# Fully Bio-Based Gelatin Organohydrogels via Enzymatic Crosslinking for Sustainable Soft Strain and Temperature Sensing

Pietro Tordi, Adrián Tamayo, Yeonsu Jeong, Bin Han, Tamer Al Kayal, Aida Cavallo, Massimo Bonini, Paolo Samorì

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To cite this version:

Pietro Tordi, Adrián Tamayo, Yeonsu Jeong, Bin Han, Tamer Al Kayal, et al., Fully Bio-Based Gelatin Organohydrogels via Enzymatic Crosslinking for Sustainable Soft Strain and Temperature Sensing. *Advanced Functional Materials*, 2025, 10.1002/adfm.202520762. [https://doi.org/10.1002/adfm.202520762](https://doi.org/10.1002/adfm.202520762)

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# Fully Bio-Based Gelatin Organohydrogels via Enzymatic Crosslinking for Sustainable Soft Strain and Temperature Sensing

Pietro Tordi, Adrián Tamayo, Yeonsu Jeong, Bin Han, Tamer Al Kayal, Aida Cavallo, Massimo Bonini,* and Paolo Samori*

Developing soft materials that integrate mechanical compliance, functional responsiveness, and environmental sustainability is key for next-generation wearable and implantable electronics. Here, a sustainable, fully bio-based organohydrogel sensor made entirely from food-grade and biodegradable components, including gelatin, microbial transglutaminase (TG), and glycerol, prepared via a simple one-pot process under mild thermal conditions, is reported. In this system, TG enzymatically crosslinks gelatin chains into a robust covalent network, while glycerol enhances flexibility, stabilizes hydration, and facilitates proton conduction. The multicomponent system reveals a tunable network morphology governed by enzymatic crosslinking density. The resulting gels exhibit remarkable stretchability (up to 450%), linear strain sensitivity up to 300%, and a high gauge factor of 2.86—placing them among the top-performing hydrogel-based strain sensors to date. In addition to strain sensing, the material shows strong thermal responsivity (0.26 °C⁻¹ in the 20–45 °C range) without being affected by variations in environmental humidity. Long-term electromechanical stability is demonstrated over 5000 cycles. Unlike conventional soft sensors that rely on synthetic polymers, fillers, or dopants, this platform entirely uses food-safe components and a simple one-pot process—offering a scalable and sustainable route to soft electronics. These findings establish enzyme-guided polymer engineering as a powerful tool for functional material design.

## 1. Introduction

The advancement of wearable and implantable electronics requires soft materials that can withstand mechanical deformation while maintaining stable electrical performance under

ambient conditions.[1–9] Conventional conductors and synthetic composites often fall short in delivering the necessary stretchability, skin compatibility, and environmental resilience needed for real-world use. This challenge has fueled growing interest in developing materials that inherently combine mechanical compliance, ionic conductivity, and sustainability—particularly for applications in health monitoring, motion recognition, and transient systems.[10–14] In this context, hydrogels have emerged as a promising class of soft conductors,[15–21] owing to their hydrated networks that support efficient ion transport.[22–24] However, standard hydrogels are prone to dehydration, mechanical fatigue, and instability under fluctuating humidity and/or temperature, limiting their suitability for long-term, on-skin sensing platforms.

To overcome these drawbacks, organohydrogels—hydrated polymer networks partially replaced with hydroscopic organic molecules such as glycerol—have been proposed as more robust alternatives.[25,26] These materials exhibit improved dehydration resistance, enhanced elasticity, and increased

resilience to mechanical stress, making them suitable for wearable sensors operating in real-world environments. However, most organohydrogels reported in literature rely on synthetic polymers or conductive fillers (e.g., reduced graphene oxide (rGO), carbon nanotubes (CNTs), polyaniline (PANI), and

P. Tordi, A. Tamayo, B. Han, P. Samori
Institut de Science et d'Ingénierie Supramoléculaires
Université de Strasbourg & CNRS
8 Allée Gaspard Monge, Strasbourg 67000, France
E-mail: samori@unistra.fr

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202520762

© 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adfm.202520762

P. Tordi, M. Bonini
Department of Chemistry “Ugo Schiff” and CSGI
University of Florence
via della Lastruccia 3, Sesto Fiorentino, Florence 50019, Italy
E-mail: massimo.bonini@unifi.it

Y. Jeong
Physics Research Institute, Department of Physics
Inha University
100 Inha-ro, Michuhol-gu, Incheon 22212, South Korea

T. Al Kayal, A. Cavallo
Institute of Clinical Physiology
National Research Council
Massa 54100, Italy
poly(3,4-ethylenedioxythiophene) (PEDOT)), which render the processing complicated, limit sustainability, and reduce key functional properties such as transparency.[27-30] As a result, there is growing interest in developing biopolymer-based organohydrogels capable of delivering multifunctional performance without compromising sustainability, safety, or fabrication simplicity.

Among available biopolymers, gelatin is particularly attractive due to its biocompatibility, biodegradability, low cost, and temperature responsiveness.[31-33] Derived from the hydrolysis of collagen, gelatin exhibits excellent film-forming properties and contains functional groups amenable to chemical or enzymatic crosslinking. However, unmodified gelatin gels suffer from poor thermal and mechanical stability, especially under hydrated conditions. To overcome these limitations, microbial transglutaminase (TG), a food-grade enzyme produced by microbial fermentation, provides an effective and environmentally benign route to covalently stabilize gelatin networks. TG catalyzes the formation of ε-(γ-glutamyl)lysine isopeptide bonds between adjacent polypeptide chains, generating robust and permanent crosslinks without toxic reagents or harsh conditions.[34-37] Despite its commercial availability, low cost, mild reactivity, and widespread use in the food industry, TG remains surprisingly underexplored in the context of soft electronics and responsive organohydrogels, where sustainable and biofriendly crosslinking strategies are in high demand.

Herein, we report a fully bio-based and transparent organohydrogel based on gelatin, glycerol, and TG, fabricated through a simple one-pot process at 60 °C. Unlike previously reported hydrogel sensors that rely on synthetic matrices or conductive additives, our system uniquely combines enzymatic covalent crosslinking and glycerol-mediated stabilization to achieve multifunctional sensing entirely from bio-derived components. Glycerol, a cheap and widely available food-grade byproduct, serves as a biocompatible plasticizer and humectant, ensuring flexibility and water retention for real-world applications. The resulting organohydrogels (GELTG) exhibit high stretchability (up to 450%), excellent strain sensitivity over a wide deformation range (0–300%) with a gauge factor of 2.86, and robust temperature sensitivity (0.26 °C⁻¹) within the physiological range. Importantly, these properties are achieved without conductive fillers, salt dopants, or synthetic monomers, and the materials display stable and robust responses under humidity fluctuations (20–80% RH). To our knowledge, this represents the most accessible strategy reported to date for fabricating multiresponsive, stretchable polymer-based sensors using only bio-based, sustainable products. Thanks to the enzyme-assisted network structure and the absence of inorganic ions, the gels maintain stable conductivity and mechanical resilience over thousands of stretching cycles. This work demonstrates how enzymatic polymer engineering can enable the preparation of green, low-cost, and high-performance soft sensors for next-generation wearable interfaces, implantable diagnostics, and transient electronics.

## 2. Results and Discussion

To develop a robust, fully bio-based soft material, we fabricated organohydrogels by combining gelatin, glycerol, and microbial transglutaminase (TG) under mild thermal conditions. As il-

lustrated in Figure 1a, TG catalyzes the formation of covalent ε-(γ-glutamyl)lysine isopeptide bonds between the side chains of glutamine and lysine residues on gelatin, establishing a permanent, enzyme-crosslinked network. Glycerol serves both as a humectant and a plasticizer, promoting water retention and enhancing elasticity. The fabrication process consists of a single step at 60 °C, using only edible, water-dispersible components—providing a sustainable and scalable route to soft functional materials (see the Experimental Section and Figure S1, Supporting Information for further details).

To systematically assess the effect of crosslinking, samples were coded based on the concentration of TG solution used during preparation, as detailed in the Experimental Section. Specifically, GELTG000 denotes the uncrosslinked control, while GELTG025, GELTG125, and GELTG250 correspond to gels prepared by mixing 5 mL of a 20% (w/v) gelatin solution formulated in a 1:2 (v/v) glycerol:water mixture with 1 mL of 0.025, 0.125, and 0.250 g mL⁻¹ (respectively) TG solution. FTIR and XPS analyses (see Figures S2 and S3, Supporting Information, respectively) were performed to probe potential chemical changes upon enzymatic crosslinking. The spectra of the gels revealed no discernible or systematic shifts in the main bands or core-level peaks, indicating that TG incorporation does not significantly alter the chemical environment of gelatin functional groups. All samples exhibit comparable thickness (≈630 µm, measured on self-standing gel matrices after removal from their casting molds; see Table S1, Supporting Information) and show high optical transparency in the visible region, as evidenced by the UV-vis spectra in Figure 1b. Below 400 nm, a progressive increase in absorbance is observed, indicating enhanced UV-blocking ability. This attenuation may provide partial protection from ultraviolet radiation, a desirable characteristic for wearable or dermal applications. Inset photographs in Figure 1b further demonstrate the transparency of the gels: each sample appears colorless and homogeneous when overlaid on a black-white grid, confirming their optical uniformity and supporting their suitability for transparent bioelectronics and optically interactive devices.

Differential scanning calorimetry (DSC) thermograms in Figure 1c reveal no detectable exothermic/endothermic transitions associated with water freezing/ice melting, indicating that all water present in the system is in a nonfreezable state.[38] This behavior stems from strong molecular interactions among gelatin, glycerol, and water, which suppress ice crystallization even at subzero temperatures. Such complete inhibition of water freezing is a defining feature of organohydrogels, crucial for maintaining mechanical flexibility and ionic mobility under cold conditions, thus enabling reliable operation across varying environments. Notably, the water content determined by thermogravimetric analysis (TGA) remains constant at ≈14 wt% across all formulations (Figure S4a and Table S1, Supporting Information), reinforcing the conclusion that observed differences in sensing behavior and material properties arise from the architecture dictated by the structural reorganization of the polymeric network rather than by changes in the hydration level. The single-step preparation process ensures precise control of the gel composition, enabling reproducibility and systematic tuning of material properties. Remarkably, the pronounced differences in physicochemical behavior discussed in the following sections stem from subtle changes in TG content (ranging
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Fabrication and basic characterization of TG-crosslinked gelatin organohydrogels. a) Schematic of the enzymatic crosslinking process using microbial transglutaminase (TG). b) UV-vis spectra of GELTG samples (GELTG000-GELTG250) showing high transparency across the visible range. Inset: photographs of gels over a black-and-white grid. c) Differential scanning calorimetry (DSC) curves, indicating the absence of water freezing/ice melting transitions and confirming that all water in the organohydrogels is nonfreezable due to strong gelatin-glycerol interactions (Exo-up).</figcaption></figure>

from 0 to 7.2 wt/wt%) - underscoring the pivotal role of enzymatic crosslinking in dictating the performance of protein-based stretchable sensors.

Furthermore, TGA analysis confirms that TG-mediated crosslinking does not affect the onset of thermal degradation (≈180 °C), which remains dictated by the decomposition of glycerol (Figure S4b and Table S1, Supporting Information). Similarly, the electrical conductivity (σ in Table S1, Supporting Information) is not significantly influenced by TG concentration, as it primarily originates from the intrinsic protic character of type A gelatin (see Figure S5, Supporting Information) - derived from the acid hydrolysis of collagen.

GELTGXXX organohydrogels exhibit remarkable electrical robustness under varying humidity conditions. Although the poly-

meric matrix is hydrophilic and capable of interacting with moisture, humidity-dependent electrical measurements (Figure S6, Supporting Information) show that the change in resistance remains nearly constant from 20% to 60% relative humidity (RH). Only a slight decrease in resistivity is observed above 70% RH, likely due to the formation of a superficial water layer that introduces a limited extrinsic conduction pathway without affecting the bulk structure.

This behavior represents a major advantage over conventional ionic hydrogels incorporating metal salts, such as NaCl, LiCl, and FeCl₃. In such systems, absorbed water promotes ion dissociation and enables proton conduction through Grotthuss-type hopping mechanisms between hydronium ions and water clusters, leading to pronounced humidity-dependent conductivity
shifts.[39-41] In our gels, by contrast, the absence of mobile metal ions eliminates the driving force for osmotic water uptake and prevents the generation of highly mobile ionic species upon hydration.

Here, conductivity arises from the protic character of type A gelatin, where protons are tightly confined within the polymer-glycerol hydrogen-bond network, ensuring stable charge transport with minimal dependence on ambient humidity. This structural and electrostatic confinement ensures a stable electrical response across a broad range of relative humidity. Such insensitivity to humidity further underscores the suitability of GELTG materials for wearable applications, where consistent signal transduction under fluctuating environmental conditions is essential.

To assess the thermal stability of GELTG organohydrogels, we examined their thermal transitions via DSC. As shown in Figure 2a, all samples exhibit a subtle endothermic inflection ≈42 °C (Table S1, Supporting Information), corresponding to the glass transition temperature ($T_g$) of the gelatin-based polymer network (Table S1, Supporting Information). The near-constant $T_g$ across all formulations suggests that enzymatic crosslinking does not significantly affect the intrinsic segmental mobility of the gelatin chains. This observation is further supported by small-angle X-ray scattering (SAXS) analysis (Figure S7, Supporting Information), which reveals nearly identical scattering profiles for all samples across the 1.3-63 nm range. These results indicate that the nanoscale architecture of the polymer network is preserved, regardless of crosslinking density.

In contrast, the macroscopic thermal stability of the gels, defined as the temperature at which structural integrity is lost, shows a strong dependence on TG concentration. As shown in Figure 2b, penetration tests reveal a marked increase in the phase transition temperature with increasing crosslinking density. GELTG000 and GELTG025 lose cohesion at ≈50 °C, while GELTG125 remains stable up to ≈68 °C. Remarkably, GELTG250 exhibits no visible collapse even above 120 °C, confirming that dense enzymatic crosslinking significantly enhances thermal robustness. Taken together with the DSC and SAXS analyses, these findings suggest that the macroscopic collapse observed in lightly crosslinked samples originates from intrinsic structural rearrangements of gelatin chains. Such reorganization likely contributes to the abrupt loss of cohesion and performance degradation observed beyond the transition threshold. This hypothesis is further corroborated by SEM characterization (Figure 2e-h), discussed in detail below.

The organohydrogels also exhibit thermally induced changes in proton conductivity, underscoring their potential for applications in environmental monitoring and electronic skins. As shown in Figure 2c, the relative variation in conductivity ($\Delta\sigma/\sigma_0$) was evaluated across the 293-318 K (20-45 °C) range, i.e., a relevant temperature window for bioelectronic systems.[42-44] Despite the mechanical robustness of highly crosslinked samples, all of the formulations display a temperature-dependent increase in conductivity, attributed to thermally activated proton transport within the gelatin-glycerol-water matrix. Notably, GELTG000 and GELTG025 show an abrupt conductivity spike ≈40 °C, aligning with the structural collapse previously mentioned. This discontinuity likely results from gel liquefaction, leading to fluid redistribution and transient ionic enhancement. In contrast, GELTG125 and GELTG250 exhibit smooth, linear increases in

conductivity, consistent with stable, intact networks. At low temperatures, proton mobility is restricted by strong hydrogen bonding with carboxylate and hydroxyl groups, whereas increasing temperature progressively weakens these interactions, enabling proton delocalization and more efficient charge transport. As temperature increases, these interactions progressively weaken, promoting proton delocalization and charge transport. The softening of the polymeric scaffold and enhanced molecular motions within the glycerol-rich phase further enhance conductivity. Importantly, this thermally responsive behavior is decoupled from ambient humidity, as the gels are fabricated without mobile salts or hygroscopic agents.

The extracted thermal sensitivity, defined as the slope of $\Delta\sigma/\sigma_0$ versus temperature in the widest linear range, is shown in Figure 2d. Among the formulations, GELTG125 exhibits the highest sensitivity (0.26 °C⁻¹), outperforming both the uncrosslinked control and GELTG250. This superior sensitivity, competitive with state-of-the-art hydrogel-based temperature sensors,[45-48] does not stem from differences in glass transition temperature, (identical across all samples as highlighted by DSC), but it can instead be ascribed to morphological variations.

To substantiate this conclusion, we conducted an SEM analysis. Since glycerol interferes with imaging, samples were washed with water prior to lyophilization. Although this step induces some swelling, the resulting micrographs reliably capture qualitative differences in network architecture. As illustrated in Figure 2e-h, increasing TG concentration progressively refines the pore morphology, size, and uniformity: GELTG000 and GELTG025 exhibit open, loosely connected networks, whereas GELTG250 forms a dense, tightly packed structure. GELTG125 displays an intermediate architecture, combining the porosity needed for water and glycerol mobility with the mechanical robustness required for stable operation.

To complement the SEM analysis, we investigated the swelling behavior of the hydrogels. As shown in Figure S8 (Supporting Information), GELTG000 and GELTG025 rapidly reach a swelling ratio (SR) of ≈1.2 within 5 min, consistent with their loose structures and eventually resulting in poor structural integrity. In contrast, GELTG125 and GELTG250 exhibit more controlled swelling behavior, stabilizing without collapsing, at SR values of ≈1.76 and ≈1.39, respectively. The higher $SR_{max}$ of GELTG125 reflects its greater structural flexibility compared to the more rigid GELTG250 network. These findings confirm that the microscopic architecture dictates macroscopic performance.[49]

This balance underlies the observed thermoresponsive behavior. Lightly or noncrosslinked gels undergo structural collapse ≈313 K (≈40 °C), as evidenced by abrupt conductivity spikes and visible failure; their porous networks enable uneven fluid migration, resulting in erratic signals. In contrast, GELTG250 remains mechanically stable but shows limited conductivity variation, likely due to restricted ionic mobility in its compact matrix. Among the tested formulations, GELTG125 stands out as the optimal composition, combining mechanical robustness with efficient ionic transport for reliable strain and temperature sensing. Its moderate crosslink density enables a smooth, linear, and reproducible conductivity response to temperature, consistent across multiple fabrication batches. Overall, these results highlight that porosity and network organization, rather than
<figure><img src="image_5.png" /><figcaption>**Figure 2.** a) DSC thermograms showing a consistent glass transition temperature ($T_g$ ≈42 °C) across all samples. b) GELTGXXX phase transition temperature assessed via probe penetration tests, indicating improved thermal stability with increasing TG content. Inset: representative image of GELTG025 morphology above the phase transition. c) GELTGXXX relative conductivity change ($\Delta\sigma/\sigma_0$) as a function of temperature (20–45 °C). d) Thermal sensitivity values extracted from the $\Delta\sigma/\sigma_0$ curves; GELTG125 shows the highest sensitivity (0.26 $\sigma^{-1}$). SEM micrographs of freeze-dried e) GELTG000, f) GELTG025, g) GELTG125, and h) GELTG250 samples, showing a gradual transition from highly porous to more compact structures with increasing TG content. Scale bars: 30 µm.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** a) Stress–strain tensile curves showing enhanced stretchability and strength upon TG-mediated crosslinking. b) Relative resistance variation (ΔR/R₀) as a function of strain, showing linear electromechanical response across all formulations. c) Young's modulus (YM) and ultimate tensile strength (UTS) values extracted from tensile tests. d) Gauge factor values derived from the linear strain range (0–300 %), with GELTG125 achieving the highest value (2.86).</figcaption></figure>

local chain mobility, dictate thermal responsiveness in these biopolymer-based organohydrogels.

We also evaluated the mechanical and electromechanical properties of GELTGXXX. As shown in Figure 3a, all samples exhibit elastomer-like behavior characterized by an initial linear elastic region, followed by strain hardening and eventual rupture. Crosslinking markedly improves both stretchability and fracture resistance compared to the uncrosslinked control (GELTG000). In particular, GELTG025 and GELTG125 display higher elongation at break and tensile strength, indicating that an optimal degree of enzymatic crosslinking yields cohesive networks capable of effectively dissipating mechanical stress. In contrast, GELTG250 fractures at a lower strain, indicating that an excessive degree of crosslinking induces brittleness and reduces deformability. The extracted mechanical parameters are summarized in Figure 3c. The Young's modulus (YM) is nearly constant across all samples at ≈0.16 MPa, confirming that TG-mediated crosslinking does not significantly affect the gel's initial stiffness. In contrast, the ultimate tensile strength (UTS) noticeably varies with crosslinking density. Both GELTG025 and GELTG125 show a pronounced increase in UTS, suggesting that moderate crosslinking improves stress transfer and mechanical resilience. Conversely, GELTG250 exhibits a drop in UTS despite higher crosslink-

ing, consistent with brittle failure modes in over-crosslinked networks.

Electromechanical behavior was assessed by monitoring the relative resistance change (ΔR/R₀) during uniaxial tensile deformation (Figure 3b). All formulations show a linear response up to 300% strain, validating their potential for stretchable sensing applications. The corresponding gauge factors (GF), reported in Figure 3d, range from 2.38 to 2.86. Among the tested formulations, GELTG125 displays the highest average GF, although the difference relative to GELTG250 lies within experimental uncertainty. Notably, the performance divergence among samples becomes more pronounced at higher strains: formulations with greater crosslinking exhibit enhanced linearity and signal stability under large deformations, due to more resilient conductive pathways and network cohesion. In contrast, GELTG000 and GELTG025 show reduced sensitivity and increasing deviation from linearity at high strain, likely due to limited structural integrity and poor modulation of conductive channels.

These differences can be directly correlated with the network morphologies observed by SEM (Figure 2e–h), where GELTG125 displays a moderately crosslinked architecture that preserves porosity for reversible deformation while maintaining structural continuity-ideal for stable electromechanical performance. In
contrast, GELTG250, with its over-crosslinked and brittle matrix, restricts dynamic rearrangement, leading to diminished sensing performance despite high cohesion.

These findings demonstrate that intermediate crosslinking yields the most favorable combination of mechanical robustness, thermal stability, and electromechanical responsiveness. Among all formulations, GELTG125 consistently emerges as the optimal composition—combining high extensibility and toughness with a reproducible, strain-dependent signal response and the highest thermal sensitivity observed across the series. This balanced performance profile renders GELTG125 particularly suitable for multifunctional, skin-compatible sensing platforms requiring reliable detection of both mechanical and thermal stimuli under dynamic conditions.

To position our material within the broader landscape of hydrogel-based strain sensors, we benchmarked the performance of GELTG125 against state-of-the-art systems reported in the literature (Figure 4a; Table S2, Supporting Information).[⁵⁰⁻⁷³] Notably, our formulation combines a high gauge factor (GF = 2.86) with a wide linear-response working strain range (up to 300 %)—a rare convergence in fully biopolymeric matrices. Most reported hydrogel sensors either operate at low strains or suffer from poor sensitivity under large deformation. For instance, gelatin- and cellulose-based systems typically exhibit GF values below 2 at strains beyond 150%, underscoring the uniqueness of our design. GELTG125 successfully bridges this trade-off, validating our strategy based on enzymatic crosslinking and glycerol-driven plasticization.

Notably, only a limited number of literature reports, highlighted by triangle symbols in Figure 4a, employ exclusively biopolymeric scaffolds, highlighting the distinctiveness of our approach. All other entries exploit the mechanical advantages of synthetic polymers such as PVA, which, while effective, compromise the material's sustainability. This comparison highlights the distinctiveness of our approach, which achieves competitive performance without resorting to synthetic or hybrid frameworks. Recent studies have elegantly shown that composite architectures and geometrical design strategies can be employed to further amplify strain sensitivity.[⁷⁴,⁷⁵] Such inspiring directions will be explored in future developments to expand the performance landscape of this fully bio-based platform.

We then assessed the hydrogel's responsiveness to different strain amplitudes. As shown in Figure 4b, GELTG125 exhibits sharp, proportional, and fully reversible resistance changes ($\Delta R/R_0$) under incremental strains of 5 %, 10 %, 15 %, and 20 %. Signals show negligible hysteresis or drift, indicating stable conductive pathways and excellent structural reversibility across repeated cycles.

Long-term durability was confirmed through cyclic loading tests. Under 10 % strain, GELTG125 maintained a highly stable response over 1000 consecutive cycles (Figure 4c), with no signal degradation. A zoomed-in region highlights the excellent reproducibility of resistance modulation, suggesting minimal mechanical or electrical fatigue. This robustness was further validated over 5000 cycles (Figure 4d), with constant amplitude, confirming preservation of the conductive network and mechanical integrity under extended use (i.e., 98% of performance retention).

To demonstrate practical functionality, we implemented a proof-of-concept Morse code transmission system. As shown in Figure 4e, programmable strain pulses encoded the sequences “HARE” (yellow trace) and “TORTOISE” (purple trace). Each character is distinctly resolved based on pulse width and spacing, with response and recovery time of ≈0.3 s, allowing accurate signal interpretation. The high contrast and temporal resolution confirm the material’s potential for interactive soft electronics requiring clear digital output and rapid responsiveness.

To further validate durability and functional stability, additional tests were performed. Cyclic tensile and compressive experiments (Figure S9, Supporting Information) showed highly reproducible profiles over 100 cycles, confirming excellent elasticity and mechanical resilience. Thermogravimetric analysis after 16 months of storage (Figure S10, Supporting Information) revealed unchanged mass loss compared to fresh samples, indicating superior water retention. Moreover, mechanical, conductive, and sensing performances remained stable after long-term storage and under low-temperature (0 °C) and high-humidity (84 %RH) conditions (Figures S11, S12 and Table S3, Supporting Information), with minimal variation in electromechanical sensitivity. These results confirm the outstanding environmental and long-term stability of the gelatin–TG organohydrogel. Real-time motion sensing was further demonstrated through finger-bending (angle-tracking) test (Figure S13, Supporting Information), both showing rapid and reproducible responses under dynamic conditions. MTT assays on L929 fibroblasts (Figure S14, Supporting Information) confirmed the biocompatibility of the crosslinked hydrogels, with cell viability above 70% in accordance with ISO 10993. These results establish the gelatin–TG organohydrogel as a sustainable and biocompatible material for wearable soft electronics.

Altogether, these results underscore the exceptional combination of mechanical resilience, electromechanical sensitivity, signal fidelity, and operational stability offered by GELTG125. Coupled with its high thermal responsiveness and humidity resistance, this formulation delivers a rare synergy of performance parameters in a fully bio-based and biocompatible matrix. These attributes position GELTG125 as a promising material for next-generation wearable, implantable, and on-skin devices aimed at motion tracking, human-machine interfacing, and health monitoring in dynamic, real-world environments.

### 3. Conclusion

We have developed a fully bio-based organohydrogel composed of gelatin, glycerol, and food-grade transglutaminase (TG), combining mechanical stretchability, thermoresponsive conductivity, electromechanical sensitivity, and environmental resilience. The fabrication process is a simple one-pot, low-temperature protocol that employs only commercially available, food-safe ingredients—placing this platform among the most accessible and sustainable soft sensor materials reported thus far.

The optimized GELTG125 formulation exhibits a gauge factor of 2.86 with a linear response up to 300 % strain and a thermal sensitivity of $0.26 \, °C^{-1}$ in the physiological range (20–45 °C). These properties are accompanied by excellent signal stability over thousands of deformation cycles and minimal sensitivity to
<figure><img src="image_7.png" /><figcaption>**Figure 4.** a) Comparison of gauge factor versus linear-response working strain range across reported hydrogel-based strain sensors, highlighting the superior combination achieved in this work (GF = 2.86, linear response up to 300 % strain). b) Relative resistance variation (ΔR/R₀) of GELTG125 under increasing tensile strains (5 %, 10 %, 15 %, 20 %), showing excellent signal responsiveness and reproducibility. c) Long-term cyclic durability test of GELTG125 under 10 % strain for 1000 cycles. Inset: zoomed-in region displaying stable and repeatable response. d) Signal retention during 5000 stretch/release cycles, confirming minimal fatigue and consistent performance. e) Proof-of-concept demonstration of Morse code detection using GELTG125, showing clear signal distinction for the sequences “HARE” (yellow) and “TORTOISE” (purple). The finger was bent by 90° to produce the spike signals shown.</figcaption></figure>
ambient humidity, due to the absence of mobile salts or hygroscopic dopants. Multiscale characterization confirms that these features stem from a finely balanced network architecture—moderately crosslinked and structurally optimized—to provide both mechanical resilience and dynamic ionic mobility.

This study underscores the power of enzyme-guided crosslinking to tune macroscopic functionality through precise structural control. By uniting performance, reproducibility, and eco-compatibility in a single edible matrix, this platform lays the foundation for a new class of wearable, implantable, and ingestible soft electronics—where sustainability, safety, and performance converge.

## 4. Experimental Section

**Materials:** Gelatin from porcine skin (Product code G2625, type A, gel strength ≈175 g Bloom; pH of a 2% (w/v) aqueous solution is equal to 5.01) and glycerol (BioXtra, ≥ 99%) were provided by Sigma Aldrich. Food-grade transglutaminase formulation was purchased from Bindly (Product n. 8437020422024, activity = 125 U g⁻¹). Deionized water was used for all solution preparations. All reagents were used without further purification.

**TG-Crosslinked Organohydrogels Preparation:** Organohydrogel samples were prepared by mixing 5 mL of a 20% (w/v) gelatin solution in a glycerol:water (1:2) mixture with 1 mL of transglutaminase (TG) solution at various concentrations (0.000, 0.025, 0.125, or 0.250 g mL⁻¹ in deionized water). The compositional details are reported in Table S4 (Supporting Information). 5 mL of the resulting mixture (6 mL) was poured into a PMMA mold (9.5 cm × 3.0 cm), and the samples were incubated at 60 °C for 2 h to allow enzymatic crosslinking to proceed. After cooling to room temperature, the gels were demolded and cut into rectangular slides (4.5 cm × 1.0 cm). Samples were coded as GELTGXXX, where XXX is 000 (Blank), 025, 125, or 250, with obvious meaning. The thickness, measured with a Mitutoyo digimatic micrometer, is ≈630 µm for all the samples (see Table S1, Supporting Information). As illustrated in Figure S1 (Supporting Information), the preparation method is readily scalable—using a 25 cm × 15 cm mold.

**Characterization Techniques—UV-Vis Spectrophotometry:** Absorbance spectra of the organohydrogels were acquired using a Cary UV-vis multi-cell spectrometer (Agilent) across the 300–800 nm range.

**Fourier-Transform Infrared Spectroscopy (FTIR):** FTIR spectra of the dried GELTGXXX samples were acquired in ATR (Attenuated Total Reflectance) mode with a Thermo Nicolet Nexus 870 FT-IR spectrophotometer (Thermo Fisher Scientific, Waltham, MA, United States) equipped with a Golden Gate accessory. All the spectra were recorded with a resolution of 2 cm⁻¹, averaging 128 scans for measurement, in the range 4000–650 cm⁻¹.

**X-Ray Photoelectron Spectroscopy (XPS):** X-ray Photoelectron Spectroscopy was performed on air-dried samples by a Thermo Scientific K-Alpha X-ray photoelectron spectrometer, equipped with an aluminum X-ray source (energy 1.4866 keV) at a vacuum level of 10⁻⁸–10⁻⁹ mbar in the main chamber. The spot size of the X-ray beam was fixed at 400 µm. All XPS spectra were calibrated to the C 1s peak at 284.8 eV.

**Differential Scanning Calorimetry (DSC):** DSC measurements were performed using a Discovery DSC 2500 (TA Instruments). All samples were sealed in Tzero aluminum hermetic pans and analyzed under two different protocols. To determine the glass transition temperature (Tg), samples were heated from 5 to 70 °C at 10 °C min⁻¹. To assess the water state, samples were first cooled from 5 to -60 °C and then heated from -60 to 25 °C at the same rate. All samples were pre-equilibrated at 40% relative humidity. Data were processed using Trios software (v5.1.1.46572, TA Instruments).

**Thermogravimetric Analysis (TGA):** TGA was carried out using a Discovery SDT650 (TA Instruments). For degradation profile analysis, samples equilibrated at 40% RH were heated from room temperature (RT) to 950 °C at 10 °C min⁻¹ under an air flow of 100 mL min⁻¹ in alumina crucibles. To quantify water content, a separate ramp from RT to 110 °C was applied (10 °C min⁻¹), followed by a 20-min isothermal step, under nitrogen flow (100 mL min⁻¹) in aluminum pans. All results were processed with Trios v5.1.1.46572.

**Small-Angle X-Ray Scattering (SAXS):** SAXS measurements were performed using a Xeuss 3HR system equipped with a GENIX-Fox 3D Cu Kr microfocus source (λ = 1.542 Å, 30 W) and an Eiger 1 m hybrid pixel detector (Dectris). Data were collected at sample-to-detector distances of 400 and 1800 mm, covering a q-range of 0.005–0.5 Å⁻¹.

**Scanning Electron Microscopy (SEM):** To assess gel morphology, samples were immersed in 100 mL deionized water for 6 h to remove glycerol, freeze-dried, and sputter-coated with a 10 nm gold layer. Micrographs were acquired using a FEI Quanta FEG 650 microscope with a 30 kV accelerating voltage, a sample-detector distance of ≈10.2 mm, and secondary electron detection.

**Swelling Experiments:** The swelling behavior of GELTGXXX was evaluated by immersing 4.5 × 1.0 cm strips in 100 mL of deionized water. At predefined time intervals, the samples were removed, gently blotted with a paper towel to eliminate surface moisture, and then weighed. The Swelling ratio (SR) was calculated using the following equation:[76]

$$SR = (W_t - W_0) / W_0 \quad (1)$$

where Wt and W0 are the mass of the swollen gel at time t and at the beginning of the experiment (i.e., dry), respectively.

**Mechanical Characterization:** Tensile mechanical properties were evaluated using a Mark-10 M7-025E digital force gauge mounted on a Mark-10 ESM-303E motorized test stand. Gel strips were clamped at the edges, with a stretchable region of 3.5 cm. For the cycling mechanical tests, an Instron universal testing machine (Model 68TM) was used, equipped with a 100 N load cell and compression plates and pneumatic clamps for compression and tensile tests, respectively.

**Temperature Sensitivity Measurements:** Temperature-dependent conductivity was assessed by placing gel samples on a Peltier heating plate connected to a Keithley 2636B source meter. The temperature gradually increased from 20 to 50 °C while recording I vs T curves under a constant 0.2 V bias. For each formulation, three independent samples were tested, and one was measured in triplicate to assess reproducibility.

**Humidity Sensitivity Measurements:** Humidity responsiveness was evaluated using 36 mm × 2 mm gel strips connected to a PCB board via iron crocodile clips. Samples were placed inside a custom-built humidity chamber, where RH was varied from 20% to 80% using wet or dry air from a commercial humidifier. Humidity was monitored with a Sensirion SMART sensor. Electrical response was measured with a Keithley 2636B under 0.2 V bias. Each formulation was tested on three samples, one of which was measured in triplicate.

**Strain Sensitivity Measurements:** Strain-sensing performance was measured using the same Mark-10 tensile setup. Gel samples (3.5 cm gauge length) were connected to the Keithley 2636B system, and current changes were recorded as I-t curves under an applied 1.0 V bias. Measurements were performed in triplicate.

**Probe Penetration Test:** GELTGXXX stripes (4.5 × 1.0 cm) were placed on copper tape attached to a thermoelectric module. A conductive probe was positioned on top of the gel. Upon heating, the gel softened, allowing the probe to penetrate and contact the copper tape, completing the circuit (see Figure S15, Supporting Information). The electrical signal was recorded to monitor the temperature-dependent conductivity response using a digital multimeter.

**Biocompatibility assessment:** In vitro cytocompatibility was conducted according to ISO 10993 with the extraction method. L929 mouse fibroblasts were cultured in RPMI medium supplemented with 10% fetal bovine serum, 1% L-glutamine, and 1% penicillin-streptomycin (defined as complete RPMI culture medium) at 37 °C and 5% CO₂. Samples were sterilized under UV light for 15 min per side.

For the extract preparation, samples of 15 × 10 mm were incubated in PBS for 1 h and after in 1 mL of complete RPMI culture medium at
37 °C for 24 h with agitation (40 rpm). A copper sample (15 × 10 mm) and an aliquot of complete RPMI culture medium with no added material were incubated in the same conditions described above and considered as positive and negative controls of toxicity, respectively.

L929 mouse fibroblasts were seeded in 96-well plates at 8 × 103 cells per well. After 24 h of seeding, the medium was replaced with the sample extracts and controls (200 µL per well). Following an exposure time of 24 h, the MTT assay was performed for cell viability quantification. The MTT is based on the reduction by viable cells with active mitochondria of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Briefly, 20 µL per well of the MTT reagent (0.5 mg mL⁻¹ in PBS) was added, and the plate was incubated at 37 °C for 3 h. The supernatant was gently removed and replaced with DMSO (100 µL per well) to solubilize the MTT tetrazolium dye. The absorbance was measured at 550 nm wavelength using a microplate reader (Spectrafluor Plus, TECAN Austria GmbH, Grödig, Austria). The percentage of cell viability was related to the negative control, assumed as 100%.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

The authors acknowledge financial support by the Agence Nationale de la Recherche through the Interdisciplinary Thematic Institute SysChem via the IdEx Unistra (ANR-10-IDEX-0002) within the program Investissement d'Avenir, the Foundation Jean-Marie Lehn, the Institut Universitaire de France (IUF). The authors are also grateful to the financial supports from the CSGI (Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase), the MUR-Italy ("Progetto Dipartimenti di Eccellenza 2023-2027" and "Progetto Dipartimenti di Eccellenza 2018-2022" (58503DIP_ECC) allocated to Department of Chemistry "Ugo Schiff"), the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 "Dalla ricerca all'impresa"-Call for tender No. 341 of 15/03/2022 of Italian Ministry of Research funded by the European Union - NextGenerationEU, CUP: B83C22004890007.

## Conflict of Interest

The authors declare no conflict of interest.

## Author Contributions

P.T., A.T., Y.J., M.B., and P.S. conceived the experiments and designed the study. P.T. performed the physicochemical characterization. P.T. and A.T. performed the electrical characterization. B.H. performed the XPS analysis. T.A.K. and A.C. performed the biocompatibility assessment. P.T., A.T., Y.J., M.B., and P.S. discussed the results and contributed to the interpretation of data. P.T., A.T., Y.J., M.B., and P.S. cowrote the paper with input from all co-authors.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

biopolymer materials, enzymatic crosslinking, gelatin, organohydrogels, strain sensing, wearable sensors

Received: August 8, 2025
Revised: October 23, 2025
Published online:

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