Zeolitic imidazolate framework-enhanced conductive nanocomposite hydrogels with high stretchability and low hysteresis for self-powered multifunctional sensors

Abstract

Conductive hydrogels have attracted increasing attention in the field of self-powered multifunctional sensors. However, simultaneously achieving high conductivity, high tensile strain, and low hysteresis remains challenging. Here, a poly(acrylamide)–polyvinylpyrrolidone nanocomposite conductive double network hydrogel with zeolitic imidazolate framework-8 nanoparticles was developed. The prepared hydrogel exhibits high stretchability (>900%), low mechanical hysteresis (<7%), high conductivity, fast response, high sensitivity, high cycling stability, and anti-freezing ability. Benefiting from such high performance, it can be utilized as a flexible electrode in a triboelectric nanogenerator for efficient energy harvesting. A self-powered multifunctional sensor is also demonstrated for human motion detection, pronunciation assessment, handwriting recognition and Morse code encryption. This work highlights the potential of conductive hydrogels for applications in self-powered wearable electronics and sensors.

---

1 Introduction

Wearable flexible sensors have attracted significant attention and are extensively employed in healthcare,^1,2 human motion detection^3–5 and energy harvesting.⁶ In particular, self-powered sensors, with their reliable power supply, excellent mechanical flexibility, and outstanding functional performance, can detect and convert mechanical energy into electrical signals without using a power supply. However, traditional self-powered sensors based on triboelectric,⁷ piezoelectric,⁸ thermoelectric,⁹ and photoelectric¹⁰ technologies are limited by complex fabrication processes, high production costs, and sensitivity to environmental conditions. Compared to other technologies, triboelectric nanogenerators (TENGs) are emerging as energy harvesting devices capable of converting mechanical energy into electrical energy by coupling contact electrification with electrostatic induction¹¹ and are widely employed for nanoscale energy harvesting and self-powered sensing.^12,13 However, the traditional electrodes of TENGs often suffer from complex fabrication, limited flexibility, and poor conductivity.^14,15

Conductive hydrogels are promising materials as the electrodes of TENGs due to their unique properties, including high conductivity, flexibility, elasticity and facile fabrication.^16,17 However, traditional conductive hydrogels typically exhibit large hysteresis due to breakage of chemical bonds or destruction and reformation of physical bonds during deformation.^18,19 Several strategies have been used to improve the hydrogels with low hysteresis through well-defined structure design strategies, such as chain entanglement,¹⁸ reducing noncovalent interaction,²⁰ and eliminating temporary entanglement.²¹ Despite the great success in these studies, some conductive hydrogels showed low tensile strain and strength.

Metal organic frameworks (MOFs) have rapidly emerged as effective nanofillers to enhance the functional diversity of hydrogels.^22–24 During the combination of MOFs with hydrogels, the side groups (including –NH₂, –OH, etc.) of the polymer chains forming the three-dimensional network of hydrogels easily coordinate with the metal ions in MOFs, which is beneficial to promote the mechanical properties and stability of the hydrogel. To the best of our knowledge, there are only a few studies on MOF-based conductive hydrogels for flexible self-powered sensors. For example, by incorporating zeolitic imidazolate framework-8 nanoparticles and lithium chloride electrolyte into a poly(acrylamide)-co-hydroxyethyl acrylate matrix, Kim developed a highly stretchable and durable conductive hydrogel for wearable TENGs in energy harvesting and sensor technology.²⁵ Therefore, MOF-based conductive hydrogels with excellent tensile properties and low hysteresis are highly significant as self-powered sensors for wearable electronics.

In this work, we introduced zeolitic imidazolate framework-8 nanoparticles (ZIF-8 NPs), which acted as multifunctional crosslinkers, to provide physical crosslinking points with a poly(acrylamide)– polyvinylpyrrolidone double network in a gel containing the typical ionic conductive sodium chloride. The prepared conductive nanocomposite hydrogel exhibits high stretchability, low mechanical hysteresis, high conductivity, high cycling stability, and anti-freezing ability. Moreover, it can be utilized as a flexible electrode in a triboelectric nanogenerator for efficient energy harvesting. A self-powered multifunctional sensor is also demonstrated for human motion detection, facial expression recognition, pronunciation assessment, handwriting recognition and Morse code encryption. This research marks a significant advancement in the design of conductive hydrogels with superior mechanical properties and multifunctionality, substantially expanding their potential applications in self-powered wearable electronics.

