Next Article in Journal
Effect of Different Aqueous Solvents with and Without Solubilized Lignin on the Swelling Behavior of Holocellulose Fibers
Previous Article in Journal
Design and Fabrication of Low-Temperature 3D-Printed Bioactive Polyurethane/MnO2 Scaffolds for Bone Repair
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Polymers
Volume 17
Issue 23
10.3390/polym17233102
polymers-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

An Anti-Freezing Ionic Conductive Hydrogel for Strain Sensing and Energy Harvesting Devices

by
Yanjie Wang
,
Wei Yu
* and
Sijun Liu
*
Advanced Rheology Institute, Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
*
Authors to whom correspondence should be addressed.
Polymers 2025, 17(23), 3102; https://doi.org/10.3390/polym17233102 (registering DOI)
Submission received: 4 October 2025 / Revised: 1 November 2025 / Accepted: 19 November 2025 / Published: 22 November 2025
(This article belongs to the Section Smart and Functional Polymers)
Browse Figures
Versions Notes

Abstract

Hydrogels with excellent flexibility and conductivity have attracted intensive attention in wearable human monitoring and energy harvesting devices. However, hydrogels containing plenty of water inevitably freeze at subzero temperatures, which deteriorates flexibility and conductivity and limits their practical applications. Herein, an anti-freezing ionic conductive hydrogel is developed by introducing Na+ into the gellan gum/hydrophobically associated polyacrylamide double network. The optimized anti-freezing hydrogel AICH3 achieves outstanding mechanical properties (fracture stress 1.1 MPa and fracture strain 1700%), remarkable conductivity (2.2 S/m), and impressive strain sensitivity (GF = 7.4) at −20 °C. Benefiting from excellent flexibility, conductivity and strain sensitivity, the assembled AICH3-based strain sensor can accurately sense the bending movement of the bionic finger at −20 °C. In addition, the AICH3 can also be used as a stretchable electrode of a triboelectric nanogenerator (TENG), and the assembled AICH3-based TENG can effectively harvest energy and power electronic devices at −20 °C. The comprehensive mechanical and conductive properties of AICH3 at subzero temperatures might be attributed to the multifunctionality of Na+, which not only promotes the fabrication of physically crosslinked gellan gum/hydrophobically associated polyacrylamide double network but also suppresses the formation of ice crystals.

1. Introduction

In recent years, flexible electronics have received increasing interest due to their extensive applications in the fields of human-machine interaction [1,2,3], smart electronic skin [4,5,6], implantable medical devices [7,8,9], and energy harvesting devices [10,11,12]. As a key component of flexible electronic devices, a stretchable conductor with remarkable strain sensitivity and stability plays a major role [13,14]. At present, most conductors for flexible devices are made by adding intrinsically conductive materials (e.g., carbon nanotubes (CNTs) [15,16], graphene [17,18], metallic nanoparticles (NPs) [19,20], etc.) or introducing conductive polymers (e.g., polyaniline (PANI) [21,22,23], polypyrrole (PPy) [24,25,26], poly(3,4-ethylenedioxythiophene) (PEDOT) [27,28,29], etc.) into a stretchable matrix, such as elastomer and rubber. However, these flexible devices usually show limited flexibility, a narrow sensing range, and a lack of long-term stability, thus limiting their widespread applications.
Conductive hydrogels are able to detect and respond to external stimuli and transform mechanical deformation into electrical signals, drawing intensive attention to flexible devices. Recently, the progress of stretchable and tough hydrogels has greatly promoted the development of conductive hydrogels in wearable electronic devices [14,30,31,32,33]. However, hydrogels containing plenty of water are inevitably frozen, which makes polymer chains difficult to move, thus resulting in invalid hydrogel devices with failed mechanical flexibility and negligible conductivity. Currently, various strategies have been developed to inhibit the formation of ice crystalline and obtain anti−freezing hydrogels. One is adding anti−freezing solvents, such as glycerol, ethylene glycol, sorbitol, or a mixture of them, into hydrogel [34,35]. Although anti-freezing solvents can effectively weaken the hydrogen bonding interaction of water molecules and prevent the formation of ice crystals, these organic solvents usually cause adverse effects on the conductivity of hydrogels and have potential harm to human health and the environment. The second strategy is introducing salts into hydrogels to inhibit the formation of ice crystals, and the presence of salts could enhance the conductivity of hydrogels [36,37,38]. However, high concentrations of ions might weaken the mechanical performance of hydrogels, owing to the oversaturated interaction (hydrogen bonding, ionic bonding, etc.) inside the network. Therefore, it is still challenging to fabricate conductive hydrogels with comprehensive mechanical properties, freezing tolerance and conductivity at subzero temperatures.
Herein, we prepared an anti-freezing ionic conductive hydrogel (AICH3) by introducing Na+ into the gellan gum/acrylamide aqueous solution. The presence of Na+ has three purposes: the first is to construct the physical double network, including the hydrophobically associated polyacrylamide (HAP) network and the ionic crosslinked gellan gum (GG) network to improve the mechanical performance of the hydrogel; the second is to provide the hydrogel with excellent ionic conductivity; and the third is to endow the hydrogel with outstanding anti-freezing capability. During the deformation process, both hydrophobic micelles and GG aggregates serve as reversible “sacrifice bonds” to consume energy, which leads to the resultant hydrogels achieving fracture stress 0.7 MPa, fracture strain 4500%, ionic conductivity 9.1 S/m, and freezing tolerance as low as −53 °C. More importantly, the AICH3 also demonstrates fracture stress 1.1 MPa, fracture strain 1700%, fatigue resistance 1000 cycles, conductivity 2.2 S/m, and strain sensitivity GF = 7.4 at −20 °C. The comprehensive properties of AICH3 ensure that the assembled AICH3-based strain sensor can accurately sense the movement of the bionic finger at −20 °C. In addition, the AICH3 can also be used as a stretchable electrode material of TENG, and the assembled AICH3-based TENG can effectively harvest energy and power electronic devices at −20 °C. Overall, the AICH3 shows an application potential in flexible devices at subzero temperatures.

