Unlocking the Power of Multicatalytic Synergistic Transformation: toward Environmentally Adaptable Organohydrogel

A sustainable and efficient multicatalytic chemical transformation approach is devised for the development of all‐biobased environmentally adaptable polymers and gels with multifunctional properties. The catalytic system, utilizing Lignin aluminum nanoparticles (AlNPs)‐aluminum ions (Al3+), synergistically combines multiple catalytic cycles to create robust, mechanically stable, and versatile organohydrogels. Single catalytic cycles alone fail to achieve desired results, highlighting the importance of cooperatively combining different cycles for successful outcomes. The transformation involves free radical crosslinking, reversible quinone‐catechol reactions, and an autocatalytic mechanism, resulting in a dual crosslinking strategy that incorporates both covalent and ionic crosslinking. This approach creates a dynamic gel system with combined energy dissipation and storage mechanisms. The engineered organohydrogels demonstrate vital multifunctionalities such as good thermal stability, self‐healing, and adhesive properties, flame‐retardancy, mechanical resilience and durability, conductivity, viscoelastic properties, environmental adaptability, and resistance to extreme conditions such as freezing and drying. The developed catalytic technology and resulting gels hold significant potential for applications in flexible electronics, energy storage, actuators, and sensors.


Introduction
Polymers and soft materials such as hydrogels, [1] organogels, [2] and organohydrogels [3] demonstrate substantial interest due to their desired and broad functional properties, and suitable applications for various technologies and industries. [4,5]Hydrogels comprising three-dimensional crosslinked hydrophilic polymeric networks with the potential of absorbing large amounts DOI: 10.1002/adma.202306657 of water have shown promising applications in tissue engineering and various biomedical challenges due to their favorable properties such as biocompatibility, [6] resembling native extracellular matrix, [7] good water absorption and retention, [8] and easy to fabricate. [9]Nevertheless, they also exhibit limitations that limit their broader applications such as poor mechanical stability (due to lack of effective energy dissipation mechanism), [10] low durability, [11] poor thermal and environmental stability (nonfreeze and dry tolerance), [12] poor compatibility with hydrophobic compounds, [13] high swelling ratio (even though the high swelling ratio is important for many applications, it can weaken the mechanical performance dramatically, moreover in tissue engineering applications high swelling ratio can compress or damage surrounding organs and tissues), [14] and sterilization challenges (Figure 1). [15]Gels that can adapt to a wide range of environmental conditions with properties such as freeze-resistant and heat-resistant are highly desirable and would further expand the technological applications of gels. [16]In lieu, organogels composed of liquid organic phase overcomes some of the limitations of hydrogels such as being anti-freezing and anti-drying, [17] and mechanically stable. [18]However, they also have their shortcomings owing to their inadequate solvent selection leading to limited solubility, in some cases flammable organic solvent, compatibility challenges between the solvent and selected materials, slow gelation kinetics due to poor interaction between the gel system and solvent, solvent evaporation that might lead to the collapse of the gel structure and limited biocompatibility of some organic solvents (Figure 1). [2]Some of the limitations of both hydrogels and organogels can be overcome by designing the unique hybrid organohydrogels, which if fabricated correctly can combine the advantages of both hydrogels and organogels, thus broadening the scope and applications of the gel system (Figure 1). [19]A plethora of organohydrogels have been reported over the years for a wide range of applications, [20,21] for instance, Gao et al. disclosed the design of a mechanically stable, environmentally adaptive, and freeze-tolerant organohydrogel through ultraviolet light in situ polymerization of oleophilic polymer network suitable as an anti-icing and anti-waxing material. [22]Zheng and Figure 1.Schematics presenting various gel materials: hydrogel composed of aqueous solvent, organogel of organic solvent, and organohydrogel containing a binary aqueous and organic solvent.Their respective limitations and the distinctiveness of the hybrid organohydrogel combine the advantages of both gels and overcome some of their limitations.
co-authors presented a mechanically robust and electrical conductive organohydrogel designed through a fishing net-inspired strategy by merging polyacrylamide hydrogel, zinc ions (Zn 2+ ), and a binary solvent system containing glycerol and water, and further combined with an electrospun fibrous mat made from poly(acrylic acid) and poly(vinyl alcohol). [23][26][27] Despite these very significant advances there is still room for further exploration and expansion of this important class of gels.
Herein, we envision designing an environmentally adaptable organohydrogel through an innovative multicatalytic synergistic transformation approach by utilizing all biomass-derived starting materials and the catalytic system Lignin aluminum nanoparticles (AlNPs)-aluminum ions (Al 3+ ) promoting a dual crosslinking chemical process.The rational design engineering strategy proceeds by combining methacrylated alginate (Alg-MA) and Lignin AlNPs in a binary solvent system in the presence of the radical generator ammonium persulfate (APS) promoting free radical crosslinking reaction prompted by the Al 3+ /Al 2+ catalytic system (Figure 2). [28]Concurrently, the lignin moiety will provide a reductive environment through the reversible quinonecatechol reaction promoting self-healing and adhesive properties of the gel. [29]Furthermore, we have previously demonstrated that the Lignin metal NPs within a hydrogel system will slowly release metal ions in situ, [30] thus promoting an autocatalytic mechanism. [31]The Lignin AlNPs are hypothesized to release additional Al 3+ in situ that further promote additional ionic crosslinking furnishing the dual crosslinked tough organohydrogel with adhesive, anti-freezing, and anti-drying properties (Figure 2).The power of the reported multicatalytic synergistic transformations can be verified by knowing that using one of the single catalytic cycles or activation modes alone would fail in endorsing fruitful results and the multifaceted properties of the engineered organohydrogel. [24]To our knowledge, this is the first example of environmentally adaptable organohydrogel engineered through the innovative multicatalytic synergistic process promoting dual crosslinking of alginate-based gel.

