Transient Dual‐Response Iontronic Strain Sensor Based on Gelatin and Cellulose Nanocrystals Eutectogel Nanocomposites

The emergence of wearable strain sensors in soft electronics has the potential to revolutionize healthcare and robotics. However, current sensors are based on petroleum‐based conductive composites that have a limited strain range. Ionic conductors such as hydrogels offer expanded strain range but have poor long‐term stability and restricted temperature operating window. Deep eutectic solvents (DESs) are promising nonaqueous electrolytes alternatives with green credentials. By combining DES electrolytes with biopolymers, transient ionic conductors are developed with high stretchability, and excellent chemical and thermal stability. Herein, cellulose nanocrystals (CNC) are incorporated, bearing ─OSO3H or ─COOH groups, to gelatin‐based eutectogels to produce nanocomposites with enhanced properties and additional functionalities. The eutectogel nanocomposite containing 1.0 wt.% COOH‐CNC demonstrate enhanced stretchability (375%) and ionic conductivity (3.0 mS cm−1) compared to the pristine gelatin‐based eutectogel (300% strain and 2.0 mS cm−1, respectively). Moreover, the spontaneous assembly of CNC within the eutectogel results in birefringence, which changes when stretching the nanocomposites. Thus, CNC incorporation provides the gelatin‐based eutectogel with a dual‐response capabilities when stretched, expanding their applications to new areas such as transient multi‐responsive strain sensors for wearable electronics, and multifunctional substrates for soft robotics, without compromising overall performance or sustainability.


Introduction
Soft electronics are devices conforming to the dynamic motions of the human body that have enabled novel intelligent systems in healthcare (i.e., smart textiles, wearable sensors, medical implants) and robotics (i.e., humanmachine interfaces). [1]Particularly, wearable strain sensors that detect motions and inform the user/machine in realtime have promising applications for the next-generation of personalized healthcare devices (i.e., assessment of prosthetics or articulations functions) and control of machines or prosthetics. [2,3][6] However, percolated networks are susceptible to structural damage at large strain, causing irreversible losses in electrical properties and in sensing functions (operation generally limited to strain ranging from 0 to 100%). [6]eanwhile, the sustained use of nonbiodegradable polymers and nanofillers has caused the accumulation of electronic waste (e-waste) in the environment. [7]With a 30% increase in the past decade, e-waste has become the fastest-growing waste stream worldwide and a threat to the environment and human health. [8,9]Hence, the next generation of soft electronics should be "transient," capable of partially, or fully, decomposing into harmless by-products after their service life.Thus, innovations in material design are paramount for fabricating the next generation of soft electronics and wearable strain sensors with expanded strain range and a minimal impact on the environment.
Iontronics have gained considerable interest due to their properties scarcely found in traditional electrical conductors, including optical transparency, stretchability, as well as self-healing and adhesive properties. [10,11]Iontronics operate owing to the coupling of electrons/ions in ionic conductors, allowing for the transmission of electrical signals due to ion migration.Moreover, when stretched, ion migration is hindered due to the lengthened conducting paths, which increases the resistance of ionic conductors. [12]Stemming from these properties, ionic conductors have emerged as attractive materials for the development of wearable strain sensors for motions detection, human-machine interfaces, and soft robotics. [13]To date, ionic conductors mostly consist of polymeric hydrogels, wherein electrical signal transmission is facilitated by the migration of ions (i.e., dissociated salts) through the water phase. [13]In such systems, the deformation of the solvent phase within the polymeric matrix will allow uninterrupted operation even at high strain (typical operation at strains above 1000%). [14]Thus, ionically conducting hydrogels have allowed the detection of expanded ranges of strain as compared to conventional percolated elastomer-conductive filler composites.However, the applicability of polymeric hydrogels as wearable strain sensors remains limited by their poor long-term stability (i.e., evaporation of water over time) and the restricted range of operating temperatures (i.e., freezing of water at low temperatures). [14,15]eep eutectic solvents (DESs) consist of mixtures of a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD) capable of associating via hydrogen bonding. [16]DESs have emerged as attractive solvents for the design of ionic conductors maintaining long-term stability thanks to its negligible volatility that makes them capable of operating over an expanded range of temperatures. [17]As a result of these properties, DESs are positioned as substitutes for solvents vulnerable to dehydration or evaporation, such as water.
[20] In the past few years, a range of high-performance eutectogel strain sensors were introduced, [21] displaying high mechanical strength (up to 31 MPa tensile strength), [22] high stretchability (up to 5100% strain), [22,23] as well as additional adhesive and/or selfhealing functionalities. [21][24] Thus, the design of high-performance eutectogel strain sensors composed solely of renewable and biodegradable components has remained a significant challenge. [19]mong biopolymer alternatives to petroleum-based polymers, gelatin, a hydrolyzed form of collagen (the most abundant protein in humans and mammals), [25] promotes dynamic non-covalent interactions (hydrogen bonding, hydrophobic, and electrostatic interactions) with DESs. [21,26,27]Capitalizing on the properties of gelatin, the Panzer group presented a gelatin-based wearable capacitive strain sensor featuring a eutectogel containing ethylene glycol : choline chloride (2:1) DES (ethaline). [28]The eutectogel was optically transparent, had high ionic conductivity (2.5 mS cm −1 ), and displayed a high stretchability (300% strain).Likewise, recent research conducted by Picchio et al. introduced tannic acid-decorated cellulose nanocrystals as nanofillers for gelatin eutectogels.The obtained nanocomposites demonstrated good mechanical properties (180% strain), a considerable conductivity (1.05 mS cm −1 ) as well as the capability to be 3D-printed, setting an unprecedent for the reinforcement of eutectogels with nanomaterials. [29][32][33] For instance, cellulose nanomaterials with a high abundance of hydroxy groups and high aspect ratio allow for the fixation of ionic species (i.e., non-covalent interactions with IL) and the formation of additional transfer paths through polymeric matrices to enhance ionic conductivity. [31,33]Moreover, in addition to being flexible, biodegradable, renewable, optically transparent, and possessing high mechanical strength, cellulose nanocrystals (CNCs) can undergo spontaneous self-assembly, allowing the design of macroscopic hierarchical structures with additional optical functionalities. [30,34,35]The nematic phase of CNCs consists of layers of nanocrystals spontaneously assembling parallel to a vector.[37] Specifically, CNC assemblies have displayed birefringence due to the small distance between two nearby and perpendicular layers (referred to as the pitch) that can alter the polarization of transmitted light.[40] Specifically, stretching the elastomeric nanocomposite can alter CNC birefringence, which is detectable by observing the material through crossed polarizers.
Herein, we report the design of gelatin-based eutectogel nanocomposites as dual-response strain sensors.The eutectogel nanocomposites were prepared using an ethaline DES, gelatin as the supporting matrix, and two types of CNCs with distinct surface chemistry as nanofillers.Specifically, CNCs containing sulfate half-ester groups (─OSO 3 H, abbreviated as S-CNC) and carboxyl groups (COOH-CNC) were incorporated in the eutectogels.The incorporation of 1.0 wt.% CNC enhanced the eutectogel ionic conductivity (by 50%) and thermal properties (+10 °C in melting temperature) as well as imparted birefringence to the nanocomposites.Notably, the incorporation of 1.0 wt.% COOH-CNC yielded more pronounced enhancement in the eutectogel ultimate tensile strain (ca.375%) as compared to 1.0 wt.% S-CNC (ca.330%), highlighting the importance of CNC surface chemistry in designing high-performance DES nanocomposites.Stretching the eutectogel nanocomposites led to changes in ionic conductivity and birefringence, which allowed the design of dual-response (resistive and optical) strain sensors.The promise of the dual-response eutectogel as a wearable sensor was further corroborated by the successful detection of various human motions (finger, elbow, knee), demonstrating the capability of these engineered systems as all-natural and transient wearable strain sensors for soft electronics.

CNC Surface Functionalization
The quality of dispersion and distribution of nanofillers in the nanocomposite depends on the interactions established at the interface between the matrix and the nanofillers.These interactions are crucial for maximizing the nanocomposite's properties, such as mechanical strength, thermal stability, and electrical/ionic conductivity.To initiate the process, commercial CNCs (S-CNC) were used as the starting material followed by the DES treatment.The importance of CNC surface chemistry was assessed by preparing eutectogel nanocomposites containing S-CNC or carboxylated CNCs (COOH-CNC).COOH-CNC were prepared using a previously reported DES treatment (choline chloride/oxalic acid dihydrate) that promoted esterification reactions between the hydroxyl groups on the surface of CNC and the carboxyl groups of oxalic acid. [41]CNC surface charge density was investigated by titration (Figure S1, Supporting Information), with S-CNC possessing 0.295 ± 0.015 mmol g −1 of OSO 3 H moieties, and COOH-CNC exhibiting 0.2 ± 0.018 mmol g −1 of carboxylic acid moieties (Figure S1, Supporting Information).COOH-CNC surface charge density agreed well with a previously reported work. [42]The functionalization of CNC surface was further corroborated by comparing the ATR FTIR spectra of S-CNC and COOH-CNC (Figure 1a).Expectedly, CNCs displayed bands at 3600-3200 cm −1 and 895 cm −1 , representative of cellulose hydroxyl groups (O─H stretching) and -glycosidic linkages (C─O stretching), respectively. [43]Carboxylic acid groups were observed in COOH-CNC, with bands at 1735, and 1429 cm −1 , attributed to C═O stretching and O─H bending, respectively. [44]S-CNC and COOH-CNC exhibited the typical diffraction peaks observed in cellulose at 2 values of 15.3, 16.5, and 22.5°, attributed to the 110, 1-10, and 200, planes, respectively (Figure 1b). [45]COOH-CNC displayed a slight reduction of the crystallinity index as compared to S-CNC (83% and 80%, respectively), which indicated that the DES treatment had only a minimal effect on the crystalline properties of CNCs.

Chemical Characterization
In all systems, the DES liquid phase constituted the largest fraction of the nanocomposite by volume.The supramolecular structure of eutectogels consists of hydrogen bonds formed between its components (ethylene glycol and choline chloride).Therefore, hydrogen bonds are the predominant non-covalent forces driving the eutectogel properties such as thermal stability, conductivity, and optical properties. [46]The nature of the chemical interactions in the eutectogel was studied by ATR FTIR (Figure 2a).Gelatin displayed bands at 1630, 1530, and 1450 cm −1 that corresponded to amide I (C=O), amide II (N─H), and carboxylic groups stretching, [47] respectively, whereas the peak at 3300 cm −1 was attributed to O─H vibrations.The eutectogel displayed more intense, and redshifted, bands (1654, 1544, and 1481 cm −1 ) as compared to gelatin.Specifically, the redshift of protein amide I band was previously explained by the denaturation of -helix into -sheet or random coil, [48] which indicated that DES incorporation produced conformational changes in the structure of gelatin.
Eutectogel nanocomposites (containing S-CNC and COOH-CNC) displayed generally similar FTIR spectral signatures (Figure 2b), except for a slight increase in the bands at 1243 and 1197 cm −1 which is characteristic of amide III groups. [49]COOH-CNC band at 1735 cm −1 , characteristic of carboxylic acid moieties, was blueshifted (1720 cm −1 ) in the eutectogel nanocomposite, which may be explained by the formation of hydrogen bonds between CNC carboxylic acids and gelatin amino, and/or hydroxy groups. [50]Moreover, the visual observation of colloids in the S-CNC eutectogel nanocomposite, which were not observed in the COOH-CNC counterpart, indicated the effectiveness of the functionalization in enabling the more homogeneous incorporation of CNC.Thus, both FTIR results and visual observation corroborated the more homogeneous dispersion and distribution of COOH-CNC in the eutectogel nanocomposites as compared to S-CNC.These results are suggested to be due to COOH-CNC carboxylic acid moieties partaking in the formation of additional hydrogen bonds with the DES (Figure 2d), which indicate the importance of surface functionalization in preparing homogeneous CNC-DES nanocomposites.

Optical Properties
The eutectogel and the eutectogel nanocomposites transparency, measured by UV-vis spectroscopy, was reduced when incorporating higher loadings of S-CNC or COOH-CNC (Figure 3a-c).Specifically, the pristine eutectogel reached 94% maximum transmittance, while the eutectogel nanocomposites with 0.3, 0.5, and 1.0 wt.% S-CNC had maximum transmittances of 75, 65, and 35%, respectively (Figure 3a).Interestingly, a slight reduction (10%) in the maximum transmittance of the eutectogel nanocomposite was observed when incorporating 1.0 wt.% COOH-CNC.These results were in good agreement with previous observations, and further corroborated the more homogeneous dispersion and distribution of functionalized CNC in the nanocomposites. [51,52]nisotropic (nano)materials split polarized light into either ordinary or extraordinary beams, which may not have the same velocities or pathways when traveling through the (nano)materials.Interference colors are produced as the two beams emerge.Typically, low-birefringent materials exhibit low-order interference colors (white, gray, and black). [53]The birefringence is magnified when a higher degree of alignment is achieved for asym-metric materials (i.e., CNC systems).The eutectogel nanocomposites displayed birefringence due to the self-assembly of CNCs in nematic liquid crystalline phases (Figure 3c). [54]Specifically, no optical birefringence is observed in the pristine eutectogel (left), and more pronounced birefringence is observed when incorporating and increasing the loading of CNCs in the eutectogel (0.3-1.0 wt.% CNC, right).Low-order interference (white color) was observed in the nanocomposites as CNC loading increased, which is characteristic of the random self-organization of CNC into an isotropic nematic phase.It is well known that in aqueous suspension, the nematic phase of CNC is observed only above a critical concentration. [55]Meanwhile, the interference color can be tuned by reducing the pitch, referred to as the perpendicular distance between nanorods after a 360°rotation.Specifically, small-enough pitches can yield high-order interference colors, which can be achieved by adjusting the electrostatic repulsions between the CNC (i.e., addition of electrolyte or reduction of the CNC surface charges). [56]The low-order interference colors observed in the eutectogel nanocomposites prepared in this study are characteristic of the nematic phase possessing high pitch, which may be explained by the favorable interactions existing between CNCs and the DES.Expectedly, CNC functionalization increased the CNC surface charge, thereby reducing the content of the nematic phase due to enhanced interactions between the CNC and the DES, and consequently lowering the birefringence of the 1.0 wt.% COOH-CNC eutectogel nanocomposite as compared to the 1.0 wt.% S-CNC counterpart.

Mechanical Properties
The pristine eutectogel demonstrated 67 kPa ultimate tensile strength and 300% strain at fracture, which agreed well with previously reported results. [41]The 1.0 wt.% S-CNC eutectogel nanocomposite displayed similar mechanical performance to the pristine eutectogel, with 60 kPa ultimate tensile strength (Figure 4a), 26 kPa Young's modulus, and 330% strain at fracture.Notably, the incorporation of 1.0 wt.% COOH-CNC reduced the tensile strength (35 kPa) and the Young's modulus (15 kPa) while a slight improvement in the strain at fracture was observed (375%, Figure 4b).We suggest that the lower ultimate tensile strength and Young's modulus of the COOH-CNC (1.0 wt.%) eutectogel nanocomposite are due to the abundant hydroxyl groups on the COOH-CNC surface.The formation of hydrogen bonds between COOH-CNCs and gelatin hydroxyl and amino groups may impede the formation of gelatin characteristic triple helix crosslinks, thereby yielding materials with lower stiffness and tensile strength. [57]igure 4c reveals the compressive loading-unloading curves of the eutectogel and of the eutectogel nanocomposites where the samples were compressed up to 50% of their original height.The hysteresis observed for all samples indicated the dissipation of mechanical energy caused by compressing the eutectogel and eutectogel nanocomposites, make them capable of withstanding repeated loading-unloading cycles (Figure 4d).While undergoing compression stress, the energy from the rupture of ionic bonds is dissipated into heat.When released, the broken bonds could be reformed, leading to the recovery of the mechanical properties of the materials.[60][61] While the pristine eutectogel demonstrated a maximum compression strength of 90 kPa, S-CNC, and COOH-CNC eutectogel nanocomposites displayed maximum values of 100 and 110 kPa, respectively, repre-senting enhancements in the compression strength of 10% and 20%.Overall, the 1.0 wt.% COOH-CNC eutectogel nanocomposite displayed higher stretchability (375%) than gelatin eutectogels previously reported for soft robotics applications (150%), [62] and slightly lower performance than petroleum-based polymer gelatin eutectogels (≈400). [63]Thus, despite the reduction in tensile strength and Young's modulus of 1.0 wt.% CNC eutectogel nanocomposites, the materials had acceptable ultimate tensile strength and sufficient stretchability (Figure 4d) to envision applications in soft electronics.

Ionic Conductivity
The pristine eutectogel displayed lower ionic conductivity than ethaline (2.0 mS cm −1 vs 7.5 mS cm −1 at RT).The reduction in ionic conductivity is due to the incorporation of gelatin (22 wt.%), which reduces the number of DES ionic charge carriers per unit volume and could also have 1) altered the DES hydrogen bonding network, or 2) increased the local DES viscosity.Particularly, lower viscosity is generally associated with higher conductivity in ionic conductors due to more efficient ion transport. [64]The incorporation of both S-CNC and COOH-CNC enhanced the eutectogel ionic conductivity (Figure 5a; Table S2, Supporting Information), with a 50% enhancement observed for the nanocomposites containing 0.5 wt.% S-CNC and COOH-CNC (3.1 and 3.0 mS cm −1 for S-CNC and COOH-CNC, respectively).The higher ionic conductivity values may be explained by the hygroscopic nature of CNCs, which would decrease the local viscosity within the eutectogel nanocomposites interface due to the introduction of small amounts of water. [65]Interestingly, higher loadings of S-CNC and COOH-CNC did not yield further enhancements in ionic conductivity, indicating that a plateau was reached.The presence of free water was not observed in the eutectogel nanocomposites (Figure S4, Supporting Information), suggesting that the enhancements in ionic conductivity may be caused by bound water on S-CNC and COOH-CNC surfaces. [66]The capability of the DES nanocomposites to serve as ionic conductors was further demonstrated by powering a LED (Figure 5b) and an LCD screen (Figure 5c).Overall, the nanocomposites displayed supe-rior ionic conductivity (more than two-fold increase) as compared to most previously reported petroleum-based, and gelatin-derived eutectogels (Table S3, Supporting Information), highlighting the promise of cellulose nanomaterials to design the next-generation, nature-derived, of transient ionic conductors with superior conductivity.

Thermal Properties
Expectedly, no transition was observed in the pristine DES within the range of tested temperatures (Figure S4, Supporting Information) as ethaline was previously reported to have an eutectic temperature at −66 °C. [67]In contrast, the pristine eutectogel displayed a melting temperature of 38 °C, which was attributed to the relaxation of the gelatin chains and is in good agreement with the results observed for a previously reported gelatin-based eutectogel. [28]The eutectogel melting temperature was higher than that of previously reported gelatin systems (20 wt.%) in water (30.8 °C) [68] due to the different interactions that gelatin can establish with DES as compared to water.However, the eutectogel possessed superior long-term stability as compared to a gelatin hydrogel, for which most of the water evaporated after one day of storage resulting in a stiff and ionically insulating material (Figure S2, Supporting Information).Interestingly, the incorporation of CNCs in the eutectogel shifted the transition from the semi-solid to a liquid state to higher temperatures.Specifically, the incorporation of 1.0 wt.% S-CNC and 1.0 wt.% COOH-CNC yielded nanocomposites with melting temperatures of 48 and 50 °C, respectively.We suggest that the higher melting point of the nanocomposites is caused by the formation of additional noncovalent interactions (i.e., hydrogen bonds) within the gelatin eutectogel network, thereby requiring additional energy to accomplish the transition from semi-liquid to a liquid state.It is important to note that the upshift of the melting temperature of the nanocomposites to temperatures of ca.50 °C was crucial, as it enabled the application of the developed systems as wearable strain sensors on the human skin.Moreover, stemming from the dynamic nature of the crosslinked network in the eutectogel and the eutectogel nanocomposites, the materials can be readily reformed after heating the samples at 50 °C (Figure S5, Supporting Information).Hence, the materials can be economically reused in terms of energy cost after structural damages (i.e., puncturing, tearing), while in the case of irreversible damage, the materials can be disposed of and left to biodegrade in the environment.

Wearable Strain-Sensor Design
Stemming from eutectogel stretchability and ionic conductivity, a range of eutectogel systems were previously explored as wearable strain sensors. [41,69,70]A useful parameter to evaluate the sensitivity of a strain sensor is the gauge factor (GF), which is defined as the slope of the plot of relative resistance versus an applied strain. [71]The GF of the pristine eutectogel, and the 1.0 wt.% S-CNC, and 1.0 wt.% COOH-CNC eutectogel nanocomposites were 1.3 ± 0.17, 1.5 ± 0.3, and 2.3 ± 0.2, respectively.Overall, the nanocomposites displayed superior GF as compared to previously reported eutectogel systems (Table S3, Supporting Information).[6] The percolated CNC network could have been altered when the nanocomposites were stretched, thereby yielding more pronounced changes in the electrical resistance of the nanocomposites when subjected to different strains as compared to the pristine eutectogel.As all the systems developed in this study displayed high sensitivity to mechanical de-formation (Figure 6a), these preliminary results suggested the promise of all-natural and transient eutectogel nanocomposites as strain sensors for wearable electronics applications.Moreover, in addition to changes in electrical resistance, the eutectogel nanocomposite displayed an optical sensing response when stretched (Figure 6f; Figure S3, Supporting Information).Similar to liquid crystals, the rearrangement of the nanocrystals can change the polarization of transmitted light due to a phase difference, which can result in significant changes in interference color when viewed between crossed or parallel polarizers. [72]Specifically, a pronounced shear-induced birefringence phenomenon was observed when stretching the eutectogel nanocomposites (Video S1, Supporting Information), which resulted in a progressive change in the interference color from green to yellow, passing through violet and orange, at higher strains.
The capability of the eutectogel nanocomposite for real-time monitoring of tensile strains was evaluated by placing the sensor on several parts of the body (finger, wrist, elbow, and knee) (Figure 6b-e).When applied in wearable configuration, the eutectogel nanocomposite was capable of detecting strain induced by most body movements.During the repeated process of stretching, bending a forefinger (mimicking typing, or playing the piano), and bending a knee (i.e., walking, or running) the electrical resistance exhibited reversible upward peaks due to the stretchinduced increase of resistance.In addition, the sensor demonstrated different magnitudes of changes in the electrical resistance according to the bending angle (including 0°, 30°, 60°, 90°, and 120°) when tested in the forefinger.As shown in Figure 6e, the ladder-shaped electrical signals are uniform and consistent to the volunteer forefinger angle of motion.Thus, the designed eutectogel nanocomposites can serve as all-natural and transient dual-responsive strain sensors for the development of the nextgeneration of wearable electronics.

Conclusion
In the present work, a new type of all-natural gelatin-based eutectogels with different CNC content were prepared.The different functional groups present on the cellulose nanocrystal surface (─OSO 3 H and ─COOH) were quantified, and the overall physicochemical and mechanical properties of the eutectogels were studied.The addition of S-CNC and COOH-CNC at a low concentration (1.0 wt.%) to the eutectogels demonstrated a positive effect on the mechanical, electrical, and thermal properties compared to the gelatin-based eutectogels due to the formation of a higher number of hydrogen bonding and other non-covalent interactions between the CNCs, gelatin, and the DES.While the S-CNC eutectogels exhibited an increase in the nanocomposite eutectogel mechanical properties, the softer COOH-CNC eutectogel could achieve an average of ≈ 375% strain, demonstrating that the functionalization of the CNC within the oxalic acid DES had a strong influence in the formation of new H-bonds.The presence of both CNC types in the eutectogels enabled optical birefringence and shear-induced birefringence phenomenon due to the different refractive index of the cellulose and the DES, where the phenomenon is more intense in the S-CNC eutectogel because the CNCs were less homogeneously dispersed within the DES, increasing the number of potential device applications.Notably, the eutectogels having CNCs demonstrated a higher  and d) knee.e) Ladder-shaped sensing response of the forefinger according to the bending angle (0°, 30°, 60°, 90°, and 120°).f) Optical birefringence was observed when stretching the sensor at the strain indicated.conductivity of up to 3.0 mS cm −1 in comparison with the pure eutectogel (2.0 mS cm −1 ), possibly because of the introduction of bound water on the nanomaterial.Finally, taking advantage on the aforementioned properties, a strain sensor was designed using the nanocomposite eutectogels, which exhibited a good linearity in response, and a gauge factor larger than ≈ 1.5 was obtained for all gels.These results confirm that the addition of cellulose nanocrystals to eutectogels is a promising approach for the design of ionically conductive soft materials, expanding the potential of designing new greener materials for biodegradable sensors, actuators, and greener flexible electronic devices.
Methods: DES Preparation: DES were prepared following a previously reported protocol. [73]Briefly, the ethaline DES was prepared by mixing ethylene glycol and choline chloride at a 2:1 molar ratio, respectively.The mixture was heated to 75 °C until a transparent, homogeneous, and colorless liquid was obtained.Separately, a dehydrated choline chloride-oxalic acid .2H 2 O DES was prepared at a 1:1 molar ratio and vigorously stirred at 60 °C until a homogeneous, transparent, and colorless mixture was obtained.The DES containers were then sealed until future use.
CNC Functionalization: CNC were functionalized following a previously reported protocol. [42]Briefly, 1.0 wt.% suspension of CNCs was placed in the oxalic acid DES.The mixture was then heated up to 60 °C for 3 h under vigorous stirring.The reacted product was then washed six times with ethanol (10 mL) and the conductivity of the solvent was measured with an EC meter (HI-2030, HANNA Instruments, USA) between each wash to ensure the removal of the DES.Finally, functionalized CNCs were resuspended in ethanol before future use.
Conductimetric Titration: 1.0 wt.% CNC suspension in water (pH 7) was used for the conductimetric titration of the functional groups on the CNC surface.Distinct titration procedures were followed due to the nature of the functional groups (weak acids or strong acids) of the two CNCs. [74]he measured conductivity was corrected for the volume of NaOH added at each data point using the following equation: Two different titrations were performed to calculate the concentration of sulfate half-ester [OSO 3 H] and carboxyl [COOH] groups on CNC surface.Additional details on the titration methods are available in the Supporting Information.The functional group concentration was calculated once the equivalence point was reached using the equation below: Eutectogels Preparation: The eutectogels were prepared following a previously reported protocol. [41]Briefly, a 22 wt.% gelatin solution was prepared in ethaline DES under slow stirring to avoid bubble formation and heated up to 75 °C for 1 h until a transparent yellow mixture was formed.The CNC eutectogel nanocomposites were prepared by placing desired amount (relative to gelatin mixture weight) of S-CNC or COOH-CNC powder into the gelatin mixture.Finally, the mixtures were cast in molds and left to gel at 4 °C for a day.
Characterizations: Infrared Spectroscopy: A Fourier Transform Infrared spectrometer (FTIR) (Spectrum Two, PerkinElmer, USA) equipped with an attenuated total reflectance (ATR) accessory was used for spectra collection within the 600-4000 cm −1 wavenumber range at a 4 cm −1 resolution.
X-ray Diffraction: X-Ray diffraction (XRD) analysis (Ultima IV, Rigaku, Japan) was performed on CNC samples (CelluForce NCC and functionalized CNC) at conditions of 30 mA and 40 kV (CuK  = 1.5406Å).The diffraction data were collected from 2 = 4 to 60°at a scanning rate of 4°min −1 and compared with the JCPDS card No. 50-2241.
Segal's equation was used to calculate the relative crystallinity index (CI).I max represents the intensity of the crystalline plane (200) and I am represents the intensity of the baseline at 2 = 18°: Optical characterization: Eutectogel transmittance (%) was measured using a UV-vis spectrophotometer (Genesys 840-208200, Thermo Scientific, USA) in the 400-800 nm range.A cylindrical sample was attached to the side of a quartz cuvette, while a pristine ethaline DES solution was used as the blank.For birefringence observation, the same samples were placed under an optical microscope (SMZ171, Motic, Spain) and observed through crossed linear polarizers.
Mechanical Characterization: A universal testing machine (Z005, Zwick-Roell, Germany) was used to determine the eutectogel and the eutectogel nanocomposites tensile performance, and an Electromechanical Tester (LPS 103, MTS Insight, USA) was used to evaluate loading-unloading cycles.A dynamic mechanical analyzer (DMA) (RSA3, TA Instruments, USA) was used to evaluate compression performance.The eutectogel and the eutectogel nanocomposites were cast on a PTFE ASTM probe mold (2.52 × 4 × 23.64 mm) for tensile test samples, while cylindrical silicone molds (8 mm diameter × 4 mm thickness) were used for compressive test samples.1.0 wt.% COOH-CNC samples (4.59 mm × 20 mm) were subjected to loading-unloading cycles at 50% stress.Tensile tests were conducted at 4 °C at 50 mm min −1 and compression tests were conducted at room temperature at 0.6 mm min −1 .
Ionic Conductivity Measurements: DES ionic conductivity was measured by electrochemical impedance spectroscopy using a potentiostat equipped with a frequency response analyzer (Versa STAT 4, Princeton Applied Research, USA).Gel precursor solutions were poured into a custom-built Teflon cell array with gold-coated electrode pins and cooled overnight at 4 °C.Room temperature impedance spectroscopy measurements were performed over the frequency range of 1 Hz-300 kHz using a sinusoidal voltage amplitude of 10 mV (with a 0 V DC offset.Experiments were conducted by triplicate. Thermal Characterization: The eutectogel and the eutectogel nanocomposite thermal properties were studied by differential scanning calorimetry (DSC3+, Mettler Toledo, USA).Scans were carried out at 10°C min −1 under nitrogen flow.The samples were first cooled to −20 °C and maintained at temperature for 10 min before heating up to 140 °C and cooling down to room temperature.
Strain Sensor Characterization: Different strain sensors were fabricated as wearable devices and tested on different parts of the body (fingers, wrist, elbow, knee).The sides of the eutectogel and the eutectogel nanocomposites were covered with copper tape.Kapton tape was used to hold the sensors on the selected body part.A layer of silver paint was deposited to bridge the copper tape and the sensors and ensure good electrical contact.The resistance of the sensors at different strains was recorded using a USB digital multimeter (SPRKC-TOL-12967, Sparkfun, China).The sensing response was reported as the relative change in the sensors' electrical resistance when stretched using the following equation: where R represents the electrical resistance at a selected strain and R 0 represents the initial electrical resistance.The gauge factor (GF) at various tensile strains was calculated using the following equation: where ΔL/L 0 is the relative strain.Ethical Statement: Informed, written consent was obtained from all participants prior to data collection for the research.

Figure 2 .
Figure 2. a) ATR spectra of the DES, gelatin, and pristine eutectogel.b) ATR spectra of the eutectogel and the eutectogel nanocomposites.c) Photograph depicting the S-CNC and the COOH-CNC dispersions in DES.d) Schematic representation depicting non-covalent interactions between DES, gelatin, and COOH-CNC.

Figure 5 .
Figure 5. a) Ionic conductivity of the eutectogel and the eutectogel nanocomposites.Photographs demonstrating the capability of an eutectogel nanocomposite to conduct electricity and power b) a LED and c) an LCD screen.

Figure 6 .
Figure 6.a) Resistance variation of the strain sensor when subjected to stretch-release cycles.Sensing response when bending the b) wrist, c) elbow,and d) knee.e) Ladder-shaped sensing response of the forefinger according to the bending angle (0°, 30°, 60°, 90°, and 120°).f) Optical birefringence was observed when stretching the sensor at the strain indicated.