2. Experimental Section

2.1. Materials

Low acyl gellan gum (GG) with the weight-average molecular weight (Mw) of 6.0 × 105 was provided by DSM Zhongken Biotechnology Co., Ltd., Tongxiang, China. The contents of metallic ions of Na+, K+, Ca2+, and Mg2+ are 0.429%, 3.917%, 0.241%, and 0.097%, respectively, measured by the inductively coupled plasma (ICP) (iCAP 6000 ICP, Thermo, Inc., Cambridge, UK). Acrylamide (AAm, 99%) was provided by Macklin Biotechnology Co., Ltd., Shanghai, China. Stearyl methacrylate (SMA, 95%) and 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (I2959, 98%) were purchased from Sigma Aldrich, Shanghai, China. Sodium chloride (NaCl, ≥99.5%) and sodium dodecyl sulfate (SDS, >85%) were obtained from Aladdin Biotechnology Co., Ltd., Shanghai, China. All of the reagents were used as received.

2.2. Synthesis of AICH

The anti-freezing ionic conductive hydrogels (AICHx) were prepared through the one-pot method, where x represents the content of NaCl (mmol/g). Taking AICH3 as an example, GG powders (1.67 × 10−4 mmol), AAm (77.1 mmol) and SMA monomers (1.54 mmol) were first added into a reaction flask containing deionized water (6 g) and stirred continuously at 85 °C for 2 h. Then, photo-initiator I2959 (0.39 mmol), SDS (4.85 mmol) and NaCl solution (6 mmol/g) were slowly added into the above aqueous solution and continuously stirred at 85 °C for 5 h. Subsequently, the obtained solution was poured into the mold and UV curing (wavelength = 365 nm, density = 400 W) for 5 min to form the hydrophobically associated PAAm network. Finally, the AICH were obtained through cooling at room temperature for 2 h.

2.3. Microstructures

The microstructures of hydrogels were obtained by a field emission scanning electron microscope (Nova Nano FESEM 450 from FEI, Hillsboro, OR, USA). The samples for FESEM were prepared as follows: the AICH were immersed in liquid nitrogen for 5 min, and then moved quickly to the lyophilizer for freeze-drying. The freeze-dried gels were fractured in liquid nitrogen. The cross-sections were sprayed with gold in a vacuum and examined by FESEM at an accelerating voltage of 10 kV.

2.4. Anti-Freezing Characteristics

The freezing tolerance of hydrogels was first characterized by measuring the freezing point based on Differential Scanning Calorimetry (TA DSC Q2000, New Castle, DE, USA). The temperature range is from 40 °C to −80 °C at a cooling rate of 10 °C/min. Meanwhile, the rheological temperature sweep (TA ARES-G2, New Castle, DE, USA) was also carried out to determine the anti-freezing property, where the phase angle tanδ is represented as a function of temperature.

2.5. Mechanical Property

The mechanical property of AICH with a dumbbell shape (a width of 5 mm in the middle, a thickness of 2 mm and a length of 50 mm) was tested at a speed of 100 mm/min by the WDW-GD universal testing machine (Shanghai Songton Instrument Co., Ltd., Shanghai, China). The mechanical property at subzero temperatures was also carried out on the universal testing machine within a low−temperature chamber. Each sample was kept at the testing temperature for 30 min to achieve a thermal equilibrium before testing.

2.6. Conductivity

The conductivity of AICH was measured by a DCR meter (UC2856, Youce Electronic Technology Co., Ltd., Changzhou, China) at an alternating current with a voltage of 1 V and a frequency of 100 kHz. The relative resistance change, ∆ R / R 0 = ( R − R 0 ) / R 0 , was determined, where R 0 is the initial resistance and R is the real-time resistance. The gauge factor (GF) was calculated from the slope of the relative resistance change ( ∆ R / R 0 ) versus strain.

2.7. Fabrication of AICH-Based Devices

The PDMS film was first prepared by mixing the pre-polymer with the curing agent at a ratio of 10:1 and then casting it into the mold at a thickness of 1 mm, followed by curing for 5 h at 60 °C. The AICH-based TENG was fabricated by assembling the AICH and two pieces of PDMS films into the sandwich structure, where the AICH acts as the electrode and the PDMS films act as the positive electrification layer and the support substrate. The copper wire was connected to the AICH for electrical output measurements. The output performance (Voc, Isc, Qsc) of AICH-based TENG was measured by an electrometer (Keithley 6514, Cleveland, OR, USA).

3. Results and Discussion

3.1. Preparation of AICH

The schematic diagram for the preparation of AICH is presented in Figure 1a. The hydrophobic SMA monomers are dissolved in the self-assembled SDS micelles under the solubilization of NaCl, and then the HAP network is formed under UV light, where the SMA micelles are used as the hydrophobic association point. As the temperature is decreased, the conformation of GG changes from random coils to double helices, and then the aggregation of double helices results in the formation of the GG network. Finally, the physically crosslinked GG/HAP double network is obtained. In the GG/HAP double network hydrogel, NaCl plays multiple roles. First of all, NaCl increases the solubility of hydrophobic SMA in aqueous solution and promotes the formation of micelles and construction of HAP; NaCl also enhances the aggregation capability of gellan gum double helices, thus increasing the crosslinking densities of HAP and GG and producing a dense network, as evidenced by FESEM images of HAP/GG hydrogels (Figure 1b,c). It is apparent that the HAP/GG hydrogels with a high content of NaCl (3 mmol/g) produce a denser network structure than those without NaCl. The second is that the presence of NaCl weakens the hydrogen bonding interaction of H2O molecules, thus greatly decreasing the freezing point of GG/HAP hydrogel. It is found from DSC measurements that the freezing point decreases from −18.8 °C to −43.4 °C as the content of NaCl increases from 0 mmol/g to 3 mmol/g (Figure 1d). The rheological temperature sweep demonstrates a similar freezing tolerance of GG/HAP hydrogels. The third is that the presence of NaCl greatly enhances the conductivity of the resultant GG/HAP double network hydrogel at subzero temperatures, which will be discussed later.

