Versatile hydrogel towards coupling of energy harvesting and storage for self-powered round-the-clock sensing

Abstract

For realizing intelligent sensing adaptive to various scenarios, flexible and integrated electronics with multiple functions, without sacrifice of electro-chemical properties, are urgently needed. In this work, we rationally design a multifunctional, high-conductivity, flexible, anti-freezing and self-adhesive double-network hydrogel with a three-dimensional (3D) interpenetrating framework. The hydrogel is capable of stretching up to approximately 1100% and can be utilized directly as a piezoresistive strain sensor, the electrode of a self-powered triboelectric nanogenerator (TENG), and the electrolyte of a supercapacitor (SC) concurrently. The self-powered and stretchable TENG generates high electrical output, thereby being capable of sensing low-level human biomechanical activities in real time. The assembled SC deliveries high capacitance at a broad range of current densities in a wide temperature as low as −20 °C. By integrating the self-powered TENG with the flexible SC into an integrated self-charging power supply system, this wearable and flexible system can harvest normal activity of the human body, realize high-sensitivity biomechanical sensing, and store the excess energy in the SC to supply continuous power for small electronic devices when needed. This work provides a promising pathway to the assembly of a wearable and self-driven system for self-powered round-the-clock health monitoring.

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1. Introduction

With the rapid development of modern society, increasing attention has been paid to keeping personally fit and healthy, which has resulted in a high demand for autonomous flexible electronics.^1–5 It is widely accepted that the real-time detection of physiological signals in daily activity would be helpful for the early detection of sub-health status and certain debilitating diseases.^2,3 Since the invention of the triboelectric nanogenerator (TENG), considerable progress has been achieved in this field in recent years.^6–13 TENGs can convert biomechanical energy into electrical signals by the coupling of triboelectrification and electrostatic induction. The unique characteristics of TENGs including high energy conversion efficiency, high sensitivity, reliability and low cost, make them promising candidates in a variety of fields, especially for biomechanical signal collection and sensing.^14–17 Although TENGs are considered as ideal candidates for biomechanical signal harvesting and sensing, the single and limited functionality of TENGs often restricts their application, especially as wearable electronics.^16–19 For real-time and continuous monitoring of health conditions, a continuous mechanical stimulus with sufficiently high strength is generally needed to trigger TENGs. Furthermore, when the human body stays in a resting state, the electrical energy from the mechanical-to-electrical energy conversion is often inadequate to support real-time sensing and signal transmission and display.

It is sensible to harvest and store excess electrical energy when the human body is in an active state and release this energy to supply the monitoring process when needed. For some subhealth status and disease conditions, real time and 24 hours monitoring of biomechanical dynamics is desired. It is promising to integrate TENGs with SCs to form a self-charging power supply system, storing the energy harvested by TENGs in the SC and releasing this energy when needed. To date, most reports have focused on the fabrication of TENGs, charging commercial SCs with rigid structures.^{16,17,20–26} For real applications, flexible charging systems, composed of wearable and flexible TENGs and SCs are urgently needed,²⁷ especially those that can be used in a wide temperature range.²⁸

Conductive polymeric hydrogels with flexible and stretchable characteristics have become promising candidates to fabricate flexible electronics, including flexible TENGs,^17,29 SCs,^30–33 sensors,^34–39 and actuators.⁴⁰ The functionalities of hydrogels can be tuned by introducing various functional and sensitive fillers into the hydrogel matrix.^41,42 However, it is still a great challenge to fabricate high-conductivity hydrogels without sacrificing their flexibility and stretchability, as well as incorporate a combination of multiple functions into one single hydrogel.

In this study, we fabricate a multifunctional double-network conductive PDA/PAM/GO hydrogel (shortened to PPG hydrogel) based on polyacrylamide (PAM) and polydopamine (PDA) with doped graphene oxide (GO) nanosheets by facile in situ polymerization. Based on this hydrogel, we assemble a piezoresistive strain sensor, a self-powered TENG and a flexible SC. A self-charging power supply and health detection system composed of a flexible TENG, a rectifier bridge and a wearable SC can harvest human biomechanical energy, realize biomechanical sensing, and support sensor devices to monitor human health, potentially realizing 24 hours health monitoring. This system guarantees intelligent, real-time, and continuous monitoring in a wide temperature range, making the system adaptable to various application scenarios.

