Multi-crosslinked strong, tough and anti-freezing organohydrogels for flexible sensors

Abstract

Hydrogels are promising sensing materials for various smart and biocompatible applications; nevertheless, it is still challenging to enhance their mechanical property and stability in wide temperature windows and under extreme conditions (such as dry and swelling states). Herein, we report a strong, tough, anti-freezing and anti-dehydration organohydrogel achieved by designing a dual-network structure with multi-crosslinking interactions. The interpenetrated poly (vinyl alcohol) (PVA) chains and poly[N,N-dimethyl(methylacrylethyl)ammonium propane sulfonate] (PDMAPS)/polyacrylamide (PAM) block copolymer chains provided abundant hydrogen bonds and cation–anion dipole interactions; besides, dimethyl sulfoxide and CaCl₂ were added to further improve the mechanical properties as well as facilitate the conductivity and anti-freezing property of the organohydrogel. By systematically optimizing the multi-interactions among these components, the organohydrogel achieved high tensile strength (2.7 MPa), high stretchability (630%), and considerable ionic conductivity (2.4 mS cm⁻¹ at RT). More importantly, it achieved remarkable stability in a wide temperature range of −40 to 80 °C. Moreover, organohydrogel sensors in resistive and triboelectric nanogenerator (TENG) modes were demonstrated for strain/temperature sensing and non-contact distance/material sensing, respectively, suggesting their great potentials in flexible electronics in the future.

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1. Introduction

Recently, smart sensors for wearable and implantable applications have been receiving growing interests, along with the rapid development of materials engineering and artificial intelligence engineering.^1–5 Developing sensing materials that can maintain multi-functionalities such as flexibility, optical transparency, biocompatibility, and mechanical strength under extreme conditions is of core importance.^6–8 Hydrogels are usually regarded as ideal materials for wearable and/or implantable smart devices owing to their excellent biocompatibility, superior ionic conductivity, tissue-like softness, designable functionalities and tunable mechanical properties.^9–11 Their unique three-dimensional hydrophilic network structure gives hydrogels solid-like performance, resulting in excellent mechanical properties and stability, whereas the aqueous phase distributed in the polymer networks endows hydrogels with liquid-like performance, enabling fast migration of carriers, notable flexibility and biocompatibility.^12,13 However, the strength and toughness of hydrogels should be further improved. Furthermore, most hydrogels tend to be fragile and lose ionic conductivity at sub-zero or hyperthermal temperature. In extremely dry or underwater environments, hydrogels can also lose water or swell significantly, respectively, leading to property degradation.¹⁴

Therefore, many efforts have been devoted to fabricating desirable gels to address these issues. For instance, gels with densely cross-linked networks, such as double networks gels,^15,16 physical gels^17,18 and topological gels,¹⁹ have been designed to ensure resistance to greater mechanical damages.²⁰ Some organic liquids (ethylene glycol, glycerin, dimethyl sulfoxide (DMSO), or their mixture) with a lower freezing point and higher vapor pressure have been utilized, endowing gels with significant anti-freezing and anti-dehydration properties.^21–26 Solvation between ions and solvent molecules can prevent the formation of ice crystals at low temperature; therefore, salt was added to gels to optimize their ionic conductivity and mechanical strength as well as enhance their corresponding properties at sub-zero temperature.²⁷ For example, a typical anti-freezing zwitterionic hydrogel with good conductivity at low temperature was prepared through co-polymerization between 2-hydroxyethyl acrylate and zwitterionic monomer sulfobetaine methacrylate.²⁸ Besides, our previous work proved that the electrostatic interaction between a polyzwitterion and salt accelerated salt dissociation into ions, and abundant charged groups on the polyzwitterion expedited ion migration, yielding excellent ionic conductivity over a wide temperature range.²⁹ Nevertheless, the combined properties with multi-environment adaptability and high mechanical strength of gels still require further investigations for expanding their applications under severe conditions.

Herein, we prepared a strong (2.7 MPa), tough (8.46 MJ m⁻³) and stretchable (630%) organohydrogel by designing a multi-crosslinked dual network structure, interpenetrated by ploy (vinyl alcohol) (PVA) chains and poly[N,N-dimethyl(methylacrylethyl)ammonium propane sulfonate] (PDMAPS)/polyacrylamide (PAM) (PDA) block copolymer chains. Large amounts of hydrogen bonds were built between the polymer chains, and abundant ionic dipole interactions were constructed between the anion–cation pairs, ensuring the mechanical properties of the organohydrogel. DMSO and CaCl₂ were added to facilitate the conductivity and anti-freezing property. Besides, a resistive-mode sensor based on the organohydrogel was designed, which could monitor external tensile strain with a gauge factor (GF) of 1.82 when stretched from 0 to 250%, and environmental temperature variations (−40 to 80 °C) could be sensed with a sensitivity of 20% °C⁻¹ at sub-zero temperatures and 0.7% °C⁻¹ at 0–80 °C. A non-contact triboelectric sensor with the organohydrogel electrode was also fabricated, which could not only recognize the distance of objects, but also could deliver specific electric signals when different objects approached.

2. Experimental

2.1 Materials

PVA 1799 (Aladdin, China), CaCl₂ (Aladdin, China), DMAPS (MERYER, China), acrylamide (AAm, Aladdin, China), N,N′-methylenebisacrylamide (Bis, Macklin, China), photo initiator 2959 (Macklin, China), DMSO (Macklin, China), 30 μm polyimide film, 30 μm FEP film and SiO₂ nano-powder with 50 nm diameter are purchased from CAS mart.

2.2 Synthesis of the hydrogels and the organohydrogels

The PVA hydrogel is synthesized by three repeating freezing-thawing cycles. Firstly, 1.5 g PVA are dissolved in 10 g deionized water using a 25 mL round bottom flask, and continuously stirring for 2 h at 90 °C to get a homogeneous and clear solution. Then this solution is poured into a mold, placed in a refrigerator set at −18 °C for 12 h, and thawed at 25 °C for 0.5 h. This freezing-thawing process is performed three times as well. The sample is named PVA_15%. The PAM and PDMAPS hydrogels are prepared by adding 1.5 g monomer, 0.75 mg cross-linking agent Bis and 0.75 mg photoinitiator 2959 in 10 g deionized water and mixing together. The precursors are then poured into a mold and placed under 365 nm ultraviolet light for 2 h. These hydrogels are denoted as PAM_15% and PDMAPS_15%.

The organohydrogel is synthesized through the synergistic effects of ultraviolet light initiation and three repeating freezing-thawing cycles. Firstly, 0.75 g PVA and 0.74 g CaCl₂ are dissolved in a mixed solvent (2.22 g deionized water and 4.44 g DMSO). The mixture is continuously stirred for 4 h at 95 °C to get a homogeneous and clear solution. Then 0.08 g DMAPS, 0.17 g AAm, 1.3 mg cross-linking agent Bis and 2.5 mg photo initiator 2959 are added into this solution, forming an initial precursor mixture. Subsequently, the precursor mixture is poured into a mold, and placed under 365 nm ultraviolet light for 2 h to form PDA block copolymer chains. This mold is placed in a refrigerator set at −18 °C for 12 h, and then thawed at 25 °C for 0.5 h. The cyclic freezing-thawing process is carried out three times to form crystals as the cross-linkers of the PVA chains. Finally, the prepared organohydrogel is peeled from the mold.

2.3 Resistive-mode sensor assembly

Two copper (Cu) tapes are fixed onto the two ends of the organohydrogel.

2.4 Non-contact TENG-mode sensor assembly

0.12 g SiO₂ is added into 2 mL ethanol, and dispersed for 10 min by ultrasound, and then 6 g PDMS precursor is added and stirred evenly. To evaporate the ethanol completely, the mixture is heated at 60 °C for more than 12 h. Afterwards, 0.6 g curing agent is added into the mixture and dispersed homogeneously. After pouring the sample into a home-made mold and curing it at 60 °C for 5 h, a 40 × 40 × 0.4 mm³ electret film is fabricated. By applying 8 kV voltage for 10 min at room temperature, the electret film is charged, where the distance between the corona pin and the surface of the film is 4 cm.

2.5 Characterization and tests

Mechanical deformations (tension and compression) are performed with a universal mechanical testing machine (YL-S71, Yuelian, China). The electrochemical workstation (CHI760E) is used to test the ionic conductivities. The resistive thermal sensor is placed in a high and low temperature alternating humid heat test chamber (QDJS-50L) to estimate the thermal sensing property. Electrical signals of the resistive strain sensors are characterized by using an LCR meter (ZM2376) at a fixed frequency of 1000 Hz. The output electrical signals of the TENG-mode sensors are measured by using a programmable electrometer (Keithley 6517). A linear motor (GKT-37×120/280×360-C-SYS-SP-02) is used to provide a stable force onto the TENG sensor, and a force gauge (DS2-200N) is used to measure this force. The microscopic morphologies of the samples are obtained by using a high-resolution field emission scanning electron microscope (Nova NanoSEM450). Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) are performed on a TA DSC Q2000 (supported by Shiyanjia Lab, https://www.shiyanjia.com) and an Xpert3 Powder, respectively. Fourier transform infrared spectroscopy (FT-IR) is performed on a VERTEX80v Bruker. Transmittance of the samples is characterized via an ultraviolet-visible-near infrared spectrophotometer (SHIMADZU, Japan). The corona process of the electret film is achieved by using a high-voltage polarization instrument (Nanjing Entai Electronic Instruments, ET 2673A).

3. Results and discussion

3.1 Structure of the organohydrogel

The organohydrogel is prepared by a one-pot synthesis method: firstly, the precursor mixture of N,N-dimethyl(methylacrylethyl)ammonium propane sulfonate (DMAPS), acrylamide (AAm), PVA and CaCl₂ was added into a mixed solvent of deionized water (DIW) and DMSO to obtain a clear solution (Fig. 1a and b), and the cross-linking agent N,N′-methylenebisacrylamide (Bis) and photoinitiator 2959 were added subsequently. Then, the mixture was placed under 365 nm ultraviolet light to form the PDA block copolymer network (Fig. S1†), while the PVA network was formed via three repeating freezing-thawing cycles at −18 °C, enabling the PVA to crystallize homogeneously (Fig. S2†). The added CaCl₂ can enhance the ionic conductivity and promote the mechanical strength by forming ion–dipole interaction with the negatively charged groups on PDMAPS. This electrostatic force also formed between the PDMAPS chains themselves because of their zwitterionic features.³⁰ Besides, multiple hydrogen bonds (PAM-PDMAPS, PVA-PDMAPS, PVA-PAM, PVA-PVA, and PAM-PAM) were also constructed in the gel (Fig. S3a†), which can improve the mechanical strength significantly (Fig. 1c). Through optimizing and balancing the hydrogen bonds, electrostatic forces, and PVA crystals, the performance of the organohydrogel can be optimized (Fig. 1d). Additionally, the formation of ice crystals at sub-zero temperatures can be suppressed, because the interactions between the water molecules and the other components, such as (i) the solvation between the water molecules and the ions (including Ca²⁺, Cl⁻ and the charged groups on PDMAPS), and (ii) the hydrogen bonds between the water molecules and DMSO, PVA and PAM, are able to destruct the hydrogen bonds between the water molecules themselves. Accordingly, the organohydrogel can be utilized as usual even at −40 °C. These interactions also ensure the anti-dehydration performance by preventing the water molecule evaporation (Fig. S3b†). Visible light transmittance of the organohydrogel reaches about 92% at 400 to 800 nm wavelength (Fig. S4†), demonstrating its application prospects as a transparent electrode in flexible electronics.

Fig. 1 Structure of the organohydrogel. Chemical structure of (a) AAm, DMAPS and (b) PDA and PVA. (c) Schematic of the organohydrogel. (d) Multi-crosslinking demonstration, including the hydrogen bond, electrostatic force, and PVA crystal.

Chemical structures of the prepared gels are confirmed by Fourier Transform Infrared spectroscopy (FT-IR) (Fig. S5†). The organohydrogel exhibits the characteristic peaks of PAM, PDMAPS and PVA hydrogels simultaneously, indicating the successful synthesis of the double network structure. The wide absorption band between 3500 and 3000 cm⁻¹ belongs to the stretching vibration of –OH, which is much wider and stronger for the organohydrogel than that for the other three curves, and shifts to a shorter wavelength, proving that more hydrogen bonds are formed in the organohydrogel. The peaks at 2935 to 2855 cm⁻¹ indicate the stretching vibration of C–H, and the peak at 1645 cm⁻¹ is assigned to the C=O stretching vibration of the PAM. The split absorption peaks of the ester groups on PDMAPS can be found at 1420 cm⁻¹ and 1000 cm⁻¹.

3.2 Property optimization of the organohydrogel

By regulating the content of the various components, the mechanical strength of the organohydrogel was optimized. The breaking stress increased from 0.17 MPa to 0.53 MPa and the strain increased from 244% to 349% when the mass ratio of AAm to DMAPS increased from 1 : 4 to 2 : 1, because the amount of the hydrogen bonds increased with the increasing content of AAm. However, the stress and strain decreased when the content of AAm increased further, as the ion dipole interaction of DMAPS was weakened, and the hydrogen bonds tended to be saturated (Fig. 2a). In this experimental process, the mass ratio of PVA to (AAm + DMAPS) was set at 1 : 1. As shown in Fig. 2b, the breaking stress of the pure PDA gel was only 0.01 MPa, and the corresponding breaking strain was 189%. Then, the breaking stress increased from 0.08 MPa to 0.92 MPa when the mass ratio of PVA to (AAm + DMAPS) increased from 1 : 3 to 3 : 1, since the PVA chain can provide more hydrogen bonds to the other components. Nevertheless, the stress reduced to 0.37 MPa in the pure PVA gel, owing to the decreased electrostatic force. The breaking strain increased with the increasing PVA : (AAm + DMAPS), and then remained at about 340%, which may be attributed to the maximum ductility of the polymer chain. Of note, the mass ratio of AAm to DMAPS was fixed at 2 : 1 in this experiment. The mass ratio of all the solid components to the solvent was 3 : 20, and the ratio of deionized water to DMSO was 1 : 1 during the optimization process as shown in Fig. 2(a and b).

Fig. 2 Property optimization of the organohydrogel. Comparisons of the breaking stress and breaking strain of the organohydrogels with a mass ratios of (a) 1 : 4, 1 : 2, 1 : 1, 2 : 1, and 4 : 1 for AAm : DMAPS, (b) 0 : 4, 1 : 3, 1 : 1, 3 : 1, and 4 : 0 for PVA : PDA and (c) 1 : 2, 1 : 1, and 2 : 1 for DMSO : deionized water. (d) Comparison of the breaking stress and breaking strain of the organohydrogels at CaCl₂ concentrations of 0.5 M, 1.0 M, 1.5 M, and 2.0 M.

Increasing DMSO can also improve the mechanical property owing to the increased amount of hydrogen bonds (Fig. 2c). The presence of CaCl₂ contributed to the improvement of mechanical strength as the ionized Ca²⁺ and Cl⁻ converted a large amount of free water to bound water because of their strong hydration ability, leading to sterically restricted hydrophilic polymer chains owing to the reduced movement range. Thus, the polymer chains are closely arranged and the hydrogen bond between the chains is further strengthened.³¹ The breaking stress rose to 2.7 MPa when 1 M CaCl₂ was added (Fig. 2d). Notably, the network structure will be negatively affected when the concentration of CaCl₂ is increased further, because the salt tends to precipitate in very high concentrations. Comparing the ionic conductivities of the organohydrogel with different CaCl₂ concentrations (Fig. S6†), 1 M CaCl₂ was selected because the ionic conductivity was saturated and the fracture strength was elevated dramatically at this concentration. Detailed strain-stress curves of the organohydrogels with different components are shown in Fig. S7.†

3.3 Mechanical properties of the optimized organohydrogel

Benefited from the double network structure and the abundant chemical bonds, the optimized organohydrogel possesses excellent mechanical properties. The normal fracture strength of the organohydrogel (σ₀) reaches up to 2.7 MPa, which is higher than those of the other hydrogels (55 kPa for pure PAM_15%, 1.3 kPa for pure PDMAPS_15% and 360 kPa for pure PVA_15%) (Fig. 3a). The corresponding true mechanical strength (σ_T) of the organohydrogel is about 20 MPa (Fig. 3b). Breaking strains of the gels are 485%, 175%, 356%, and 630% corresponding to the PAM, PDMAPS, PVA hydrogels, and the organohydrogel in this work. The hysteresis of the strain-stress curve is comparatively low, but it becomes more discernible during stretching at high tensile strains from 100% to 400% because larger deformations will result in increasing energy dissipation (Fig. 3c). The hysteresis rate is calculated from the integral area ratio between the hysteresis loop and the tensile loading curve: . The hysteresis rate of the first tensile cycle is 31.8%, which is larger than those of the following nine cycles (∼18%), because most unstable covalent crosslinking bonds have been broken in the first cycle, dissipating most of the energy (Fig. 3d). The compressive strength of the organohydrogel at 90% compression strain was 4.1 MPa (Fig. 3e), and the sample maintained a good self-recovery ability when it was compressed to 80% strain repeatedly (Fig. 3f), revealing the good compressibility of the organohydrogel.

Fig. 3 Mechanical properties of the organohydrogel. (a) Uniaxial tensile test of different gels. (b) Stress–strain curves of the different gels. (c) Stress–strain curves of the organohydrogel when stretched to different lengths. (d) Cyclic stress–strain curves of the different gels. (e) Compressive stress–strain curves of the different gels. (f) Cyclic compressive stress–strain curves of the different gels. (g) Toughness histograms of the different gels. (h) Fracture energy of the different gels. (i) Ashby plots of stress and strain values of the reported gels.

The toughness of this organohydrogel is up to 8.46 MJ m⁻³ (Fig. 3g), and the detailed derivation process is illustrated in Note S1. The fracture energy of the organohydrogel was calculated from the Greensmith method, which is 6.84 kJ m⁻² (Fig. 3h). Fig. 3i illustrates the comparison of the fracture energy and tensile strength between the organohydrogel in this work and the gels in the previous works (PAM-peptibe-Zn²⁺,³² P(NaSS-co-MPTC),¹⁷ PAMPS-PAAm,³³ PS-DN,¹⁶ Agar/HPAM,¹⁵ NPs-P-PAA,³⁴ PDA-pGO-PAM,³⁵ P-DN(PAMPS/PAM),³⁶ and ionogel³⁷), from which it is observed that our organohydrogel exhibits obvious advantages in mechanical properties compared to the others. The detailed values of the tensile strength and fracture energy for comparison are listed in Table S1.† Sectional scanning electron microscope (SEM) images of the freeze-dried gels indicate that the organohydrogel has a much denser network structure than the pure PVA hydrogel, which further accounts for the observed mechanical performances (Fig. S8†). Based on the remarkable mechanical properties, the organohydrogel can maintain integrity even when tied in a knot and stretched up to 3 times of its original length. Besides, the organohydrogel with a width of 3 mm can easily hold a 500 g weight and recover to the initial state after being pressed dramatically (Fig. S9†).

3.4 Anti-swelling, anti-dehydration, and anti-freezing properties of the organohydrogel

Swelling, dehydration, and freezing are the major drawbacks of the gel materials and greatly restrict their application. However, the organohydrogel prepared in this work features significant anti-swelling, anti-dehydration, and anti-freezing properties. The swelling rate of the organohydrogel (30%) is much lower than that of the PVA_15% hydrogel (90%) when soaked in deionized water for 12 h (Fig. 4a). Obvious swelling can be observed for the PVA_15% hydrogel, while the shape of the organohydrogel is maintained for a long time after being soaked for 0 h, 2 h, and 12 h (Fig. S10†). The anti-swelling ability of the organohydrogel is mainly attributed to the inherent denser network.³⁸ The dehydration rate of the organohydrogel (16%) is also lower than that of the PVA_15% hydrogel (81%) when exposed to air for 100 h, which is of great significance for the use of gels in various scenarios (Fig. 4b). Moreover, after being exposed to air for up to 30 days, the breaking elongation and breaking stress of the organohydrogel show almost no decay compared to its original state (Fig. 4c) because of the forementioned multiple interactions between the components (Ca²⁺, Cl⁻, DMSO, and PDMAPS) and the water molecules.

Fig. 4 Anti-swelling, anti-dehydration, and anti-freezing properties of the organohydrogel. (a) Swelling rate of the PVA_15% hydrogel and the organohydrogel upon soaking in deionized water for different times. (b) Dehydration rate of the PVA_15% hydrogel and organohydrogel upon exposure to an ambient environment for different times. (c) Breaking elongation and breaking stress of the organohydrogel upon exposure to an ambient environment for different times. (d) Results of the differential scanning calorimetry (DSC) test of the PVA_15% hydrogel and organohydrogel. (e) X-ray diffraction (XRD) patterns of the PVA_15% hydrogel and organohydrogel. (f) Ionic conductivity of the organohydrogel at −40 to 80 °C. (g) Comparison of the LED bulb's luminance using the PVA_15% hydrogel or organohydrogel as a conductor of the circuit at 25 °C and −40 °C. (h) Flexibility demonstration of the PVA_15% hydrogel and organohydrogel at −40 °C and 25 °C.

By using differential scanning calorimetry (DSC), an obvious ice melting point at approximately 2.5 °C is observed in the PVA_15% hydrogel, while there is no ice melting point in the organohydrogel, indicating that the water molecules in the organohydrogel don't crystallize (Fig. 4d). To explore the crystallinity, X-ray diffraction (XRD) patterns are tested as well (Fig. 4e). A sharp high peak at around 2θ = 22° is observed in the PVA_15% hydrogel, while the peak of the organohydrogel is broader and lower, because the anti-freezing property of the organohydrogel suppresses the crystallization process of the PVA chains.

The ionic conductivities of the organohydrogel in the temperature range of −40 to 80 °C are 2 mS cm⁻¹ at 20 °C and 0.13 mS cm⁻¹ at −40 °C (Fig. 4f). The PVA_15% hydrogel and the organohydrogel were then used as a conductor in an electric circuit connected to an LED bulb, and the bulb in the organohydrogel circuit was much brighter than that in the PVA_15% hydrogel circuit at −40 °C, although their brightnesses at room temperature were similar (Fig. 4g). Besides, the organohydrogel exhibits flexibility even at −40 °C, while the PVA_15% hydrogel is fragile (Fig. 4h).

3.5 Applications of the organohydrogel in flexible electronics

Dynamic physical cross-linked gels with ionic conductivity are desirable candidates for the strain sensor and temperature sensor;^39–42 therefore, a strain and temperature dual-functional sensor based on the organohydrogel was fabricated. The organohydrogel is the main sensing part, and two Cu tapes are fixed on the ends of the gel to connect to the external circuit (Fig. 5a). Initially, ions are distributed disorderly in the organohydrogel, while during stretching, the free ions transporting along the polymer chains will take a longer path from a Cu electrode to the other (Fig. S11†). The resistance of the organohydrogel increases continuously with the increasing level of stretching; therefore, the strain sensor can be utilized to monitor the tensile strain, for which the gauge factor (GF) is approximately 1.82 and the fitting coefficient (R²) is 0.99 during stretching from 0 to 260% (Fig. 5b). When the strain sensor is used to monitor the human finger motion, an increasing resistance signal can be measured as the finger is bent from 30° to 90° (Fig. 5c), and further the device still remains sensitive even when operating at −20 °C (Fig. S12†). Besides, there is no obvious signal drift or fluctuation when the strain sensor is stretched from 0 to 20% for more than 1000 cycles, which is attributed to the significant elasticity of the organohydrogel (Fig. 5d).

Fig. 5 Resistive-mode sensor based on the organohydrogel. (a) Structure illustration of the resistive-mode sensor. (b) Gauge factor of the resistive strain sensor. (c) Resistive variations of the strain sensor upon bending of a human finger to 30°, 60° and 90°. (d) Resistive response as the strain sensor is stretched from 0% to 20% for more than 1000 cycles. (e) Dynamic temperature response of the thermistor at an ambient temperature of −40 to 80 °C. (f) Resistance variation in the thermistor upon holding cold water or hot tea for two cycles, and the insert pictures are the photographs of holding cold water or hot tea.

As we know, the amount of ion migration increases with the increasing temperature because the molecular dynamics of the solution is enhanced at higher temperatures. Accordingly, the collision frequency between the ions increases, leading to the acceleration of the ion migration rate. For the organohydrogel in this work, the migration rates of Ca²⁺ and C⁻ increase when the environmental temperature increases, and the conductivity of the gel is subsequently enhanced. Contrastively, when the temperature decreases, the migration rates of both of the ions will slow down, and the conductivity will be impaired. According to this analysis, this resistance sensor can also be used as a thermistor. In Fig. 5e, the resistance of the thermistor decreases dramatically when the temperature increases from −40 to 25 °C, while the attenuation rate becomes slower when the temperature is above 25 °C. Defining the resistance at 25 °C as the original resistance, the sensitivity of the thermistor is about 20% °C⁻¹ at sub-zero temperatures, and it is 0.7% °C⁻¹ above the freezing point. When the device is attached on the belly of a human finger, while grasping a cup of cold water or hot tea alternately, an obvious increasing resistance can be detected for the cold water, but a decreasing resistance was obtained for the hot tea (Fig. 5f). The thermal sensing capability of the device is mainly attributed to the accelerated ion dissociation at higher temperatures, and the dissociated ions can migrate more rapidly at higher temperatures.⁴³

Using the organohydrogel as the electrode and the negatively charged electret as the dielectric layer, a non-contact triboelectric nanogenerator (TENG)-mode sensor was prepared, which can recognize distance and objects because of the enormous injected charges on the electret. Taking an electropositive object as an example, when it is far away from the non-contact sensor, the amounts of the injected negative charges are equal to the positive ions near the surface of the organohydrogel, and the negative ions near the bottom electrode are balanced to the positive charges in the Cu electrode. When the electropositive object approaches the sensor, partial positive ions in the organohydrogel will be neutralized with the negative ions, and electrons flow from the ground. Conversely, the electrons flow back to the ground when the object is moved away from the sensor (Fig. 5a).

Closer distance easily causes higher output for the same object; therefore, the voltage changes from −0.3 V to −50 V when the distance between the non-contact sensor and the PI decreases from 100 mm to 3 mm. Similarly, the voltage of the TENG increases from 0.03 V to 4.9 V when butyronitrile approaches (Fig. 6b). Different electrical properties as well as different charge quantities of the objects also lead to different outputs of the non-contact TENG sensor. For the electropositive materials (metal and butyronitrile), the direction of the signals is upward, while it is reversed when the electronegative materials (paper, PE, silk, Ecoflex, PTFE, PI, and PET) approach, and higher outputs can be observed during the motion process of the highly charged objects (Fig. 6c). The full titles of PE, PTFE, PI and PET are illustrated in Table S3.† When PTFE is selected as the sensed object, the non-contact TENG sensor is able to stably operate for more than 5000 cycles without obvious performance decay at the frequency of 1 Hz (Fig. 6d). Besides, benefited from the remarkable and reliable distance sensing and object recognizing ability, the non-contact sensor is hoped to be utilized in future robotic systems, such as non-contact buttons, exit switches, etc. For instance, by setting a set of incremental voltage thresholds on the data acquisition software, when objects begin to approach the sensor, the nethermost indicator light will be lighted up. Afterward, as the distance gets closer, the uppermost indicator lights will be lighted up in turn (Fig. 6e).

Fig. 6 Non-contact TENG-mode sensor based on the organohydrogel. (a) Working principle of the non-contact TENG sensor. (b) Output voltage of the non-contact sensor when the electropositive and electronegative material approach. (c) Output comparison of the non-contact sensors as different materials approach. (d) Anti-fatigue property of the non-contact TENG sensor. (e) Application demonstration of the non-contact TENG sensor in a robotic system.

4. Conclusions

A strong and tough organohydrogel was fabricated in this work, which can be stretched to 630% with a fracture stress of 2.7 MPa. The mechanical performance mainly benefits from the balance between the ionic dipole interactions, hydrogen bonds, and PVA crystals within the polymer networks. Meanwhile, the organohydrogel also achieves a good anti-freezing ability, which can remain flexible and conductive at −40 °C. Furthermore, there is no obvious swelling in the organohydrogel when soaked in deionized water for 12 h owing to its dense network structure, and it can maintain the initial mechanical property even when exposed to the ambient environment for 30 days. Afterward, sensors in the resistive- and non-contact TENG-mode are fabricated based on this organohydrogel, where the resistance sensor can realize strain sensing and temperature sensing, while the TENG sensor can recognize distance and materials. In view of the multiple properties of this organohydrogel, we expect that it can be utilized in flexible electronics in the future.

Author contributions

Jing Wang and Longwei Li contributed equally, they finished the main investigation, data collection and original draft. Zi Hao Guo and Chongxiang Pan took part in the discussion. Professor Pu was the supervisor. The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

Data availability

Data are available from the authors upon reasonable request.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors thank the supports from the grants from National Natural Science Foundation of China (52173274), Chinese Academy of Sciences (124GJHZ2023031MI), the National Key R&D Project from the Ministry of Science and Technology (2021YFA1201603) and the Fundamental Research Funds for the Central Universities.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr03363a

‡ These authors contributed equally.

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