3.2. Mechanical Property of AICH

The mechanical properties of AICH at room temperature (25 °C) are first evaluated by tensile measurements. The presence of NaCl greatly enhances the mechanical performance of hydrogels. The fracture strain and fracture stress increase from 530% to 4500% and from 0.05 kPa to 0.71 kPa, respectively, as the content of NaCl is increased from 0.1 mmol/g to 3 mmol/g (Figure 2a,b). This is because the solubility of hydrophobic SMA in water is very low (10−9 mL/mL), and SMA is unable to enter SDS micelles through the continuous aqueous phase in the absence of salt [39]. Increasing NaCl content enhances the solubility of SMA and leads to the formation of micelle crosslinking points, thus producing a dense GG/HAP network. This inference is also confirmed by the FESEM images in Figure 1b,c.
The hydrogels without NaCl or with low NaCl content (such as AICH0.5) lose their flexibility when the temperature is decreased to −20 °C (Figure S2), because the formation of ice crystals limits the movement of network strands. However, a high content of NaCl (such as 3 mmol/g) greatly improves the stretchability of hydrogels at −20 °C (Figure S2). To quantitatively evaluate the mechanical property of AICH3 at subzero temperatures, the universal testing machine equipped with a low-temperature chamber is used for tensile measurement. Before testing, the sample was equilibrated in the chamber for 30 min. In contrast to AICH0.1, the AICH3 can be stretched to a strain of 1700% at −20 °C (Figure 2c,d). Moreover, the stress–strain curves of AICH3 at different subzero temperatures are shown in Figure 2e. With the decrease in temperature, the elastic modulus of AICH3 increases and the ductility decreases. However, the AICH3 can still be stretched to a strain of 830% even at −35 °C (Figure 2f).
The cyclic compressive tests with a strain of 20% were further performed at −20 °C to evaluate the anti-fatigue of AICH3 at subzero temperatures. It is found that the stress-strain curves basically remain constant over 1000 cycles (Figure 3a), suggesting that the AICH3 has an excellent anti-fatigue performance at subzero temperatures. In addition, the AICH3 also shows a self-healing capability at subzero temperatures. The self-healing experiment was carried out as follows: The dumbbell-shaped sample was cut into two pieces. Subsequently, the cut surfaces were brought together to form a contact and sealed into a polyethylene bag and then moved into the chamber of −20 °C and stored for 2 h. The cut sample was found to be well healed (Figure 3b), and the self-healed sample could be stretched to about four times of original length. The excellent anti-fatigue and self-healing capability of AICH3 at subzero temperatures is mainly attributed to the design of the physically crosslinked double network, where the ionic crosslinked GG network and HAP network synergistically contribute to improving the anti-fatigue and self-healing of AICH3 at subzero temperatures.

3.3. Conductivity and Strain Sensitivity of AICH

The presence of NaCl not only improves the mechanical property but also endows AICH3 with superior conductivity and strain-sensitivity at subzero temperatures. AICH3 achieves a conductivity of 2.2 S/m at −20 °C, which approaches 9.1 S/m at 25 °C (Figure 4a). In comparison, the conductivity of hydrogels with low NaCl content (<1 mM/g) at −20 °C is reduced by more than one order of magnitude than that obtained at 25 °C. The various conductivities of hydrogels with varying NaCl contents at subzero temperatures are mainly attributed to the freezing tolerance. For example, AICH0.5 has a freezing point of −18.5 °C, meaning that there are plenty of ice crystals formed at −20 °C, which greatly inhibits the movement of ions and results in an obvious decrease in conductivity. However, the freezing point of AICH3 is low to −43.6 °C, which ensures that there are plenty of free ions existing in the AICH3 at −20 °C, thus leading to an ignorable decrease in conductivity.
The strain sensitivity is usually studied through the relative resistance variation (∆R/R0 = (R − R0)/R0) under different applied strains. The gauge factor (GF), defined as the slope of ∆R/R0 vs. strain curve, is commonly employed to assess the strain sensitivity of a flexible conductor. Within tensile strain of 1000% (Figure 4b), the change in GF of AICH3 at −20 °C can be divided into three regions: (I) ε < 100%; (II) 100% < ε < 500%; (III) 500% < ε < 1000%, and the corresponding GF values are 1.4, 4.7 and 7.4, respectively. The increase in GF may be attributed to the fact that the ion migration channel becomes narrower and the migration path becomes longer when the AICH3 undergoes stretching deformation, thus resulting in the increase in ∆R/R0. It is noted that the conductivity and GF of AICH3 at subzero temperature overmatch the most reported conductive hydrogels [40,41,42,43,44,45,46] (Table 1).
In addition, the stable strain sensitivity is essential for the flexible strain sensors. To investigate the conductive stability, the AICH3 is cyclic loaded from tiny deformation (0.5% to 10%) to large strain (20% to 500%) at −20 °C (Figure 4c,d). It is apparent that the curves of ∆R/R0 versus time almost overlap at a given tensile strain, suggesting that AICH3 demonstrates an excellent repeatable and stable strain response in a wide range of strain (0.5–500%) at subzero temperatures, which ensures that AICH3 can be used to detect various deformations at subzero temperatures.

3.4. AICH3-Based Flexible Devices

The outstanding mechanical property, conductivity, and strain sensitivity at subzero temperatures endow the AICH3-based strain sensor with the ability to monitor different deformations. As shown in Figure 5, the AICH3-based strain sensor was attached to the bionic finger and placed in the low-temperature chamber (−20 °C) to detect the bending deformation of the bionic finger. It can be seen that the AICH3-based strain sensor can accurately sense the tiny deformation of the bionic finger. Importantly, the resistance immediately fell back to the original level when the bionic finger was completely relaxed. At the same time, the AICH3-based strain sensor is capable of distinguishing the different bending angles of the bionic finger. When the bionic finger was held at a certain angle, the resistance almost remained at a constant value and returned to the original value after straightening the finger. These results indicate that the AICH3 has potential for fabricating the wearable electronic skin.
Recently, triboelectric nanogenerators (TENGs) have been rapidly developed for converting mechanical energy into electrical energy. Herein, the AICH3 is utilized as an electrode to fabricate a TENG with a single-electrode working mode (Figure 6a). When a material touches PDMS, an equal number of positive and negative charges are generated at the interfaces (Figure 6b, I), owing to the difference in electronic affinity between PDMS and the contact material. When the contact material is separated from PDMS, the negative charges at the surface of PDMS induce the migration of ions in AICH3 owing to the electrostatic equilibrium, leading to electrons flowing from the AICH3 electrode to ground and the generation of electric current (Figure 6b, II). When the contact material and PDMS are separated completely, a new electrostatic equilibrium is established (Figure 6b, III). Once the contact material is back close to PDMS, electrons flow back from ground to AICH3, generating a reverse current (Figure 6b, IV). Repeating the contact–separation process will generate a detectable alternating current. In contrast to traditional hydrogels, the AICH3 utilized as an electrode will greatly extend the application range of TENG owing to its excellent conductivity at subzero temperatures.
The output performance of AICH3-based TENG using commercial PET, PE, PVC, PTFE, BOPP, and paper as contact materials (length × width = 3 cm × 4 cm) is evaluated. Due to the different electronic affinities between PDMS and contact materials, the AICH3-based TENG can output various electrical signals. Among them, the PET film used as contact material produced a maximum output voltage (Figure 6c). Therefore, in the following quantitative evaluation, the PET film is used as the contact material to measure the output performance of AICH3-based TENG. As shown in Figure 6d, the peak values on the open-circuit voltage (Voc), short-circuit current (Isc) and short-circuit charge (Qsc) are 111 V, 324 nA and 161 nC, respectively. Through loading external resistances from 50 MΩ to 10 GΩ, the output power of the AICH3-based TENG is obtained. When the resistance is 1 GΩ, the output areal power density reaches a maximum value of 4.43 mW/m2. In addition, the output performance of the AICH3-based TENG at subzero temperatures is also evaluated. It is found that the Voc of AICH3-based TENG almost maintains a constant value at −20 °C (Figure 6f). Decreasing the temperature from 25 °C to −20 °C shows a negligible influence on the Isc. These results indicate that the AICH3-based TENG has an outstanding and stable energy conversion efficiency, even at subzero temperatures.
To evaluate the potential application of AICH3-based TENG in energy harvesting, a self-charging power system was designed. The equivalent circuit is shown in Figure 7a; it is composed of an electronic, a commercial capacitor, a bridge rectifier, and the AICH3-based TENG. The capacitor with varied capacitance values is charged by the AICH3-based TENG. The capacitor of 1 μF can be charged to 2.0 V within 5 s, but the charging rate decreases with the increase in capacitance value (Figure 7b). To demonstrate the performance of energy harvesting, a digital watch is connected to the circuit to form a self-charging and powering circuit (Figure 7c). The commercial capacitor with the capacity of 22 μF is charged to a voltage of 3 V within 57 s and can power the digital watch for about 10 s (Figure 7d). Furthermore, the capacitor can be charged and discharged repeatedly to power the digital watch.

4. Conclusions

In this work, we synthesized an anti-freezing ionic conductive hydrogel with a fracture stress of 1.1 MPa, fracture strain of 1700%, conductivity of 2.2 S/m, and gauge factor of 7.4 at −20 °C. The excellent mechanical, conductive, and strain-sensitive properties at subzero temperatures are mainly attributed to the multifunctionality of Na+. That is to say, the presence of Na+ not only promotes the construction of a dynamic crosslinked GG/HAP double network but also inhibits the formation of ice crystals at subzero temperatures. Benefiting from excellent mechanics, conductivity, and strain sensitivity, the assembled AICH3-based strain sensor can accurately detect the bending of the bionic finger at subzero temperatures. In addition, the AICH3 can also be used as a stretchable electrode, and the assembled AICH3-based TENG can effectively harvest energy and power electronic devices at −20 °C. Overall, this work provides a feasible strategy for preparing anti-freezing ionic conductive hydrogel and expands the application of hydrogels at subzero temperatures.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/polym17233102/s1, Figure S1: The relationship of phase angle (tanδ) with temperature; Figure S2: The photographs of tensile tents of AICH0.5 and AICH3 at −20 °C.

Author Contributions

Writing—original draft, Y.W.; Writing—review and editing, S.L.; Supervision, W.Y. and S.L.; Funding acquisition, S.L. All authors have read and agreed to the published version of the manuscript.