2. Results and discussion

Scheme 1a illustrates the synthetic process of the versatile, flexible and conductive hydrogels. Using ammonium persulfate (APS) and N,N′-methylenebisacrylamide (BIS) as the initiator and crosslinker of the polymerization reaction, AM and DA polymerize into their corresponding polymers PAM and polydopamine (PDA), which also crosslink with each other via a Schiff-based reaction.⁴³ The interpenetrating double network endows the resultant hydrogel with high mechanical strength and flexibility. Deep in the polymeric chains, two-dimensional GO nanosheets are doped. Under alkaline conditions, abundant functional groups of GO such as hydroxyl and carboxyl groups can bond with the functional groups of PDA and PAM, facilitating the uniform dispersion of GO in the framework, reinforcing the inner structure of the double network and enhancing the conductivity of the hydrogel. The hydrogen bonding and π–π stacking interactions between the PDA and PAM chains endow the PPG hydrogel with exceptional stretchability and the functional groups in PDA guarantee the self-adhesive properties (Scheme 1b). Given the remarkable properties of the hydrogel including high conductivity, flexibility and excellent mechanical properties, we utilized it as a functional layer to construct a TENG device and a flexible and wearable supercapacitor concurrently. Furthermore, these devices can be integrated together to form a self-charging power system for round-the-clock biomechanical sensing without additional energy supply (Scheme 1c).

Scheme 1 Schematic diagram of the fabrication process, chemical structure, and applications of the PPG hydrogel. (a) Schematic diagram of the fabrication process of the PPG hydrogel, (b) schematic diagram of the inner chemical structure of the hydrogel, and (c) versatile applications of the PPG hydrogel for assembling TENGs, SCs and strain sensors.

To investigate the inner microstructure of the PPG hydrogel, we observed the hydrogel using SEM. As shown in Fig. 1a, the hydrogel possesses a 3D porous structure with a pore size of 1–3 μm, which can facilitate fast transportation of ions within the porous structure. When assembling TENGs, a mismatch between the electrodes and the substrates always leads to a discontinuous and unstable output, resulting in an imprecise signal when used for sensing. Attributed to the presence of PDA in the interpenetrating matrix, the hydrogel exhibited excellent self-adhesion properties towards a variety of substrates (Fig. 1b). It can adhere to wood, plastic, glass, and skin firmly and the tensile deformation during finger bending did not influence its adhesion. Compared to the pure single-network of the PAM hydrogel with conductivities of 0.023 s cm⁻¹, the conductivity of the PPG hydrogel significantly increased to 0.173 s cm⁻¹ (Fig. S1†). In addition, the PPG hydrogel exhibited extraordinary stretchability, and could be stretched to 1100% (Fig. 1c). It displayed a positive response of the electrical resistance to the stretching ratio, and this can be divided into two ranges, 0–500% and 500–1000%, for which the GF was 0.0688 and 0.0133, respectively (Fig. 1d and e). The hydrogel as a conductor can light a light-emitting diode (LED) in its stretching, bending and twisting states (Fig. 1f).

Fig. 1 Characterization of the PPG hydrogel. (a) SEM images, (b) photographs showing the adhesive properties, and (c) stretchability of the PPG hydrogel. (d) ΔR/R₀ variations under stretching and releasing from 0 to 1100% strain, (e) relative resistance change of the hydrogel under various applied strains, and (f) flexibility of the hydrogel as a conductor to light an LED in the stretching, bending and twisting states.