Fabrication and Characterization of the Lignin Metal NPs and the Polymer
To obtain the Lignin AlNPs a thorough study was performed by screening the influence of reaction time and temperature, and by adding various amounts of the base ammonium hydroxide (NH 4 OH) (Figure 3a, Table S1, Supporting Information).The best combination of reaction conditions within the tested parameter frame was: a reaction solution of water and NH 4 OH (6:4), room temperature (≈24 °C), and a reaction time of 4 h (1 h under sonication and 3 h under stirring) (Table S1, entry 8, Supporting Information).The Lignin AlNPs were obtained with a size of ≈119 nm and with a zeta potential of -28.5 (Figure 3b).Ultraviolet-visible (UV-vis) analysis verified the quinone formation and the formation of AlNPs (Figure S1a, Supporting Information). [32]The X-ray diffraction (XRD) patterns of the Lignin AlNPs showed some of the characteristic peaks of AlNPs at 2 ≈ 38.6°and 44.8°, corresponding to the (111) and (200) planes, [33] and characteristic peaks of Al 2 O 3 and its corresponding NPs (both -Al 2 O 3 and -Al 2 O 3 phases) at 2 (-Al 2 O 3 ) ≈ 35.3°, 43.4°, and 57.5°, corresponding to the (104), (311), and (113) planes, and at 2 (-Al 2 O 3 ) ≈ 22.8°, 31.5°, and 46.5°corresponding to the (111), (220), and (400) planes (Figure S1b, Supporting Information). [34]Two unsigned peaks at 29.3°a nd 47.7°were also detected that might correspond to organic compounds in the lignin. [30,35]The X-ray photoelectron spectroscopy (XPS) confirmed the presence of the AlNPs and some traces of Al 3+ in the Lignin AlNPs sample (Figure S1c, Supporting Information). [36]Conversely, the Lignin AgNPs were prepared with a similar method without sonication but with 1 h of stirring, generating a material with a size of ≈55 nm, a zeta potential of -3.55, and the presence of AgNPs was confirmed by XRD analysis (Figure S2a-c, Supporting Information).Further characterization of the Lignin AlNPs and Lignin AgNPs was performed through Fourier transform infrared spectrometry (FTIR) analysis confirming the presence of the lignin moiety and its functional groups in accordance with our previous report (Figure S3a, Supporting Information). [30]The thermal properties  S2, Supporting Information).The slightly lower thermal stability of the Lignin metal NPs is most likely due to the oxidation of the lignin during the NP formation process thus slightly decreasing its thermal stability.Several studies have demonstrated that lignin under oxidative conditions can be degraded and therefore loses its stability. [37]Nevertheless, the glass transition temperature (T g ) was highest for the Lignin Ag-NPs (62.5°C), followed by Lignin AlNPs (58.4 °C) and lastly the unmodified lignin (54.1 °C) (Figure S3c, Table S2, Supporting Information). [38]The morphology and chemical analysis of the Lignin AlNPs were investigated through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis demonstrating evenly distributed Lignin AlNPs and the presence of the element Al throughout the material (Figure 3c-e and Figure S4, Supporting Information).SEM imaging of the Lignin AgNPs also demonstrated nanosized particles evenly distributed throughout the lignin template (Figure S5, Supporting Information). [30]he alginate (Alg) employed to engineer various gels was modified through methacrylation (Figure S6a, Supporting Information) to promote dual crosslinking (covalent and ionic crosslinking).The structure of the synthesized methacrylated Alg (Alg-MA) was verified through proton nuclear magnetic resonance spectroscopy ( 1 H NMR) and FTIR analysis confirming the presence of the methacrylic group (Figure S6b,c, Supporting Information).The 1 H NMR spectrum confirmed the presence of the double bond (C = C) in the methacrylic moiety at a chemical shift of  = 5.68 and 5.36 ppm, and the methyl (-CH 3 ) group at  = 1.86 ppm. [39,40]FTIR analysis verified the signal at 1718 cm −1 corresponding to the carbonyl ester within the methacrylic group (stretching -COO-) and the double bond (C = C) at 813 cm −1 . [40,41]Alg-MA was obtained without interfering with the thermal properties of the initial starting material (Alg) (Figure S7 and Table S2, Supporting Information).

Rational Design Engineering of Various Gels, Their Mechanism, and Characterization
To investigate the advantages of the dual crosslinking strategy, we devised a rational design for engineering various gels through several chemical strategies.The chemical strategies comprise: 1) dual crosslinking triggered by dual catalysis combined with autocatalysis mechanism [30,31] (multicatalytic synergistic strategy) generating both chemical crosslinks and ionic crosslinks, 2) solely chemical crosslinks, and 3) solely ionic crosslinks (Figure 4a-c).The initial multicatalytic synergistic process triggering dual crosslinking was performed by treating Alg-MA with Lignin AlNPs in the presence of APS in both water solution providing a hydrogel (LAlNPsH) and in a binary solvent system comprising glycerol and water providing the organohydrogel (LAlNPsOH) (Figure 4a).This devised multicatalytic synergistic strategy includes the activation of quinone-catechol redox process providing a redox environment, the free radical crosslinking of the Alg-MA substrate, and subsequently further ionic crosslinking rendered from the released Al 3+ within the gel system promoted through an autocatalytic mechanism (Figure 2).The engineering of an organohydrogel (LAgNPsOH) via the solely chemical crosslinking strategy was performed by using Lignin AgNPs as the catalyst (Figure 4b).Monovalent cations such as silver ions (Ag + ) will not promote any ionic crosslinking, therefore the Ag + released in due course will not promote any ionic crosslinking within the Alg-MA gel system (Figure 4b). [42]Lastly, the gel comprising solely ionic crosslinks was engineered by treating the Alg with aluminum nitrate (Al(NO 3 ) 3 ) providing a crosslinked hydrogel (ICAlgH) (Figure 4c).All the gels containing lignin demonstrated adhesive properties due to the quinone-catechol redox environment within the gel system promoted by the Lignin AlNPs and Lignin AgNPs. [30]Moreover, all the gels provided crosslinked and free-standing gels (Figure 4a-c).The ionic crosslinked ICAlgH, however, was weak and brittle (Figure 4c).The as-prepared gels were characterized by FTIR analysis and the signals for the organohydrogels showed the presence of glycerol peaks at 2932 and 2878 cm −1 (C-H symmetric and asymmetric stretch) and also an increased strong absorption at about 3300 cm −1 from the hydroxyl (-OH) stretching of the glycerol (Figure S8, Supporting Information). [43]The success of the chemical crosslinking could be confirmed by the disappearance of the signal at 813 cm −1 corresponding to the methacrylate group (C = C).Lignin presence was observed in the LAlNPsOH, LAlNPsH, and LAgNPsOH samples at about 631 cm −1 corresponding to the C-H deformation. [44]This peak was absent in the ICAlgH and Alg-MA samples.The phenolic (-OH) in lignin was also detected at 1236 cm −1 in the LAlNPsOH, LAlNPsH, and LAgNPsOH samples (Figure S8, Supporting Information). [45]The possible coordination bonds formed between the Al 3+ and lignin in our gel system are difficult to detect in the FTIR (Figure S8, Supporting Information) due to the complexity of our gel system and the possible superimposing with other peaks (Figure S9, Supporting Information). [28,46]The plausible chemical and physical bonds and interactions within the various devised gels (excluding the interactions rendered from the lignin moiety) [30] are presented in Figure 4d, where the ICAlgH formulation comprises many ionic crosslinks promoting energy dissipation. [47]Generally, a gel fabricated through solely ionic crosslinks provides a weak and brittle gel without further manipulations such as dual crosslinking. [48]The LAgNPsOH formulation contains mainly covalent crosslinks, with some plausible physical interactions from the glycerol, thus generally, lacking a mechanism to dissipate mechanical energy to further promote the toughness and recoverability of the gel system (Figure 4d). [9,49]On the other hand, combining both covalent and ionic crosslinks endorse energy storage and dissipation mechanism promoting a tough hydrogel as in the case of the LAlNPsH formulation. [50]To design a tough, formable, and self-healing gel the system must comprise covalent crosslinks and many physical crosslinks that promote deformation and energy dissipation. [51]The LAlNPsOH organohydrogel formulation containing glycerol adds further hydrogen bonding to the system and therefore increases its durability (Figure 4d). [52]e made an initial investigation on the hypothesis that the Ag + /Ag 2+ catalytic system might promote the free radical crosslinking reaction in the presence of APS in the LAgNPsOH gel.It is well known that APS can oxidize Ag + to Ag 2+ . [53,54]The reduction potential of Ag + /AgNPs is about 0.278-0.391V depending on the sizes of the AgNPs, [55] whilst the reduction potential of Ag 2+ /Ag + is about 1.89-2.10V. [56] Based on this information it should be preferable for APS to oxidize the AgNPs/Ag + catalytic system compared to the Ag + /Ag 2+ catalytic system.Therefore, in the LAgNPsOH gel most likely the AgNPs/Ag + catalytic system is more preferable to promote the crosslinking of Alg-MA.To investigate the above assumption that the crosslinking of Alg-MA could also proceed via the Ag + /Ag 2+ catalytic system, we performed experiments where the Alg-MA was reacted with AgNO 3 and APS in a water/glycerol solvent system.The reaction yielded a crosslinked and self-standing organohydrogel (Figure S10a, Supporting Information).To ensure that the reaction proceeds via free radical crosslinking of the methacrylic moieties and not through oxidative decarboxylation reaction, [30,53] a control experiment was performed: Alg was reacted with AgNO 3 and APS in a water/glycerol solvent system.The reaction did not provide any crosslinked and self-standing gel (Figure S10b, Supporting Information), thus the oxidative decarboxylation reaction failed to promote a successful outcome.Based on these results we can assume that the Ag + /Ag +2 catalytic system in the presence of APS could promote the free radical crosslinking.

Thermal, Rheological, and Mechanical Performance of the Various Engineered Gels
The thermal properties of the various gels investigated through TGA analysis demonstrate that the T onset for the organohydrogels (LAlNPsOH and LAgNPsOH) and LAlNPsH was between 62 and 63 °C, whilst for Alg-MA and ICAlgH a slightly higher temperature was observed (≈68 °C) (Figure 5a,b).The T max occurred at ≈230 °C for the organohydrogels and at about 240 °C for the hydrogels (LAlNPsH and ICAlgH) and AlgMA.Interestingly, the weight loss for the organohydrogels at T max was 93% (LAlNPsOH) and 91% (LAgNPsOH), respectively.The hydrogels only showed a weight loss of 51% (LAlNPsH) and 61% (ICAlgH) (Table 1).The organohydrogels comprise high content of glycerol which will evaporate during the measurement thus leading to increased weight loss meanwhile the freeze-dried hydrogels only contain the metal ion (Al 3+ ), Lignin AlNPs, and the Alg or Alg-MA, therefore have higher solid content.Similarly, the final degradation temperature (T final ) was lower for the  1).
The evaluated rheological properties are presented in Figure 5c-f.Initially, a strain sweep was performed and the storage/elastic (G′) and loss/viscous modulus (G″) were recorded to evaluate the viscoelastic behaviors of the various engineered gels. [57]This evaluation provides important information about molecular structure and intramolecular interactions. [58]The linear viscoelastic region (LVR, indicated in the gray region) and the gel point (tan () = 1), [59] indicate the transition point from a solid-like behavior to a gel-like behavior was determined from the strain sweep test (Figure 5c, and Figure S11a-c, Supporting Information). [60]The organohydrogel LAgNPsOH sample demonstrated the highest G′ with 7200 Pa confirming a more rigid gel compared to the organohydrogel LAlNPsOH (1123 Pa) and the hydrogels (LAlNPsH = 356 Pa and ICAlgH = 800 Pa) (Figure 5c, Figure S11a-c, Supporting Information and Table 2).Similar behavior could also be observed in the frequency sweep () evaluation, where LAgNPsOH, followed by LAlNPsOH, demonstrated the highest G′ values at the initial  values (Figure 5d).Nevertheless, the LAlNPsOH sample showed a much larger LVR (up to strain (%) = 166) than all the other formulations (LAgNPsH up to strain (%) = 13 and ICAlgH up to strain (%) = 4), and the LAgNPsOH demonstrated lowest value (up to strain (%) = 0.8) (Figure 5c, Figure S11a-c, Supporting Information and Table 2).The gel formulation LAlNPsOH with the highest LVR value demonstrates the gel's resistance to high strain without deformation. [61]The LAgNPsOH formulation, on the other hand, demonstrates almost immediate gel structure changes (such as ruptured bonds or entanglement) due to its very low LVR. [62]The LAlNPsOH formulation comprises dual crosslinking with both covalent bonds and ionic crosslinks a) The G′ (elastic modulus) was determined at the initial strain (%) from the strain sweep graph; b) LVR is the strain range where the gel can resist without deformation; c) Gel point is when tangent () = G″/G′ = 1; d) The G′ of LAlNPsH at first high strain cycle is 368 Pa and at the last high strain cycle 142 Pa, thus recovered to 39% of its original modulus; e ) The G′ of ICAlgH at the first high strain cycle is 396 Pa and at the last high strain cycle 299 Pa, thus recovering to 76% of its original modulus.(combining energy dissipation and storage mechanism), whilst the LAgNPsOH is solely covalently crosslinked, thus having no reversible physical bonds providing dynamic behavior that can impact and improve the viscoelastic properties. [63]The self-healing or recoverability of the gels was evaluated through rheological measurement conducted at 1 Hz by applying 300% strain, a value outside of the LVR, followed by 1% strain, a value inside of the LVR, for 5 min to monitor gel recovery.Both the organohydrogels demonstrated recoverability in all the evaluated consecutive cycles, whilst the LAlNPsH hydrogel did not show any self-healing ability and only recovered up to 39% of its initial G′ value.The ICAlgH hydrogel showed slightly better recoverability by recovering up to 76% of its initial G′ value (Figure 5e, Figure S11d-f, Supporting Information and Table 2).
The glycerol within the organohydrogels adds a vitrification effect such as hydrogen bonds to the gel system thus promoting self-healing ability. [52,64]The deformation or change in the shape of a gel is provided from the strain sweep experiments, whilst the information about the internal resistance of a gel against deformation is extracted from the stress experiments. [65]herefore, we performed stress sweep tests of the various gel formulations.The LAlNPsOH organohydrogel had the highest stability and was the only gel without any transitional change from a solid-like behavior to a gel-like, thus its elastic nature (tan () < 1) dominates over the viscous (Figure 5f).Whereas for the other gels, the gel point was obtained at some point during the stress sweep measurements, indicating a transition to a more viscous-like behavior (Figure 5f, and Figure S11g-i, Supporting Information).Furthermore, the mechanical integrity was evaluated for cylindrically shaped gels through a compression test on an Instron mechanical tester (Figure 5g-j).
The mechanical testing further corroborated the impact of the multicatalytic synergistic chemical strategy prompting the dual crosslinking process.The organohydrogel LAlNPsOH demonstrated superior mechanical integrity with a significantly higher compressive modulus (832.0 ± 44.7 kPa) and compressive stress (9.5 ± 0.1 MPa), followed by LAgNPsOH (compressive modulus: 399.0 ± 23.5 kPa and compressive stress: 6.6 ± 0.8 MPa), and LAl-NPsH (compressive modulus: 79.4 ± 17.3 kPa and compressive stress: 2.0 ± 0.6 MPa), and lastly the ionic crosslinked hydrogel ICAlgH (compressive modulus: 73.1 ± 6.9 kPa and compressive stress: 1.2 ± 0.4 MPa).These results also verify our suggested mechanistic chemical interactions within the engineered gels promoting their superior properties and the importance of combining an abundance of physical crosslinks promoting energy dissipation with covalent crosslinks endorsing energy storage mechanism within a constructed gel (Figure 4d). [51]he self-recovery, mechanical resilience, and durability of the LAlNPsOH gel were further investigated by subjecting the gel to cyclic deformation (stretching and retraction) at 150% and 300% strain without resting (Figure 5j).The gel successfully recovered during six uninterrupted cycles without tearing apart, and the first cycle demonstrated a larger hysteresis loop indicating a large amount of energy available for dissipation. [66]These experiments verify the gels' energy dissipation, self-recovery, and mechanical durability.The self-healing was additionally investigated through further mechanical and rheological experiments of the self-healed gel after cutting the gel and putting the pieces in direct contact, upon which they immediately self-healed.The compressive stress recovered to 60% of its original value and the compressive modulus to 33%, respectively (Figure 5k, and Figure S12a,b, Supporting Information).The rheological results demonstrated recovery of about 30% in strain, 20% in frequency, and 18% in stress (Figure 5l, and Figure S12c,d, Supporting Information).Despite the low recovery in some instances, the self-healing was performed immediately without any external activation such as heat, or allowing the gel to fully heal with more time, thus the gel demonstrated good recoverability.

Environmental Adaptability
The evaluation of the environmental adaptability of the engineered organohydrogel (LAlNPsOH) and the hydrogel (LAl-NPsH) was performed by exposing the gels to harsh conditions and further immediate manipulation of the materials.After exposing the organohydrogel to a cold environment (-30 °C) for 3 days, the gel did not freeze and remained intact, reshapable, and adhesive (Figure 6a).In contrast, the hydrogel did not show any anti-freezing property and thus froze and broke upon manipulation with no adhesive property (Figure 6b).Exposing the organohydrogel to a hot environment (+60 °C) for one day did not impose any drying and the gel showed to be undamaged, reshapable, and adhesive (Figure 6c).The hydrogel formulation, on the other hand, dried and broke upon manipulation and further lost its adhesion (Figure 6d).These experiments demonstrate the environmental adaptability of the engineered organohydrogel without any loss of its properties or functionalities.

Impact from Glycerol Content, Adhesion, and Conductivity Analysis
The impact of varying the ratios between water and glycerol within the LAlNPsOH gel on the mechanical and rheological properties was evaluated.Increasing the amount of glycerol demonstrated a pronounced impact on the mechanical property increasing significantly compared to the gel without any glycerol (Figure 7c,d).For instance, the compressive stress results were: 2.0 ± 0.6 MPa (Glycerol:Water  The adhesion performance was evaluated by measuring the shear strength through lap shear experiments of the various LAlNPsOH-based gels containing various amounts of glycerol and the gels after exposure to various temperatures (Figure 7f,g, and Figure S15a,b, Supporting Information).The adhesion strength of the LAlNPsOH: +60 °C gel (shear strength: 72.5 ± 13.2 kPa) demonstrated a significant decrease compared to the LAlNPsOH (shear strength: 116.9 ± 6.8 kPa) and LAl-NPsOH: -30 °C (shear strength: 98.8 ± 1.6 kPa), however no significant differences were observed between LAlNPsOH and LAlNPsOH: -30 °C gels (Figure 7f).The gels containing various amounts of glycerol demonstrated increased adhesion strength with increasing amounts of glycerol (108.1 ± 6.2 kPa (Glycerol:Water (0:100), 122.3 ± 17.7 kPa (Glycerol:Water (25:75), and 116.9 ± 6.8 kPa (Glycerol:Water (50:50)), nevertheless the gel with the highest glycerol content showed a decrease in shear strength (82.2 ± 2.7 kPa (Glycerol:Water (75:25)) (Figure 7g).However, only significant differences were observed between the Glycerol:Water (25:75) and (Glycerol:Water (50:50) gels, and the Glycerol:Water (75:25) gel.Overall, all the gels demonstrated good shear strength (Figure 7f,g). [30]Ionic conductivity is an important property of gels for flexible electronics applications. [68]Therefore, the electrochemical property evaluation was performed on the LAlNPsOH and LAlNPsH formulations.The conductivity for the organohydrogel LAlNPsOH formulation was lower (8.8 μS cm −1 ) than the LAlNPsH hydrogel (32.3 μS cm −1 ).The obtained conductivity is in harmony with a previous report, [69] and lower than a similar reported hydrogel. [28]The LAlNPsOH was successfully used as a conductor to light a light-emitting diode (LED) lamp (Figure 7h).After cutting the gel in half and putting the pieces in contact the gel rapidly healed back and retained its conductivity (Figure 7h).

Flame-Retardant Ability
Fire safety is an important aspect of materials used in close contact with the human body, such as wearable electronics and energy storage devices, since they might contain liquids and other components that are volatile, flammable, and can ignite fire and explosion. [70]Hence, their flame-retardant performance is a very important and desired property.Since the glycerol content of the LAlNPsOH gel is ≈50 wt%, we envision the gel demonstrating flame-retardant ability. [71]This property was investigated through combustion tests.The burning experiments were performed using porcine skin to mimic the human skin and further expose solely the skin and the skin covered with the LAlNPsOH gel to a butane torch (≈1430 °C) for 5, 10, and 15 s (Figure 8).The burned surface of the porcine skin exposed to the flame showed a temperature of 280.0 °C after 15 s and the burned skin could easily be detected (Figure 8a-d).However, when the skin was covered with the gel only a temperature of 37.1 °C was recorded after 15 s  and c) the burned porcine skin after the experiment.d) Thermal infrared images of the porcine skin exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s.e) Image of the porcine skin covered with the LAlNPsOH gel prior to exposure to the high-intensity flame.f) The protected porcine skin exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s, g) and after the experiment.h) The porcine skin protected with larger-sized gel to remove the impact from the surrounding flame on the porcine skin, and further exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s, and i) after the experiment.j) Thermal infrared images of the protected porcine skin exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s.
on the burned surface of the material, and the skin was significantly less burned (Figure 8e-j).We observed that the surrounding of the high-intensity flame also impacted the performance of the skin covered with the gel, thus to avoid this background impact the gel was made bigger relative to the skin to provide better protection and coverage, which showed improved performance by showing less burning (Figure 8h-i).The obtained results demonstrate that the devised gel potentially could improve the safety performance and fire-proof requirements of wearable devices and electronics.Even though the initial flame-retardant evaluation demonstrates promising results a more comprehensive and systematic analysis and evaluation need to be performed.Experiments such as limiting oxygen index (LOI), UL-94 tests, cone calorimeter measurements, thermal conductivity analysis, and comparison with commercially available flame-retardant materials (e.g., 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10oxidized (DOPO), and Tris(chloropropyl) phosphate (TCPP)).
These additions would provide a more comprehensive understanding of the materials' superiority and potential as flameretardant material.
Despite the very beautiful reports on organohydrogels in recent years, [2] demonstrating superior characteristics and functionalities, [3] most of the reports use non-biobased components in their organohydrogels, [5,22,66] such as acrylamide, [25,27] acrylic acid, [23,26] and polyvinyl alcohol (PVA). [20,72]Nevertheless, the multi-characteristics and functionality of the organohydrogel presented herein provide numerous advantages compared to several already disclosed organohydrogels, and therefore, make it a very attractive and suitable material for flexible electronics, wearable devices, and energy storage.Besides the good adhesion strength (higher than most reported organohydrogels), [25] and mechanical properties (higher than some reports), [31] the presented gel is flame-retardant, conductive, resilient, durable, and environmentally adaptable.The LAlNPsOHs could easily be attached to human skin with good adhesion and flexibility, and removed without promoting any irritation or leaving any residues on the skin (Figure S16a-d, Supporting Information).Furthermore, the most important and inimitable innovation of the disclosed report is the sustainable and highly efficient multicatalytic synergistic chemical transformation and the employment of allbiobased and environmentally friendly materials.
Nevertheless, despite the promising obtained experimental results suggesting that the devised organohydrogel might be a suitable candidate for flexible electronics, wearable devices, and energy storage applications, further experiments such as in human motion monitoring (e.g., monitoring of arm, wrist and finger bending movements), energy supply, strain sensors adhering onto human arms and joint sites, employed for pressure sensing and pulse signal monitoring, monitoring resistance changes during usage of human body and arms etc. need to be performed to support and corroborate the above claims and to obtain a broader and more profound fundamental understanding and insight into the specific applications.