Funding

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (52473015).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are contained within the article or Supplementary Material.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Wang, P.; Wang, G.Q.; Sun, G.F.; Bao, C.C.; Li, Y.; Meng, C.Z.; Yao, Z. A Flexible-Integrated Multimodal Hydrogel-Based Sensing Patch. Nano-Micro Lett. 2025, 17, 156. [Google Scholar] [CrossRef]
  2. Wu, Z.J.; Zhang, L.Y.; Wang, M.; Zang, D.F.; Long, H.Y.; Weng, L.; Guo, N.; Gao, J.G.; Liu, Y.H.; Xu, B.B. A wearable ionic hydrogel strain sensor with double cross-linked network for human-machine interface. Adv. Compos. Hybrid Mater. 2025, 8, 17. [Google Scholar] [CrossRef]
  3. Xue, C.; Zhao, Y.R.; Liao, Y.T.; Zhang, H.Y. Bioinspired Super-Robust Conductive Hydrogels for Machine Learning-Assisted Tactile Perception System. Adv. Mater. 2025, 37, 2416275. [Google Scholar] [CrossRef]
  4. Guo, X.K.; Zhang, S.L.; Patel, S.; Sun, X.L.; Zhu, Y.L.; Wei, Z.C.; Wang, R.G.; He, X.D.; Wang, Z.K.; Yu, C.J.; et al. A skin-mimicking multifunctional hydrogel via hierarchical, reversible noncovalent interactions. Sci. Adv. 2025, 11, eadv8523. [Google Scholar] [CrossRef]
  5. Lin, W.J.; Zhang, J.Y.; Pan, G.Y.; Lin, Y.B.; Chen, D.Q.; Lu, J.H.; Li, J.X.; Huang, J.H.; Li, Z.H.; Lin, X.F.; et al. Multifunctional Wide-Temperature Threshold Conductive Eutectic Hydrogel Sensor for Shoulder Pain Monitoring and Photothermal Therapy. Adv. Funct. Mater. 2025, e09779. [Google Scholar] [CrossRef]
  6. Shi, H.R.; Kuang, F.; Huo, H.X.; Gao, Y.H.; Duan, X.; Shen, J.J.; Wan, J.Y.; Li, Y.M.; Du, G.B.; Yang, L. Temperature-Responsive Cellulose-Based Janus Hydrogel as Underwater Electronic Skin. Nano Lett. 2025, 25, 7835–7844. [Google Scholar] [CrossRef]
  7. Zhang, K.Q.; You, H.B.; Liu, Y.X.; Lin, F.L.; Li, Y.N.; Jia, L.N.; Bao, J.X.; Ge, X.; Pan, G.F.; Yu, H.T.; et al. Development of a multifunctional and biocompatible hydrogel for flexible implantable sensors. Chem. Eng. J. 2025, 522, 167403. [Google Scholar] [CrossRef]
  8. Choudhury, S.; Wang, Z.L.; Kim, S.W. Hydrogel-based piezoelectric materials and devices for implantable bioelectronics. Biomaterials 2026, 327, 123768. [Google Scholar] [CrossRef] [PubMed]
  9. Chen, Y.Q.; Zhang, H.; Yao, J.; Cao, X.; Luo, Y.; Wang, Q.G.; Yu, Y.; Su, R.; Yao, Y.D.; Li, G.H.; et al. Biodegradable Anti-Stenosis Microsurgical Implantable Hydrogel Stent for Efficient Vas Deferens Recanalization. Adv. Funct. Mater. 2025, e15388. [Google Scholar] [CrossRef]
  10. Liu, H.; Deng, Y.; Li, J.H.; Liu, Q.H.; Mu, L.H.; Zhang, R.Z.; Sun, C.L.; He, J.M.; Qu, M.N. Stretchable, anti-freezing, self-healing, and degradable high-performance conductive hydrogel-based triboelectric nanogenerator for energy harvesting and human activity recognition. Mater. Today Energy 2025, 53, 102013. [Google Scholar] [CrossRef]
  11. Chen, H.; Zu, G.Q.; Ju, Q.Q.; Yang, X.J. Hydrogel-based sandwich-structured triboelectric nanogenerator energy harvesting storage system for multi-functional sensing and monitoring. Chem. Eng. J. 2025, 523, 168739. [Google Scholar] [CrossRef]
  12. Zhao, J.W.; Yang, J.H.; Chen, J.; Hou, C.L.; Wang, Y.Z.; Wang, Y.J. A stretchable triboelectric nanogenerator with an organohydrogel as electrode for biomechanical energy harvesting and self-powered sensing. Mater. Today Commun. 2025, 49, 113704. [Google Scholar] [CrossRef]
  13. Yang, B.H.; Li, Y.W.; Zheng, K.; Xiong, Y.; Jin, X.K.; Zhu, L.X.; Cai, R.; Hu, B. Bi-modal synergistic hydrogel sensors coupled with machine learning enable gesture parsing and identity recognition. Chem. Eng. J. 2025, 523, 168174. [Google Scholar] [CrossRef]
  14. Chen, J.Y.; Liu, F.F.; Abdiryim, T.; Liu, X. An overview of conductive composite hydrogels for flexible electronic devices. Adv. Compos. Hybrid Mater. 2024, 7, 35. [Google Scholar] [CrossRef]
  15. Yim, Y.J.; Yoon, Y.H.; Kim, S.H.; Lee, J.H.; Chung, D.C.; Kim, B.J. Carbon Nanotube/Polymer Composites for Functional Applications. Polymers 2025, 17, 119. [Google Scholar] [CrossRef]
  16. Wu, Z.J.; Deng, X.S.; Yu, X.; Gu, J.W.; El-Bahy, Z.M.; Mersal, G.A.M.; Zhang, J.; Alhadhrami, A.; Xu, H.Y.; Guo, N.; et al. Electrospun thermoplastic polyurethane membrane decorated with carbon nanotubes: A platform of flexible strain sensors for human motion monitoring. Polymer 2024, 303, 127120. [Google Scholar] [CrossRef]
  17. Ran, J.; Liu, Y.F.; Feng, H.X.; Shi, H.X.; Ma, Q. A review on graphene-based electrode materials for supercapacitor. J. Ind. Eng. Chem. 2024, 137, 106–121. [Google Scholar] [CrossRef]