Since the PPG hydrogel exhibited high conductivity attributed to the synergy of electronic and ionic conduction, we applied it as the electrode to fabricate flexible single-electrode sandwich-structured PPG-TENG by encapsulating the hydrogel with Ecoflex silicone rubber. The operating mechanism and electrical output performance of the PPG-TENG are shown in Fig. 2a. Once the dielectric material such as Kapton moves towards the silicone rubber layer, electrification occurs at the contact interface, and an equal number of charges with opposite polarities are generated (Fig. 2a(i)). When the dielectric layer moves away from the PPG-TENG, positive charges are generated in the PPG hydrogel (Fig. 2a(ii)) and electrons flow from the hydrogel to the ground through an external circuit and generate an electrical pulse. When the dielectric layer is completely separated from the PPG-TENG, an electrostatic equilibrium is achieved without electrical output (Fig. 2a(iii)). When the dielectric layer moves towards the PPG-TENG again, the whole process will be reversed, and the potential difference will gradually decrease. Finally, the electrons will flow from the ground back to the hydrogel electrode through the external circuit (Fig. 2a(iv)). The corresponding potential during the continuous contact and separation process was simulated by COMSOL software (Fig. 2b). The whole PPG-TENG was flexible, stretchable and twistable, as shown in Fig. 2c(i–iii). We compared the output including open-circuit voltage V_oc, short-circuit current I_sc, and transferred charge Q_sc of the TENG made from the PPG and PAM hydrogels (Fig. 2d–f and S2†). When using the PPG hydrogel as the electrode and Cu as the triboelectric layer, the resultant PPG-TENG generated a V_oc of 143 V, I_sc of 1.8 μA, and Q_sc of 47 nC, substantially higher than those of that made with the PAM hydrogel (V_oc of 60 V; I_sc of 0.68 μA; and Q_sc of 19 nC) (Fig. S2†) with a frequency of 1 Hz.

Fig. 2 Working principle and output performance of the PPG-TENG. (a) Schematic diagram of the working principle of the PPG-TENG. (b) Finite element simulation of the potential distribution of the TENG during friction using COMSOL Multiphysics. (c) Flexibility of the PPG-TENG. (d–f) Electrical outputs (V_oc, I_sc and Q_sc) of the TENG. (g–i) Electrical outputs (V_oc, I_sc and Q_sc) of the TENG in contact with different materials.

Based on the basic contact electrification principle, the PPG-TENG can sense and identify different contact materials when the silicone rubber layer is contacting these different materials, related to the output signals. When Kapton, polytetrafluoroethylene (PTFE), Cu and Al contacted the PPG-TENG, the generated V_oc values were 85 V, 121 V, 143 V and 156 V, respectively, and the generated I_sc and Q_sc displayed a similar increasing trend (Fig. 2g–i).

To act as wearable sensors for detecting human biomechanical activities with various frequencies, we measured the output of the PPG-TENG under various operating frequencies (Fig. S3†). When the frequency increased from 0.5 to 2 Hz, V_oc and Q_sc were generally maintained at approximately 143 V and 47 nC, respectively, while I_sc showed a positive correlation to the frequency using Cu as the triboelectric layer. Furthermore, we measured the output of the PPG-TENG under various mechanical forces. The PPG-TENG can respond to even an extremely small force of 0.1 N. As shown in Fig. S4,† the electric potential of the PPG-TENG device increased from 5.6 V to 13.3 V when the force increased from 0.1 N to 5 N, indicating that the device was sensitive enough to detect minor deformation. In addition, the PPG-TENG had a fast response to external force, with a response time within 50 ms and a recovery time within 70 ms (Fig. S5†), suitable for real-time monitoring. When it was stretched, the output decreased correspondingly (Fig. S6†), which should be attributed to the slight decrease in resistance when stretching.

With the positive relationship between the stretching ratio and conductivity of the hydrogel (as shown in Fig. 1e), we also applied the PPG hydrogel as a wearable strain sensor and self-powered e-skin. The PPG-TENG was first worn at various positions of the human body to monitor the biomechanical motions (Fig. 3). When attached at the finger joints, it can sense finger bending and estimate the bending degree (Fig. 3a). The output signal increased with increasing bending degree, presenting good repeatability. As a wearable device, the PPG-TENG was capable of distinguishing pressures applied to the device. As a result, it can clearly identify the number of fingers touching the PPG-TENG (Fig. 3b). The device was also sensitive enough to detect the frequency of human motion, such as arm swinging, breathing and coughing (Fig. 3c–e), exhibiting potential in human motion monitoring. Attributed to the high sensitivity and flexibility, the sensor can monitor tiny human motions in real time, including face movements and expressions, mouth opening, swallowing, blowing and frowning (Fig. 3f–i).

Fig. 3 Applications of the self-powered PPG-TENG and PPG-based piezoresistive sensors. PPG-TENG output with (a) different bending degrees of fingers, (b) different numbers of touching fingers, and (c) different frequencies of arm swing, and pathological monitoring of (d) breathing and (e) coughing. The PPG hydrogel as a piezoresistive sensor to detect (f) an opening mouth, (g) swallowing, (h) blowing and (i) frowning.

The PPG hydrogel, featuring good mechanical properties and electrical conductivity, was also used as an electrolyte for assembling a flexible all-solid-state SC to store energy generated from the PPG-TENG and provide continuous power supply for the sensors (Fig. 4a). The electrode of the SC was fabricated by the electrochemical polymerization of pyrrole on carbon fiber at a constant voltage of 0.8 V. From the SEM images, polypyrrole was evenly coated on the surface of CF (Fig. S7†). Before use as an electrolyte, the PPG-hydrogel was immersed in 3 M KOH with ethylene glycol (EG) solution, resulting in a PPGK hydrogel. The counterpart treated from the PAM-hydrogel was denoted as PAMK. Then, the supercapacitor was prepared by clamping the hydrogel with two polypyrrole-functionalized carbon cloths as electrodes (PPy/CF), forming a sandwich structure. From the cyclic voltammetry (CV) curve, the SC made from the PPGK hydrogel exhibited a higher capacitance than that assembled from the PAMK hydrogel (Fig. 4b). This was further confirmed by the galvanostatic charge–discharge (GCD) curves of the electrolytes (Fig. S8†). Electrochemical impedance spectroscopy (EIS) results revealed the mechanism of the enhanced electrochemical performance of the PPGK hydrogel, based on the frequency response characteristics of the electrodes. As shown in Fig. 4c, the PPGK hydrogel possessed smaller internal resistance and larger slope in the low-frequency region than the PAMK hydrogel, suggesting the low internal conductivity of the PPGK hydrogel electrode and fast ion diffusion to the electrode interface in the electrolyte. The assembled SC exhibited a typical rectangular CV curve (Fig. 4d) and nearly triangular GCD curve (Fig. 4e) even under different sweep rates and current densities, revealing excellent retention of its capacitive behavior. The capacitance was calculated to be 123, 121, 96, and 73.2 A g⁻¹ at current densities of 0.5, 1, 1.5 and 2 A g⁻¹, respectively (Fig. 4f), confirming its good rate capability. The excellent performance of the PPGK hydrogel was mainly attributed to the combination of the 3D interconnected framework of the hydrogel and the 2D GO nanosheets, resulting in fast electron transport and an unobstructed ion diffusion pathway. As shown in Fig. 4g–i, the SC exhibited superior stability under various conditions. The CV curves measured at different bending degrees (0, 30, 60, and 90°), twisting states and pressing states exhibited almost identical capacitive behavior (Fig. 4g–i). These results proved that the electrochemical performance of the SC was not affected even with large deformation. The good maintenance can be attributed to the combination of the flexibility of the whole SC and strong adhesion between the electrodes and the electrolyte.

Fig. 4 Performance of the SC. (a) Structure and optical photograph of the SC, (b) CV and (c) EIS curves of the supercapacitors using the PAM, PPG and PPGK hydrogels as the electrolyte, (d) CV and (e) charge–discharge curves of the supercapacitor using the PPGK hydrogel as the electrolyte at different sweep rates and current densities and (f) corresponding capacitances at different current densities. CV curves of the SC under (g) different bending degrees, and (h) twisting and (i) pressing states.