2 Results and discussion

2.1 Preparation and characterization of the hydrogel

The preparation process of the hydrogel is summarized as follows. First, ZIF-8 NPs, the monomer acrylamide (Am), polyvinylpyrrolidone (PVP), the crosslinker (N,N′-methylenebisacrylamide) and the initiator (Irgacure 2959) were added into NaCl solution. Then, the mixture was poured into a polytetrafluoroethylene mold and irradiated under UV light (365 nm) to obtain the ZIF-8@PAm/PVP hydrogel. More details about the synthesis of the conductive hydrogel, including the amounts and concentrations of materials, are provided in the ESI.† The internal interactions between ZIF-8 NPs and polymer networks are shown in Fig. 1a, in which the networks were closely associated via ZIF-8 NPs with multipoint coordination bonding including hydrogen bonds and electrostatic interactions. The as-prepared ZIF-8@PAm/PVP hydrogel can be stretched to 900.0% strain and recover to its initial length (Fig. 1b). The columnar sample can withstand compression and quickly recover after the removal of the compressive force (Fig. 1c). In addition, the hydrogel can lift a weight of 200.0 g and had a good static puncture resistance capacity (Fig. 1d and e).

Fig. 1 (a) Schematic illustration of the ZIF-8@PAm/PVP hydrogel. (b and c) Photographs of the ZIF-8@PAm/PVP hydrogel under the stretch-release and the compression-recovery cycles (scale bar = 1.0 cm). (d and e) Photographs of the ZIF-8@PAm/PVP hydrogel showing load-bearing capacity and puncture-resistant capacity (scale bar = 1.0 cm).

Fig. 2a depicts the scanning electron microscopy (SEM) image of ZIF-8 NPs. It shows that all ZIF-8 NPs consist of nanosized crystals with a rhombohedral dodecahedral structure, and the average size of ZIF-8 NPs is around 450 nm. Fig. 2b and c show the SEM images of freeze-dried PAm/PVP hydrogels and ZIF-8@PAm/PVP hydrogels, respectively. The surface of the PAm/PVP hydrogels was observed to be smooth, while that of ZIF-8@PAm/PVP hydrogels was rough, which indicated that ZIF-8 was uniformly integrated with PAm/PVP hydrogels. The average pore size of ZIF-8@PAm/PVP hydrogels was reduced from 1.83 μm to 1.72 μm (Fig. 2d). The chemical structures of ZIF-8@PAm/PVP hydrogels were investigated by Fourier-transform infrared spectroscopy (FTIR). As shown in Fig. 2e, when PVP was introduced, the N–H bending vibration peak was absent, and the C=O stretching vibration peak shifted to 1639 cm⁻¹, indicating the formation of hydrogen bonds between PAm and PVP in the PAm/PVP hydrogel.²⁶ The peaks in the spectrum of ZIF-8 NPs were attributed to the stretching vibration of the C–H group at 2927 cm⁻¹, the C=N group at 1585 cm⁻¹, and the stretching vibration of the C–N group at 1141 cm⁻¹ and 991 cm⁻¹, respectively.²⁷ The characteristic peaks mentioned in the ZIF-8 were also observed in the ZIF-8@PAm/PVP hydrogel. X-ray diffraction (XRD) was further employed to investigate the internal morphological structure of ZIF-8 NPs, PAm/PVP hydrogels, and ZIF-8@PAm/PVP hydrogels, as shown in Fig. 2f. The characteristic peak of the synthesized ZIF-8 NPs was consistent with previous research results.²⁵ The PAm/PVP hydrogel displayed a broad peak at approximately 26°, which corresponded to the characteristic peak of the polymer. The diffraction peaks of ZIF-8 NPs were also observed in ZIF-8@PAm/PVP hydrogels, which confirmed the successful integration of ZIF-8 NPs into the hydrogel matrix.

Fig. 2 (a–c) Scanning electron microscopy images of ZIF-8 nanoparticles, PAm/PVP hydrogels and ZIF-8@PAm/PVP hydrogels. (d) Average pore diameter of the PAm/PVP hydrogel and ZIF-8@PAm/PVP hydrogel. (e and f) Fourier-transform infrared spectroscopy and X-ray diffraction patterns of ZIF-8, PAm/PVP hydrogels and ZIF-8@PAm/PVP hydrogels. (g) Differential scanning calorimetry curves of the initial hydrogel without NaCl and ZIF-8 NPs and the ZIF-8@PAm/PVP hydrogel. (h) Weight change rate curves of PAm/PVP and the ZIF-8@PAm/PVP hydrogel. (i) UV-visible spectra of the ZIF-8@PAm/PVP hydrogel with different contents of ZIF-8 NPs.

The structural stability of ZIF-8@PAm/PVP hydrogels in an extremely cold environment is another crucial factor in determining the stable work for wearable flexible sensors. As shown in Fig. 2g, the anti-freezing properties of the hydrogels were evaluated by differential scanning calorimetry (DSC). The initial hydrogel without NaCl and ZIF-8 NPs exhibited a peak at −13.8 °C, which shifted to −28.9 °C for the ZIF-8@PAm/PVP hydrogel, showing excellent anti-freezing ability. Furthermore, the mass of the ZIF-8@PAm/PVP hydrogel showed minimal changes after 24 hours and stabilized over 120 hours at 20.0 °C and 40.0% relative humidity (Fig. 2h).