  18. Zhang, X.Y.; Sun, Q.; Liang, X.; Gu, P.Z.; Hu, Z.Y.; Yang, X.; Liu, M.X.; Sun, Z.J.; Huang, J.; Wu, G.M.; et al. Stretchable and negative-Poisson-ratio porous metamaterials. Nat. Commun. 2024, 15, 44707. [Google Scholar] [CrossRef]
  19. Zhang, J.Y.; Wang, X.R.; Zhao, X.Y.; Chen, H.L.; Jia, P. Significantly Enhanced Acidic Oxygen Evolution Reaction Performance of RuO2 Nanoparticles by Introducing Oxygen Vacancy with Polytetrafluoroethylene. Polymers 2025, 17, 59. [Google Scholar] [CrossRef] [PubMed]
  20. Zou, L.G.; Li, Y.X.; Feng, S.Y.; Wang, Z.Y.; Xiao, H.; Chen, S.X.; Wang, Y.Q.; He, L.C.; Mao, X. Innovations and Applications of Composite Hydrogels: From Polymer-Based Systems to Metal-Ion-Doped and Functional Nanomaterial-Enhanced Architectures. Small 2025, 21, 03147. [Google Scholar] [CrossRef] [PubMed]
  21. Liu, J.K.; Lu, Z.; Jiang, Y.C.; Gao, Y.T.; Liu, W.; Hu, W.F.; Xin, B.J. Hierarchical MXene/PANI porous film electrodes with improved ion transport for supercapacitors. Int. J. Electrochem. Sci. 2025, 20, 101199. [Google Scholar] [CrossRef]
  22. Setiawan, K.M.; Humaidi, A.R.; Hidayat, F.; Elsoraya, N.; Hanifah, A.; Alfiah, M.; Steky, F.V.; Milana, P.; Fukutani, K.; Rizal, C.; et al. Synthesis of Polyaniline/Sulfonated Polyaniline (PANI:SPAN) Acid-Base Pair Mimicking PEDOT:PSS Intermolecular Structure as a Hole-Transporting Layer. ACS Appl. Electron. Mater. 2025, 7, c01316. [Google Scholar] [CrossRef]
  23. Khan, M.; Inamuddin. Electrochemical characterization of recycled polyethylene terephthalate-polyaniline bioanode for enzymatic biofuel cell applications. Electrochim. Acta 2025, 539, 146934. [Google Scholar] [CrossRef]
  24. Zhang, C.Y.; Guo, C.J.; Ji, A.S.; Zhou, H.L.; Zhou, W.L.; Liu, W.Z.; Wu, T.; Qu, J.P. Economical approach to thermoelectric cooling: Development of conductive polyethylene/polypyrrole@constantan composite using extensional rheological technology. Sci. China Mater. 2025, 68, 4125–4134. [Google Scholar] [CrossRef]
  25. Hu, C.N.; He, J.; He, Y.T.; Peng, Y.J. Anti-Freezing Conductive Gelatin Hydrogel Reinforced with Polypyrrole-decorated Cellulose Nanofibers for Strain Sensors and Triboelectric Nanogenerators. Chin. J. Polym. Sci. 2025, 43, 2083–2093. [Google Scholar] [CrossRef]
  26. Tao, S.X.; Yang, Q.; Su, J.; Zhang, N.; Xu, L.H.; Pan, H.; Zhou, T.C.; Zhang, H.J.; Jin, K.L.; Wang, J.P. High-performance flexible conductive films based on bacterial cellulose/polypyrrole: Synergistic effects of polyvinyl alcohol and fluorinated graphene in surface, interface and functional optimization. Colloids Surf. A Physicochem. Eng. Asp. 2025, 724, 137447. [Google Scholar] [CrossRef]
  27. Hou, S.Y.; Fan, X.Y.; Jin, T.Y.; Li, H.X.; Han, Y.C. A Highly Conductive, Stretchable, and Healable Sensing Electrode Consisting of PEDOT Conductive Pathway and Semi-Interpenetrating Network. Adv. Funct. Mater. 2025, 19643. [Google Scholar] [CrossRef]
  28. Saghir, S.; Ramirez-Sarabia, J.; Imenes, K.; Schiavone, G. Electropolymerised PEDOT:Polydopamine enables high-performance bioelectrode coatings. Sci. Rep. 2025, 15, s41598. [Google Scholar] [CrossRef]
  29. Fan, Z.; Pan, X.Y.; Gu, Z.M.; Gao, Y.H.; Gao, X.Y.; Pan, L.J.; Gao, W.J.; Jiang, D.Y. Highly Conductive and Mechanically Robust PEDOT:PSS-Based Fibers: Fabrication Methods, Structure-Property Relations, and Potential Applications. Macromol. Rapid Commun. 2025, 00639. [Google Scholar] [CrossRef] [PubMed]
  30. Mo, F.; Zhou, P.C.; Lin, S.H.; Zhong, J.W.; Wang, Y. A Review of Conductive Hydrogel-Based Wearable Temperature Sensors. Adv. Healthc. Mater. 2024, 13, 01503. [Google Scholar] [CrossRef]
  31. Imani, K.B.C.; Dodda, J.M.; Yoon, J.; Torres, F.G.; Bin Imran, A.; Deen, G.R.; Al-Ansari, R. Seamless Integration of Conducting Hydrogels in Daily Life: From Preparation to Wearable Application. Adv. Sci. 2024, 11, 06784. [Google Scholar] [CrossRef]
  32. Guo, W.Y.; Ma, M.G. Conductive nanocomposite hydrogels for flexible wearable sensors. J. Mater. Chem. A 2024, 12, 9371–9399. [Google Scholar] [CrossRef]
  33. He, Q.H.; Cheng, Y.; Deng, Y.J.; Wen, F.; Lai, Y.K.; Li, H.Q. Conductive Hydrogel for Flexible Bioelectronic Device: Current Progress and Future Perspective. Adv. Funct. Mater. 2024, 34, 08974. [Google Scholar] [CrossRef]
  34. Wang, J.; Li, L.W.; Guo, Z.H.; Pan, C.X.; Pu, X. Multi-crosslinked strong, tough and anti-freezing organohydrogels for flexible sensors. Nanoscale 2025, 17, 1400–1410. [Google Scholar] [CrossRef]
  35. Guo, R.Y.; Bao, Y.; Zheng, X.; Chen, J.; Zhang, W.B.; Liu, C.; Ma, J.Z. Organohydrogel Based Electronic Skin Reinforced by Dual-Mode Conduction and Hierarchical Collagen Fibers Skeleton. Adv. Sci. 2025, 12, 12934. [Google Scholar] [CrossRef] [PubMed]