In addition to the flexible properties, the SC was also capable of coping well with extreme conditions. When stored at −20 °C, the PPGK hydrogel still retained excellent flexibility and transparency. The PPGK hydrogel was used as a conductor to light an LED bulb at −20 °C, and the LED still remained operational (Fig. 5a). The hydrogel can be easily stretched and twisted at −20 °C (Fig. 5b). This result indicated that low temperature did not cause obvious structural change of the hydrogel. The anti-freezing properties of the PPGK hydrogel were attributed to the presence of the H₂O/EG binary solution, in which molecular clusters formed to facilitate the reduction of the saturated vapor pressure, thereby inhibiting the crystallization. In addition, the functional carboxyl and carbonyl groups of the GO nanosheets can increase the density of crosslinking points of the hydrogel network. Next, the capacitance performances of the PPGK hydrogel-based SC at 15 °C to −20 °C were measured (Fig. 5c and d). As shown in Fig. 5e, the capacitance only slightly decreased upon decreasing the temperature, and the system can work well even at −20 °C.

Fig. 5 Anti-freezing and washing properties of the SC. (a) Photographs of the PPGK hydrogel at 25 °C and −20 °C, and the PPGK hydrogel as a conductor to light an LED at −20 °C, (c) CV, (d) charge–discharge curves and (e) corresponding capacitance of the SC based on the PPGK hydrogel at different temperatures, and (f) the retention of the capacitance of the SC after different washing cycles.

To save cost and resources, part of SC can be reused. After the whole SC had been immersed in water, the electrode could be easily peeled off from the hydrogel and reused to be assembled into a new SC. We measured the electrochemical performance of the reassembled SC. As shown in Fig. 5f and S9,† the CV curves of the SC deviated slightly from those for the pristine state after being reassembled for five repeated cycles. Compared with the original state, the capacitance remained approximately 90% after five cycles, indicating the good stability and practicability (Fig. 5f).

The flexible SC can be connected in parallel or in series to reach a higher current or voltage, and used as a wearable power supplement for powering electrical devices (Fig. 6a). Additionally, the operation voltage of the SC can be tuned by connection of the SCs in parallel. As shown in Fig. 6b and c, the potential range increased to 1.6 V when two SCs were connected in series, and the discharging time was prolonged when two identical SCs were connected in parallel. Furthermore, the SCs can be charged by the PPG-TENG to realize energy harvesting and storage when real time sensing based on the PPG-TENG is not needed. Fig. 6d shows the equivalent circuit of the self-charging power supply system, including a PPG-TENG, a rectifier bridge and a SC. In the system, the PPG-TENG was used to collect the biomechanical energy, and the energy can be stored in the SC, and the stored electrical energy can power external electronic devices. As shown in Fig. 6e–h, after being charged, the SC can light up a LED or medical electronic device such as a pedometer as a demonstration. These results proved the potential applications of the system in flexible and wearable personalized healthcare.

Fig. 6 (a) Illustration of the wearable self-charging power supply system for electrical energy collection, storage and monitoring. GCD curves of a single SC and two SCs connected (b) in series, and (c) in parallel at the same current density. (d) Circuit diagram of the energy supply mode. (e) Charging curve of the SC charged by the PPG-TENG and powering a watch. (f–h) Demonstration of the connected SCs as a power supply to drive an LED or electronic device.

3. Conclusion

In summary, we have proposed a versatile, high-conductivity, flexible, anti-freezing and self-adhesive interpenetrating double-network hydrogel based on a 3D PDA and PAM copolymerized framework doped with GO nanosheets. Based on this hydrogel, we assemble a self-charging power supply system which can harvest and transfer energy to a wearable supercapacitor, realizing self-powered biomechanical sensing. This system realized the complementation of the functions of the TENG and SC, which can realize the daily real-time detection of physiological signals, providing personalized real-time human healthcare.

