# Multiresponsive Ionic Conductive Alginate/Gelatin Organohydrogels with Tunable Functions

Pietro Tordi, Adrián Tamayo, Yeonsu Jeong, Massimo Bonini, Paolo Samorì

► To cite this version:

Pietro Tordi, Adrián Tamayo, Yeonsu Jeong, Massimo Bonini, Paolo Samorì. Multiresponsive Ionic Conductive Alginate/Gelatin Organohydrogels with Tunable Functions. Advanced Functional Materials, 2024, 34 (52), 10.1002/adfm.202410663. hal-04866334

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Submitted on 6 Jan 2025

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# Multiresponsive Ionic Conductive Alginate/Gelatin Organohydrogels with Tunable Functions

Pietro Tordi, Adrián Tamayo, Yeonsu Jeong, Massimo Bonini,* and Paolo Samori*

Materials combining stretchability and sensitivity to external stimuli are key for wearable electronics applications to enable the emergence of disruptive technologies in biosensing, health monitoring, photodetection and human motion recognition. Conductive organohydrogels have gained significant attention due to their high sensitivity and cost-effective preparation. Biopolymers like gelatin and alginate offer unique opportunities for developing responsive wearable devices, owing to their biocompatibility and sensitivity toward environmental factors. Here sustainable bio-inspired method is presented to produce alginategelatin organohydrogels combining transparency in the visible range, ionic conductivity, high stretchability, and multiresponsiveness. The controlled alginate's crosslinking with various metal cations like Mn²⁺, Cu²⁺, Fe³⁺, and Zr⁴⁺ enables modulating ionic conductivity as well as finely tuning the material's thermal and mechanical properties. These organohydrogels show responsiveness to temperature (from 10 to 50 degrees, with a sensitivity of 0.19 K⁻¹), relative humidity (from 20 to 80%, with a sensitivity of 0.022 RH(%)⁻¹), and strain (gauge factor >1.6), enabling real-time monitoring of environmental and physiological parameters. Remarkably, they also exhibit photoresponsivity of 9.2 µA W⁻¹ under visible light, a feature rarely reported in literature. The ease of tuning responsiveness to the chosen stimuli and the high sensitivities open perspectives for applying these materials as wearable stretchable sensors.

## 1. Introduction

The immense application potential of wearable electronics has triggered intensified research efforts in the development of new multifunctional materials combining stretchability and

sensitivity to external stimuli paving the way for their applications in health monitoring, biosensing, photoresponsivity, and human motion recognition.[1-7] Among flexible and stretchable materials, conductive hydrogels have gained a great research interest during the last decade because of their superior mechanical properties when compared to inorganic conductors.[8-13] The use of ionic conductivity for sensing is recently emerging as a powerful strategy enabling the use of polymeric matrices that are not intrinsically conductive, without the addition of carbon-based electron conductive fillers that affect the transparency of the final products.[14-16] Ionically conductive hydrogels operate on the basis of the movement of ions within their 3D structure, in analogy to the ionic conduction observed in biological tissues.[17-20] This characteristic, together with the high sensitivity and stable conductivity, make ionic conductive hydrogels exceptionally well-suited as wearable sensors for health monitoring.

The use of ionic conductivity opens the door to the design of stretchable sensors employing biopolymeric compounds,

which are notorious for their poor electrical conductivity. Biopolymers offer in fact interesting opportunities for the development of responsive wearable devices, since they are sustainable, usually biocompatible/biodegradable and often particularly sensitive to the surrounding environment. Among biopolymers, gelatin and alginate are well-suited to this purpose, when properly processed. Gelatin is a polypeptide obtained from the hydrolysis of collagen, capable of forming hydrogels extremely sensitive to temperature.[21,22] Alginate, on the other hand, is an exopolysaccharide extracted from brown algae that possesses high reactivity and selectivity towards metal cations with various oxidation states:[23-30] in fact, cations interact with both the hydroxyl and carboxylate groups on the polymer backbone, producing crosslinked hydrogel structures with characteristic porosities and mechanical properties. Gelatin and alginate hydrogels are well known for their compatibility with human tissues, making them commonly used as scaffolds for cell proliferation and as materials for wound dressings.[31-34]

Among hydrogels, those able to absorb a large amount of organic molecules retaining their structural integrity are attracting more and more interest in the last few years:[35,36] in fact, compared to conventional hydrogels, they can be endowed with

<footnote>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</footnote>

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

<footnote>ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202410663</footnote>

<footnote>© 2024 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.</footnote>

<footnote>DOI: 10.1002/adfm.202410663</footnote>
<figure><img src="image_3.png" /><figcaption>Thermal sensor</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Photodetector</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Humidity sensor</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Strain sensor</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_10.png" /></figure>

**Figure 1.** a) 3D cartoon of the multiresponsive alginate/gelatin organohydrogel crosslinked with a metal cation (MAlgGel), placed on human skin. The scheme shows the potential applications of MAlgGel, as thermal sensor, photodetector, humidity sensor and strain sensor. b) UV-Vis spectra, c) optical image of the prepared MAlgGel samples placed on top of a printed ISIS Strasbourg logo, and d) small angle X-ray scattering curves. Samples' coding: – = AlgGel = black; Mn = MnAlgGel = pink; Cu = CuAlgGel = blue; Fe = FeAlgGel = red; Zr = ZrAlgGel = green. SAXS curves were offset for clarity.

additional functionalities while retaining extremely high stability against stresses and enhanced sensitivity to environmental conditions. These emerging materials, known as organohydrogels, have recently found application as carriers for drug delivery or sensors (e.g., temperature, strain, and humidity).[37,38] The introduction of molecules such as glycerol[39] or ethylene glycol[40] is commonly exploited to impart high flexibility to the gel structure and to enhance its sensitivity (e.g., to strain) in a dehydrated environment. In this regard, You et al. developed gelatin/polypyrrole/reduced graphene oxide porous organohydrogel for pressure and strain sensing.[41] The sensitivity in this case relied on changes in the electrical conductivity of the matrix resulting from structural changes involving the porous architecture. Highly stretchable ionic conductive organohydrogels were also realized, with good sensitivities and durabilities.[42,43] Interestingly, Feng et al. developed ionic conductive strain sensors that can function as supercapacitors by doping the polymeric scaffold with alizarin red S.[44] It is worth mentioning that the introduction of light sensitive and conductive polymer fillers, such as polypyrrole, enables UV detection. However, it unfortunately de-

termines a loss of transparency and results in a high magnitude of dark state current, which is undesirable for practical device applications.[45,46] The organohydrogel responsiveness to multiple stimuli usually arises from the presence of fillers which however strongly hampers the reproducibility of the preparation and the tuning of the performances.

Aiming at fostering the use of biopolymer-based stretchable sensors for application in wearable electronics (e.g., e-skins) and environmental monitoring, here we report an unprecedented sustainable and reproducible preparation method for the development of alginate-gelatin organohydrogels simultaneously displaying ionic conductivity, stretchability and transparency in the visible range. Gelatin was used to impart stretchability and thermal responsiveness, whereas alginate was crosslinked with di-, tri- and tetravalent cations (namely Mn²⁺, Cu²⁺, Fe³⁺, Zr⁴⁺) to modulate both the mechanical properties and the ionic conductivity of the material. The possibility to compare between different ions (going beyond the common Ca²⁺-mediated crosslinking of alginate) adds novelty to our study, since no similar approaches have been reported in the literature. **Figure 1a** portrays
<table><caption>**Table 1.** Physicochemical properties of the MAlgGel organohydrogels.</caption><thead><tr><th></th><th>AlgGel</th><th>MnAlgGel</th><th>CuAlgGel</th><th>FeAlgGel</th><th>ZrAlgGel</th></tr></thead><tbody><tr><td>Thickness [µm]</td><td>511 ± 15</td><td>817 ± 24</td><td>805 ± 24</td><td>829 ± 25</td><td>847 ± 26</td></tr><tr><td>Polymer content [w/w%]</td><td>18.3 ± 1.0</td><td>28.4 ± 1.5</td><td>24.7 ± 1.0</td><td>21.9 ± 0.6</td><td>21.4 ± 1.1</td></tr><tr><td>Glycerol content [w/w%]</td><td>74.3 ± 0.8</td><td>57.9 ± 1.2</td><td>60.7 ± 0.7</td><td>63.2 ± 0.3</td><td>63.7 ± 0.8</td></tr><tr><td>Water content [w/w%]</td><td>7.4 ± 0.2</td><td>13.7 ± 0.3</td><td>14.6 ± 0.3</td><td>14.9 ± 0.3</td><td>14.9 ± 0.3</td></tr><tr><td>Degradation onset [°C]</td><td>148.9</td><td>171.3</td><td>168.7</td><td>167.8</td><td>169.6</td></tr><tr><td>M<sup>X+</sup> content [× 10<sup>-2</sup> mmol g<sup>-1</sup>]</td><td>—</td><td>7.1 ± 0.4</td><td>5.9 ± 0.3</td><td>7.5 ± 0.4</td><td>6.3 ± 0.3</td></tr><tr><td>T<sub>g</sub> [°C]</td><td>41.7 (57.1)<sup>a)</sup></td><td>42.4</td><td>42.5</td><td>52.5</td><td>62.8</td></tr><tr><td>YM [MPa]</td><td>1.87 ± 0.14</td><td>0.52 ± 0.08</td><td>2.44 ± 0.06</td><td>2.57 ± 0.16</td><td>2.46 ± 0.06</td></tr><tr><td>UTS [MPa]</td><td>1.45 ± 0.06</td><td>0.64 ± 0.02</td><td>1.88 ± 0.04</td><td>1.17 ± 0.11</td><td>1.42 ± 0.05</td></tr></tbody></table>

a schematic illustration of the multi-responsive alginate-gelatin organohydrogel (AlgGel) crosslinked with a generic metal cation (M), applied on human skin. These matrices proved responsiveness to temperature, humidity and strain, thus endowing our gels with a huge potential for real-time monitoring of multiple stimuli related to the environment and the human body (e.g., solar irradiation, tissue inflammation and motion). Significantly, the electrical properties of bottom-contacted two-terminals organohydrogel devices exhibited a markedly high photocurrent (I<sub>PHOTO</sub>) (ca. 20 nA) in response to visible light, a feature scarcely reported in the literature. When considering the incident light intensities, ranging from few µW cm<sup>-2</sup> to tens of mW cm<sup>-2</sup> in common polymer-based wearable photodetectors,<sup>[45-47]</sup> our device demonstrated fast and sensitive photoswitching behavior, with response and recovery times amounting to 2.23 s and 3.13 s, respectively, under 3.47 mW cm<sup>-2</sup> of white LED illumination. This unique photoresponsivity is attributed to the light sensitivity of gelatin.<sup>[45]</sup> and the efficient charge injection from metal electrodes to hydrogel upon exposure to white LED. The possibility to easily tune the responsive nature as a function of the chosen crosslinker and the demonstrated high sensitivities to multiple stimuli suggest promising perspectives for the use of our devices as wearable stretchable sensors.

## 2. Results and Discussion

### 2.1. Physicochemical Characterization of the MAlgGel Organohydrogels

The results of the analysis on the compositional, structural and optical properties of the effect of crosslinking of AlgGel samples upon addition of specific cationic species are reported in Figure 1b-d, Table 1, and Figure S1 (Supporting Information). They reveal that crosslinked samples possess comparable thickness (i.e., around 800 µm, see Table 1 and Figure S1, Supporting Information) exceeding that of AlgGel. Importantly, the sample's crosslinking endows robustness to the hydrogel structure, hindering its collapse during the drying process through a strong modification of the gel's structure, by enhancing its packing density (see Figure S2, Supporting Information). Figure 1b,c provide unambiguous evidence of the high transparency of MAlgGel

to visible light. The crosslinking ion does not particularly affect the amount of glycerol loaded by the gels during the last step of the preparation (between 58 and 64 (w/w%), Table 1). Likewise, Table 1 and Figure S3 (Supporting Information, TGA water evaporation thermogram) reveal that the water content inside the organohydrogels is constant at values of around 14% (w/w%) upon crosslinking. The polymer content of the samples is also reported in Table 1 and clearly varies as a function of the ion. This finding derives from the different shrinkage experienced by the polymer network upon the final dehydration step and strongly influences the light responsiveness of our materials (see Section 2.4). The absence of peaks in the DSC thermograms displayed in Figure S4a (Supporting Information) testifies that only non-freezable water is present in the samples,<sup>[48,49]</sup> suggesting that our materials can retain their flexibility below 0 °C, thereby extending their applicability beyond the limit of common hydrogels. TGA analysis (Figure S5 and Table S1, Supporting Information) highlights that the addition of glycerol to the uncrosslinked polymer matrix lowers the degradation onset of the polymer (corresponding to the first massive mass loss in Figure S5, Supporting Information, AlgGel thermogram) to approximately 150 °C, while the crosslinking increases the stability of the latter. The MAlgGel samples also display similar thermal stability, undergoing the first degradation around 170 °C. DSC curves reported in Figure S4b (Supporting Information) show that the addition of glycerol lowers the glass transition temperature (T<sub>g</sub>) of the polymer, increasing the chain's degrees of freedom. Interestingly, AlgGel shows two inflection points that can be ascribed to T<sub>g</sub> (at 41.7 °C and 57.1 °C, respectively): in fact, the absence of a crosslinker may lead to an easier reorganization of the polymer chains during the heating, resulting in a second glass transition. In the case of both MnAlgGel and CuAlgGel only one T<sub>g</sub> can be identified at 42.5 °C, in accordance with the previously hypothesized role of the crosslinker. In the case of FeAlgGel and ZrAlgGel the T<sub>g</sub> shifts to higher temperatures (i.e., 52.5 °C and 62.8 °C, respectively) evidencing that the freedom of the polymer chains is reduced by the crosslinking, probably due to the protonation that occurs in the acid Fe<sup>3+</sup> and Zr<sup>4+</sup> crosslinking bath. Such a reduced freedom leads to a higher rigidity of FeAlgGel and ZrAlgGel. The different mechanical properties of the MAlgGel samples arise from the different strength of the interactions between the M<sup>X+</sup> ions

<footnote>a) Second peak resulting from the re-assembling of the chains during the first thermal event. T<sub>g</sub>, YM, and UTS stand for glass transition temperature, Young's modulus, and ultimate tensile strength, respectively.</footnote>
<table><caption>**Table 2.** Conductivity, mechanical response, thermal-response, humidity and photoresponse of AlgGel and MAlgGel samples. The sensitivity parameters are averaged from three different measurements.</caption><thead><tr><th></th><th>AlgGel</th><th>MnAlgGel</th><th>CuAlgGel</th><th>FeAlgGel</th><th>ZrAlgGel</th></tr></thead><tbody><tr><td>Conductivity [mS m<sup>-1</sup>]<sup>a)</sup></td><td>0.17 ± 0.06</td><td>1.3 ± 0.3</td><td>3.7 ± 0.5</td><td>6.0 ± 1.4</td><td>6.1 ± 0.9</td></tr><tr><td>Temperature response [K<sup>-1</sup>]</td><td><sup>b)</sup></td><td>0.19 ± 0.02</td><td>0.16 ± 0.03</td><td>0.06 ± 0.01</td><td>0.13 ± 0.02</td></tr><tr><td>B [k] mol<sup>-1</sup>]<sup>c)</sup></td><td>6.4 ± 0.7</td><td>2.7 ± 0.2</td><td>2.2 ± 0.3</td><td>2.3 ± 0.6</td><td>2.2 ± 0.2</td></tr><tr><td>Humidity response [x 10<sup>-2</sup> RH(%)]<sup>-1</sup></td><td>(2.1 ± 0.2)</td><td>2.2 ± 0.3</td><td>2.1 ± 0.3</td><td>1.8 ± 0.1</td><td>1.5 ± 0.1</td></tr><tr><td>RH sensitivity range [%]</td><td>20–60</td><td>20–60</td><td>20–60</td><td>20–70</td><td>20–80</td></tr><tr><td>Photo-responsivity [µA W<sup>-1</sup>]</td><td>0.6</td><td>9.2</td><td>5.1</td><td>4.3</td><td>5.8</td></tr><tr><td>Photo switching speed [s]</td><td>3.42/4.35</td><td>2.23/3.13</td><td>3.31/3.61</td><td>4.00/4.28</td><td>3.76/3.92</td></tr><tr><td>GF1 [Region I]</td><td>2.80 ± 1.15</td><td>2.25 ± 0.57</td><td>1.67 ± 0.19</td><td>2.29 ± 0.22</td><td>2.38 ± 0.26</td></tr><tr><td>GF2 [Region II]</td><td>2.18 ± 0.38</td><td>2.02 ± 0.40</td><td>1.73 ± 0.18</td><td>2.15 ± 0.07</td><td>2.27 ± 0.18</td></tr></tbody></table>

and the polymer network, as also highlighted by the correlation lengths ($\xi$) extracted by small angle X-ray scattering (SAXS) investigation (Figure 1d and Table S2, Supporting Information). These aspects are discussed in greater detail in the next sections. As shown in Table 1, the inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis reveals that the amount of crosslinking ions in the MAlgGel samples is particularly low, yet sufficient to guarantee an easily detectable ionic conductivity and provide an excellent structural integrity under stress. The samples prepared using different cations (Mn$^{2+}$, Cu$^{2+}$, Fe$^{3+}$ and Zr$^{4+}$) and the non-crosslinked AlgGel exhibit significantly different conductivities, which can be attributed to the different ionic density, retention in the gel matrix and kinetic energy. The AlgGel sample exhibits the lowest conductivity (0.17 ± 0.06 mS m$^{-1}$), being one order of magnitude lower than that of MnAlgGel (1.3 ± 0.3 mS m$^{-1}$). The conductivity is even superior in the case of CuAlgGel (3.7 ± 0.5 mS m$^{-1}$), FeAlgGel (6.0 ± 1.4 mS m$^{-1}$) and ZrAlgGel (6.1 ± 0.9 mS m$^{-1}$) (Table 2). The higher glycerol content can contribute to generate the particularly low conductivity of AlgGel. However, differences in conductivity are not determined simply by the glycerol content within the samples (see Table 1), but they are rather related to the ions interaction with the polymer matrix (which is dependent on charge and dimension of the M$^{X+}$ species[50] and usually involves electrostatic forces). The ionic conductivity values of the MAlgGel samples are comparable with state-of-the-art ionic conductive hydrogel sensors (1–1000 mS m$^{-1}$).[51–54]

## 2.2. MAlgGel Organohydrogels Sensitivity to Thermal Variations

Our gels possess thermoresponsive properties that arise from factors such as ionic mobility and viscosity of the polymer network, which are highly dependent on the temperature.[55,56] The thermo-responsivity of the organohydrogels can be profitably used for environmental monitoring or biomedical application such as the real time observation of temperature variations induced by various external phenomena (e.g., sun exposition or water evaporation) or monitoring infection and inflammation evolution.

The temperature-dependence of the conductivity ($\sigma$) was extracted between 283 and 323 K, which are regarded as the optimal

range for electronic skins and generic wearable electronics.[57–59] The MAlgGel samples were connected to a home-made heater, equipped with two copper electrodes. As shown in Figure 2a and Figure S6 (Supporting Information), the electrical conductivity of the MAlgGel samples is highly dependent on the temperature. The charge transport mechanism in the hydrogel arises from the free diffusion of ions and their interaction with the hydrogel matrix: the ionic conductivity of the sample increases with the temperature. At low temperature, protons and ions tend to interact via dipole-dipole forces with the carboxylate and hydroxyl groups of the alginate/gelatine matrix, yielding low ionic conductivity for all the different MAlgGels. On the other hand, at higher temperatures, the interaction between ions and matrix is weakened and the water molecules are less prone to interact with glycerol: the resulting increase in ion mobility combined with the decrease of viscosity determines an enhancement in the ionic conductivity. The temperature sensitivity of the organohydrogels was extracted from the slope of the $\Delta\sigma/\sigma$ versus T curves reported in Figure 2b (see values in Table 2). MnAlgGel, CuAlgGel and ZrAlgGel provided thermal sensitivity values of 0.19, 0.16, and 0.13 K$^{-1}$, respectively, while FeAlgGel exhibited the lowest thermal sensitivity (i.e., 0.06 K$^{-1}$). This evidence may indicate that, in this specific case, the interaction between the crosslinking ions and the polymer matrix is the strongest among the tested samples, significantly hampering the ionic movement. The “long-distance” correlation lengths ($\xi_1$) of FeAlgGel (i.e., ≈6.3 nm, see Section S3 and Table S2, Supporting Information), which is determined by small angle X-ray scattering, is the smallest among the investigated samples, and it supports our hypothesis. Interestingly MnAlgGel exhibited a change of the thermal sensitivity above 307 K, which can be attributed to a phase transition of the gelatin component in the polymeric matrix.[60] The weak interaction between Mn$^{2+}$ and alginate (already reported in literature[25,26,61]) is sufficient to maintain the structural integrity of the material during the heating, over the examined temperature range: the net change in the ionic conductivity may arise from nonlinear variations of the polymer chains’ mobility as the temperature increases. AlgGel displayed a nonlinear dependence with high thermal sensitivity (>0.2 K$^{-1}$). It is worth mentioning that the onset of AlgGel samples melting was observed at ca. 300 K, which was accompanied by a drastic increase of the ionic conductivity (Figure S6a, Supporting Information). By and large, because of its

<footnote>a) Extracted using Cu electrode; b) Nonlinear temperature responsiveness; c) Activation energy of the ionic movement.</footnote>
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 2.** The influence of the temperature on a) conductivity and b) relative change of conductivity of AlgGel and the MAlgGels. The influence of relative humidity on c) resistivity and d) relative change of resistivity of AlgGel and the MAlgGels. All the measurements were conducted on three different batches for each sample (Figures S6 and S8, Supporting Information, full dots), demonstrating excellent reproducibility in terms of fabrication and temperature/humidity sensitivity.</figcaption></figure>

low linearity of the electrical response and the drastic change in the physicochemical properties over the thermal region of interest, AlgGel was therefore not considered as a promising platform for temperature monitoring. The crosslinked samples were subjected to multiple cycles of heating and cooling, to check their robustness to cyclability. We found a high reversibility in the range of temperature from 283 to 323 K (Figure S6, Supporting Information, empty dots) for all of them. Conversely, AlgGel did not recover its original performances after the melting at 300 K. In order to study in depth the ion conduction mechanism of the alginate/gelatin organohydrogels, the conductivity of the samples as a function of the temperature was fitted according to the typical Arrhenius plot (Figure S7, Supporting Information). The temperature dependence on the MAlgGel conductivity could not be accurately fitted using the aforementioned model: the Vogel-Tamman-Fulcher (VTF) approach, incorporating the influence of various physical parameters, (e.g., viscosity, temperature, ion charge) on the conductivity,[62,63] provided better results. The obtained B parameters (activation energy of the ionic movement, reported in Table 2), obtained using the VTF equation described in Section S4 (Supporting Information) (Equation S2, Supporting Information), are similar for all the crosslinked matrices (between 2.7 and 2.2 kJ mol⁻¹), while AlgGel reaches the highest value among the analyzed samples (i.e., 6.4 kJ mol⁻¹): this behavior

can be attributed to the lower amount of ions in the AlgGel matrix.

### 2.3. MAlgGel Organohydrogels Sensitivity to Relative Humidity Variations

The MAlgGel organohydrogels are able to reversibly absorb water due to the hydrophilic nature of both the polymeric matrix and the glycerol molecules, the latter being the main component of our materials (≈60% w/w, see Table 1). This characteristic makes our products an ideal platform for the development of high performance wearable humidity sensors.[64–66] The response to humidity changes was measured inside a home-made box, supplying air at different relative humidity (RH) levels inside the samples’ chamber. The resistivity (ρ) of the samples was recorded in the RH range between 20% to 80% (see Figure 2c), which is considered being the optimal range for their use in the field of wearable electronics.[64,66,67] Figure 2d portrays relative changes of resistivity (Δρ/ρ) of the samples at different RH. It shows that AlgGel, MnAlgGel, and CuAlgGel are characterized by identical humidity responses (≈0.020 RH(%)⁻¹) in the range from 20% to 60% RH, reaching a plateau of the electrical response for RH exceeding 60%. In contrast, FeAlgGel and
<figure><img src="image_15.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>(e)</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>(f)</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 3.** a) Photoresponse of MnAlgGel and AlgGel upon exposure to white LED. Schematic inset image displaying a side-view of MnAlgGel photodetector. b) Secondary-electron cutoff of Al (blue squares), Au (black), Cu (red), and MnAlgGel (pink). c) Current versus voltage characteristics of the MnAlgGel photodetector with a channel length of 3 cm, receiving the photons focused on the local area: a center of hydrogel (Position A) and a metal/hydrogel interface (Position B). d) Band diagram of the MnAlgGel/Cu interface. e) Time-resolved photoresponse of the MnAlgGel and AlgGel photodetector with a channel length of 1 cm. f) Normalized $I_{\text{Photo}}$ versus time plots of MnAlgGel. All the illuminated area by the LED in this experiment was fixed as a circle with a 1 cm radius.</figcaption></figure>

ZrAlgGel exhibited similar humidity-sensitivity (0.018 RH(%)⁻¹ and 0.015 RH(%)⁻¹, respectively) in a wide range of RH values corresponding to 20–70% and 20–80%, respectively. The broadening of the sensing range can be ascribed to the different water retention capacities among the samples resulting from the higher glycerol content (+3%, w/w) and stiffness of the FeAlgGel and ZrAlgGel samples with respect to the other M²⁺-crosslinked samples. In general, MAlgGel samples exhibited humidity sensitivity in the same order of magnitude (0.022–0.015 RH(%)⁻¹, see Table 2), which suggests that glycerol is ruling the sensing process by imparting high hydrophilicity to the matrix. The mechanism behind the sensing comprises the absorption of water into the structure that triggers an increase in the conductivity[⁶⁸] by proton/ion transport. After the water absorption, a proton may be transferred to a water molecule to form H₃O⁺ and migrate by hopping from site to site across the glycerol moieties along the organohydrogel. Compared with traditional humidity sensing materials, gels feature additional characteristics such as the high stretchability and adaptability that are key for the design of next-generations of e-skins.[⁶⁴] To assess the robustness of the crosslinked samples to the exposure at different RHs, they have been exposed to multiple cycles of humidity changes. All samples revealed a complete reversibility in the range of RH from 20% to 80% (Figure S8, Supporting Information, empty dots).

## 2.4. MAlgGel Organohydrogels as Photodetectors

A side-view scheme of MAlgGel photodetector under white LED illumination is illustrated in the inset of Figure 3a. Throughout this study, the illuminated area on the hydrogel devices remains constant at 3.14 cm², forming a circle with a radius of 1 cm. A polycarbonate substrate, which holds promise for wearable device applications (e.g., e-skins) due to its transparency and flexibility, was exploited to assess the photoresponsive characteristics of the MAlgGel organohydrogel. The copper electrodes (Cu in the inset image of Figure 3a) were patterned onto the polycarbonate substrate as bottom contact electrodes. To optimize the contact electrodes, we tested three different metals (i.e., aluminum, gold, and copper), while employing MnAlgGel as the active channel. Interestingly, MnAlgGel with Cu electrodes exhibited light sensitivity under white LED illumination (Figure 3a), while the other devices with Al and Au electrodes hardly showed photoresponsive characteristics (Figure S9a–c, Supporting Information). Considering the ability of all three metals to facilitate ionic conduction in conventional electrolyte systems,[⁶⁹,⁷⁰] the efficiency of electron injection at the MnAlgGel/Cu interface may promote enhanced ionic conduction within the hydrogel, thereby explaining the observed nontrivial photoresponse. To substantiate this interpretation, the secondary-electron cutoff was estimated using photoelectron yield spectroscopy in air (PYSA), as shown in
Figure 3b. The varying work function values from depending on the employed metals result in the formation of a variable built-in electric field in the organohydrogel at the metal/hydrogel interface. The MnAlgGel/Cu contact exhibits the lowest electric field value, while the MnAlgGel/Al contact shows the highest value, thus hindering the injection of photoexcited electrons into MnAlgGel. See Figures S9d–f and S10 (Supporting Information) for more details.

To investigate the interface effect on photoresponse, we meticulously controlled the position of light irradiation, as illustrated in the inset of Figure 3c. We selected two different light exposure positions: i) Position A—placed at the center of hydrogel channel, not including a metal/hydrogel interface—and ii) Position B—located at the metal/hydrogel interface. Upon illumination at Position A, the current displayed a behavior similar to the one observed in the dark. On the other hand, when the LED was illuminating at Position B, a clear photocurrent ($I_{\text{PHOTO}}$) response was observed, demonstrating a 30% increase in ionic current. The importance of the built-in electric field generated at the interface between an ionic conductor and a metal in contributing to $I_{\text{PHOTO}}$ has been reported in the literature.[71,72] In particular, the photoresponse in our MnAlgGel device can be ascribed primarily to the trapped electrons at the metal/hydrogel interface and the resulting built-in electric field. Figure 3d illustrates the band diagram of the MnAlgGel/Cu interface, which is based on the Fermi energy levels of each component, and shows the trap-induced photoresponse. Figure 3a,e demonstrate a distinct difference in $I_{\text{PHOTO}}$ between $M^{2+}$-crosslinked hydrogel and non-crosslinked hydrogel. The MnAlgGel device shows the best photoresponsivity compared to the others, as it has the highest polymer content (see Table 1) in the hydrogel, which suggests that MnAlgGel possesses a higher tendency to trap electrons within its polymer matrix.

While factors such as the type of metal, the light irradiation spot, and the active channel length (see Figure S11, Supporting Information) are expected to play an important role in the photoresponse of the materials, the presence of $M^{2+}$ ions in gel matrix were found to be also essential to improve the durability/stability of the gel matrix, ensuring suitable ionic conduction. The excellent long-term stability of our hydrogel was confirmed by comparing $I_{\text{PHOTO}}$ in its pristine state and after one week of storage under ambient conditions (see Figure S12, Supporting Information). For a channel length ($L_{\text{Ch}}$) of 3 cm, MnAlgGel exhibited an $I_{\text{PHOTO}}$ exceeding 3.5 nA, whereas AlgGel showed no photoresponse. Even with a channel length shortened from 3 to 1 cm, AlgGel's $I_{\text{PHOTO}}$ reaches about 1.4 nA, as indicated by the black arrow in Figure 3e. However, MnAlgGel displayed a much greater photoresponse, with a 15-fold higher $I_{\text{PHOTO}}$ compared to the uncrosslinked AlgGel. Additionally, the photoswitching rate of MnAlgGel was benchmarked with the MAlgGel variants containing different cations in Figure 3f. MnAlgGel displayed the fastest photoswitching response (rise time: 2.23 s and fall time: 3.13 s, respectively), while the other MAlgGels exhibited over 3 s of rise time and 3.5 s of fall time. To estimate photoswitching rate, 10% and 90% of normalized $I_{\text{PHOTO}}$ were utilized, as represented by the black dashed line in Figure 3f. All estimated values are summarized in Table 2.

## 2.5. Mechanical Properties and Strain Sensitivity of the MAlgGel Organohydrogels

Figure 4b shows that the mechanical properties of the organohydrogels depend on the nature of the crosslinking cation. All the MAlgGel samples display lower stretchability with respect to the AlgGel sample due to the higher rigidity imparted by the crosslinking. Among the $M^{2+}$-crosslinked samples, CuAlgGel and MnAlgGel feature the highest stretchability (≈140%), while FeAlgGel and ZrAlgGel samples are more rigid (maximum stretchability around 45% and 55% respectively). This phenomenon can be ascribed to the protonation of the alginate's polymer chains during the crosslinking, as a result of the acid pH of the $Fe^{3+}$ and $Zr^{4+}$ solutions. The higher rigidity contributes to increase the linearity of the stress versus strain curves with respect to AlgGel, the latter resembling stress versus strain curves reported in literature for gelatin.[73] CuAlgGel shows the same behavior, while the interaction between alginate and $M^{2+}$ ions is not strong enough to impart linearity. The Young's modulus (YM) of the crosslinked samples was extracted from the slopes of the stress versus strain curves in the 0–0.04 strain range (Figure S13, Supporting Information). The results shown in Table 2 indicate that the YM follows the order $M^{2+} < Cu^{2+} \approx Zr^{4+} < Fe^{3+}$, in analogy with the “long-distance” correlation lengths ($\xi_1$) extracted by SAXS analysis (see Section S3 and Table S2, Supporting Information). The ultimate tensile strength (UTS) of all the samples was assessed and the results are reported in Table 2. CuAlgGel shows the highest value of UTS (1.88 ± 0.04 MPa), while manganese the lowest (0.64 ± 0.02 MPa). This evidence, along with the YM and $\xi_1$ values obtained, is in agreement with the $M^{2+}$-alginate affinity scale reported in literature.[25,26,61] The changes in the ionic pathway induced by stretching the organohydrogels are responsible for significant variation in the conductivity, endowing our products with a huge potential for motion recognition application. We chose the 0-0.25 strain region to assess the electrical response (in terms of relative change of resistance, $\Delta R/R_0$) of the samples, to prove the high sensitivity of the device for low applied stresses. Figure 4c indicates that all the samples undergo a $\Delta R/R_0$ variation close to 50%, but it is important to keep in mind that the strain value does not contain information regarding the stress applied. The Gauge factor (GF) of the organohydrogels was determined using the following equation:[74]

$$GF = (\Delta R/R_0)/\epsilon \quad (1)$$

in which $\Delta R/R_0$ is the relative change of resistance, while $\epsilon$ is the strain. GF was calculated in the strain ranges 0–0.04 (GF1) and 0–0.25 (GF2) in order to assess that this property is constant for all the examined strain values. The results, reported in Table 2, show that the response of AlgGel sample strongly fluctuates for very low strain values. This can be attributed to the lack of linearity of the stress versus strain curve (Figure S13, Supporting Information). Taking into account the error associated with the GF1 and GF2 (see Table 2), it is clear that CuAlgGel, FeAlgGel and ZrAlgGel are the most reliable among the crosslinked samples. Higher GFs were obtained for the $Fe^{3+}$ and $Zr^{4+}$-crosslinked samples (2.15 ± 0.07 and 2.27 ± 0.18,
<figure><img src="image_21.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>(e)</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 4.** a) Photograph of the experimental setup for the samples connected using two copper electrodes. b) Stress versus strain curves and c) $\Delta R/R_0$ versus strain of AlgGel (black), MnAlgGel (pink), CuAlgGel (blue), FeAlgGel (red) and ZrAlgGel (green). Images of the finger bending steps for d) the strain sensing test and e) related electrical response.</figcaption></figure>

respectively). The values of GF obtained for the MAlgGel (between 1.7 and 2.3) are comparable if not superior to most of the ionic conductive hydrogel strain sensors (typically showing GF falling in the range 0.5–1.7).[52,54,75] However, considering the high rigidity of FeAlgGel and ZrAlgGel, CuAlgGel is preferable to detect movements that imply small stresses. CuAlgGel was tested to detect the bending of a finger, adopting the setup illustrated in Figure 4d. The organohydrogel proved to be particularly sensitive to the bending angle, as shown in Figure 4e. Furthermore, they demonstrated high durability, maintaining their sensitivity over 2500 stretching cycles (Figure S14, Supporting Information). This evidence, along with the low applied direct voltage required (i.e., 0.2 V), suggests promising perspectives for the use of our products for real time motion monitoring.

### 3. Conclusion

In summary, we successfully developed a novel class of transparent, stretchable, and multi-responsive organohydrogels, consisting of alginate crosslinked with various transition metal cations ($Mn^{2+}$, $Cu^{2+}$, $Fe^{3+}$, $Zr^{4+}$) and gelatin. These MAlgGel organohydrogels exhibit remarkable properties required for wearable electronics applications to enable environmental monitoring and human motion recognition. Through a systematic investigation of their mechanical, electrical, thermal, humidity/temperature sensing characteristics, we demonstrated the versatility and potential of these materials. Our results indicate that the addition of glycerol and crosslinking with

metal ions significantly enhance the stability, stretchability, and ionic conductivity of the hydrogels. The ability to modulate mechanical properties and ionic conductivity by varying the crosslinking ions offers flexibility in tailoring the performance of the organohydrogels for specific applications. Moreover, the temperature and humidity sensitivity of the MAlgGel samples (reaching $0.19 K^{-1}$ and $0.020 RH(\%)^{-1}$, respectively) make them promising candidates for real-time monitoring of physiological/environmental conditions. Our MAlgGel organohydrogels displayed a remarkable photoresponsivity ($9.2 \mu A W^{-1}$ upon exposure to visible light), which is a unique feature not commonly reported in the literature. This advancement opens up new opportunities for integrating wearable sensors with photodetection functionalities for diverse applications, including environmental monitoring and biometric authentication. Furthermore, our investigation into the electrical response of the MAlgGel samples under mechanical strain highlights their potential for motion recognition applications. The high sensitivity (gauge factor $>1.6$) and reliability of these sensors make them suitable for detecting subtle movements, such as finger bending, with low applied direct voltage (0.2 V) requirements. Overall, our findings contribute to the advancement of wearable electronics by introducing a new class of transparent, stretchable, and multiresponsive hydrogel-based sensors. The tunable properties of these MAlgGel organohydrogels and the biocompatible/biodegradable nature of their organic matrix offer promising avenues for the development of next-generation wearable devices with enhanced functionality and performance. Additional research endeavors in this direction are needed
to fully harness the potential of these materials in real-world applications.

**4. Experimental Section**

**Materials:** Alginic acid sodium salt was obtained from Sigma Aldrich (Product code 180947). Mannuronate/guluronate = 1.50 ± 0.04, determined through FT-IR spectroscopy.[24] Molecular weight ≈ 132 kDa, determined through viscosimetric analysis[24]. Gelatin from porcine skin (Product code G2625, type A, gel strength ≈ 175 g Bloom) and glycerol (BioXtra, ≥ 99%) were also provided by Sigma Aldrich. Manganese chloride tetrahydrate (MnCl₂·4H₂O, ACS reagent ≥ 99.8%) and copper chloride dihydrate (CuCl₂·2H₂O, ACS reagent ≥ 99.0%) were purchased from Sigma Aldrich. Iron chloride hexahydrate was obtained by Thermoscientific (FeCl₃·6H₂O, 99+%, for analysis), while zirconium chloride (ZrCl₄, ≥ 99.5% trace metals basis) was purchased from Aldrich. Deionized water was used to prepare all the solutions. All reagents were used without any further purification.

**Preparation of MAlgGel Organohydrogels:** 1.36 g of gelatin were dissolved in 11.3 mL of water, gently stirring for 10 min at 50 °C. 3.80 g of an alginate 4% (w/v) water solution were then added, and the mixture was kept stirring at 50 °C for 10 min. The solution was poured into a petri dish (diameter = 9 cm) and let cool down for 20 min at room temperature. The petri dish was then kept in a fridge (T = 4 °C) for 10 min. Then, 20 mL of MClₓ (MnCl₂, CuCl₂, FeCl₃ and ZrCl₄) 0.5 M water solutions (pH 4.9, 3.1, ≈0, ≈0, respectively) were poured into the petri dish. After 1 h the gels were removed from the petri dish and dipped in 200 mL of deionized water for 1 h. The gels were then cut into squares (l = 6 cm) and dip into 30 mL of glycerol 25% (w/v) water solution for 3 h. At that point the gels were removed and kept drying overnight in the fume hood, before cutting 4.5 cm x 1.0 cm stripes. The obtained samples were named MAlgGel (M = Mn, Cu, Fe and Zr). The non-crosslinked sample (i.e., AlgGel) was prepared following the previously reported procedure, skipping the crosslinking-washing sections.

**Characterization Glycerol Content Determination:** The points below were followed, assuming that the amount of ions is not relevant:

1) The gels, in their pre-glycerol treatment stage, were dried under fume hood and then under vacuum. Their weight before and after the dehydration was recorded.

2) In this way, the ratio between the dry polymer content and the initial hydrogel mass was determined.

3) Wet pre-glycerol gels were weighted before undergoing the glycerol addition step. In their post-glycerol treatment stage, the gels were dried under fume hood and then under vacuum. Their weight after the dehydration was recorded.

4) The mass of dry polymer in the tested samples derives from point 2.

5) The glycerol amount is obtained by subtracting the dry polymer mass and the water content (determined by TGA) from the weight of the dried organohydrogels recorded at point 3.

**UV–Vis Spectrophotometry:** UV–Vis absorbance spectra of the gels were collected with a Cary UV–Vis multi-cell spectrometer (Agilent) in the 200–1000 nm range, simply sticking the gels to the wall of a quartz cuvette perpendicular to the light path.

**Thermogravimetric Analysis (TGA):** Discovery SDT650 (TA Instruments) was used to perform Thermogravimetric analysis. All the analyzed samples were previously equilibrated at RH = 33%. A heating ramp from room temperature (RT) to 1000 °C, with a rate of 10 °C min⁻¹, was applied to determine the degradation profile of the samples. Measurements were performed under air flux (100 mL min⁻¹), using aluminum pans. For the water content determination, a heating ramp from RT to 120 °C (rate of 10 °C min⁻¹), followed by a 10 min isotherm was applied. Measurements were performed under N₂ flux (100 mL min⁻¹), using aluminum pans. Data were analyzed with Trios v5.1.1.46572 software (TA Instruments). Measurements were performed in duplicate.

**Differential Scanning Calorimetry (DSC):** DSC analysis was carried out by means of Discovery DSC 2500 (TA Instruments). Tzero aluminum hermetic pans (TA Instruments) were used for all the measurements. For the determination of the glass transition temperature (T<sub>g</sub>), the samples were analyzed by applying a heating ramp from 5 °C to 80 °C, with a heating rate of 10 °C min⁻¹. In order to define the state of the water in the samples, the latter were subjected to a cooling ramp from 5 °C to -60 °C, followed by a heating ramp from -60 °C to 25 °C (in both cases with a heating rate of 1 °C min⁻¹). Data were analyzed with Trios v5.1.1.46572 software (TA Instruments). Measurements were performed in duplicate.

**Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES):** The content of crosslinking ions (Mn²⁺, Cu²⁺, Fe³⁺, Zr⁴⁺) in the MAlgGel samples was determined using an Agilent 720-ES Inductively coupled plasma-atomic emission spectrometer (ICP-AES) equipped with a pneumatic nebulizer and a cyclonic spray chamber. Prior the analysis, approximately 10 mg of each sample was kept in 2 mL of a Na₂EDTA 0.05 M + NaOH 10% (w/v) water solution (2.00 mL and 0.100 mL respectively) for 1 h at 80 °C, under magnetic stirring. After dissolution, 1 mL of the solution was diluted 6 times with MilliQ water. The calibration lines were obtained by preparing the standard solutions starting from commercial standards certified at 1000 ppm. After each measurement, the system was washed with a 2% HNO₃ solution. Measurements were performed in triplicate.

**Scanning Electron Microscopy (SEM):** The morphology of the gels (prior to the glycerol addition) was assessed using a FEI Quanta FEG 650 scanning electron microscope. Samples were lyophilized at the end of the washing step of the preparation procedure. A 20 nm thick gold layer was sputtered on the samples. The micrographs were acquired with an accelerating voltage of 30 kV, sample-detector distance around 9.5 mm and using a detector for secondary electrons.

**Small-Angle X-Ray Scattering (SAXS):** Small-angle X-ray scattering analysis was carried out using Xeuss 3.0 (Xenocs). Cu Kα radiation with a wavelength (λ) of 1.542 Å at a power of 30 W was obtained using an ultrabrilliant point microfocus X-ray source (GENIX-Fox 3D, Xenocs, Grenoble). Two sample-to-detector distances of 450 and 1800 mm (calibrated using silver beaker) provided a measurable q-range between 0.006 and 0.6 Å⁻¹ (where q, the scattering vector, is given by q = 4π/λ sin θ, and 2θ is the scattering angle). Measurements were performed under vacuum using hermetic gel holders. Data were analyzed using Sasview software (https://www.sasview.org/).

**Mechanical Properties:** The mechanical properties of the organohydrogels were assessed using a Mark-10 M7-025E digital force gauge mounted on a Mark-10 ESM-303E motorized test stand. Samples were clamped at the edges: the length of the stretchable region was equal to 3 cm.

**Temperature Sensitivity Test:** The temperature sensitivity measurements were carried out using rectangular hydrogel stripes of 33 mm (length) × 10 mm (width) that were first connected with conductive copper tape on top of the Kapton/Al₂O₃ cylinder. Subsequently, the samples were transferred to a home-made temperature test chamber, and the cylinder was filled with water at different temperatures. Finally, the samples were connected to the probe workstation (Keithley 2636B). The sensitivity of the samples was tested increasing the temperature from 10 °C to 50 °C. The related *l* versus *t* curves were recorded applying a direct voltage of 1 V. For the sensitivity evaluation, the conductivity versus temperature was plotted to obtain linear correlation. For each hydrogel sensor, three samples were tested and one of them was measured three times to assess the self-stability of the samples.

**Humidity Sensitivity Test:** The humidity sensitivity measurements were carried out using rectangular hydrogel stripes of 36 mm (length) × 2 mm (width), connected with iron crocodiles on a PCB board. A homemade humidity chamber was employed for the RH sensing test at room temperature. Finally, the samples were connected to the probe workstation (Keithley 2636B). During the testing, the RH was changed by adding either wet or dry air from a commercial humidifier. The humidity inside the chamber was monitored using a commercial humidity sensor (Sensirion SMART). The relative humidity of the test chamber was increased from 20% to 80% and the related *l*-*t* curves were recorded applying a direct voltage of 1 V. For the sensitivity evaluation, the resistivity versus relative humidity was plotted to obtain linear correlation. For each hydrogel sensor, three
samples were tested and one of them was measured three times to assess the self-stability of the samples.

**Photoresponsivity:** Optoelectrical measurements of all organohydrogel-based photodetectors were performed using a Keithley dual-channel 2636B sourcemeter at room temperature under dark and white LED, with an intensity of 3.47 mW cm⁻² (ambient condition).

**Photoelectron Yield Spectroscopy:** The work function (WF) was measured by photoelectron yield spectroscopy in air (PYSA), using a AC-2 photoelectron spectrometer (Riken-Keiki Co.). The ambient photoelectron yield measurements were performed onto an Au/glass pattern substrate for the organohydrogels, which was prepared by photolithograph and thermal evaporation, and placing the gel directly on the metal electrodes. The ultraviolet (UV) incident light powers were of 300 nW for the metal electrodes, and 1000 nW for the organohydrogels with a counting time of 10 s per point.

**Strain Sensitivity Tests:** The sensitivity of the MAlgGel samples to strain was assessed using a Mark-10 M7-025E digital force gauge mounted on a Mark-10 ESM-303E motorized test stand. Samples were clamped at the edges (the length of the stretchable region was equal to 3 cm) and then connected to the probe workstation (Keithley 2636B) to measure current variations. The related *I*–*t* curves were recorded applying a direct voltage of 1 V. For the sensitivity evaluation, the normalized change in resistance versus strain to obtain a linear correlation was plotted. The experimental setup is illustrated in Figure 4a.

**Supporting Information**

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

**Acknowledgements**

The authors acknowledge financial supports 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 CSGI (Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase) and MUR-Italy ("Progetto Dipartimenti di Ecellenza 2023-2027" allocated to Department of Chemistry "Ugo Schiff"). Prof. Mirko Severi is gratefully acknowledged for the ICP-AES measurements. This work was also supported by National Research Foundation of Korea (NRF) grant funded by Korea government (MSIT) (No. RS-2023-00251360) and by 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., and Y.J. contributed equally to this work. P.T., A.T., Y.J., M.B., and P.S. conceived the experiments and designed the study. P.T., A.T., and Y.J. performed all electrical characterization. All authors discussed the results and contributed to the interpretation of data. P.T., A.T., Y.J., M.B., and P.S. cowrote the paper.

**Data Availability Statement**

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

**Keywords**

alginate, gelatin, multiresponsive sensors, organic electronics, wearable electronics

Received: June 18, 2024

Revised: July 22, 2024

Published online: August 1, 2024

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