Fig. S1† shows that there was an increase in bound water compared to free water, and the characteristic peak of the bound water shifted from 3351 cm⁻¹ to 3363 cm⁻¹ with the addition of NaCl. The ions attracted surrounding water molecules, formed stable bonds and hindered the escape of water molecules, thereby effectively improving water retention. As shown in Fig. 2i, the transmittance of the ZIF-8@PAm/PVP hydrogel decreased to 81.1% when 0.1% ZIF-8 NPs was added, which was still sufficient to observe and detect the state of human skin in real time (Fig. S2 ESI†).

2.2 Mechanical properties of the hydrogel

The mechanical properties of the PAm/PVP hydrogel with different contents of NaCl were first studied. As the mass fraction of NaCl increased from 1.0% to 10.0%, the fracture strength of the PAm/PVP hydrogels increased from 296.3 kPa to 327.5 kPa (Fig. S3, ESI†). Moreover, the addition of NaCl improved the fracture strain of the PAm/PVP hydrogels, possibly because the polymer chains became entangled under the influence of NaCl.²⁸ The hydrogel with 10.0% NaCl was further studied in this work. The stress–strain curves of PAm/PVP hydrogels with varying contents of ZIF-8 NPs were measured (Fig. 3a). As shown in Fig. 3b, the hydrogel with 0.1% ZIF-8 NPs exhibited excellent tensile strain, with a maximum elongation at break of 944.2% and a toughness of 1569.4 kJ m⁻³. This enhancement was attributed to ZIF-8 NPs providing physical crosslinking points and facilitating hydrogen bond formation with polymer chains, which allowed for rapid energy dissipation and stress elimination, improving tensile strength. When content exceeded 0.1%, the hydrogels showed decreased breaking tensile strain (782.2%) and tensile strength (372.6 kPa). This reduction was likely due to excessive crosslinking, which shortened the distance between polymer chains and restricted molecular chain mobility to reduce stretchability.²⁹ Furthermore, ZIF-8@PAm/PVP hydrogels can still be stretched by 711.5%, with a tensile strength of 297.9 kPa at low temperatures of −20.0 °C (Fig. S4, ESI†). Thus, the hydrogel with 0.1% ZIF-8 NPs was further studied in this work.

Fig. 3 (a) Tensile stress–strain curves of ZIF-8@PAm/PVP hydrogels with different contents of ZIF-8 NPs. (b) The fracture strain and toughness of ZIF-8@PAm/PVP hydrogels with different contents of ZIF-8 NPs. (c) Tensile loading–unloading curves of ZIF-8@PAm/PVP hydrogels at different strains. (d) The corresponding dissipated energy and hysteresis ratio of ZIF-8@PAm/PVP hydrogels at different strains. (e) Stability test of ZIF-8@PAm/PVP hydrogels for 500 consecutive cycles at 200.0% strain. (f) Comparison of the hysteresis and strain of ZIF-8@PAm/PVP hydrogels with those of previously reported hydrogels. (g) Schematic illustration of the crosslinked network structure of the ZIF-8@PAm/PVP hydrogel during the stretch-release cycle. (h) The G′ and G′′ of the PAm/PVP hydrogel and ZIF-8@PAm/PVP hydrogel under a frequency sweep from 0.1 to 100.0 rad s⁻¹. (i) Compressive stress–strain curves of ZIF-8@PAm/PVP hydrogels with different contents of ZIF-8 NPs.

Although the dissipated energy increased, the hysteresis was not significantly increased during loading–unloading cycles with different strains (Fig. 3c and d). This can be attributed to the rapid dissociation and reorganization of dynamic hydrogen bonds within the hydrogel network during tensile cycles.³⁰ As shown in Fig. 3e, 500 continuous loading–unloading cyclic experiments were conducted at 200.0% tensile strain to evaluate the tensile fatigue resistance of the ZIF-8@PAm/PVP hydrogel. The energy dissipation of the ZIF-8@PAm/PVP hydrogel remained basically unchanged. In addition, the hysteresis behaviour of the ZIF-8@PAm/PVP hydrogel was nearly independent of the stretching rate (Fig. S5, ESI†). Notably, the ZIF-8@PAm/PVP hydrogel exhibited low hysteresis (6.50%) at 800.0% tensile strain, which was significantly lower compared to various previously reported hydrogels (Fig. 3f).^{20,21,31–37} The low hysteresis of the ZIF-8@PAm/PVP hydrogel could be attributed to the energy dissipation mechanism. As shown in Fig. 3g, the ZIF-8 NPs acted as physical cross-linkers to prevent polymer chain disentanglement during stretching. The stress was transmitted along the PAm/PVP chains and through entanglements to adjacent chains, while the breaking of hydrogen bonds dissipated elastic energy. Upon releasing the stretch, the fractured sacrificial bonds rapidly reformed, nearly completely reconstructing the dynamic crosslinking network. This process returned the energy absorbed by the PAm/PVP elastic chains during loading back to the system, resulting in negligible hysteresis. To get insight into the dynamic viscoelastic behaviour of the hydrogel, we measured the storage modulus (G′) and loss modulus (G′′) using a modular compact rheometer. As shown in Fig. 3h, no significant changes in G′ and G′′ of hydrogels were observed under a frequency sweep from 0.1 to 100.0 rad s⁻¹, showing the extremely high stability of the polymer network. Fig. 3h showed the frequency-dependent storage modulus (G′) and loss modulus (G′′) of the hydrogel before and after the addition of ZIF-8 nanoparticles. The addition of ZIF-8 NPs resulted in an increase in G′ and a decrease in loss modulus G′′ of the hydrogel, indicating enhanced elastic behavior and reduced viscous behavior of the hydrogel. This is likely due to the interactions between ZIF-8 NPs and the hydrogel matrix, which increase the rigidity of the material and reduce energy dissipation. Additionally, as the oscillation frequency increased, the tan δ of the hydrogels remained stable (Fig. S6, ESI†).

The mechanical compression properties of ZIF-8@PAm/PVP hydrogels with different contents of ZIF-8 NPs were also investigated. As shown in Fig. 3i, the compression stress increased as the ZIF-8 NP content increased to 0.1%. However, when the ZIF-8 NP content reached 0.5%, the mechanical properties of the hydrogel weakened, possibly due to the presence of free and un-interacted ZIF-8 NPs. Furthermore, the hydrogel showed a small dissipative energy of 4.3 kJ m⁻³ at a cyclic compression strain of 60.0% (Fig. S7 and S8, ESI†), indicating that this hydrogel could withstand large compressive deformations. The excellent elasticity of ZIF-8@PAm/PVP hydrogels was evaluated by successive cyclic compression experiments at 40.0% strain for 100 cycles (Fig. S9, ESI†). The ZIF-8@PAm/PVP hydrogel with outstanding mechanical properties showed potential for use in flexible wearable electronic devices.

2.3 Electrical properties of the hydrogel sensor

An ideal conductive hydrogel sensor should possess high stretchability, high sensitivity, and stable electrical performance. The porous structure of PAm/PVP hydrogels facilitated efficient ion transport, while the addition of environmentally friendly NaCl significantly enhanced the ionic conductivity and electrical sensitivity of the hydrogels. Electrochemical impedance spectroscopy (EIS) was used to evaluate the ionic conductivity of the ZIF-8@PAm/PVP hydrogel with different contents of ZIF-8 NPs. The addition of 10% NaCl increased the ionic conductivity of the PAm/PVP hydrogel to 1.40 S m⁻¹. With a ZIF-8 NP content of 0.1%, the conductive ZIF-8@PAm/PVP hydrogel still showed a high ionic conductivity of 1.36 S m⁻¹ (Fig. S10, ESI†). A comparative analysis of ZIF-8@PAm/PVP hydrogels with different contents of ZIF-8 NPs, including fracture strain, strength, hysteresis, toughness, and conductivity, is summarized in Fig. S11, ESI†.

To verify the potential of the ZIF-8@PAm/PVP hydrogel as a sensor, the electrical sensitivity was evaluated by measuring the relative resistance change (ΔR/R₀) and calculating the gauge factor (GF) based on the ratio of strain. As shown in Fig. 4a, the GF of the hydrogel sensor can be divided into three response ranges, 2.34 (0.0–200.0%), 4.76 (200.0–600.0%), and 6.38 (600–900.0%). Due to the low hysteresis and mechanical stability, the hydrogel-based strain sensor exhibited negligible mechanical relaxation and low electrical hysteresis over a wide strain range during cyclic loading and unloading (Fig. S12, ESI†). As depicted in Fig. 4b, the response time and recovery time of the ZIF-8@PAm/PVP hydrogel sensor were both 140.0 ms, which was beneficial for real-time monitoring of human motion. Moreover, the hydrogel sensor showed stable and repeatable performance under both small strain (2–10%) and large strain (100–500%), as shown in Fig. 4c and d. During progressive tensile cycling (5–200%), the ZIF-8@PAm/PVP hydrogel sensor generated continuous, stable, and easily distinguishable electrical signals (Fig. S13, ESI†). Additionally, the hydrogel sensor performed excellent segment stability loaded with the step-wise strains of 50% to maximum 200% and then unloaded to the initial state (Fig. 4e). The resistance changes of the hydrogel sensor were independent of the stretching rate during cyclic stretching at 200% strain (Fig. 4f). As shown in Fig. 4g, the GF of the hydrogel sensor in this work exceeded that of most previously reported hydrogel sensors and was comparable to the GF of several other advanced sensors.^35,38–45 More importantly, the hydrogel sensor exhibited excellent stability during 500 tensile cycles at 200% strain (Fig. 4h). Furthermore, the hydrogel sensor retained stable electrical performance at low temperatures (Fig. S14, ESI†). These results indicated that ZIF-8@PAm/PVP hydrogel sensors showed high sensitivity and excellent stability as flexible wearable sensors. With high stretchability, high sensitivity, anti-freezing capabilities, and stable sensing performance, ZIF-8@PAm/PVP hydrogels are highly suitable for flexible wearable sensors capable of detecting various human activities. As shown in Fig. S15a,† the assembled hydrogel strain sensor was mounted on a volunteer's finger to monitor changes in finger bending angles. When attached to the throat, the hydrogel strain sensor generated specific and repeatable signals in response to spoken words such as “Hello” and “Good morning” (Fig. S15b and c, ESI†). As shown in Fig. S15d,† wrist joint bending speed was accurately detected through variations in electrical signals. Similarly, hydrogel strain sensors installed on the neck and knee effectively monitored neck and knee movements (Fig. S15e and f, ESI†). Furthermore, the hydrogel strain sensor was integrated with programming and additional hardware to manipulate various lighting systems by utilizing resistance variations induced by mechanical strain as a trigger (Fig. S16 and Video S1, ESI†). When the hydrogel strain sensor was progressively stretched, the LED illuminated in distinct patterns, and as tension was released, the LED gradually dimmed. Therefore, this hydrogel-based sensor provided a promising way to fabricate real-time wearable devices for practical applications.

Fig. 4 (a) The relative resistance changes of the ZIF-8@PAm/PVP hydrogel sensor under different strains. (b) Response and recovery times of the ZIF-8@PAm/PVP hydrogel sensor. (c and d) The relative resistance changes of the ZIF-8@PAm/PVP hydrogel sensor during cyclic loading and unloading under small and large strains. (e) The dynamic response of the ZIF-8@PAm/PVP hydrogel sensor to sequential step-stretching of 50.0% and releasing back. (f) The relative resistance changes of the ZIF-8@PAm/PVP hydrogel sensor during stretching to 200.0% strain at different strain rates (from 10 to 150 mm min⁻¹). (g) Comparison of the maximum gauge factor of the ZIF-8@PAm/PVP hydrogel sensor with those of previously reported hydrogels. (h) The cycling sensing behaviour of the ZIF-8@PAm/PVP hydrogel sensor for 500 cycles at a strain of 200.0%.

2.4 Performance of the hydrogel-based TENG

Owing to its excellent ionic conductivity and mechanical flexibility, the conductive ZIF-8@ PAm/PVP hydrogel was utilized to construct the working electrode of a TENG for energy harvesting. Fig. 5a illustrates the working mechanism of the hydrogel-based TENG, which consisted of a nitrile layer, an Ecoflex triboelectric layer, and a conductive ZIF-8@PAm/PVP hydrogel electrode. When the Ecoflex layer was in contact with the nitrile rubber layer at equilibrium, an equal number of charges was generated on their surfaces in a balanced state. Upon separation, the negative charge on the nitrile rubber surface induces the movement of cations within the conductive hydrogel to balance the negative charge, forming a layer of excess cations at the interface. Meanwhile, an electric double layer forms at the interface between the copper wire and hydrogel, resulting in excess negative ions at that interface. The electrons then flowed to the ground through the copper wire, generating an electrical signal until electrostatic equilibrium was reached. When the Ecoflex layer was in contact with the nitrile rubber layer again, the electrons flowed back from the ground to the interface between the copper wire and hydrogel, thereby producing an electrical signal in the opposite direction.⁴⁶ To further analyze the energy harvesting mechanism, computational simulation using COMSOL Multiphysics was conducted to visualize the electrostatic potential distribution within the system (Fig. 5b).

Fig. 5 (a) The working mechanism of the ZIF-8@PAm/PVP hydrogel-based TENG. (b) COMSOL simulation of the potential distribution of the ZIF-8@PAm/PVP hydrogel-based TENG. (c–e) The open-circuit voltage, short-circuit current and charge of the ZIF-8@PAm/PVP hydrogel-based TENG at different frequencies. (f and g) The output voltage, current, and power density of the ZIF-8@PAm/PVP hydrogel-based TENG at different load resistances. (h) The response time of the ZIF-8@PAm/PVP hydrogel-based TENG. (i) The open-circuit voltage of the ZIF-8@PAm/PVP hydrogel-based TENG at 25.0 and −20.0 °C. (j) The output voltage durability of the ZIF-8@PAm/PVP-based hydrogel TENG for 500 cycles.

A hydrogel-based TENG with a contact area of 25.0 mm × 25.0 mm was used to investigate the effect of frequency on its triboelectric properties. As shown in Fig. 5c, the open-circuit voltage remained at 85.0 V as the working frequency increased from 0.5 Hz to 3.0 Hz. The short-circuit current and transferred charge at different frequencies are shown in Fig. 5d. The current gradually increased from 0.9 μA to 1.6 μA as the frequency increased, while the charge quantity remained nearly constant at 39.0 nC (Fig. 5e). As shown in Fig. 5f, the output voltage increased from 0 to 85 V as the load resistance increased from 10⁰ kΩ to 10⁶ kΩ, while the current decreased in accordance with Ohm's law. The maximum power density of the TENG was 344.1 mW m⁻² at a load resistance of 5.0 × 10⁴ kΩ (Fig. 5g). The TENG showed an ultra-fast response time of about 20 ms for motions (Fig. 5h). As shown in Fig. 5i, the TENG showed enhanced electrical output performance at low temperatures, and the voltage of the TENG reached 45 V at −20.0 °C. It also exhibited ultra-stable electrical output performance, with the voltage remaining essentially constant during more than 500 consecutive contact-separation cycles (Fig. 5j). Alternating current generated by the TENG was converted into direct current using a bridge diode, and the resultant electric current was utilized to charge a commercial capacitor. The charging capability was demonstrated by charging a series of commercial capacitors at 3 Hz (Fig. S17 and S18, ESI†). A 47 μF capacitor can be charged to a voltage over 3 V within 120 s (Fig. S19 and Video S2, ESI†). Moreover, 100 commercial LEDs were simultaneously illuminated by tapping the TENG (Fig. S20 and Video S3, ESI†). These results indicated that the hydrogel-based TENG could have significant promise in self-powered flexible and wearable devices.

2.5 Application of the hydrogel-based TENG

The practical applications of the hydrogel-based TENG in self-powered systems were next investigated. The self-powered sensor demonstrated the ability to recognize handwriting by converting it into unique electrical signals as different letters were written on the ZIF-8@PAm/PVP hydrogel-based TENG (Fig. 6a). As shown in Fig. 6b and c, the self-powered sensor produced stable and repeatable voltage signals, allowing letters to be distinguished based on the shape and number of peaks in the voltage curve. Additionally, the self-powered sensor was employed to transmit encrypted information using Morse code, where short-term touches generated electrical signals as “dots” and long-term touches were represented as “dashes” (Fig. 6d and e). Information could be encrypted and decrypted using a Morse code table representing the 26 English letters. Critical messages such as “SOS”, which convey human states and emotions, can be accurately encrypted and transmitted, providing potential for sending distress signals in emergencies, particularly for individuals with limited mobility and expression (Fig. 6f, ESI†). The self-powered sensor also can detect mouse click frequencies, such as single-click, double-click, and triple-click events (Fig. 6g). Additionally, the self-powered sensor monitored different walking states and arm movements when attached to the sole of a shoe or the outer side of an elbow (Fig. 6h and i).

Fig. 6 The application of the ZIF-8@PAm/PVP hydrogel-based TENG as a self-powered sensor. (a) Schematic representation of employing it as a hand-writing sensor. Voltage changes for (b) “TENG” and (c) “OK”. (d) Schematic diagram of the Morse code. (e) The voltage signals generated for Morse code: the short-term touch represented as “dot” and the long-term touch represented as “dash”. (f) Using the self-powered sensor to send the message “SOS” by finger clicks. (g–i) Detection of clicking the mouse, arm movements, and walking.

4 Conclusion

This study presented the development of a highly stretchable and low-hysteresis conductive nanocomposite hydrogel by incorporating ZIF-8 NPs into a PAm/PVP hydrogel matrix, where the ZIF-8 NPs acted as multipoint coordination bonding including hydrogen bonds and electrostatic interactions. The resulting ZIF-8@PAm/PVP hydrogel exhibited high stretchability, low mechanical hysteresis, high conductivity, fast response, high sensitivity, high cycling stability, and anti-freezing ability. Moreover, the ZIF-8@PAm/PVP hydrogel can be utilized as a flexible electrode in a triboelectric nanogenerator for efficient energy harvesting. A self-powered multifunctional sensor was also demonstrated for human motion detection, pronunciation assessment, handwriting recognition and Morse code encryption. This study demonstrated the significant potential of conductive hydrogels for use in self-powered wearable technology and sensory applications.

Data availability

All the data related to the current submission entitled “Zeolitic imidazolate framework-enhanced conductive nanocomposite hydrogels with high stretchability and low hysteresis for self-powered multifunctional sensors” are available upon reasonable request directly from the authors via email to the corresponding authors.

Author contributions

R. W., W. Z., Z. L and J. X. conceived the project. All authors contributed to the experiments, data analysis, and writing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 52103146, 52225306, 52350120, 22405134 and 52461160302), the Young Elite Scientists Sponsorship Program by CAST (Grant No. 2021QNRC001), the Tianjin Natural Science Foundation (Grant No. 24JCYBJC00920), the National Key Research, Development Program of China (Grant No. 2022YFB3807103 and 2022YFA1203304), the Beijing-Tianjin-Hebei Basic Research Cooperation Project (Grant No. J230023), and Key Projects of Anhui Province’s Science and Technology Innovation Tackle Plan (Grant No. 202423i08050057). We would like to thank the Analytical & Testing Centre of Tiangong University for rheological measurements, scanning electron microscopy and differential thermal scanning analysis.

References

1. J. Yi and Y. Xianyu, Adv. Funct. Mater., 2022, 32, 2113012.
2. Z. Xu, Y. Yang, X. Pang, B. Jiang, P. Mao, L. Gong, B. Wang, L. Peng, L. Tang and S. Li, Chem. Eng. J., 2025, 503, 158243.
3. L. Ma, R. Wu, A. Patil, S. Zhu, Z. Meng, H. Meng, C. Hou, Y. Zhang, Q. Liu, R. Yu, J. Wang, N. Lin and X. Liu, Adv. Funct. Mater., 2019, 29, 1904549.
4. Y. Zou, G. Liu, H. Wang, K. Du, J. Guo, Z. Shang, R. Guo, F. Zhou and W. Liu, Small, 2024, 20, 2404435.
5. Z. Zeng, Y. Yang, X. Pang, B. Jiang, L. Gong, Z. Liu, L. Peng and S. Li, Adv. Funct. Mater., 2024, 34, 2409855.
6. R. Wu, L. Ma, A. Patil, Z. Meng, S. Liu, C. Hou, Y. Zhang, W. Yu, W. Guo and X. Y. Liu, J. Mater. Chem. A, 2020, 8, 12665–12673.
7. S. Tushar, A. Sayam, M. Uddin, T. Dip, H. Anik, M. Aktar and S. Sharma, J. Mater. Chem. A, 2023, 11, 19244–19280.
8. P. Li, Z. Zhang, W. Shen, C. Hu, W. Shen and D. Zhang, J. Mater. Chem. A, 2021, 9, 4716–4723.
9. G. Li, Y. Hu, J. Chen, L. Liang, Z. Liu, J. Fu, C. Du and G. Chen, Adv. Funct. Mater., 2023, 33, 2303861.
10. J. Shin, Y. Kim, J. Park, J. Lee, S. Park, S. Lee, J. Lee and K. Lee, J. Mater. Chem. A, 2021, 9, 25694–25705.
11. S. Liu, W. Tong, C. Gao, Y. Liu, X. Li and Y. Zhang, J. Mater. Chem. A, 2023, 11, 9270–9299.
12. C. Zhang, X. Lin, N. Zhang, Y. Lu, Z. Wu, G. Liu and S. Nie, Nano Energy, 2019, 66, 104126.
13. H. Xiang, L. Peng, Q. Yang, Z. L. Wang and X. Cao, Sci. Adv., 2024, 10, eads2291.
14. R. Wang, S. Gao, Z. Yang, Y. Li, W. Chen, B. Wu and W. Wu, Adv. Mater., 2018, 30, 1706267.
15. H. Wang, C. Jeong, M. Seo, D. Joe, J. Han, J. Yoon and K. Lee, Nano Energy, 2017, 35, 415–423.
16. Y. Wu, Y. Luo, T. Cuthbert, A. Shokurov, P. Chu, S. Feng and C. Menon, Adv. Sci., 2022, 9, 2106008.
17. B. Zhang, R. Wang, R. Wang, B. Chen, H. Li, A. Shen and Y. Mao, Mater. Chem. Front., 2024, 8, 4003–4028.
18. J. Kim, G. Zhang, M. Shi and Z. Suo, Science, 2021, 374, 212–216.
19. J. Wang, F. Tang, C. Yao and L. Li, Adv. Funct. Mater., 2023, 33, 2214935.
20. T. Li, X. Li, J. Yang, H. Sun and J. Sun, Adv. Mater., 2023, 35, 2307990.
21. X. Meng, Y. Qiao, C. Do, W. Bras, C. He, Y. Ke, T. Russell and D. Qiu, Adv. Mater., 2022, 34, 2108243.
22. W. Sun, X. Zhao, E. Webb, G. Xu, W. Zhang and Y. Wang, J. Mater. Chem. A, 2023, 11, 2092–2127.
23. W. Cao, Z. Lin, D. Zheng, J. Zhang, W. Heng, Y. Wei, Y. Gao and S. Qian, J. Mater. Chem. B, 2023, 11, 10566–10594.
24. J. Wychowaniec, H. Saini, B. Scheibe, D. Dubal, A. Schneemann and K. Jayaramulu, Chem. Soc. Rev., 2022, 51, 9068–9126.
25. M. Rahman, M. Rahman, H. Kumar, K. Kim and S. Kim, Adv. Funct. Mater., 2023, 33, 2303471.
26. H. Zhang, W. Niu and S. Zhang, ACS Appl. Mater. Interfaces, 2018, 10, 32640–32648.
27. T. Zhu, S. Xu, F. Yu, X. Yu and Y. Wang, J. Membr. Sci., 2020, 598, 117681.
28. G. Chen, J. Huang, J. Gu, S. Peng, X. Xiang, K. Chen, X. Yang, L. Guan, X. Jiang and L. Hou, J. Mater. Chem. A, 2020, 8, 6776–6784.
29. H. Liu, H. Peng, Y. Xin and J. Zhang, Polym. Chem., 2019, 10, 2263–2272.
30. E. Kamio, M. Minakata, H. Nakamura, A. Matsuoka and H. Matsuyama, Soft Matter, 2022, 18, 4725–4736.
31. H. Wang, B. Liu, D. Chen, Z. Wang, H. Wang, S. Bao, P. Zhang, J. Yang and W. Liu, Mater. Horiz., 2024, 11, 2628–2642.
32. R. Du, T. Bao, T. Zhu, J. Zhang, X. Huang, Q. Jin, M. Xin, L. Pan, Q. Zhang and X. Jia, Adv. Funct. Mater., 2023, 33, 2212888.
33. Z. Shen, Z. Zhang, N. Zhang, J. Li, P. Zhou, F. Hu, Y. Rong, B. Lu and G. Gu, Adv. Mater., 2022, 34, 2203650.
34. Y. Liu, G. Tian, Y. Du, P. Shi, N. Li, Y. Li, Z. Qin, T. Jiao and X. He, Adv. Funct. Mater., 2024, 34, 2315813.
35. J. Liu, X. Chen, B. Sun, H. Guo, Y. Guo, S. Zhang, R. Tao, Q. Yang and J. Tang, J. Mater. Chem. A, 2022, 10, 25564–25574.
36. S. Han, Y. Hu, J. Wei, S. Li, P. Yang, H. Mi, C. Liu and C. Shen, Adv. Funct. Mater., 2024, 34, 2401607.
37. G. Chen, O. Hu, J. Lu, J. Gu, K. Chen, J. Huang, L. Hou and X. Jiang, Chem. Eng. J., 2021, 425, 131505.
38. J. Zou, X. Jing, Z. Chen, S. Wang, X. Hu, P. Feng and Y. Liu, Adv. Funct. Mater., 2023, 33, 2213895.
39. F. Hu, Z. Huang, C. Luo and K. Yue, Mater. Horiz., 2023, 10, 5907–5919.
40. J. Wen, J. Tang, H. Ning, N. Hu, Y. Zhu, Y. Gong, C. Xu, Q. Zhao, X. Jiang, X. Hu, L. Lei, D. Wu and T. Huang, Adv. Funct. Mater., 2021, 31, 2011176.
41. X. Yu, H. Zhang, Y. Wang, X. Fan, Z. Li, X. Zhang and T. Liu, Adv. Funct. Mater., 2022, 32, 2204366.
42. M. Wang, L. Li and T. Zhang, Nano Energy, 2024, 126, 109586.
43. Y. Long, B. Jiang, T. Huang, Y. Liu, J. Niu, Z. Wang and W. Hu, Adv. Funct. Mater., 2023, 33, 2304625.
44. J. Zou, X. Jing, S. Li, P. Feng, Y. Chen and Y. Liu, Small, 2024, 20, 2401622.
45. X. Yu, Y. Zheng, H. Zhang, Y. Wang, X. Fan and T. Liu, Chem. Mater., 2021, 33, 6146–6157.
46. Y. Long, Z. Wang, F. Xu, B. Jiang, J. Xiao, J. Yang, Z. Wang and W. Hu, Small, 2022, 18, 2203956.

---

Footnote

† Electronic supplementary information (ESI) available: All the information about the materials and methods. See DOI: https://doi.org/10.1039/d4ta08994d

---