4. Experimental

4.1. Preparation of the hydrogels

To fabricate the PPG hydrogel, 3 g of acrylamide (AM) powder was dissolved in 10 mL of H₂O under continuous magnetic stirring for 2 min. Subsequently, graphene oxide (GO) (5 mg mL⁻¹) and 3 mg of dopamine (DA) were added. Then, 150 mg of ammonium polysulfate (APS) and 3 mg of N,N′-methylenebisacrylamide (BIS) were added into the above solution and stirred for another 5 min, and then 25 μL of N,N,N′,N′-tetramethylethylenediamine (TMEDA) were rapidly injected into the reaction mixture. The above solution was transferred into a culture dish for further polymerization to produce the final PPG hydrogel. The thickness of the PPG hydrogel was tuned by controlling the volume of the transferred solution. Pure PAM hydrogel with a single polymeric network was also fabricated.

4.2. Fabrication of the hydrogel-based PPG-TENG

The PPG-TENG with a sandwich structure was prepared by encapsulating the PPG hydrogel (20 mm × 30 mm × 2 mm) between two pieces of silicone rubber (40 mm × 40 mm × 1 mm). The PP-TENG was fabricated using a similar method using the PAM hydrogel as the electrode instead.

4.3. Fabrication of the hydrogel-based SCs

Before the fabrication of the SC, the PPG and pure PAM hydrogels were immersed in 3 M KOH in H₂O/glycol (H₂O/EG) solution first, and named PPGK and PAMK hydrogel. Then the PPGK-SC with a sandwich structure was prepared by encapsulating the PPGK hydrogel (20 mm × 40 mm × 2 mm) between two pieces of carbon cloth coated with polypyrrole (PPy), which were fabricated by electrochemical deposition. The PAMK hydrogel-based SC was fabricated similarly to the PPGK-based SC using PAMK as the electrolyte.

4.4. Characterization

The morphology and structure of the hydrogels were characterized using a field emission scanning electron microscope (SEM) (Nova 450). The V_oc, I_sc, and Q_sc of the TENGs were measured using a Keithley 6514 electrometer. The electrochemical performances of the hydrogel and SC were analyzed using an electrochemical workstation (CHI 660E, Chenhua).

Data availability

The data are available from the corresponding author on reasonable request.

Author contributions

Zhuo Wang: data curation, investigation and project administration and writing – original draft; Quanhong Hu, Shaobo Wang and Chuyu Tang: resources and software; Zhirong Liu and Linlin Li: funding acquisition; Linlin Li: supervision, writing – review & editing. All authors discussed the results and revised the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Nature Science Foundation (No. 82072065, 82202333), the Fundamental Research Funds for the Central Universities (E2EG6802X2), and the National Youth Talent Support Program.

Notes and references

1. X. Fan, T. Ke and H. Gu, Multifunctional, Ultra-Tough Organohydrogel E-Skin Reinforced by Hierarchical Goatskin Fibers Skeleton for Energy Harvesting and Self-Powered Monitoring, Adv. Funct. Mater., 2023, 33, 2304015,  DOI:10.1002/adfm.202304015.
2. S. H. Chou, H. W. Lu, T. C. Liu, Y. T. Chen, Y. L. Fu, Y. H. Shieh, Y. C. Lai and S. Y. Chen, An Environmental-Inert and Highly Self-Healable Elastomer Obtained via Double-Terminal Aromatic Disulfide Design and Zwitterionic Crosslinked Network for Use as a Triboelectric Nanogenerator, Adv. Sci., 2023, 10, 2202815,  DOI:10.1002/advs.202202815.
3. C. Wang, X. Qu, Q. Zheng, Y. Liu, P. Tan, B. Shi, H. Ouyang, S. Chao, Y. Zou and C. Zhao, et al., Stretchable, Self-Healing, and Skin-Mounted Active Sensor for Multipoint Muscle Function Assessment, ACS Nano, 2021, 15, 10130,  DOI:10.1021/acsnano.1c02010.
4. D. Yang, Y. Ni, X. Kong, S. Li, X. Chen, L. Zhang and Z. L. Wang, Self-Healing and Elastic Triboelectric Nanogenerators for Muscle Motion Monitoring and Photothermal Treatment, ACS Nano, 2021, 15, 14653,  DOI:10.1021/acsnano.1c04384.
5. M. T. Rahman, M. S. Rahman, H. Kumar, K. Kim and S. Kim, Metal-Organic Framework Reinforced Highly Stretchable and Durable Conductive Hydrogel-Based Triboelectric Nanogenerator for Biomotion Sensing and Wearable Human-Machine Interfaces, Adv. Funct. Mater., 2023, 33, 2303471,  DOI:10.1002/adfm.202303471.
6. F.-R. Fan, Z.-Q. Tian and Z.-L. Wang, Flexible triboelectric generator, Nano Energy, 2012, 1, 328,  DOI:10.1016/j.nanoen.2012.01.004.
7. K. Dong, J. Deng, Y. Zi, Y. C. Wang, C. Xu, H. Zou, W. Ding, Y. Dai, B. Gu and B. Sun, et al., 3D Orthogonal Woven Triboelectric Nanogenerator for Effective Biomechanical Energy Harvesting and as Self-Powered Active Motion Sensors, Adv. Mater., 2017, 29, 1702648,  DOI:10.1002/adma.201702648.
8. W. Jiang, H. Li, Z. Liu, Z. Li, J. Tian, B. Shi, Y. Zou, H. Ouyang, C. Zhao and L. Zhao, et al., Fully Bioabsorbable Natural-Materials-Based Triboelectric Nanogenerators, Adv. Mater., 2018, 30, 1801895,  DOI:10.1002/adma.201801895.
9. D. W. Shin, M. D. Barnes, K. Walsh, D. Dimov, P. Tian, A. I. S. Neves, C. D. Wright, S. M. Yu, J. B. Yoo and S. Russo, et al., A New Facile Route to Flexible and Semi-Transparent Electrodes Based on Water Exfoliated Graphene and their Single-Electrode Triboelectric Nanogenerator, Adv. Mater., 2018, 30, 1802953,  DOI:10.1002/adma.201802953.
10. Y. Jie, J. Ma, Y. Chen, X. Cao, N. Wang and Z. L. Wang, Efficient Delivery of Power Generated by a Rotating Triboelectric Nanogenerator by Conjunction of Wired and Wireless Transmissions Using Maxwell's Displacement Currents, Adv. Energy Mater., 2018, 8, 1802084,  DOI:10.1002/aenm.201802084.
11. X. He, H. Zou, Z. Geng, X. Wang, W. Ding, F. Hu, Y. Zi, C. Xu, S. L. Zhang and H. Yu, et al., A Hierarchically Nanostructured Cellulose Fiber-Based Triboelectric Nanogenerator for Self-Powered Healthcare Products, Adv. Funct. Mater., 2018, 28, 1805540,  DOI:10.1002/adfm.201805540.
12. S. Hao, J. Jiao, Y. Chen, Z. L. Wang and X. Cao, Natural wood-based triboelectric nanogenerator as self-powered sensing for smart homes and floors, Nano Energy, 2020, 75, 104957,  DOI:10.1016/j.nanoen.2020.104957.
13. Y. J. Hao, C. G. Zhang, W. Su, H. K. Zhang, Y. Qin, Z. L. Wang and X. H. Li, Sustainable materials systems for triboelectric nanogenerator, SusMat, 2024, e244.
14. Z. Wang, Z. Liu, G. Zhao, Z. Zhang, X. Zhao, X. Wan, Y. Zhang, Z. L. Wang and L. Li, Stretchable Unsymmetrical Piezoelectric BaTiO₃ Composite Hydrogel for Triboelectric Nanogenerators and Multimodal Sensors, ACS Nano, 2022, 1661,  DOI:10.1021/acsnano.1c10678.
15. Z. Wu, P. Wang, B. Zhang, H. Guo, C. Chen, Z. Lin, X. Cao and Z. L. Wang, Highly Durable and Easily Integrable Triboelectric Foam for Active Sensing and Energy Harvesting Applications, Adv. Mater. Processes Technol., 2020, 6, 2000737,  DOI:10.1002/admt.202000737.
16. X. Peng, K. Dong, C. Ning, R. Cheng, J. Yi, Y. Zhang, F. Sheng, Z. Wu and Z. L. Wang, All-Nanofiber Self-Powered Skin-Interfaced Real-Time Respiratory Monitoring System for Obstructive Sleep Apnea-Hypopnea Syndrome Diagnosing, Adv. Funct. Mater., 2021, 31, 2103559,  DOI:10.1002/adfm.202103559.
17. Y. Long, Z. Wang, F. Xu, B. Jiang, J. Xiao, J. Yang, Z. L. Wang and W. Hu, Mechanically Ultra-Robust, Elastic, Conductive, and Multifunctional Hybrid Hydrogel for a Triboelectric Nanogenerator and Flexible/Wearable Sensor, Small, 2022, 2203956,  DOI:10.1002/smll.202203956.
18. G. Zhao, X. Zhang, X. Cui, S. Wang, Z. Liu, L. Deng, A. Qi, X. Qiao, L. Li and C. Pan, et al., Piezoelectric Polyacrylonitrile Nanofiber Film-Based Dual-Function Self-Powered Flexible Sensor, ACS Appl. Mater. Interfaces, 2018, 10, 15855,  DOI:10.1021/acsami.8b02564.
19. F. Sheng, J. Yi, S. Shen, R. Cheng, C. Ning, L. Ma, X. Peng, W. Deng, K. Dong and Z. L. Wang, Self-Powered Smart Arm Training Band Sensor Based on Extremely Stretchable Hydrogel Conductors, ACS Appl. Mater. Interfaces, 2021, 13, 44868,  DOI:10.1021/acsami.1c12378.
20. J. P. Lee, J. W. Lee, B.-K. Yoon, H. J. Hwang, S. Jung, K. A. Kim, D. Choi, C. Yang and J. M. Baik, Boosting the energy conversion efficiency of a combined triboelectric nanogenerator-capacitor, Nano Energy, 2019, 56, 571,  DOI:10.1016/j.nanoen.2018.11.076.
21. S. Qin, Q. Zhang, X. Yang, M. Liu, Q. Sun and Z. L. Wang, Hybrid Piezo/Triboelectric-Driven Self-Charging Electrochromic Supercapacitor Power Package, Adv. Energy Mater., 2018, 8, 1800069,  DOI:10.1002/aenm.201800069.
22. J. Wang, Z. Wen, Y. Zi, L. Lin, C. Wu, H. Guo, Y. Xi, Y. Xu and Z. L. Wang, Self-Powered Electrochemical Synthesis of Polypyrrole from the Pulsed Output of a Triboelectric Nanogenerator as a Sustainable Energy System, Adv. Funct. Mater., 2016, 26, 3542,  DOI:10.1002/adfm.201600021.
23. J. Wang, Z. Wen, Y. Zi, P. Zhou, J. Lin, H. Guo, Y. Xu and Z. L. Wang, All-Plastic-Materials Based Self-Charging Power System Composed of Triboelectric Nanogenerators and Supercapacitors, Adv. Funct. Mater., 2016, 26, 1070,  DOI:10.1002/adfm.201504675.
24. Q. Jiang, C. Wu, Z. Wang, A. C. Wang, J.-H. He, Z. L. Wang and H. N. Alshareef, MXene electrochemical microsupercapacitor integrated with triboelectric nanogenerator as a wearable self-charging power unit, Nano Energy, 2018, 45, 266–272,  DOI:10.1016/j.nanoen.2018.01.004.
25. Z. Li, K. Hu, M. Yang, Y. Zou, J. Yang, M. Yu, H. Wang, X. Qu, P. Tan and C. Wang, et al., Elastic Cu@PPy sponge for hybrid device with energy conversion and storage, Nano Energy, 2019, 58, 852–861,  DOI:10.1016/j.nanoen.2018.11.093.
26. Y. Yang, L. Xie, Z. Wen, C. Chen, X. Chen, A. Wei, P. Cheng, X. Xie and X. Sun, Coaxial Triboelectric Nanogenerator and Supercapacitor Fiber-Based Self-Charging Power Fabric, ACS Appl. Mater. Interfaces, 2018, 10(49), 42356–42362,  DOI:10.1021/acsami.8b15104.
27. