Introducing Dynamic Bonds in Light‐based 3D Printing

Light‐based 3D printing has received significant attention due to several advantages including high printing speed and resolution. Along with the development of new technologies, material design is key for the next generation of light‐based 3D printing. Conventional printable polymeric materials, also known as photopolymers or photoresins, often lead to thermosets–polymer networks cross‐linked by permanent covalent bonds which bring limited adaptability and restricted reprocessability. Dynamic bonds that can reversibly break and reform enable network rearrangement, thereby offering unprecedented properties to the materials such as adaptability, self‐healing, and recycling capabilities. Hence, introducing dynamic bonds into materials for light‐based 3D printing is a promising strategy to further expand and meet the diverse application scenarios of 3D printed multi‐functional materials and moreover meet more demanding sustainable and nature‐inspired design considerations (e.g., adaptability and self‐healing). Herein, an overview of recent advances in dynamic photopolymers for light‐based 3D printing, aiming to bridge these two promising research fields is presented. Importantly, the current challenges are also analyzed and perspectives for further developing dynamic photopolymers for light‐based 3D printing and their potential applications are provided.


Introduction
9][10][11][12] The latter includes lightbased 3D printing technologies such as stereolithography (SLA), digital light processing (DLP), computed axial lithography (CAL), and twophoton laser printing (TLP).[22][23] The formulations usually con tain bifunctional or multifunctional monomers, which undergo irreversible photocrosslinking during the 3D printing process.[26][27] Although this permanent crosslinking offers superior mechanical, thermal and chemical stability, it limits multi functionality, such as adaptability, healability and recyclability, which restricts the diverse and more demanding application scenarios of 3D printed objects.Also, it causes a great concern to the environment due to their nonprocessable, singleuse nature.Therefore, the development of new multifunctional and reprocessable photopolymers for lightbased 3D printing mate rials is needed to meet the progressively expanding applications and sustainability implications.
36][37][38][39] Given the versatility and potential of dynamic bonds, the development of novel dynamic bonds and the exploration thereof in materials science is a promising research topic.42][43][44][45][46] However, the emergence of dynamic polymers as the mate rial candidate for 3D printing has only recently gained atten tion, especially for lightbased 3D printing with the advantages of high printing resolution and printing speed.It is therefore timely to analyze the potential of introducing dynamic bonds in lightbased 3D printing to fabricate new multifunctional mate rials that exhibit unprecedented properties and expand their applications (Figure 1).
Herein, we present a comprehensive review including recent advances in dynamic photopolymers for lightbased 3D printing, with a focus on material design, aiming to guide researchers in the design and fabrication of novel 3D structures based on dynamic polymers.This review first introduces some of the most widely adopted lightbased 3D printing technolo gies, followed by a brief discussion of conventionally employed dynamic covalent and noncovalent bonds in polymeric mate rials.Afterward, recently developed dynamic photopolymers to prepare 3D printed materials with covalent and noncovalent bond exchange reactions are highlighted.To conclude, the chal lenges and future perspectives of lightbased 3D printing based on dynamic polymers are discussed.

Light-based 3D Printing Technologies
In general, lightbased 3D printing techniques utilize light to initiate the chemical crosslinking of liquid photoresins consisting of (macro)monomers, initiators and light absorbers (e.g., dyes, quenchers).[49] Based on different light sources, sev eral types of lightbased 3D printing technologies have been developed.
Stereolithography (SLA), first patented by Hull et al. in 1986, [50] is known to be one of the first original 3D printing technologies and utilizes a laser beam or UV light as the light source to induce the photopolymerization of the photoresins.As shown in Figure 2A, an SLA 3D printing system typically consists of a laser as light source, a liquid resin bath and a build platform.During the SLA printing process, the liquid photo resin undergoes a rapid liquidtosolid transformation within seconds of light exposure and patterned solid sheets with the designed thickness (typically 5-200 micrometers) are built point by point until the entire 3D object is completed.Two key parameters for successful SLA printing are the speed of photo curing and the viscosity of the resin. [48,51,52]Fast photocuring speed ensures that each layer of the object is rapidly cured from liquid to solid (on the order of seconds), while the viscosity of resins should be minimized to allow the resins to flow quickly across the build platform and form new layers (typically ten to hundreds of mPa•s).
Digital light processing (DLP) 3D printing uses patterned light to initiate the crosslinking of photoresins.Different from the pointbypoint curing in SLA, a light image is pro jected onto the liquid photoresin, and an entire layer can be cured at once during the DLP printing (Figure 2B).As a result, DLP 3D printing shows much faster printing speeds than SLA printing. [7,21,49,51]  a variation of DLP called continuous liquid interface printing (CLIP), which uses oxygen inhibition to create a reaction "dead layer" at the bottom of the resin bath. [15]This dead layer allows for a continuous printing approach increasing vertical print speeds by two orders of magnitude (1000 mm h −1 in the zdirection) while simultaneously removing material defects intrinsic to the layerbylayer printing.In another study, Walker and coworkers introduced a mobile liquid interface (a fluori nated oil) at the bottom of the resin bath to reduce the adhesive forces between the interface and the printed objects, allowing for a continuous rapid printing process. [13]Unlike CLIP, the size of the printing area was not restricted by thermal limita tions because the flowing oil enabled direct cooling across the entire print area.The group demonstrated continuous vertical print rates exceeding 430 millimeters per hour with a volu metric throughput of 100 liters per hour, as well as the ability to print 3D structures made from hard plastics, ceramic precur sors and elastomers.Similarly, Wu and coworkers proposed a slippery surface for DLP printing to reduce vertical solidsolid interfacial adhesion and increase the refilling speed of liquid photoresin and further improve the printing speed. [53][56][57] Unlike the previously described printing technol ogies that build objects in a layerbylayer manner, volumetric 3D printing creates objects by curing materials in 3D space, allowing for the creation of complex, highly detailed shapes.
Pioneering work in the field was done by Kelly and coworkers, who developed a new printing method, called computed axial lithography (CAL) (Figure 2C), which allowed for the fabrica tion of arbitrary geometries volumetrically through photopoly merization. [54]They demonstrated the concurrent printing of all points within a 3D object by illuminating a rotating volume of photoresin with a dynamically evolving light pattern.Acrylate based polymers and soft structures with exceptionally smooth surfaces were engineered into a gelatin methacrylate hydrogel and then printed with features as small as 0.3 millimeters.Regehly and coworkers presented a volumetric 3D printing approach by combining a novel dualcolor photoinitiator with a new projection light system for dualcolor photopolymerization, which achieved rapid and highresolution printing. [55]Com pared to stateoftheart volumetric printing methods, this tech nique has a resolution about ten times higher than computed axial lithography without feedback optimization, and a volume generation rate of four to five orders of magnitude higher than twophoton photopolymerization.
When moving to smaller scales, twophoton laser printing (TLP), also termed as direct laser writing (DLW), is the tech nique of choice.60][61][62][63] The laser is tightly focused inside a droplet of photosensitive resins, allowing for the initiation of twophoton polymeriza tion in a very small volume referred to as a voxel.Due to the Adapted with permission. [54]Copyright 2019, Science.D) Two-photon laser printing (TLP).
nonlinearity of the process, this technology allows for sub micron resolution.Recently, many efforts are being undertaken in making this technology faster and more scalable, for example through parallelization approaches.Saha and coworkers over came the pointbypoint writing scheme of TLP by spatially and temporally focusing an ultrafast laser to implement a projectionbased layerbylayer parallelization. [14]This increased the throughput up to three orders of magnitude and expanded the geometric design space.Wegener and coworkers also pre sented multifocus TLP, [64] allowing for a voxel printing rate by more than a factor of 30.In ≈48 h (i.e., total time including all settling times, etc.), more than one hundred thousand 3D unit cells were achieved, composed of about three hundred billion voxels, which was more than one hundred times faster than commercial printing technology using only one focus.More recently, we contributed to a new concept relying on the use of twostep absorption replacing twophoton absorption as the pri mary optical excitation process, [65] which significantly increased printing speed while keeping high resolution.In a followup work, lightsheet 3D laser microprinting that combined image projection with an optical nonlinearity based on twocolor two step absorption was reported. [66]In this case, continuouswave laser diodes at 440 nm wavelength for projection and a contin uouswave laser at 660 nm for the light sheet were utilized and a peak printing rate of 7 × 10 6 voxels s −1 at a voxel volume of 0.55 µm 3 was achieved.

Dynamic Bonds in Polymeric Materials
[69][70][71] These dynamic bonds can be covalent or noncovalent, depending on the under lying bond exchange mechanism (Figure 3).By introducing dynamic bonds into a polymeric architecture, the obtained polymeric materials are capable of rearranging their polymeric networks via reversible bond exchange processes under a stim ulus, thus tuning their macroscopic shapes and mechanical properties.In the last decades, polymers containing dynamic bonds have emerged as smart materials to achieve functional features such as shape memory, selfhealing and recycling.3]46,76] These dynamic covalent polymers behave as classical thermosets, showing better resistance to solvents or environments than thermoplastics.However, under an appro priate stimulus (heat, catalysts, light, and pH), the dynamic covalent bonds can be activated to repeatedly rearrange the polymer network topology through reversible bond exchanges.This bond exchange process typically proceeds through either an associative or dissociative mechanism, although more com plex pathways have also been introduced. [42,43,76]For the associa tive mechanism, the bond breakage and new bond formation occur simultaneously in a single step, maintaining a constant net degree of crosslinking throughout the exchange process (Figure 3A(i)).[79][80] Many of the resulting dynamic covalent networks crosslinked by associative dynamic covalent bonds are also known as vitrimers, which was introduced by Leibler and coworkers [37] and inspired by the remarkable temperature dependence of the viscosity with an Arrhenius relationship similar to vitreous silica and other glass formers.Unlike the additionelimination bond exchange in associative processes, dynamic covalent polymers based on dissociative mechanisms first undergo a bond breakage step yielding (transient) reactive groups in the network which can subsequently recombine or reform a new covalent bond con nection with a complementary reactive group (Figure 3A(ii)).Representative dissociative dynamic covalent bond exchanges include DielsAlder cycloadditions, thiol/aza Michael addition, boronic ester cycloaddition and silanol bonds. [35,38,40,42,43]Dif ferent from the constant crosslinking density obtained during the associative bond exchange, the crosslinking density of dis sociative covalent polymer networks is largely reduced when the dynamic covalent bonds are activated.As a result, dynamic polymer networks based on associative bonds exhibit higher thermal stability, improved solvent resistance and better resist ance to material deformation than dissociative counterparts.It is important to note that the actual exchange mechanisms of many dynamic covalent bonds, such as imine bonds and disulfides, may be a combination of both or even more com plex reaction pathways and are highly dependent on substrate structures, reaction conditions and matrix effects. [44]It should be noted that despite the realm of dynamic chemistry platforms and significant progress in the fabrication of dynamic covalent polymer networks (including covalent adaptable networks and vitrimers), some limitations still exist when designing repro cessable materials.Indeed, a reduction in material properties is often observed following reprocessing, while the number of reprocessing cycles is finite. [76]Moreover, ensuring high repro cessability typically coincides with a lower dimensional stability under usage conditions (e.g., material creep). [46]For example, triggering the associative transesterification for reprocessing purposes often relies on the introduction of catalysts at suffi ciently high concentrations.However, the presence of catalysts can accelerate aging phenomena which may lead to compro mised longterm material stability. [37,82]Similarly, the repro cessing of materials based on dissociative thiolene adducts can be affected by molecular weight, the functionality of thiol moiety, and the vinyltothiol ratio. [83]Therefore, further investi gations into new mechanisms that allow to compensate for this property degradation after reprocessing are highly desired.
86][87] Therefore, dynamic polymers containing these noncovalent interactions are more likely to develop good stimulusrespon siveness, selfhealing (typically at ambient temperature) or recycling properties.In fact, nature commonly utilizes these noncovalent interactions to assemble extremely complex biological macromolecules while keeping their interactions dynamic. [73,88,89]For example, hydrogen bonds are ubiquitous in biomolecules, imparting specific physiological or biochemical functionality to structures such as muscle protein titin.[92] The Aida group reported a family of etherthiourea linear polymers crosslinked by dense hydrogen bonding.These polymers were stiff, showing the strength of the hydrogen bonding interactions, yet also enabled selfhealing when compressed. [93]Similarly, Dong and coworkers devel oped transparent elastomers with a titinmimicking molecular structure that exhibited not only the selfhealing ability but also enhanced mechanical properties such as toughness and elastic recovery. [89]Super toughness (345 MJ m −3 ) and high tensile strength (44 MPa) after selfhealing could be achieved through the incorporation of a motif containing hierarchical hydrogen bonds (urethane, urea, and UPy).Metalligand coordination has also been used to develop strong and tough selfhealing and recycling of dynamic polymers for conductive composites/ adhesives and energy absorption. [94,95]Hence, including such dynamic, reversible bonds in polymer systems enables the engineering of lifeinspired materials.

Dynamic Covalent Bonds in Light-based 3D Printing
The ability of dynamic covalent bonds to respond to external stimuli such as heat, light, magnetic field and pH allows for subsequent material (re)shaping, reprocessing and beyond.Typically, the introduction of shapechanging behavior during 3D printing is referred to as 4D printing.The resulting 4D printed structures are capable of changing functions or prop erties over time (the socalled fourth dimension), mimicking lifelike behavior.98][99][100][101][102][103][104] Beyond the singleshape deformability of these polymers, polymers crosslinked by dynamic covalent chemistries have shown addi tional features such as selfhealing, recycling and reprocessing, which can extend a material's life cycle and reduce negative environmental impact.Therefore, the introduction of novel dynamic covalent bonds in lightbased 3D printing can further expand the scope of materials for 4D printing in applications where the system needs to be adapted on demand.To achieve this aim, the careful design of new materials that ensure printa bility using lightbased 3D printing techniques but also include the desired functionality (dynamic bonds) is required.
In general, there are two basic requirements for the photoresins applicable to most lightbased 3D printing tech niques: rapid photocuring and moderate flowability (vis cosity). [21,22,27,48,51]Rapid photopolymerization reactions allow liquid monomers to change from liquid to solid under light irradiation.Moreover, to ensure that the liquid photoresins can evenly spread in the printable area during SLA or DLP printing process, a good fluidity (low viscosity) is necessary (typically ten to hundreds of mPa•s).For TLP, resins having higher photoreactive group densities as well as high viscosity are favorable. [99,105]Therefore, the design of dynamic covalent polymers for lightbased 3D printing can be carried out in two steps.The first step is to functionalize the nonphotopolymer izable dynamic (macro)monomers with specific photopolymer izable functional groups to achieve a fast photopolymerization capability.The second step is to adjust the component formula tions to achieve a suitable viscosity in view of the envisioned lightbased 3D printing technology.In the following sections, we discuss recent examples of the emerging field with a special focus on the material design that introduces reshaping, self healing and recycling functionality.

Reshaping
The ability to reshape given by the introduction of dynamic covalent bonds in printable materials has been recently exploited.[108] The thi oesteranhydride networks are commonly synthesized by a ringopening reaction of the corresponding anhydride upon thiol addition, which is facilitated by a catalyst (bases and nucleophiles) and can be made to be reversible at elevated temperatures, typically above 80 °C.Podgórski and coworkers developed a dynamic thiol−ene photoresin based on commer cially available anhydride, thiol and ene monomers for SLA (Figure 4A-C). [107]The thioesteranhydride photopolymer was synthesized by the reaction of allyl succinic anhydride and trimethylolpropane tris(3mercaptopropionate) to prepare the initial resin mixture (Figure 4A).The resin's rheological and curing properties were then optimized by the inclusion of less viscous diallyl monomers to enable 3D printing using the SLA technique.To achieve a desirable curing depth of 200 µm, a combination of radical photoinitiator (BAPO) and inhibitor (pyrogallol) were used at a weight ratio of 0.5 to 0.05, resulting in more than 90% thiol−ene conversion within 12 s of curing time.The dynamic reversible exchange was characterized by a series of stress relaxation and creep experiments, showing rapid exchange rates ranging from minutes to seconds at tem peratures of 80-140 °C.Various 3D geometries were printed (Figure 4B), which could be reconfigurable above 80 °C while being depolymerizable at or above 120 °C (Figure 4C).Further, the stimuli responsiveness was demonstrated as the structures could be erased by the deactivation of the exchange catalyst.These features of this deactivation postprinting open new opportunities in 4D material development with twoway or multiway shape memory effects.
Besides the shape change, dynamic covalent bonds also allow reversible assembly and disassembly under specific external stimuli to change the size and mechanical proper ties.Recently, our group prepared "living" microstructures using TLP based on the inclusion of dynamic alkoxyamine bonds into the polymeric network. [109]This living process was based on nitroxide exchange reactions and chain exten sion via nitroxidemediated polymerization (Figure 4D,E).We designed a resin system based on poly(ethylene glycol) dia crylate (PEGDA, average M n = 700 g mol −1 ) as a crosslinker, 4methacryloyloxy2,2,6,6tetramethylpiperidine1oxyl (TEMPO) methacrylate as nitroxide source and 7diethylamino 3thenoylcoumarin (DETC) as photoinitiator.The concentration of photoinitiator as well as the ratio between TEMPO methacrylate and photoinitiator were critical to achieving good printability.By the nitroxide exchange reaction (Figure 4D), a reduction of mechanical properties of ≈50% for the reduced Young's Modulus and hardness was achieved while main taining structural quality.By chain extension via nitroxide mediated polymerization using styrene as a monomer (Figure 4D), a dramatical increase of the volume (≈8 times) of the 3D printed microstructures was observed.An increase by two orders of magnitude in the mechanical properties, i.e., Young's modulus (from 14 MPa to 2.7 GPa), was also observed.Meanwhile, the initial 3D shape was well preserved, including the retention of its fine structural details (Figure 4E).The approach we developed introduced a new dimension by ena bling the creation of microstructures with dynamically tunable sizes and mechanical properties.B) 3D printed objects.C) Square-shaped 3D print reshaped into two consecutive permanent shapes, then fixed, and finally restored to its original shape by deformation and heating at 140 °C.Adapted with permission. [54]Copyright 2021, American Chemical Society.D) Post-printing modification of covalent adaptable microstructures (CAMs) via nitroxide exchange reaction and nitroxide mediated polymerization.E) Top view SEM images of CAM octopus and gecko (left) after printing and after 2 h (middle) and 4 h (right) of post-printing reaction time.Scale bars: 20 µm.Adapted with permission. [79]Copyright 2022, Wiley-VCH.

Self-healing
Polymers are prone to (mechanical) damage caused by external forces, leading to the degradation of the material properties and performance.Selfhealing, which is inspired by the repair func tionality of biological systems, has been successfully proven to enhance the durability and service life of materials. [73][71][72][73][74][75][76] Extrinsic self healing relies on externally added healing agents in the polymer matrix, while intrinsic selfhealing is typically based on revers ible interactions (noncovalent and covalent) in the polymer networks.The most widely exploited extrinsic selfhealing route is the incorporation of healing agents containing microcap sules.However, the instability of healing agents and the lack of repetitive healing capability in most cases prohibit the prac tical applications of extrinsic selfhealing materials.In contrast, intrinsic selfhealing facilitated by dynamic reversible bonds in principle can proceed infinitely without much concern for longterm performance degradation.The intrinsic selfhealing process is based on the breaking and reforming of dynamic bonds in the damaged areas, which generally requires some level of external interference, such as a force to push the dam aged surfaces together, and external stimuli to activate dynamic bonds. [76]By adjusting the healing conditions according to the activation conditions of the embedded dynamic bonds, a recovery of close to 100% mechanical properties after self healing is achievable.[120][121] The type of dynamic bond mainly affects the healing conditions rather than the recovered mechanical properties after selfhealing.The ideal dynamic system would enable 100% healing efficiency to be achieved under ambient conditions, even without external interference, thus displaying spontaneous healing behavior at room temperature, albeit without impacting material stability.
Although various dynamic noncovalent polymers have been employed to achieve selfhealing properties, many of the mate rials are prone to creep due to the intrinsic weakness of most noncovalent bonds and the fact that they typically undergo a bond breaking/reformation equilibrium at room temperature.[115][116][117] As dis cussed in Section 3, the exchange of dynamic covalent bonds is typically categorized into either associative or dissociative mechanisms.In the following, we discuss recent strategies for the design of selfhealing dynamic covalent polymers for lightbased 3D printing based on these two bond exchange mechanisms.

Self-Healing based on Associate Exchange Mechanism
[124][125] The reason is that reactive groups are omnipresent in common building blocks and commercial thermoset plastics, including the widely used epoxy resins.Triggering transesterification for selfhealing purposes often requires the introduction of catalysts.Zhang and coworkers developed DLP printing of reprocessable ther mosets that could repair a broken part by simply 3D printing new material on the damaged site based on a transesterifica tion reaction between the hydroxyl and ester functional groups (Figure 5A,B). [126]The printing resins were prepared by mixing 2hydroxy3phenoxypropyl acrylate as the monomer, bisphenol A glycerolate diacrylate as a crosslinker, and diphenyl(2,4,6 trimethylbenzoly) phosphine oxide as photoinitiator, and zinc acetylacetone hydrate as the catalyst to accelerate the trans esterification reaction (Figure 5A).Once photopolymerized during DLP printing, subsequent heating to 180 °C promoted the transesterification reaction between the ester and hydroxyl groups of the material.Based on this system, a 3D printed rabbit structure that had lost its ears could be repaired by first polishing the damaged area to a flat surface, and then DLP printing new material on the flat surface to rebuild the missing part of the rabbit (Figure 5B).After printing, the rabbit struc ture was heated to 180 °C for 4 h to rearrange the dynamic polymer networks across the interface and further restore the mechanical performance.
Disulfide bonds are ubiquitous in living organisms, pri marily to maintain the tertiary structure of proteins.[129][130][131] As a weak dynamic covalent bond, the disulfide bond can confer selfhealing properties to the materials at a lower temperature.For instance, Li and coworkers fabricated a type of polyurethane elastomer with the excellent selfhealing ability for DLP 3D printing. [132]A type of polyurethane acrylate containing disulfide bonds (PUSA) was first synthesized through two steps: a prepolymer was prepared initially by the reaction of polyethylene glycol (PEG1000), bis(2hydroxyethyl) disulfide and isophorone diisocyanate and then capped with hydroxyethyl acrylate to get polyurethane acrylate.The obtained PUSA was then compounded with reactive diluents and photo initiators to obtain a printable resin.Due to the good fluidity and high curing rate, the resin could be applied to DLP printing of various 3D objects with complex structures, high printing accu racy, and remarkable selfhealing ability (Figure 5C).The tensile strength and elongation at break of the polyurethane elastomer were 3.39 ± 0.09 MPa and 400.38 ± 14.26%, respectively, and the healing efficiency reached 95% after annealing at 80 °C for 12 h and could be healed for multiple times.With the ease of fabrication and excellent performance, the polyurethane elas tomers for DLP 3D printing were noted to hold great potential applications in flexible electronics, soft robotics, and sensors.Similarly, Yu and coworkers reported selfhealable elastomers, relying on a molecularly designed photocrosslinkable elastomer resin synthesized from vinylterminated polydimethylsiloxanes, [4-6% (mercaptopropyl)methylsiloxane]dimethylsiloxane and iodobenzene diacetate with both thiol and disulfide groups. [133]he former facilitated a thiolene photopolymerization during the 3D printing process and the latter enabled a disulfide  [126] Copyright 2018, Springer Nature.C) Self-healing of the DLP printed honeycomb structure based on disulfide bonds.Adapted with permission. [132]Copyright 2019, American Chemical Society.D) Self-healing process of the 3D soft actuator based on disulfide bonds.Adapted with permission. [133]Copyright 2019, Springer Nature.E) Schematic illustration of modular 4D printing.Various modules are assembled by interfacial welding via dynamic covalent bond exchange.Adapted with permission. [134]Copyright 2020, Elsevier.
metathesis reaction during the selfhealing process.The com petition between the thiol and disulfide groups governed the photocuring rate and selfhealing efficiency.Single and multi material selfhealable structures for 3D soft actuators, mul tiphase composites, and architected electronics were fabricated using an SLA system (Figure 5D).
In addition to the ability to repair the damage, the self healing capability based on dynamic covalent bonds can also be used to assemble different printed small parts into large and complex structures.This assembly is based on an interfacial bond exchange process activated under a certain stimulus.This assembly strategy can reduce the time required for layerbylayer printing and improve the designability of 3D structures.Along this line, Fang and coworkers assembled 4D printed struc tures based on dynamically crosslinked polymers in a modular fashion by interfacial bond exchange, greatly extending the scope of the technology by combining 4D printing and mod ular assembly (Figure 5E). [134]The functional system relies on dynamic hindered urea linkage and was utilized for the suc cessful fabrication of various complex shape memory devices, including 3D Miurapatterned structures with zero Poisson's ratio and cylindrical structures with Kresling patterns for supe rior mechanical stability.This approach could extend the pos sibilities for the future development of multifunctional devices with seamless integration of materials, structure and function.

Self-Healing Based on Dissociate Exchange Mechanism
DielsAlder (DA) cycloadditions are wellknown thermally reversible reactions.[137] Recently, several DA systems have been applied in 3D printing.DurandSilva and coworkers investigated the effect of a thermally reversible Diels−Alder crosslinker on the shape stability of printable resins and their selfhealing properties (Figure 6A). [138]For resins containing different concentrations of dynamic covalent crosslinkers in a polyacrylate network, the content of dynamic crosslinkers was shown to play a key role in balancing shape stability with selfhealing ability.The shape stability of printed objects was assessed by measuring the dimensional changes after heat treatment and the selfhealing efficiency of the 3D printed objects was characterized by scratch healing and tensile testing.It was demonstrated that a dynamic covalent crosslinker concentration of 1.8 mol% was sufficient to provide 99% selfhealing efficiency without disrupting the shape stability of the printed objects.
The borate ester bond is a pHresponsive dynamic covalent bond.141][142] Robinson and coworkers used boronate esters as a key building block for developing catalystfree 3Dprinting resins that were capable of room temperature exchange of the crosslinking sites (Figure 6B-D). [143]The orthogonality of boronate esters were exploited in fastcuring, oxygentolerant thiol−ene resins, which could modulate the dynamic properties of 3D printed objects by adding static covalent crosslinking without under going roomtemperature bond exchange (Figure 6B).The mechanical properties of printed parts varied between a tradi tional thermoset and a vitrimer.DLP printed objects exhibited a balance of structural stability showing a residual stress of 18% and rapid exchange with a characteristic relaxation time of 7 s, allowing for interfacial welding and postprint functionalization (Figure 6C,D).Hamachi and coworkers demonstrated that the incorporation of carbamate bonds capable of undergoing dis sociative exchange reactions provided improved interlayer net work formation in printed urethane acrylate polymers. [144]In the presence of a dibutyltin dilaurate catalyst, the exchange of the carbamate bonds enabled rapid stress relaxation with an activation energy of 133 kJ mol −1 , consistent with a dissociative bond exchange process.Annealed tensile samples printed in the zaxis direction with layers perpendicular to the tensile load demonstrated an increase in elongation at break, indicative of selfhealing.

Recycling
Plastic waste continues to grow and has become a major con cern for the development of a sustainable society.Recycling offers a potential solution to tuning plastic waste into valuable products. [29,30,76,145,146]Traditional plastic recycling relies on the physical reprocessing of thermoplastics.While this is a well established industry practice, it is only applicable to thermo plastic polymers and its practical value is severely limited by degraded product properties.Thermoplastics recycling gener ally requires high temperatures, and various aging processes, including thermal degradation, oxidation and hydrolysis, which are often occurring during recycling, resulting in polymer chain scissions and further performance deterioration.In con trast, slight chain scissions in thermosets have little effect on the network structures and properties.Despite this advanta geous feature, conventional thermosets cannot be recycled and reprocessed due to their permanent covalent crosslinking.Dynamic covalent polymers have the potential to overcome this since they can be reprocessed similarly to thermoplastics.As discussed above, lightbased 3D printed materials mainly con sist of unreprocessable thermosets based on the photopolym erization of dual and multifunctional monomers, which can only be disposed of as waste at the end of their life and hence pose a threat to the environment.The introduction of dynamic covalent polymers in lightbased 3D printing is promising to endow printed 3D objects with recycling capabilities.Below, recent work on this emerging field is discussed.As in the pre vious section, the examples have been classified depending on the exchange mechanism of the materials employed.

Recycling based on Associate Exchange Mechanism
In addition to the selfhealing ability, transesterification reac tions have also been utilized for recycling and reprocessing purposes.The dynamic topological exchange can be assisted by the presence of catalysts or hydroxyl groups present in the networks.These dynamic polymers can be reprocessed by compression molding at elevated temperatures.As a pio neering example, Zhang and coworkers developed 3D printable reprocessable thermosets that allowed to reform a printed 3D structure into a new arbitrary shape and recycled unwanted printed parts based on the transesterification reaction (Figure 5A and Figure 7A). [126]The printed 3D object was recy cled by grinding it into powders and then pouring the powders into a mold.After the thermal treatment, a thermosetting sheet was formed due to the transesterification reaction (Figure 7A).This process was repeated for up to three cycles, but certain mechanical degradation occurred in each step.Recently, the same group upcycled vitrimer waste by developing a UVcurable Adv.Funct.Mater.2024, 34, 2300456 Figure 6.A) Diels-Alder (DA) system and schematic representation of the printed system and its shape stability and self-healing properties.Adapted with permission. [138]Copyright 2021, American Chemical Society.B) 3D printable material containing dynamic boronate esters.C) Images of 3D printed and welded ice cream, and D) Demonstration of a printed starfish sample that was subsequently welded to a printed rock, yielding a mixed fluorescent/ nonfluorescent system.Adapted with permission. [143]Copyright 2021, American Chemical Society.
recycling solution system based on the transesterification reac tion (Figure 7B). [147]Conventional unprintable vitrimer powders were mixed with the printing resins, and the resulting mixture was compatible with DLP 3D printing to fabricate 3D struc tures with high resolution (up to 20 µm) and high geometric complexity.Heat treatment triggered bond exchange reactions in the printed structures, and greatly enhanced the mechanical properties.This method allowed for multiple printing cycles of vitrimer waste.Moreover, the resulting mixture could be applied as an adhesive to bond printed small parts together to build a larger and more complex structure that could not be printed.Similarly, Cui and coworkers introduced the dynamic thiocarbamate bonds into the photocurable methacrylate to prepare reprocessable and selfhealable 4D printable poly thiourethane with Young's modulus of 1.2 GPa and tensile strength of 61.9 MPa (Figure 7C,D). [148]The shapememorized 4D objects featured high shape fixity and shape recovery, and reconfigurable permanent shapes brought by the solidstate plasticity.The potential application was further demonstrated in smart alarms for laser exposure or fire caused by the incor poration of carbon nanotubes to obtain a dualmode triggered alarm (discussed in Section 6).Moreover, the surface wettability and cell adhesion performance of printed objects with excel lent biocompatibility could be readily adjusted through the exchange reaction with sulfhydryl compounds.

Recycling Based on Dissociate Exchange Mechanism
DielsAlder (DA) reactions have also been exploited for 3D printable systems exhibiting recyclability.Li    [126] Copyright 2018, Springer Nature.B) Illustration of a full cycle that reprocesses vitrimers into complex 3D geometry through DLP-based 3D printing and dynamic transesterification interaction.Adapted with permission. [147]Copyright 2022, Wiley-VCH.C) Dynamic exchange reaction of thiocarbamate bonds.D) Reprocessing process of DLP printing polythiourethane based on dynamic thiocarbamate bonds.Scale bar: 1 cm.Adapted with permission. [148]Copyright 2022, Wiley-VCH.
between diphenylmethanebismaleimide and furfuryl alcohol (Figure 8). [149]A polyurethane containing the thermorespon sive DAadduct was first synthesized (Figure 8A) and then compounded with reactive diluent and photoinitiators to get a series of photoresins.Benefiting from the high curing rate and good fluidity, various complex 3D structures were printed by a DLP 3D printer (Figure 8B).High mechanical properties with tensile strength higher than 30 MPa were obtained.Based on the dynamic reversibility of the DA/retroDA system, the printed objects exhibited good recycling ability.Indeed, recy cling through hot pressing was demonstrated by obtaining a transparent, although slightly yellower, sheet from the initially made samples cut in pieces (Figure 8C).After the recycling pro cess, the tensile strength of the reprocessed samples recovered close to that of the pristine samples.In addition, the printed samples exhibit remarkable shape memory performance with shape fixity and recovery ratios of 96.4% and 99.3%, respectively.
In general, recycling or upcycling is seen as transforming waste into valuable products and represents a feasible approach toward more sustainable use of materials.Thus, introducing recycling capability allows 3D printed complex structures to be considered as valuable raw materials rather than waste at the end of their life.As discussed above, a range of recyclable dynamic polymers for lightbased 3D printing have been devel oped.However, it is important to note that most of the recy cling capability is achieved by hot pressing, which comes at the cost of losing the 3D complexity of the printed structures.Only limited studies demonstrated the successful reprinting via solvolysis, chemical modification and/or adding extra photo reactive monomers to maintain the 3D complex structures.For instance, Chen and coworkers proposed a twostage curing approach involving dynamic reactions by using the acrylate epoxy hybrid resin for the recyclable DLP 3D printing of highperformance thermosetting polymers. [150]The fabricated thermosetting parts were depolymerized by ethylene glycol via dynamic bond exchange reactions displacing the initial crosslinks.After the evaporation of excessive reactant, the con centrated solution was used for the next round of 3D printing.However, since the depolymerized oligomers cannot be pho topolymerized again due to the lack of photopolymerizable acrylate groups, the addition of large amounts of new acrylate resin (70%) and photoinitiator were required to ensure actual reprinting.The mechanical performance decreased, from 2.2 to 1.6 GPa for Young's modulus and from 46 to 38 MPa for the tensile strength, partly due to residual ethylene glycol mole cules arising from the recycling process.It should be noted that such reprinting does not enable a complete closedloop sustain able process.

Dynamic Non-covalent Bonds in Light-based 3D Printing
Compared with dynamic covalent polymers, dynamic polymers containing dynamic noncovalent interactions (e.g., hydrogen bonding, metalligand coordination, hostguest complexation Adv.Funct.Mater.2024, 34, 2300456 1 cm.C) Recycling process of the cured samples.Adapted with permission. [149]Copyright 2020, Elsevier.[86][87] However, dynamic noncovalent bonds have been even less explored for lightbased 3D printing.The main reasons are the challenging formulations.Indeed, one needs to maintain stable noncovalent interactions during the entire printing process, while rather poor mechanical prop erties of the materials are obtained.Nonetheless, dynamic non covalent interactions are regaining increased attention in the design of dynamic polymer networks as a result of their com plementary and synergistic material properties when combined with more robust dynamic covalent systems.In this section, an overview of some remarkable examples of dynamic noncova lent bonding interactions in 3D printed materials is given.
Hydrogen bonding is a common physical interaction in nature.Under certain conditions, such as high tempera tures, hydrogen bonding interactions are lost, while at low temperatures hydrogen bonds reorganize hence allowing for the preparation of selfhealing polymers. [73,88,89]Wang and coworkers reported a selfhealing polymer gel based on poly(ethylene glycol) (PEG) as solvent (PEGgel) for DLP printing (Figure 9A,B). [151]The developed PEGbased gels with poly(hydroxyethyl methacrylatecoacrylic acid) physically crosslinked by hydrogen bonding demonstrated high stretcha bility and toughness, rapid selfhealing, and longterm stability.The tensile strength of the PEGbased gels varied from 0.22 to 41.3 MPa, fracture strain from 12% to 4336%, modulus from 0.08 to 352 MPa, and toughness from 2.89 to 56.23 MJ m −3 .Based on this system, a selfhealing pneumatic actuator was fabricated by DLP 3D printing.Caprioli and coworkers used commercially available materials (including poly(vinyl alcohol), poly(acrylic acid) and poly(ethylene glycol) diacrylate, along with a watercompatible photoinitiator based on diphenyl (2,4,6tri methylbenzoyl)phosphine oxide (TPO)) and a commercial DLP printer to fabricate 3D hydrogels with the selfhealing ability (Figure 9C). [152]These hydrogels were based on a semiinterpen etrated polymeric network, enabling selfhealing of the printed objects relying on hydrogen bonding.The selfhealing occurred rapidly at room temperature without any external trigger.After reattachment, the damaged samples could withstand defor mation and recovered 72% of their initial strength after 12 h.Similarly, Gao and coworkers developed a type of mechani cally robust and reprocessable 3D printed thermoset by com bining hydrogen bonds and exchangeable βhydroxyl esters into acrylate vitrimers. [153]The resin formulations for 3D printing comprised the synthesized diacrylate prepolymer together with acrylamide generated exchanged βhydroxyl esters and pen dent amides in crosslinked networks, while hydrogen bonds resulting from the amide groups acted as sacrificial bonds that dissipated vast mechanical energy under an external load.With the inclusion of 20 wt.% acrylamide, the average tensile strength and Young's modulus were up to 40.1 and 871 MPa, which increased by ≈4.4 and 3.9 times, respectively.The net work rearrangement of crosslinked vitrimers could be achieved through the dynamic ester exchange reactions with the gradual disappearance of hydrogen bonds at elevated temperatures, imparting reprocessability into the printed structures.
The temperaturedependent nature of metal coordination reactions is also beneficial for healing, recycling and reshaping upon heating.Zhu and coworkers reported a family of dynamic noncovalent polymers with highly tailorable mechanical prop erties for DLP printing (Figure 9D). [22]The dynamic polymers crosslinked by coordination bonding and hydrogen bonding endowed printed objects with excellent selfhealing and recy cling ability.The mechanical properties of printed objects could be easily tailored from soft elastomers to rigid plastics to satisfy practical applications.Taking advantage of the dynamic cross linking, various assembling categories, including 2D to 3D, small to large 3D structures, toward different materials, and the fabrication of functional devices with a selfhealing capacity could be realized.Similarly, Wu and coworkers designed an interpenetratednetwork hydrogel based on noncovalent interactions, affording fast solidliquid separation for DLP printing. [154]Poly(acrylic acid (AA)Nvinyl2pyrrolidone (NVP)) and carboxymethyl cellulose (CMC) were crosslinked via Zn 2+ ligand coordination and hydrogen bonding with the resulting mixed AANVP/CMC solution being used as a printing resin.The printed poly(AANVP/CMC) hydrogel exhibited high ten sile toughness (3.38 MJ m −3 ) and superior selfhealing ability (healed stress: 81%; healed strain: 91%).
Finally, dynamic noncovalent interactions between host and guest molecules can allow polymers to display stimulusrespon sive properties.Based on such hostguest interaction, we intro duced stimuliresponsive composite scaffolds fabricated by TLP to simultaneously stretch large numbers of single cells in tailored 3D microenvironments (Figure 10). [155]The key material was a stim uliresponsive photoresin containing noncovalent crosslinking between cyclodextrin (host) and adamantane (guest) (Figure 10A).This allowed for reversible actuation under physiological condi tions by applying soluble competitive guests (Figure 10B,C).Cells adhering to these scaffolds established an initial traction force of ∼80 nN.After the application of an equiaxial stretch of up to 25%, the cells remodeled their actin cytoskeleton, doubled their trac tion forces, and equilibrated at a new dynamic set point within 30 min.When the stretch was released, traction forces gradually decreased until the initial set point was retrieved.Such a process wellmonitored the cell response to force.

Applications of Light-based 3D Printed Dynamic Polymers
The above described the different covalent and noncovalent dynamic polymers that have been employed in lightbased 3D printing to implement reshaping, selfhealing and recycling functionality.In the current section, we will focus on the applica bility of lightbased 3D printed dynamic polymers.In particular, two main areas have been identified that will be discussed in the following by selected examples: soft robotics and sensing devices.
The introduction of dynamic polymers can impart self healing and reprocessing properties to 4D printed shape memory polymers.This ability to reconfigure permanent shapes is an important feature for its application in soft robotics. [36,116]For example, as discussed in Section 4.3, Cui and coworkers introduced dynamic thiocarbamate bonds to prepare reprocessable and selfhealing 4D printable polythio urethane (Figure 7C,D). [148]In addition to selfhealing and recycling capabilities, the solidstate plasticity of the dynamic polythiourethanes allowed the printed gripper to reconfigure Figure 9. A) Schematic diagram and photographs of a soft pneumatic actuator prepared via DLP printing.B) Healing of the actuator after total separation, and bending under 0.5 MPa air pressure after 10 min of healing.Scale bars: 5 mm.Adapted with permission. [151]Copyright 2022, Wiley-VCH.C) Self-healing process of DLP printed 3D hydrogels through reversible hydrogen bonding.Adapted with permission. [152]Copyright 2021, Springer Nature.D) DLP printing of healable and recyclable dynamic non-covalent polymers with tailorable mechanical properties based on metal coordination bonding and hydrogen bonding.Adapted with permission. [22]Copyright 2021, American Chemical Society.its permanent shape and change the direction of shape recovery.An opened gripper was printed and deformed into a closed temporary shape while holding a remarkable 500 g weight load (Figure 11A(a,b)).After cooling to room tempera ture and fixing the temporary shape, this gripper was able to lift the weight up.Once the closed gripper was reheated to 100 °C, the closed gripper could recover to the opened shape and release the weight (Figure 11A(c)).The network topology of the dynamic polythiourethane material could be reorganized and adapted, causing the permanent shape of the gripper to reconfigure into a new permanent closed shape (Figure 11A(d)).Subsequently, the deformed gripper was able to recover to the closed state and lift the weight (Figure 11A(e)).Further more, the authors also exploited their system as a sensor.For this, carbon nanotubes (CNTs) were introduced to endow printed structures with responsiveness to nearinfrared (NIR) light through the photothermal effect.The sensor ability was exploited in a smart fire alarm that could be activated by heat or NIR (Figure 11B(a-d)). [148]The power switch was connected to a branched circuit with a green light emitting diode, which indi cated a "safe" response signal (Figure 11B(c)).In the event of a fire, as the temperature rises, the shape memory of the switch was triggered, thereby switching the electric circuit to a second branched circuit connected to a red light and buzzer sign aling the occurrence of danger and providing an early warning (Figure 11B(d)).In addition, the 4D printed structures showed good blood compatibility, cytocompatibility, histocompatibility, and cell adhesion properties, which provides opportunities in clinical application such as skull repair, vascular and tracheal stents that would benefit from high degrees of customization.
[158][159] 3D printing techniques have the ability to fabricate wearable sensors with customized and complex designs. [98,152,160,161]As discussed in Section 5, Wu and coworkers printed an interpenetratednetwork hydrogel based on noncovalent interactions (Zn 2+ ligand coordination and hydrogen bonding), which exhibited high tensile tough ness and superior selfhealing ability. [154]The presence of metal ions and water endowed the printed hydrogels with electrical conductivity, which made them applicable as flexible sensors.The researchers designed and assembled a manipulator using different hydrogels with varying water content, which exhib ited different resistance responses.Four types of finger cots and stretch belts for fitting four different fingers on one hand were printed and assembled into a manipulator.When the assembled manipulator was deformed by external stimuli, the change in the strength of the resistance response was uniform (Figure 11C(a)).Additionally, the gestures of the hand can be monitored owing to the different sensitivities of the different parts on each finger resulting from the varying water content (Figure 11C(b)).

Summary and Future Perspectives
Copyright 2022, Wiley-VCH.C) Wearable flexible sensors: (a) resistance responses of assembled manipulator flexible sensors to finger bending, (b) hand gestures are monitored by resistance responses of the assembled manipulator.Adapted with permission. [154]Copyright 2021, Wiley-VCH.
More recently, lightbased 3D printing has entered the stage of advanced material design and preparation based on application requirements, and novel materials have gradually become the core element and control bottleneck that restricts the progress of 3D printing technology.At the same time, dynamic polymers have also been extensively studied for lifeinspired materials, including properties such as selfhealing and recycling.This desire is driven by the strong need to reuse polymers and/or increase their service life.Notable progress has been made on multiple fronts: new dynamic covalent chemistries, mecha nisms for network recyclability and selfhealing, and extension toward device applications.The combination of lightbased 3D printing and dynamic polymers is promising to manufacture 3D multifunctional and reprocessable material structures that meet the demands for sustainable future applications.
Although much progress has been made in light 3D printing of stimuliresponsive, healable and recyclable 3D objects based on dynamic bonds, the practical applications of these printed materials require further design of novel dynamic polymers and overcome critical challenges to introduce unprecedented features of 3D printed materials, including intrinsic reprinting.
First, dynamic polymers with highly tailorable mechanical properties and multiple functions are still rare.Currently, most lightbased 3D printing relies on a single resin leading to lim ited material performance and functionality for printed objects.For example, currently printed 3D macro or microstructures mostly can respond only to one stimulus (e.g., temperature or pH) and their properties can no longer be changed on demand.How to use the developed lightbased 3D printing technologies to manufacture multimaterial or multiproperty reprocessable 3D objects at once remains a question.An effective strategy may be to develop new dynamic and versatile polymer networks for lightbased 3D printing relying on novel dynamic bonds and their incorporation with functional molecules or fillers.
Second, currently reported dynamic polymers for lightbased 3D printing require external stimuli to initiate their healing, such as an additional force to push the fractured surfaces together and external stimulation (e.g., temperature and light) to activate dynamic bond exchanges.Compared to conventional selfhealing dynamic polymers with simple structures (usu ally thin films or bulk), 3D printed complex structures exhibit greater difficulty in healing damage, i.e., in reverting both their complex structure and their material performance.By a strict definition, "selfhealing" implies that the material can repair defects spontaneously without any external interference.This is ideal for the healing of complex structures that are 3D printed.However, such selfhealing without applied stimuli is difficult to achieve, so for the healing of 3D printed objects, the extra interference should be optimized to the minimum level.In addition, most of the reported selfhealing 3D objects are soft and show limited functionality, limiting their practical appli cations.Introducing dual/multi crosslinkers and fillers is an effective strategy to enhance the mechanical performance of 3D printed objects while combining selfhealing polymers with functional fillers such as silver nanowires, graphene and liquid metal is a promising solution to achieve multifunctional self healing 3D structures.
Finally, the current recycling of 3D printed objects mainly by hot pressing suffers from loss of complex shapes and degradation of performance.The ideal recycling of lightbased 3D printed objects should be a closedloop 3D printtoreprint process that can recover 100% of the original complex 3D struc tures or even create entirely new 3D structures.[150][151] However, these polymers with infinite chemical recyclability to liquid monomers only can be polymerized on a gram scale, and large amounts of acidic or basic solvents need to be used to recycle and purify the monomers.Meanwhile, all the monomers lack photoactive groups and rapid liquid-solid transition ability.Therefore, developing completely reversible photopolymers for closedloop lightbased 3D printing remains a grand challenge, which requires further exploration at the fundamental research level to design completely recyclable photopolymers based on novel dynamic bonds for lightbased 3D printing.
The everincreasing interest and production form a real driving force to overcome the outlined challenges and tran sition to a circular material economy and multifunctional applications for lightbased 3D printing, which offers prom ising prospects when combined with stateoftheart dynamic polymer systems.

Figure 1 .
Figure 1.The combination of 3D printing and dynamic polymers enables diverse functionality and the sustainable design of 3D printed objects.

Figure 3 .
Figure 3. Overview of dynamic bonds introduced in polymeric materials: A) Dynamic covalent bonds via i) associative exchange mechanism and ii) dissociative exchange mechanism, and B) Dynamic non-covalent bonds.

Figure 4 .
Figure 4. A) Reactions scheme of the thiol-anhydride reaction, thiol−ene polymerization and crosslinking and thioester-anhydride reversion above 80 °C.B) 3D printed objects.C) Square-shaped 3D print reshaped into two consecutive permanent shapes, then fixed, and finally restored to its original shape by deformation and heating at 140 °C.Adapted with permission.[54]Copyright 2021, American Chemical Society.D) Post-printing modification of covalent adaptable microstructures (CAMs) via nitroxide exchange reaction and nitroxide mediated polymerization.E) Top view SEM images of CAM octopus and gecko (left) after printing and after 2 h (middle) and 4 h (right) of post-printing reaction time.Scale bars: 20 µm.Adapted with permission.[79]Copyright 2022, Wiley-VCH.

Figure 5 .
Figure 5. A) Transesterification reaction involved in dynamic covalent networks.B) Demonstration of the ability of the material to repair flawed printed rabbit structure based on transesterification reaction.Adapted with permission.[126]Copyright 2018, Springer Nature.C) Self-healing of the DLP printed honeycomb structure based on disulfide bonds.Adapted with permission.[132]Copyright 2019, American Chemical Society.D) Self-healing process of the 3D soft actuator based on disulfide bonds.Adapted with permission.[133]Copyright 2019, Springer Nature.E) Schematic illustration of modular 4D printing.Various modules are assembled by interfacial welding via dynamic covalent bond exchange.Adapted with permission.[134]Copyright 2020, Elsevier.

Figure 7 .
Figure 7. A) Demonstration of recycling of a DLP printed structure based on transesterification reaction.Scale bar: 5 mm.Adapted with permission.[126]Copyright 2018, Springer Nature.B) Illustration of a full cycle that reprocesses vitrimers into complex 3D geometry through DLP-based 3D printing and dynamic transesterification interaction.Adapted with permission.[147]Copyright 2022, Wiley-VCH.C) Dynamic exchange reaction of thiocarbamate bonds.D) Reprocessing process of DLP printing polythiourethane based on dynamic thiocarbamate bonds.Scale bar: 1 cm.Adapted with permission.[148]Copyright 2022, Wiley-VCH.

Figure 8 .
Figure 8. A) Chemical structure of the polyurethane diacrylate based on a dynamic Diels-Alder (DA) system.B) DLP 3D printed structures.Scale bars:1 cm.C) Recycling process of the cured samples.Adapted with permission.[149]Copyright 2020, Elsevier.

Figure 10 .
Figure 10.A) Chemical structure of components and printed products containing dynamic non-covalent host-guest interactions using TLP.B) Schematic image of the microscopic cell scaffolds, fabricated by TLP with three different materials.The addition of 1-AdCA leads to a swelling of the hydrogel and displacement of the beams.C) Optical micrographs of the two states with and without 1-AdCA.The red arrows indicate the displacement of the individual beams scaled by a factor of 3. Adapted with permission.[155]Copyright 2020, Science.

Figure 11 .
Figure 11.A) Soft robotic example of a reconfigurable gripper of 4D printed polythiourethane: (a) printing model of the gripper, (b) photographs of an as-printed open gripper, (c) the process of unloading a 500 g weight by the gripper, (d) reconfiguration of the permanent shape of the gripper, (e) the process of grabbing and lifting a 500 g weight with the gripper.Scale bar: 1 cm.B) Dual-mode trigger alarm for sensing applications: (a) printing model of a dual-mode triggered alarm, (b) circuit diagram of the trigger alarm, (c) green light indicated safe state, (d) red light and buzzing sound activated warning response indicating a dangerous state.Adapted with permission.[148]Copyright 2022, Wiley-VCH.C) Wearable flexible sensors: (a) resistance responses of assembled manipulator flexible sensors to finger bending, (b) hand gestures are monitored by resistance responses of the assembled manipulator.Adapted with permission.[154]Copyright 2021, Wiley-VCH.
and coworkers fabricated a type of polyurethanes through DLP 3D printing with high mechanical performance, excellent recycling ability and shape memory properties based on DA reactions Adv. Funct.Mater.2024,34,2300456