Conclusion
Herein, we present a rational approach for the engineering of environmentally adaptable organohydrogels through a multicatalytic chemical strategy.The use of all-biomass-derived starting materials, including alginate, lignin, and glycerol, combined with a sustainable one-step catalytic process, offers significant advantages.The multicatalytic process is facilitated by Lignin Al-NPs, which trigger an autocatalytic mechanism generating Al 3+ ions that catalyze a free radical crosslinking reaction through the Al 3+ /Al 2+ catalytic system promoting covalent crosslinking.Simultaneously, the reversible quinone-catechol reaction is initiated, creating a reductive environment, and the released Al 3+ ions further contribute to the dual crosslinking process by promoting ionic crosslinking within the gel system.The successful outcome of this approach relies on the synergistic combination of all the catalytic cycles rather than relying on individual cycles.
Different types of gels were fabricated via the proposed strategy: organohydrogels with either dual crosslinking or solely chemical crosslinking, and hydrogels formed through dual crosslinking or solely ionic crosslinking.The properties of the organohydrogels were found to be superior.The covalent crosslinks formed via free radical crosslinking contribute to the structural integrity and energy storage mechanism of the gel system.The reversible quinone-catechol redox reaction enhances properties such as adhesion and self-healing, while energy dissipation is achieved through physical crosslinks such as ionic crosslinks and hydrogen bonding.
The leading candidate organohydrogel formulation, LAl-NPsOH, exhibits good thermal stability and superior mechanical properties compared to other gels.It demonstrates resistance to deformation, self-healing ability, and high mechanical integrity.These properties arise from the combination of energy dissipation and storage mechanisms within the gel system, achieved through the dual crosslinking strategy involving covalent bonds and physical crosslinks (ionic crosslinks and hydrogen bonds).
The organohydrogel proved to be highly adaptable to environmental conditions, displaying anti-freezing and anti-drying properties when exposed to high temperatures.The gel maintained its robustness, adhesiveness, and moldability under these conditions, whereas the hydrogel formulation froze and dried.
In summary, all the obtained characteristics of the devised organohydrogels demonstrated respectable performance such as adhesion strength (up to 122.3 ± 17.7 kPa), flame-retardancy, mechanical resilience (compressive stress up to 16.7 ± 0.47 MPa, and compressive modulus up to 901.4 ± 15.0 kPa) and durability (confirmed through multi-cycle uniaxial tensile demonstrating stability for all the tested six consecutive cycles at 150% and 300% strains), conductivity, viscoelastic properties, recoverability and self-healing properties.Furthermore, the mechanical and rheological properties could be tuned by adjusting the glycerol content, and characteristics improved significantly after exposing the gel to extreme temperatures.This multicatalytic synergistic chemical transformation for engineering alginatebased organohydrogels with dual crosslinking presents a groundbreaking strategy that can serve as a valuable technological tool for the sustainable development of various gels for applications such as flexible electronics, energy storage devices, actuators, and sensors.
Synthesis of Methacrylated Alginate: The methacrylated alginate was prepared following the method in a previous report with minor modifications. [39]Briefly, alginate (400 mg) was dissolved into 20 mL of deionized (DI)-water to produce a 2% (w/v) solution.Then, methacrylic anhydride (20 mL) was added slowly at 4 °C to the alginate solution under N 2 -gas, and the mixture was maintained under continuous stirring for 3 days at room temperature.The pH was adjusted periodically to 8-9 using a NaOH aqueous solution (5 m).After the 3-day reaction period, the resulting solution was poured into 100 mL of cold ethanol to precipitate the methacrylated alginate product.Next, the precipitate was vacuum filtered, washed with ethanol (40 mL × 3), and then dried under vacuum and stored at -20 °C until use.From the addition of the methacrylic anhydride the reaction flask was covered with aluminum foil to provide dark conditions and the reaction was maintained under N 2 -gas during the entire time.The methacrylated alginate (MA-Alg) was obtained as a white solid (0.777 g).
Preparation of Lignin AlNPs: The Lignin AlNPs were prepared following the methods in a previous report with minor modifications. [30]An aqueous solution of 20 mg mL −1 lignin was prepared by adding the lignin (100 mg) to DI-water (5 mL) and vortexing and sonicating until it was fully dissolved.In parallel, (Al(NO 3 ) 3 •9H 2 O (50 mg) was dissolved in DI-water (1 mL) then 4 mL aqueous ammonium solution was added (5 m), and sonicated for 5 min.Afterward, the lignin solution was added dropwise to the Alsolution under sonication and continuously sonicated for 1 h, followed by stirring at room temperature for an additional 3 h.
Preparation of Lignin AgNPs: The Lignin AgNPs were prepared following a method in a previous report. [30]An aqueous solution of 20 mg mL −1 lignin was prepared by adding the lignin (100 mg) to DI-water (5 mL) and vortexing and sonicating until it was fully dissolved.In parallel, AgNO 3 (50 mg) was dissolved in DI-water (4 mL), and 1 mL of aqueous ammonium solution was added (5 m).The lignin solution was added dropwise to the AgNO 3 solution and stirred at room temperature for 1 h.
Preparation of the Lignin Metal NPs-Alg-MA-Based Organohydrogel: A glass vial containing Alg-MA (100 mg) was charged with the Lignin AlNPs solution (0.5 mL) or the Lignin AgNPs solution (0.5 mL) and mixed, followed by the addition of glycerol (0.5 mL).The mixture was mixed and sonicated until fully dissolved.Afterward, the mixture was purged with N 2gas for 5 min and then a solution of APS (10 mg) in DI-water (0.2 mL) was added.Then the mixture was mixed and sonicated (for 30 s) and purged with N 2 -gas for 5 min.Next, the mixture was incubated at room temperature for 24 h, providing a self-standing and crosslinked organohydrogel.
Preparation of the Lignin AlNPs-Alg-MA-Based Hydrogel: A glass vial containing Alg-MA (100 mg) was charged with DI-water (0.5 mL) and mixed.Afterward, the Lignin AlNPs solution (0.5 mL) was added, mixed, and sonicated until fully dissolved.Then, the mixture was purged with N 2gas for 5 min and then a solution of APS (10 mg) in DI-water (0.2 mL) was added.Then the mixture was mixed and sonicated (for 30 s) and purged with N 2 -gas for 5 min.Next, the mixture was incubated at room temperature for 24 h, providing a self-standing and crosslinked hydrogel.
Preparation of the Ionically Crosslinked Alginate Hydrogel: The alginate sodium salt (20 mg) was loaded into a glass vial followed by the addition of DI-water (2.0 mL) to produce a 1% (w/v) solution and mixed until a homogenous solution was obtained.Afterward, a solution of Al(NO 3 ) 3 ⋅9H 2 O (12 mg, 0.6 w/v%) in DI-water (2.0 mL) was added dropwise under shaking (Tamro, IKA KS 130 Basic, 320 min −1 ).Next, the mixture was continuously shaken for 24 h, providing an ionically crosslinked hydrogel.
Proton Nuclear Magnetic Resonance Spectroscopy ( 1 H NMR): 1 H NMR spectra were recorded at 400 MHz and 298 K with a Bruker Avance, using deuterium oxide (D 2 O) as the solvent.Spectra were based on 128 scans and reported in ppm relative to the solvent residual peak at 4.79 ppm for D 2 O. MestReNova 9.0 software was used for data analysis.
Fourier Transform Infrared Spectroscopy (FTIR): FTIR analysis was carried out on a Perkin Elmer Spectrum 100 FT-IR Spectrometer equipped with a single reflection (attenuated total reflection: ATR) accessory unit (Golden Gate) from Graseby Specac LTD (Kent, England) and a TGS detector using the Golden Gate setup.The spectra were collected based on 32 scans averaged in transmittance mode and at regions between 4000 and 600 and with 4 cm −1 resolutions.Data were recorded and processed using the software PerkinElmer Spectrum (2015).

X-Ray Diffraction Analysis (XRD):
The Lignin AlNPs and Lignin AgNPs samples were characterized using the powder X-ray diffractometer PANalytical X'Pert Pro equipped with a Cu K source.The samples were freezedried before analysis.

X-Ray Photoelectron Spectroscopy (XPS):
The oxidation state of the aluminum within the Lignin AlNPs was determined through XPS analysis.The XPS spectra were recorded on a Kratos AXIS Supra+ XPS.ESCApe software was used for the data acquisition and processing.
Ultraviolet-Visible (UV-Vis) Absorbance: The measurements were carried out on a Shimadzu UV-2550 UV-vis spectrophotometer and recorded between 200 and 700 nm.
Dynamic Light Scattering (DLS) Analysis: The NP sizes and zeta potential were characterized using a ZETASIZER Nano Series Nano-ZS MALVERN INSTRUMENTS.All measurements were conducted at 25 °C with at least three replicates.
Thermogravimetric Analysis (TGA): TGA was performed with a Mettler Toledo TGA 1 instrument.Samples having masses of ≈8 mg were used and the experiment was performed at a heating rate of 10 °C min −1 under an N 2 -atmosphere with a purge rate of 50 mL mi −1 n at the temperature range of 30−600 °C.The samples were kept isothermally at 120 °C for 10 min to remove solvent residues and then cooled to 30 °C, followed by heating at 600 °C, and the starting degradation temperature (T onset ), temperature with the highest degradation rate (T max ), final degradation temperature (T final ) and residual amount at 600 °C were determined.Data analysis was performed on Mettler STARe evaluation software.
Differential Scanning Calorimetry (DSC): DSC analyses were performed by using a Mettler Toledo DSC instrument.Samples having masses of ≈6 mg were inserted in 100 μL aluminum pans with pierced lids in an N 2 -atmosphere.The applied heating rate was 10 °C min −1 in an N 2 -atmosphere (rate 50 mL min −1 ).The thermal behavior of the samples was investigated by using two repeated heating-cooling cycles.The temperature program was: first ramp from 25 to 200 °C and stayed at 200 °C for 2 min, followed by a cooling cycle from 200 to −30 °C.After an isotherm at −30 °C for 2 min, a second heating cycle was performed from −30 to 200 °C.Glass transition temperature (T g ) was determined from the second heating curve.Data analysis was performed on Mettler STARe evaluation software.
Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS) Analysis: The morphology and chemical analysis of the material was performed on a High-resolution high-vacuum cold fieldemission (FE)-SEM Hitachi S-4800 (Japan), equipped with SE, BSE, STEM and EDS detectors (X-Max 80 SDD EDS detector from Oxford Instruments, UK).
Rheological Analysis: Samples were stored at room temperature until characterized with a TA Instruments model DHR-2.Gels with cylindric shapes (4 mm height, 15 mm diameter) were prepared for the tests.The storage (G′) and loss modulus (G″) were registered using a parallel plate, plate steel (25 mm diameter) with a gap of 2500 μm.Samples were loaded onto the plate and allowed to equilibrate for 1 min.For oscillatory rheology, the linear viscoelasticity regimen was confirmed by conducting oscillatory strain sweeps from 0.01 to 1000% at 1 Hz.Oscillatory stress sweeps were conducted at ≈0.1−1000 Pa and 1 Hz, and an oscillatory frequency sweep was performed at 0.1−100 Hz and 10 Pa.The G′ and G″ versus strain (%), shear stress, and angular frequency were registered.Recovery testing was conducted at 1 Hz by applying 300% strain, a value outside of the LVR range, followed by 1% strain, a value inside of the LVR, for 5 min to monitor gel recovery.
Mechanical Properties: Mechanical testing was performed on an Instron 5944 Universal Testing Machine with single column, load of 500 N, and speed of 10 mm min −1 .The compression test was performed according to the Standard ASTM D695 with cylindric specimens and dimensions of 25.4 mm height and 12.7 mm diameter.The multi-cycle uniaxial tensile experiments were performed by preparing a LAlNPsOH gel specimen with a rectangular shape (length = 20 mm, width = 10 mm, and thickness = 4 mm).The uniaxial tensile rate was 10 mm min −1 and cyclic loading and unloading were performed for six consecutive cycles at 150% and 300% strains.All the samples were conditioned at 23 °C and 50% relative humidity for 2 days before testing.The results of the compression tests were based on the average of 3 measurements.The maximum compressive stress was obtained at the maximum compression, and the compressive modulus was determined from the slope of the stress−strain curve.Bluehill software was used for test control and data acquisition.
Adhesion Property [30] Lap shear test was performed on an Instron 5944 Universal Testing Machine with a single column, a load of 500 N, and according to the modified ASTM F2255-05 standard.Fresh porcine skin was purchased from a local store.The experiments were performed by taking two pieces of porcine skin (length = 40 mm, width = 25 mm, and thickness = 5 mm) and glue separately with superglue onto glass slides (75 × 25 mm).The LAlNPsOH gel (≈50 mg) was applied to the surface of one of the skin samples with a bond are of 5 cm 2 (20 × 25 mm), and the second skin sample was immediately placed over the gel.Subsequently, force was applied with a clamp for 5 min at 37 °C in a wet environment before measurements.The results were based on the average of three measurements.Afterward, the samples were pulled apart until failure with a cross-head speed of 10 mm min −1 .The shear strength was used as the indication of adhesion strength.
Electrical Measurement: The conductivity of the organohydrogel (LAl-NPsOH) and hydrogel (LAlNPsH) were evaluated by preparing the gels in rectangular shapes with 20 mm length, 10 mm width, and 4 mm thickness, and using the measurement setup with a digital multimeter.The conductivity was calculated according to Equation (1), [66] where L and A represent the length and cross-sectional area of the gels.R is the resistance that is measured by the electrochemical workstation.
The conductivity of the LAlNPsOH formulation was further investigated by employing the gel as a conductor in the circuit to light a light-emitting diode (LED) indicator with a constant voltage of 12 V (digital power supply, PeakTechP 6225 A).
Flame-Retardance: The experiments were performed through an open fire test by using a butane torch (≈1430 °C).The porcine skin and the skin covered with the LAlNPsOH gel were exposed to the flame for 5 s, removing the material from the flame for 1 s, and then back for additional 5 s (10 s), removing and a further 5 (15 s), and the temperature on the surface exposed to the flame was recorded using an infrared camera.
Statistical Analysis: The experiments were conducted in triplets and analyzed using analysis of variance (ANOVA) followed by Tukey's multiple comparison tests to determine statistical differences between mean values.For the experiments containing two groups, the results were analyzed using Student's t-test and compared the means of the two groups.The software GraphPad Prism 6 was used to calculate the statistics.Values are means ± SD, where (*) p < 0.05, (**) p < 0.01, (***) p < 0.005, and (****) p < 0.001, indicating statistically significant differences, ns = not significant.

Figure 2 .
Figure 2. Schematic presenting the overall chemical strategy, design, and multifunctionality of the environmentally adaptable organohydrogel engineered through a multicatalytic synergistic process prompted by the Lignin AlNPs-Al 3+ catalytic system.Lignin AlNPs = Lignin aluminum nanoparticles; Alg-MA = Methacrylated alginate.

Figure 3 .
Figure 3. a) Schematic presenting the preparation of the Lignin AlNPs.b) Size of the NPs, size distribution, and zeta potential.SEM images and EDS mapping images of the Lignin AlNPs: c) SEM image, and i) zoomed area of the SEM image.d) Microscopy image of the EDS mapped area, and e) EDS mapping of the Al element.Al(NO 3 ) 3 = Aluminum nitrate; H 2 O = Water; NH 4 OH = ammonium hydroxide; R.t. = Room temperature; Lignin AlNPs = Lignin aluminum nanoparticles; SEM = Scanning electron microscopy; EDS = Energy-dispersive X-ray spectroscopy.

Figure 5 .
Figure 5. Thermal, rheological, and mechanical properties.a) TGA curves and b) derivative thermogravimetric curves.c) The LVR of the LAlNPsOH formulation (indicated in the gray region) was determined through G′ and G″ versus strain sweep, also in the same graph the tangent () (green graph) demonstrates the gel point of the various formulations.d) The viscoelastic properties of the various gels were determined by a frequency sweep ().e) The recoverability of the LAlNPsOH formulation was determined by recording the G′ and G″ during multiple cycles at the resting state, low strain (1%), and at high strain (300%), the rapid recovery to its original modulus is indicated in the gray region.f) Stress sweep of the gel LAlNPsOH.g) Representative examples of the stress-strain curves of the various gel formulations.h) Compressive moduli, and i) Compressive stress of the various gels.The data given are mean values ± SD (N = 3); P-values were determined by a one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison test, where (***) p < 0.005, (****) p < 0.001, indicating statistically significant differences, ns = not significant.j) Six continuous stretching and releasing cycles of the LAlNPsOH at 150% and 300% of tensile strain.k) Compressive stress of the original LAlNPsOH after immediate self-healing.Values are means ± SD (N = 3); P-values were calculated using the student's t-test comparing the two groups, and p < 0.05 indicating significant differences.l) Strain sweep of the original LAlNPsOH after immediate self-healing.TGA = Thermogravimetric analysis; LVR = Linear viscoelastic region; G′ = Storage/elastic modulus; G″ = Loss/viscous modulus; SD = Standard deviation.

Figure 6 .
Figure 6.Photos demonstrating the environmental adaptability of the organohydrogel compared to the hydrogel.a) The organohydrogel maintained its properties after incubation at -30 °C for three days and immediate testing showed it was reshapeable and adhesive.b) The hydrogel was frozen and non-adhesive after incubation at -30 °C for three days.c) The organohydrogel maintained its properties after incubation at +60 °C for one day and immediate manipulation showed it was reshapable and adhesive.d) The hydrogel was dry and non-adhesive after incubation at +60 °C for one day.

Figure 7 .
Figure 7. Mechanical, rheological, adhesion, and conductivity properties.a) Compressive stress of the LAlNPsOH gel after exposure to various temperatures.b) Strain sweep curves of the LAlNPsOH gel after exposure to various temperatures.c) Compressive stress and d) compressive moduli of the LAlNPsOH gel containing various amounts of glycerol.e) Frequency sweep () of the LAlNPsOH gel containing various amounts of glycerol.f) Adhesion strength based on the shear strength of the LAlNPsOH gel after exposure to various temperatures.g) Shear strength of the LAlNPsOH gel containing various amounts of glycerol.h) Photos demonstrating the conductivity of the LAlNPsOH formulation by acting as a conductor in a circuit to light a LED lamp.i) The original gel demonstrating conductivity.ii) Disconnected gel with no conductivity property.iii) Upon fusing the two pieces the gel rapidly self-healed and acted as a conductor.iv) Zoomed area of the healed region.The data given are mean values ± SD (N = 3); P-values were determined by a one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison test, where (*) p < 0.05, (**) p < 0.01, (***) p < 0.005, (****) p < 0.001, indicating statistically significant differences, ns = not significant.

Figure 8 .
Figure 8. Flame-retardant evaluation of the LAlNPsOH gel.a) Image of solely the porcine skin prior to exposure to a high-intensity flame.b) The porcine skin exposed to a high-intensity flame for various time points, 0, 5, 10, and 15 s,and c) the burned porcine skin after the experiment.d) Thermal infrared images of the porcine skin exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s.e) Image of the porcine skin covered with the LAlNPsOH gel prior to exposure to the high-intensity flame.f) The protected porcine skin exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s, g) and after the experiment.h) The porcine skin protected with larger-sized gel to remove the impact from the surrounding flame on the porcine skin, and further exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s, and i) after the experiment.j) Thermal infrared images of the protected porcine skin exposed to the high-intensity flame for various time points, 0, 5, 10, and 15 s.

Table 2 .
Rheological parameters of the various devised gel formulations.