  36. Zhang, X.Y.; Lin, Y.; Shen, S.T.; Du, Z.H.; Lin, Z.Q.; Zhou, P.P.; Huang, H.L.; Lyu, X.L.; Zou, Z.G. Intrinsic Anti-Freezing, Tough, and Transparent Hydrogels for Smart Optical and Multi-Modal Sensing Applications. Adv. Mater. 2025, 37, 13856. [Google Scholar] [CrossRef]
  37. Yu, Y.F.; Wang, S.; Yu, H.T.; Liao, X.J.; Feng, W. Anti-freezing conductive hydrogels with exceptional mechanical properties and stable sensing performance at-30 °C. Mater. Horiz. 2025, 12, 2679–2688. [Google Scholar] [CrossRef]
  38. Zhang, D.; Chen, H.; Zhang, Y.X.; Yang, J.T.; Chen, Q.; Wu, J.; Liu, Y.L.; Zhao, C.; Tang, Y.J.; Zheng, J. Antifreezing hydrogels: From mechanisms and strategies to applications. Chem. Soc. Rev. 2025, 54, 00718b. [Google Scholar] [CrossRef]
  39. Huang, J.; Morin, F.J.; Laaser, J.E. Charge-Density-Dominated Phase Behavior and Viscoelasticity of Polyelectrolyte Complex Coacervates. Macromolecules 2019, 52, 4957–4967. [Google Scholar] [CrossRef]
  40. Liu, H.Y.; Wang, X.; Cao, Y.X.; Yang, Y.Y.; Yang, Y.T.; Gao, Y.F.; Ma, Z.S.; Wang, J.F.; Wang, W.J.; Wu, D.C. Freezing-Tolerant, Highly Sensitive Strain and Pressure Sensors Assembled from Ionic Conductive Hydrogels with Dynamic Cross-Links. ACS Appl. Mater. Interfaces 2020, 12, 25334–25344. [Google Scholar] [CrossRef]
  41. Hu, C.X.; Zhang, Y.L.; Wang, X.D.; Xing, L.; Shi, L.Y.; Ran, R. Stable, Strain-Sensitive Conductive Hydrogel with Antifreezing Capability, Remoldability, and Reusability. Acs Appl. Mater. Interfaces 2018, 10, 44000–44010. [Google Scholar] [CrossRef]
  42. Ye, Y.H.; Zhang, Y.F.; Chen, Y.; Han, X.S.; Jiang, F. Cellulose Nanofibrils Enhanced, Strong, Stretchable, Freezing-Tolerant Ionic Conductive Organohydrogel for Multi-Functional Sensors. Adv. Funct. Mater. 2020, 30, 03430. [Google Scholar] [CrossRef]
  43. Sui, X.J.; Guo, H.S.; Chen, P.G.; Zhu, Y.N.; Wen, C.Y.; Gao, Y.H.; Yang, J.; Zhang, X.Y.; Zhang, L. Zwitterionic Osmolyte-Based Hydrogels with Antifreezing Property, High Conductivity, and Stable Flexibility at Subzero Temperature. Adv. Funct. Mater. 2020, 30, 07986. [Google Scholar] [CrossRef]
  44. Wu, J.; Wu, Z.X.; Lu, X.; Han, S.J.; Yang, B.R.; Gui, X.C.; Tao, K.; Miao, J.M.; Liu, C. Ultrastretchable and Stable Strain Sensors Based on Antifreezing and Self-Healing Ionic Organohydrogels for Human Motion Monitoring. ACS Appl. Mater. Interfaces 2019, 11, 9405–9414. [Google Scholar] [CrossRef] [PubMed]
  45. Zhang, L.; Wang, J.Q.; Zhang, J.; Hou, L.X.; Ye, D.Z.; Jiang, X.C. Preparation of tough and anti-freezing hybrid double-network carboxymethyl chitosan/poly(acrylic amide) hydrogel and its application for flexible strain sensor. J. Mater. Sci. 2022, 57, 19666–19680. [Google Scholar] [CrossRef]
  46. Chen, D.W.; Bai, H.Y.; Zhu, H.Y.; Zhang, S.W.; Wang, W.; Dong, W.F. Anti-freezing, tough, and stretchable ionic conductive hydrogel with multi-crosslinked double-network for a flexible strain sensor. Chem. Eng. J. 2024, 480, 148192. [Google Scholar] [CrossRef]
Polymers 17 03102 g001
Figure 1. (a) Schematic diagram of AICH preparation. FESEM images of AICH with various NaCl concentrations, (b) 0 mmol/g and (c) 3 mmol/g. (d) DSC curves of AICH with various NaCl contents.
Figure 1. (a) Schematic diagram of AICH preparation. FESEM images of AICH with various NaCl concentrations, (b) 0 mmol/g and (c) 3 mmol/g. (d) DSC curves of AICH with various NaCl contents.
Polymers 17 03102 g001
Polymers 17 03102 g002
Figure 2. (a) The tensile stress-strain curves of AICH with various NaCl contents at room temperature, and (b) the corresponding effect of NaCl on the fracture strain and stress. (c) The tensile stress−strain curves of hydrogels with various NaCl contents at −20 °C, and (d) the corresponding effect of NaCl on the fracture strain and stress. (e) The tensile stress-strain curves of AICH3 at different subzero temperatures, and (f) the corresponding effect of temperature on the fracture strain and stress.
Figure 2. (a) The tensile stress-strain curves of AICH with various NaCl contents at room temperature, and (b) the corresponding effect of NaCl on the fracture strain and stress. (c) The tensile stress−strain curves of hydrogels with various NaCl contents at −20 °C, and (d) the corresponding effect of NaCl on the fracture strain and stress. (e) The tensile stress-strain curves of AICH3 at different subzero temperatures, and (f) the corresponding effect of temperature on the fracture strain and stress.
Polymers 17 03102 g002
Polymers 17 03102 g003
Figure 3. (a) The cyclic compressive tests of AICH3 at a constant strain of 20% at −20 °C. (b) The self−healing property of AICH3 at −20 °C.
Figure 3. (a) The cyclic compressive tests of AICH3 at a constant strain of 20% at −20 °C. (b) The self−healing property of AICH3 at −20 °C.
Polymers 17 03102 g003
Polymers 17 03102 g004
Figure 4. (a) Conductivity of AICH with various NaCl contents at 25 °C and −20 °C. (b) The change in GF of AICH3 with increasing tensile strain at −20 °C. The stability of AICH3 during cyclic loading-unloading with various tensile strains of (c) 0.5−10% and (d) 20−500%.
Figure 4. (a) Conductivity of AICH with various NaCl contents at 25 °C and −20 °C. (b) The change in GF of AICH3 with increasing tensile strain at −20 °C. The stability of AICH3 during cyclic loading-unloading with various tensile strains of (c) 0.5−10% and (d) 20−500%.
Polymers 17 03102 g004
Polymers 17 03102 g005
Figure 5. (a) The change in relative resistance when the AICH3-based strain sensor is fixed on the bionic finger and repeatedly bends at −20 °C, and (b) the change in relative resistance when the bending degree of the bionic finger gradually increases and then gradually recovers different bending degrees.
Figure 5. (a) The change in relative resistance when the AICH3-based strain sensor is fixed on the bionic finger and repeatedly bends at −20 °C, and (b) the change in relative resistance when the bending degree of the bionic finger gradually increases and then gradually recovers different bending degrees.
Polymers 17 03102 g005
Polymers 17 03102 g006
Figure 6. (a) Scheme of AICH3-based TENG. (b) Working mechanism of AICH3-based TENG. (c) Comparison of output voltage as the various contact materials are used. (d) The open-circuit voltage (Voc) in black color, short-circuit current (Isc) in dark cyan color and short-circuit charge in olive-green color (Qsc) of AICH3-based TENG. (e) Instantaneous current and area power density of AICH3-based TENG at various resistances. (f) The Voc of AICH3-based TENG at −20 °C. (g) The Qsc of AICH3-based TENG changes as the temperature decreases from 25 °C to −20 °C.
Figure 6. (a) Scheme of AICH3-based TENG. (b) Working mechanism of AICH3-based TENG. (c) Comparison of output voltage as the various contact materials are used. (d) The open-circuit voltage (Voc) in black color, short-circuit current (Isc) in dark cyan color and short-circuit charge in olive-green color (Qsc) of AICH3-based TENG. (e) Instantaneous current and area power density of AICH3-based TENG at various resistances. (f) The Voc of AICH3-based TENG at −20 °C. (g) The Qsc of AICH3-based TENG changes as the temperature decreases from 25 °C to −20 °C.
Polymers 17 03102 g006
Polymers 17 03102 g007
Figure 7. (a) The equivalent circuit and (c) the photograph of a self-charging power system. (b) Charging curves of various commercial capacitors using the AICH3-based TENG as the power source. (d) Voltage profile when the commercial capacitor with a capacity of 22 μF is charged by the AICH3-based TENG and powers a digital watch.
Figure 7. (a) The equivalent circuit and (c) the photograph of a self-charging power system. (b) Charging curves of various commercial capacitors using the AICH3-based TENG as the power source. (d) Voltage profile when the commercial capacitor with a capacity of 22 μF is charged by the AICH3-based TENG and powers a digital watch.
Polymers 17 03102 g007
Table 1. Comparison of the conductivity and GF for the AICH3 and the reported ionic conductive gel at subzero temperatures.
Table 1. Comparison of the conductivity and GF for the AICH3 and the reported ionic conductive gel at subzero temperatures.
ReferencesTemperature (°C)Conductivity (S/m)GF at Strain Range
This work−202.21.4 at 0–100%
This work−202.24.7 at 100–500%
This work−202.27.4 at 500–1000%
Ref. [40]−200.013.9 at 0–280%
Ref. [41]−200.32-
Ref. [42]−701.11.2 at 120%
Ref. [43]−402.7-
Ref. [44]−181.91.9 at 0–200%
Ref. [45]−200.11.5 at 0–150%
Ref. [46]−300.320.6 at 0–100%
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Wang, Y.; Yu, W.; Liu, S. An Anti-Freezing Ionic Conductive Hydrogel for Strain Sensing and Energy Harvesting Devices. Polymers 2025, 17, 3102. https://doi.org/10.3390/polym17233102

AMA Style

Wang Y, Yu W, Liu S. An Anti-Freezing Ionic Conductive Hydrogel for Strain Sensing and Energy Harvesting Devices. Polymers. 2025; 17(23):3102. https://doi.org/10.3390/polym17233102

Chicago/Turabian Style

Wang, Yanjie, Wei Yu, and Sijun Liu. 2025. "An Anti-Freezing Ionic Conductive Hydrogel for Strain Sensing and Energy Harvesting Devices" Polymers 17, no. 23: 3102. https://doi.org/10.3390/polym17233102

APA Style

Wang, Y., Yu, W., & Liu, S. (2025). An Anti-Freezing Ionic Conductive Hydrogel for Strain Sensing and Energy Harvesting Devices. Polymers, 17(23), 3102. https://doi.org/10.3390/polym17233102

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop