaches hinder the large-scale production of mechanically strong aerogels.Therefore, Zhu et al. designed lightweight and porous glass fibers-reinforced PI aerogels using 4,4-oxydianiline (ODA) combined with rigid p-phenylene diamine. [234]It is well known that glass fibers are widely used ceramic fibers in the matrix of aerogel composites owing to their low thermal expansion coefficient and high mechanical resilience. [8]The compatibility of these fibers with a porous aerogel network is highly beneficial for these composites.The glass fiber-filled PI aerogels confirm the good impregnation of fiber/aerogel composites. [234]Fiber felts or mats act as skeletal support, reduce shrinkage, and enhance the mechanical strength of aerogels.As discussed previously, the mechanical strength depends on the type of cross-linkers used during synthesis. [58,252]218d] Furthermore, the impregnation of glass fibers enhances the Young's modulus of PI aerogel to as high as 113.5 MPa (at 0.177 g cm −3 ) compared to the density.
In recent years, researchers have focused on the synthesis of polymer composite aerogels by combining them with 1D fibers, 2D sheets, or 3D foams, which endow them with various intriguing properties and enable their wide-range applications. [25,253]iu et al. [253] constructed MXene/PI (2D/3D composite) aerogels by interconnecting and bridging MXene sheets with PI macromolecules, and confirmed the outstanding compressibility of the aerogels.The mechanical properties of these composites strongly depend on the intersheet interactions between MXene sheets and PI aerogels, which transfer the mechanical load between adjacent sheets.They observed that the MXene/PI aerogel can rebound 42.23 g steel block (1055.8times heavier than aerogel).These types of composites have broad advantages in shock-absorbing applications.Zhang et al. developed CNTs cross-linked PI aerogels using the "grafting-from" method. [226]Surface functionalization is a facile way to improve the interfacial interactions between nanosheets/tubes/fibers and polymer matrix, thus promoting the formation of well-dispersed polymer nanocomposites.The homogeneous dispersion of CNTs and tailoring CNTpolymer interface bonding are two important aspects of shifting the nanoscale mechanical properties to the macroscopic dimensions.The cross-linking of CNTs with PI polymer result in significant enhancement in the strength and modulus without losing their physical properties.The as-prepared PI aerogels possess a Young's modulus of 2.5 MPa, which increased to 46.5 MPa after the addition of 2 wt% CNT.These results were attributed to the strong cross-linking between CNT and PI aerogels (molecular type cross-linking), which resulted in the transfer of stress of aerogel network to the surface of the modified CNTs.The physical and mechanical properties of synthetic polymer-based aerogels and their synthesis methods are summarized in Table 5.The use of nanostructured conductive polymers is limited by their low solubility, reduced mechanical integrity, and production complexity. [254]

Natural and Biopolymer-Based Aerogels
Polysaccharide aerogels (e.g., cellulose, chitosan, alginate, starch, pectin, and agar) are bio-aerogels made from natural, semisynthetic, and synthetic sources that have fascinating biomedical applications. [4,7,10,255]Protein-based aerogels (such as gelatin, whey protein, soy, egg white, and silk fibroin) are an example of these aerogels, which have promising life science and biomedical applications. [256]Despite the lack of early commercial potential, biopolymer aerogels are among the first synthesized aerogels and have never totally disappeared from the research community's radar.Aerogels consisting of polysaccharides account for 87% of fabricated biopolymer aerogels, among which 45% accounts for cellulose alone, 12% for alginate, 10% for chitosan, 9% for pectin, and 7% for starch.Protein and nucleic acid aerogels account for 8% of the samples investigated, with composite aerogels accounting for 19% of the total, including 7% silicabiopolymer composites.In traditional polymer aerogel systems, oil-derived precursors are mostly used, and these are to be replaced by more sustainable precursors in biopolymer aerogels, motivating researchers to focus on natural or biopolymer-based aerogels.Furthermore, biopolymers frequently have a high concentration of surface functional groups, enabling new uses and handy anchoring places for extra functionality. [54,257]The utilization of bio-derived materials generated from renewable resources, such as proteins, polysaccharides, or plant oils, minimizes the overall carbon footprint of the products and their environmental impact.Using bio-macromolecules as templates for in situ polymerization and blending techniques, several attempts have been made to facilitate their synthesis, and examples of these templates include cellulose, alginate, gellan gum, and DNA. [254,258]Compared to carbon nanotubes, nano-fibrillated cellulose (NFC) has attracted special attention because of its extraordinary physical features, such as a high elastic modulus of 100-200 GPa. [259]The degree of cross-linking between the polymeric chains of the polysaccharide can generally govern the 3D network of polysaccharide-based aerogels.Furthermore, compared to physically cross-linked gels, chemical cross-linking enables the improved control of the porous structure of chemically cross-linked gels, offsetting the costs for crosslinker and precursors, as well as the time-consuming gel purification operations for eliminating unreacted starting components.The effect of different cross-linking networks on the morphology and mechanical properties is comprehensively described in Figure 14.The schematic representation of different crosslinking agents used in cellulose aerogels is shown in Figure 14a.It is well known that the network structures play an important role in altering the mechanical properties of cellulose aerogels.The effect of different cross-linking conditions on the morphology and mechanical properties of cellulose aerogels is presented in Figure 14b,c.Each cross-linking method provides different advantages to cellulose-based aerogels.The combination of low and high molecular weight cross-linkers can improve the crystallinity and mechanical properties of extremely porous cellulose aerogels.Hence, regenerated cellulose aerogels with superior mechanical properties can be designed using a dual crosslinking method. [287]Cellulose aerogel nanopaper is formed from native cellulose nanofibers and is produced using the same approach as traditional wood fiber paper. [288]Improved mechanical capabilities, optical clarity, reduced thermal expansion, and  [287a] b,c) Effect of different crosslinked networks on the morphology and mechanical properties of aerogels. [287,295]287a] Copyright 2020, Wiley-VCH.For the subpanels here: SEM image of chemical crosslinking, and SEM image and stress-strain profile for chemical dual crosslinking: Reproduced with permission. [295]Copyright 2018, American Chemical Society.287b] Copyright 2018, American Chemical Society.287c] Copyright 2018, American Chemical Society.287d] Copyright 2016, Wiley-VCH.287e] Copyright 2017, American Chemical Society.
oxygen barrier features of cellulose nanopaper structures have resulted in a diverse set of applications.Despite its 28% porosity, cellulose nanopaper has a unique combination of Young's modulus (13.2 GPa), tensile strength (214 MPa), and strain-tofailure (10%; Figure 14c). [289]The potential applications of cellulose nanopaper can be expanded by widening the spectrum of possible porosities, as well as increasing and tuning the specific surface area of the nanopaper structures.The porous feature of NFC aerogel is shown in Figure 15a, and thefigure confirmed the entanglement of the nanofibers with each other, and the tendency to form a highly porous network.The tensile modulus and strength of an NFC nanopaper with 86% porosity are 150 and 7.4 MPa, respectively, and this increases to 470 and 20 MPa, respectively, in NFC nanopapers regenerated in liquid CO 2 with a porosity of 74% (Figure 15b). [290]Additionally, researchers have reported that the modulus of NFC nanopapers with 40% porosity prepared via ethanol and acetone evaporation is 7-9 GPa. [290]The mechanical properties of high-porosity TEMPO-oxidized NFC nanopaper are intriguing (Figure 15c).The modulus, tensile strength, and strain-to-failure of TEMPOoxidized NFC nanopaper with a 56% porosity are 1.4 GPa, 84 MPa, and 17%, respectively.These qualities are similar to those of common commodity thermoplastics.However, the density is significantly lower, at 640 kg m −3 .These advantageous properties can be attributed to the nanofiber network topology and random-in-the-plane NFC orientation distribution.The repulsive force results in the formation of "scarce" network, generating large pores.However, a lamellar structure is the result of (partial) crystallization.Pores can be generated in aerogel materials using FD.The submergence of stable NFC gel into liquid nitrogen leads to the formation of the solid phase, and the size of the ice crystal generated is smaller, resulting in a significantly more uniform 3D structure.The morphology of cellulose/boron nitride nanosheets (BNNS) composite aerogel, in which BNNS is stacked with the cellulose nanofibers, is presented in Figure 15d.
in ethanol, then subjected it to SCD and FD before preparing cellulose aerogels and cryogels from cellulose solution in mixed solvents of 1-ethyl-3-methylimidazolium acetate ionic liquid ([Emim][OAc]) and 8% NaOH in water. [293]Several synthesis parameters could influence the shape, porosity, and density of the resultant aerogels.When compared to cellulose aerogels made at the same concentration, the cellulose cryogels' compressive strength (>4 MPa) and fracture strain (>70%, respectively) were higher.Liu et al. explored the effect of an electrolyte on the characteristics of regenerated cellulose hydrogels [294] They utilized different coagulating baths to regenerate cellulose dissolved in LiOH/urea solution: CH 3 COOLi, HCOOLi, LiCl, and LiNO 3 , CH 3 COONa, CH 3 COONH 4 , and CH 3 COOK.In the coagulating bath, CH 3 COO-and Li + with large B-coefficients in the Hofmeister series resulted in a more uniform network topology, resulting in resilient cellulose hydrogels.The cellulose hydrogels that were rejuvenated in CH 3 COOLi aqueous solution had a tensile strength of 2.34 MPa.In order to produce strong functional cellulose hydrogels, they also used an organic base (benzyltrimethyl ammonium hydroxide) to dissolve cellulose and water evaporation to induce tight packing.With an increase in the evaporation duration from 0-200 min, the tensile strength, elongation at break, and toughness of the hydrogels rose increased 6.2 MPa, 73.8%, and 1.87 MJ m −3 to 11.7 MPa, 87.1%, and 4.31 MJ m −3 , respectively.Epichlorohydrin and poly(ethylene glycol) diglycidyl ether, respectively, are low-and high-molecularweight cross-linkers that were used to create chemically dualcross-linked cellulose hydrogels. [295]Both the relatively short-and long-chain cross-linking were shown to retain network elasticity and successfully absorb mechanical energy, boosting their ten-sile strength and compressive strength from 65 and 112 kPa to 1.7 and 9.4 MPa, respectively.
The microscopic morphology and porous structure investigations presented thus far demonstrate the dependency of systematic changes in aerogels on the filler content.However, composite aerogels inherit the physical integrity and flexibility of cellulose aerogel in relation to the macroscopic mechanical behavior.In addition, the difference in their stress-strain curves is minimal, indicating that the cellulose network provides mechanical stability.This implies the very small contribution of the brittle silica component to the mechanical strength of composite aerogels. [101]The compression modulus of composite aerogels (7.9 MPa) is higher than two orders of magnitude greater than silica aerogel and 50 times more than that of bacterial cellulose aerogel.It was due to a difference in the mode of deformation and a possible anisotropy in the cellulose gel structure.By employing water glass as a silica source, Chen et al. presented a simple method for the synthesis of silica in the pores of structured cellulose aerogels. [298]This approach enabled the incorporation of different-sized nanoparticles into cellulose scaffolds, with the produced silica preserving both the chemical cellulose integrity and the open-porous cellulose aerogel network structure.Compared to the previously discussed efforts, this strategy is relatively inexpensive, and it would offer a fresh perspective on the design and manufacture of cellulose aerogels with enhanced mechanical characteristics for practical use.The role of silica in strengthening the impact was quite significant: the Young's modulus of the cellulose aerogel increased by approximately two times from 49 to 84 MPa after the addition of 4.16 wt% silica, and it increased with increasing silica content in the cellulose matrix.In addition, the aerogel exhibited a linear Figure 16.a) Variation in the morphology of cellulose aerogels with increasing compression strain, [15] b) stress-strain curves of CNF-polymer composite aerogels, [300] c) comparative chart of the compressive modulus of different aerogels, [300] and d) relative density vs relative modulus of different aerogels compared to NFC aerogels. [15]15b] Copyright 2017, Wiley-VCH.b,c) Adapted with permission. [300]Copyright 2013, American Chemical Society.elastic stress-strain behavior at low stresses (5%), and its deformation was projected to be predominantly caused by elastic cell wall bending.The elastic cell wall can endure the pressure owing to its high silica moduli.
Ge et al. developed graphene oxide nanosheet-reinforced borate-cross-linked carboxymethyl cellulose (GO/CMC) aerogels with controlled morphologies and improved environmentally acceptable mechanical qualities. [297]Figure 15e,f shows the morphology and compressive strength of GO/CMC composite aerogels with varying GO content, and it was observed that the inclusion of GO significantly enhanced the compressive strength and Young's modulus of composite aerogels.At 80% strain, the compressive stress of the composite aerogel containing 5% GO was 349 kPa, which was 62% greater than the compressive stress of the pure CMC aerogel.In comparison to previously reported cellulose aerogel reinforced with nanocrystalline cellulose (489.1, 18.25 kPa) [273] and nanocellulose/GO/sepiolite composite foam (570 kPa), the Young's modulus of the composite aerogel with 5 wt% GO was 1029 kPa. [299]The addition of boric acid induced the cross-linking of carboxymethyl cellulose and GO, thus improving the connectivity between both components and strengthening the skeleton structure of the composite aerogel, and the high Young's modulus of the composite aerogel was attributed to the enhancement effect of GO.
The change in the morphology of cellulose aerogels under compressions indicates that it tended towards elastic buckling mechanism (Figure 16a).Studies have reported the synthesis of biopolymer composite aerogels by combining silica aerogels with various biopolymers to overcome the brittleness of silica aerogels.Shanyu et al. [102] synthesized robust, superinsulator aerogels using a silylated NFC scaffold and MTMS-derived network.The NFC scaffold enhanced the mechanical characteristics of silica aerogels, resulting in the interpenetration of hybrid aerogel networks with low density (0.12-0.14 g cm −3 ), high porosity (93%), and high specific surface area (450 m 2 g −1 ).Additionally, they created strong aerogels based on silica-pectin hybrid matrices, where the pectin was added by a controlled parameter one-pot synthesis approach to the silica network.To improve the mechanical properties of the pore walls, CNFs were cross-linked using 3-glycidoxypropyltrimethoxysilane (GPTMS) and branched polyethyleneimine (b-PEI).To achieve this, first, GPTMS was hydrolyzed to produce silanol groups, which are then covalently cross-linked with the hydroxyl groups of cellulose.Simultaneously, the reactive epoxide groups in GPTMS react with the amine groups in b-PEI to enhance the cross-linking density of the network.These reactions endow the pore walls of the porous materials with the desired mechanical characteristics, allowing them to withstand the stress imbalance caused by capillary pressure during solvent evaporation in the drying process.The compression stress-strain curves of the aerogels prepared using APD and FD procedures, were used to assess their mechanical properties, and the aerogels were compressed to 50% of their original thickness.Although the compression stress of the atmospheric-dried and freeze-dried cellulose aerogels was similar, the compression modulus of the freezedried cellulose aerogels was higher.This was attributed to the aggregation of CNFs in the freeze-dried aerogel, which resulted in the sheet-like topology of the pore walls, resulting in the relatively high compression resistance of the aerogel.The compressive behaviors of the cross-linked and non-cross-linked CNF, PVA, and graphene oxide nanosheets (GONS) aerogels are presented in Figure 16b.The image confirmed the influence of different parameters on the mechanical properties of aerogels.Figure 16c confirms the effect of different cross-linkers on the specific compressive strength of CNF-, PVA-, and GONS-based aerogels.Particularly, cross-linking was confirmed to improve the mechanical properties of these aerogels.Deuber et al. developed open cell-like porous aerogels interconnected with particles, and compared the aerogel to other reported aerogels using the powerlaw relationship for density and modulus (Figure 16d). [15]In addition, it has been proposed that the pore structure of aerogels (e.g., closed cell (slope = 1), open cell (slope = 2) and honeycomb structure (slope = 3)) can be determined using the slope of density vs. modulus.The physical and mechanical properties of the synthesis methods of natural or biopolymer-based aerogels are summarized in Table 6.

Carbon-Based Aerogels
Carbon-based aerogels (CAs) are widely used in different applications, such as electrolytic capacitors, [301] electrosorption, [302] and wearable piezoresistive sensors [303] owing to their functional characteristics (high SSA, good electrical conductivity, chemical inertness, thermal stability, and low thermal conductivity). [304]s are commonly designed using a carbonization method after the polymerization reaction of organic monomers, followed by solvent exchange, and drying.Alternatively, monoliths are prepared using carbon-based materials, such as graphene and CNTs. [305]Different pore characteristics are recognized by employing different phenol-type precursors, such as resorcinol, melamine, cresol, polymeric isocyanate, and phenol.The type of solvent, catalyst, molar ratio of the precursor, conditions of the solvent exchange, temperature, and the drying method also affect the properties of CAs. [306]In the case of RF aerogel, which is the main candidate material for CAs, the gelation of the sol at a low temperature limits the number of effective nucleation sites because certain catalysts do not have sufficient energy to initiate the polymerization reaction.Studies have observed that the proportion of effective nucleation sites at a low gelation temperature is small (Figure 17a).At a medium gelation temperature, only a limited percentage (2%) of effective nucleation sites are present.Typically, CAs can be classified based on the gelation temperature of the RF aerogel, as shown in Figure 17a-d, and the numbers from 1 to 5 were expressed in 15 °C intervals from 30 to 90 °C. Figure 17b shows the TEM image of the neck morphologies of CAs-C1.The connectivity between particles was improved with a uniform network with an increase in the gelation temperature and, changing the network from a pearl-necklace to a semi-fibril-like structure.When carbonaceous agglomerates are subjected to a compressive force, only their spines are impacted without disturbance in the carbon particle network.It is necessary to reinforce the neck of carbon particles to achieve robust CAs without a noticeable increase in density.This can be accomplished by increasing the gelation temperature within a suitable range.The mechanical strength improvement of the CAs can be controlled using different processes as discussed.
Previous studies have been reported that CAs-C1 prepared at the lowest gelation temperature exhibits a big particle size and pore diameter (Figure 17c).Figure 17d shows the compression stress-strain curves of various CAs.CAs-C1 sample exhibited the lowermost compressive strength and modulus among all the CAs, which can be ascribed to its low bulk density and decreased structural uniformity.In addition, despite their similar bulk density, the compressive strength and modulus of the CAs increased significantly with an increase in the gelation temperature from 45 to 90 °C.Although the densities are nearly identical, the compressive strength of CAs-C3 (21.4 MPa) was approximately two times higher than that of CAs-C2 (10.1 MPa) (11.5 MPa), which can be attributed to their structural distinctions.There were also changes in their pore properties, as the particle size and pore size decreased with a decrease in the gelation temperature and concentrations of formaldehyde, water, and catalyst. [307]Particularly, after carbonization, the mean size distribution of micro-and mesopores, density, and the specific surface area changed as a function of the molar ratio of the components in the synthesized polymer. [308]The properties of CAs can also be affected by the activation method (physical (CO 2 or air with high temperature) or chemical (KOH, NaOH, HCl, H 2 SO 4 , and K 2 CO 3 ) method). [306]he physical and mechanical properties for different synthesis methods of carbon-based aerogels are summarized in Table 7.
The porous structures of RF-based carbon aerogels can be oriented under freeze-casting conditions to improve their mechanical strength of 126 MPa. [323]In addition, high mechanical strength (210.5 MPa), good oxidation resistance, and good thermal stability can be achieved by utilizing various types of linkers and polymers that can be copolymerized with phenolic resins. [304]Compared to general aerogels, some polymer-based CAs can be controlled to achieve a high mechanical strength owing to the thick necking between particles or particle agglomeration through a spinodal decomposition mechanism by optimizing the ratio of solvent, catalyst, and polymer precursor. [306,318,324]n important key approach for the fabrication of continuous pore skeleton networks is the reduction of the surface tension using surfactants or alcoholic solvents. [116,325]CA with a skeleton network can achieve high mechanical strength (1.5−2 GPa) because it compensates for the weak necking structure that causes the low mechanical strength of the aerogel. [306]CA with compressible or flexible properties has received tremendous attention in various research fields.To achieve desired properties, studies have attempted the preparation of CA using flexibletype polymer foam [326] or grafting elastic fiber or sheet-type materials. [327]ecently, renewable, eco-friendly, and economical biomass material-based aerogels came in the spotlight for the preparation of CAs via carbonization. [301,328]In addition, studies have been conducted on elasticizing hollow carbon by constructing macropores of controllable size inside the carbon framework by incorporating polystyrene spheres into the RF gels as a sacrificial template before carbonization.This enhanced the mechanical strength by more than two times without exerting any effect on the density. [189,329]The mechanical strength of aerogels can be improved by subjecting the aerogel to more than one million cycles at 30% strain, while also achieving super-thermal-insulation properties (23 mW m −1 K −1 ), as its cycles cause the CA to imitate various kinds of biostructures, such as stems, human hair, or polar bear hair, and the aerogel is manufactured in the form of nanotubes. [330]Another way to further improve the mechanical strength of CAs is to coat the surface of the aerogel with organic material, after which it is subjected to carbonization to achieve a more than twofold increase in the mechanical strength compared to that of existing RF aerogels. [331]The overall effect of different synthesis conditions on each kind of aerogel is systematically schematized in Figure 18.

Hierarchically Porous Aerogels
The chemical composition, porosity, and cell morphologies of cellular materials are largely responsible for their functional qualities.The materials can be returned to their original dimensions after distortion and storage in a temporary shape according to the shape-memory (SM) effect. [332]Temporary-topermanent shapeshift can be inducted by external stimulation, such as heat, light, humidity, pH, or electric and mag-netic forces. [333]SM materials can be used in various fields, including medical technology, implantable and wearable devices, robotics, and unaided deployable structures used in aerospace platforms. [233,334]n many of these applications, the conventional tradeoff is between mass density and mechanical strength. [333]ightweight, high-toughness structural components are necessary for a variety of applications, from delicate devices of interest to reconstructive orthopedic surgery and implantable cardiovascular devices to enormous structures of significance in the aviation and aerospace industries.An earlier work examined commercial aliphatic isocyanurate-based triisocyanate (Desmodur N3300A) and triethylene glycol assisted sol-gelderived poly(isocyanurate-urethane) (PIR-PUR) aerogels with shape-memory properties. [335]The PIR-PUR sol was cast in suitable molds created by the 3D printing of the negative metastructure using a computer-aided design (CAD) model. [333]After gelation and demolding, wet gels were dried to create monolithic meta-aerogels that had the required cross-sectional area, shape, and periodicity.A thermomechanical compressive stress was used to create such meta-aerogels, which began at a temperature above the glass transition point of PIR-PUR aerogel.The PIR-PUR meta-aerogels shrank during compression, indicating their negative Poisson's ratio, and under pressure, meta-aerogels were chilled to a temperature significantly below their glass transition.
Metal-organic frameworks (MOFs)-, and covalent-organic frameworks (COFs)-based aerogels are increasing interest owing to their good mechanical properties (rigid structure of MOFs) and high SSAs (microporous structure of MOFs) compared to conventional aerogels. [336]Researchers have focused on synthesizing a composite of MOFs with other materials to achieve highly functional MOF-based aerogel composites, namely, MOF@silica aerogel, MOF@graphene aerogel, MOF@cellulose aerogel, and MOF@organic polymer composites. [337]The classic sol-gel process involves the polymerization of the monomers into a strongly cross-linked gel network.Particularly, the incorporation of MOFs-based nanoparticles during the synthesis process of aerogels can enhance the dispersion of MOFs-based nanoparticles and strengthen the mechanical properties of the aerogels, resulting in a hierarchical porous structure that enables the widespread application of aerogels in energy storage and con-version, building and industrial insulation, and phase change materials.

Aerogel Fibers
Aerogel fibers are one of the well-transformed types for achieving enhanced commercialization, but their fragility has restricted their further application.In addition, the drying step can be avoided during the fabrication of fibers as it has an in-situ drying approach during its processing.There are four major techniques used to synthesize aerogel fibers, including wet reaction spinning, extraction/injection, freeze-spinning, and wet coaxial spinning. [5]The wet reaction spinning method is broadly used for the synthesis of various fibers based on silica, cellulose, Kevlar, and graphene-based materials. [338]Alternatively, many researchers have employed the extraction/injection approach for the synthesis of cellulose, aramid, and silk fibroin aerogel fibers.The combination of spinning technology and an ice-templating technique, known as freeze spinning, presents a new path for the production of aerogel fibers. [5]Various aerogels have been synthesized by combining an ice-templating approach with FD.In addition, the coaxial fibers can be synthesized using the wet coaxial spinning approach, which is similar to wet reaction spinning.However, there are certain complexities associated with the use of this process, as it requires the use of solutions for the core and shell.Zhang et al. developed various aerogel fibers using spinning with sol-gel transition method. [338,339]

Additive Manufactured or 3D-Printed Aerogels
In 2015, the first study combining printing technology with aerogel to produce 3D-printed aerogel scaffolds was published. [340]ince then, printed aerogels have attracted increasing attention, and they currently represent an interdisciplinary study topic that combines aerogel and additive manufacturing technology. [15]his quick expansion can be explained by either of two simple factors.Firstly, the process for making aerogels is always a liquidto-solid sol-gel one that is easily adaptable to the ink compositions used in liquid jetting or slurry extrusion printing methods.Second, the low density and strength of aerogels makes it challenging to shape-machine them, which restricts the options for normal post-machining.The "bottom-up" printing technologies provide an alternative method for fabricating sophisticated shapes without the need for time-consuming post-processing.Furthermore, current formulation and printing methods can be customized.Because of the increasing demands of contemporary society, the global market for plastic foam materials is substantial (annual sales of 341.3 billion USD) and expanding at a rate of 4.8% yearly.The bulk of current foam materials is made of plastics, which take hundreds of years to decompose, resulting in severe worldwide pollution problems. [341]he size restrictions imposed by supercritical dryers prevent the fabrication of complex-shaped, large-scale aerogel structures for thermal insulation.One possible solution to these fabrication problems is to create modular, thermally insulating bricks that are simple to make regardless of length scales (for example, from a few millimeters to a few tens of centimeters).This would involve combining 3D printing technology with one of the aerogel synthesis mechanisms. [342]The load-bearing element in the modular brick is made up of 3D-printed solid polymer structures, whereas the aerogel components provide thermal insulation.Some research articles on the 3D printing of complexshaped aerogels, particularly on the use of the direct ink writing (DIW) approach, have been published. [343]In these conditions, a high volume percent of solid precursor components or the use of a low temperature were both used to achieve the right viscosity of the liquid ink.Zhao et al. [343] employed DIW for the 3Dprinting of 4-20 μm silica aerogel particles in pentanol with at least 40% particle loading.Cheng et al. [344] reported the 3D printing of Kevlar aerogel articles on cold plates using DIW to promote solvent freezing.They observed that not all aerogel systems can be 3D-printed into complex shapes using the DIW approach because of the inability to always identify viscosity modifiers, or the inability to compromise the porosity and bulk density (e.g., by utilizing a high solid fraction).This is particularly true for thermal insulation applications, as increasing the solid content of aerogel structures reduces the total porosity, which increases the thermal conductivity.
Mechanical metamaterials have auxetic physical properties such as negative Poisson's ratios, and the most prominent expression of these properties is their preference for shrinkage rather than expansion in the transverse direction when subjected to uniaxial compression. [333]Auxetic material designs have been used in biomedical devices like artificial arteries, whose wall thickness increases rather than decreases under the tension produced during pulse-driven blood flow, enhancing the structural integrity of the arteries and lengthening its lifespan. [345]Materials with a negative Poisson's ratio are typically more impact-and indentation-resistant, which makes them desirable for body armor applications such as bulletproof helmets and vests. [346]The load-bearing element in the modular brick is made up of 3Dprinted solid polymer structures, whereas the aerogel components provide thermal insulation. [342]Multifunctionalities can be generated for a wide range of applications using the 3D-printed multiscale porous structure.The design of the structure, material, and process is always guided by the structure-propertyperformance principle.Researchers are devoting efforts to merging the features of porous structures and additive manufacturing technology to take advantage of additive manufacturing technologies for the 3D printing of porous materials. [347]Despite the significant growth and advancement in recent decades, there are two fundamental hurdles in this scientific field.i) The most significant difficulty is the inability of additive manufacturing methods to fabricate submicron and nanoscale porosity features, but only a multiscale network.ii) The majority of building blocks cannot be directly assembled using additive manufacturing techniques.Some intrinsically porous materials, such ceramic aerogels, cannot be handled or assembled with current AM processes.
To date, porous structures have been studied using a variety of additive manufacturing techniques, and their further development is becoming an increasingly important pillar in the manufacturing industry. [347,348]However, scaling up production and achieving the commercialization of this cutting-edge technology remain a hurdle.A scalable AM technique with low cost and high efficiency must be created in the future for mid-volume or mass production.By enhancing the ink composition and tying the ink rheology, the porosity and pore hierarchy can be managed. [349]n recent years, sustainable manufacturing has been confirmed to be beneficial to the current economy, and also to the environment and future civilization.In the 3D-printed porous aerogel network, the creation of multiscale porous structures inspired by nature has a bright future.In reality, despite the fact that 3D printed porous networks have been demonstrated to significantly outperform their equivalents in various applications (e.g., energy storage, [350] catalysts, [351] and water treatments, [352] ) the ink is still not completely optimized for 3D printing.According to research, an ideal ink design and rheology behavior may help to further improve the performance of a 3D-printed multiscale porous network. [353]Last but not least, close cooperation between numerous disciplines, including manufacturing, material science, chemistry, physics, biology, industrial, and civil engineering, would be required for the development of additive manufacturing for porous structures.The remarkable advancement in the subject is highlighted in this section, and we think that in the future the significance of the effects of porous architectures on the environment, economy, and society will only increase.Figure 19 provides an overview of recent trends in multiscale aerogels with morphology for a different type of features.-g) SEM images for: b) cellulose, [355] c) silica, [356] d) carbon, [357] e) polyurea, [358] f) polyimide, [359] and g) MOF [336] aerogels.h-j) Photographs for 3D printed aerogels of: h) graphene, [360] i) silica, [361] and j) carbon [362] aerogels.k-p) SEM imagess for 3D-printed: k) CNF, [363] l) silica, [361] m) carbon, [362] n) graphene, [360] o) polyimide, [364] and p) MXene [365] aerogels. q-s)Photographs for: q) polyimide, [366] r) Kevlar, [367] and s) SF-GO [368] aerogel fibers.t-y) SEM images for: t) cellulose, [369] u) silica, [339] v) SF-GO, [368] w) graphene, [370] x) polyimide, [366] and y) Kevlar [367] aerogels.a) Adaped under the terms of CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[354] Copyright 2020, The Authors, published by MDPI. b) Adpted with permission.[355] Copyright 2020, American Chemical Society. c) Adpted with permission. [356] Copyrght 2019, American Chemical Society.d) Adapted with permission. [357]Copyright 2021, American Chemical Society. e) Adpted with permission. [358]Copyright 2017, Elsevier.f) Adapted with permission.[359] Copyright 2020, American Chemical Society. g) Ad Copyright 2022, Elsevier. h,n) apted with permission.[360] Copyright 2016, Wiley-VCH. i,l) dapted with permission.[361] Copyright 2021, Elsevier. j,m) dapted with permission.[362] Copyright 2022, American Chemical Society. k) Aapted with permission.[363] Copyright 2018, American Chemical Society. o) Adpted with permission.[364] Copyright 2023, Elsevier. p) Adpted with permission.[365] Copyright 2021, Wiley-VCH. q,x)Adapted with permission.[366] Copyright 2023, American Chemical Society.r,y) Adapted with permission.[367] Copyright 2023, American Chemical Society. s,v) dapted with permission. [368] Copyrght 2020, American Chemical Society. t) Adpted with permission. [369] Copyrght 2015, Elsevier.u) Adapted with permission.[339] Copyright 2020, American Chemical Society.w) Adapted with permission. [370]Copyright 2012, American Chemical Society.

Future of Aerogels: From Laboratory to Commercialization
In the last couple of years, significant breakthroughs have been achieved in the production of mechanically strengthened and multifunctional aerogels and their composites owing to advanced synthesis technologies.This recent methodology has enabled great improvement in the properties of aerogels, considerably expanding their scope for various applications.Aerogel research became more exciting and fascinating in the past 5 years.For example, the application range of aerogels has been extended beyond thermal insulation to the energy and technology fields.Researchers have developed various pioneer methods to design high-mechanical strength aerogels using 0D, 1D, and 2D nanomaterials, polymerized silica precursors, and polymers.Additive manufacturing technology is progressing rapidly, and 3D-printed aerogels have emerged in the past 5 years.Therefore, despite the aforementioned technological hurdles in the future, we are optimistic that new technology adoption will result in the development of a new generation of inexpensive, adaptable aerogels that have the potential to add new functionality and broaden current application areas, particularly in the fields of energy, sensing, and biomedicine.
Nevertheless, the mechanical brittleness of aerogels limits their field of applications.It has already been approximately 90 years since the first development of aerogel, and many researchers have devoted tremendous efforts to enhancing the mechanical properties of aerogels, which is yet to be completely achieved.Therefore, based on existing research, we are proposing two different perspectives (compositional and dimensional control) for enhancing the mechanical strength of aerogels.Generally, silica aerogel has been confirmed to exhibit outstanding properties.However, their functionalities vary with their compositions.For example, semiconducting aerogel could be used as a highly efficient catalyst material with a high specific surface area.Furthermore, the dimensional variation could endow it with enhanced mechanical properties.Typical aerogels consist of a 0D-based colloidal necking structure.However, if higherdimensional materials of 1D and 2D are used rather than the colloidal structure, the mechanical stability of aerogels, which is derived from the intrinsic properties of the low-dimensional materials used, can be significantly enhanced.Aerogels with a network structure of, 1D nanowires, and nanopores based on 2D nanosheets are synthesized through the process development of various materials, such as inorganic, organic, carbon, metal, and their hybrids.
Increased attention should be devoted to the control of particle interaction, roles of surfactants in interfacial control, cluster growth control, and growth direction control through the use of a capping agent for the synthesis of low-dimensional nanomaterials.Through these considerations, a pore structure material with nano dimensions of 0D, 1D, and 2D be developed.In the second stage, the introduction of condensation-inducing materials, the control of surface polarity, and facile bonding between molecules using click chemistry should be considered to fabricate aerogels with multidimensional and multicompositional pore structure, as well as to develop a process for large area/mass production.In the last stage, various applications, such as filters, insulation materials, artificial bones, catalysts, and aerospace, should be explored using aerogels with extreme mechanical strength that can be fabricated by overcoming the limitations of the necking structure and introducing self-organizing processes.We propose the most suitable strategies and future methods to synthesize mechanically enhanced aerogels, and this is schematically illustrated in

Figure 1 .
Figure 1.Number of published papers on aerogels (data source: from Web of Science) and mechanical properties of aerogels vs density.

Figure 4 .
Figure 4. Range of modulus of different types of aerogels with their corresponding network structure.SEM and visible photo of silica (organically modified silica) aerogel: Adapted with permission.[43a]Copyright 2018, American Chemical Society.SEM and visible photo of silica-based composite aerogel: Adapted with permission.[12]Copyright 2022, Elsevier.SEM and visible photo of non-silicate aerogel: Adapted with permission.[121c]Copyright 2020, American Chemical Society.SEM and visible photo for synthetic polymer aerogel: Adapted with permission.[121a]Copyright 2020, American Chemical Society.SEM and visible photo of natural/biopolymer aerogel: Adapted with permission.[121b]Copyright 2021, American Chemical Society.SEM image of carbon aerogel: Adapted with permission.[121d]Copyright 2022, Elsevier.

Figure 5 .
Figure 5. a) Pearl necklace microstructure and necking growth of silica aerogels, b,c) stress vs strain curves of varying-density silica aerogels, [16b]d) compressive stress vs compressive strain profiles for different types of aerogels,[123] e) variation in the elastic modulus of silica aerogels compared to those obtained using MD simulation and in available literature,[124] and f) comparison of the mechanical strength and thermal conductivity ranges of different aerogels.[16,102]a-c) Adapted with permission.[16b]Copyright 2021, Elsevier.d) Adapted with permission.[123]Copyright 2013, American Chemical Society.e) Adapted with permission.[124b]Copyright 2019, Elsevier.f) Adapted with permission.[102]Copyright 2015, Wiley-VCH.

Figure 9 .
Figure9.a) All-alumina aerogels exhibit sheet-like morphology, b) after annealing, the specific surface area decreased from the original value (750 m 2 g −1 ) to a constant value of ≈480 m 2 g −1 ,[113] c) photos of semi-transparent molded bodies before and after the compression test,[113] d) a significantly increased slope of Young's modulus and compression strength by enhancing the alumina scaffold through zirconia addition,[113] e) a photograph of X-VO x , f) compression test under different temperature conditions,[208] and g) native vanadia (VO x ) aerogel (density 0.078 g cm −3 ) and a cross-linked vanadia (X-VO x ) aerogel (density 0.428 g cm −3 ).[208] a-d) Adapted with permission.[113]Copyright 2018, American Chemical Society.f,g) Adapted with permission.[208]Copyright 2008, Springer Nature.

Figure 10 .
Figure10.a) Illustration of the meta-structure design of boron nitride (BN) aerogels, b) uniaxial compression of hBNAGs with repeatable strain up to 95% with inset showing the experimental snapshots of one compression cycles,[210] c) experimental snapshots of the cross-sectional views and the corresponding scanning electron microscopy (SEM) images of the negative Poisson's ratio behavior of the hBNAGs under uniaxial compression,[210] d) compression stress-strain curve of the SiC-SiO 2 core-shell nanowire aerogel in the axial direction showing four deformation regions,[211] e) comparison of the specific modulus of the AH-SSCSNWA with those of other aerogels with random structure,[211] f) SEM image and optical photograph of ZrO 2 -Al 2 O 3 nanofibrous aerogel membranes showing excellent flexibility,[121]  and g) compressive stress vs strain curves during loading-unloading cycles and experimental snapshots of a cycle.[121]a-c) Adapted with permission.[210]Copyright 2019, The Authors, published by The American Association for the Advancement of Science.d,e) Adapted with permission.[211]Copyright 2020, American Chemical Society.f,g) Adapted with permission.[121c]Copyright 2020, American Chemical Society.

Figure 14 .
Figure 14.a) Schematic representation of the chemical and physical cross-linking strategies of cellulose-based aerogels.[287a]b,c) Effect of different crosslinked networks on the morphology and mechanical properties of aerogels.[287,295]Figure adapted with permission.[287a]Copyright 2020, Wiley-VCH.For the subpanels here: SEM image of chemical crosslinking, and SEM image and stress-strain profile for chemical dual crosslinking: Reproduced with permission.[295]Copyright 2018, American Chemical Society.Stress-strain graph for chemical crosslinking: Reproduced with permission.[287b]Copyright 2018, American Chemical Society.SEM image for physical crosslinking: Reproduced with permission.[287c]Copyright 2018, American Chemical Society.Stress-strain profile of physical crosslinking: Reproduced with permission.[287d]Copyright 2016, Wiley-VCH.SEM image and stress-strain profile for chemically and physically double crosslinking: Reproduced with permission.[287e]Copyright 2017, American Chemical Society.

Figure 18 .
Figure 18.Schematic representation of different aerogels synthesis strategies for enhancing the mechanical properties of aerogels and their possible applications.

Figure 20 .
Figure 20.Summary of most suitable synthesis strategies for different kinds of aerogels and new strategies to improve the mechanical properties of aerogels.

Figure 20 .
This image presents multicompositional and multidimensional strategies, resulting from flexible synthesis techniques for 0D-1D, 0D-2D, 1D-2D, and 0D-1D-2D combined aerogels.Hyung-Ho Park obtained his B.S. (1981) in metallurgical engineering from Hanyang University, his M.S. (1984) from KAIST, South Korea, and his Ph.D. (1988) from Bordeaux I University of France.He was a postdoctoral researcher at National Scientific Research Center (CNRS) in Bordeaux I University of France for 2 years.By 1995, he had been a senior researcher at Electronic Telecommunication Research Institute (ETRI) in Korea.After that, he obtained a position as professor in Material Science and Engineering at Yonsei University in 1995.Presently he is serving as a Director of Aerogel Materials Research Centre as well as Yonsei University-Industry Collaboration Centre.His research activities are concerned with design of material characterization, various oxide nanoporous structures based on solution process, aerogels, microstructure control, surface properties and heat-resistant mesoporous materials.

Table 2 .
Overview of various aerogels and their relationship with multicompositional, multidimensional, and multiscalable networks.

Table 4 .
Physical and mechanical properties of non-silicate based aerogels.

Table 5 .
Physical and mechanical properties of synthetic polymer-based aerogels.

Table 6 .
Physical and mechanical properties of natural/biopolymer-based aerogels.

Table 7 .
Physical and mechanical properties of carbon-based aerogels.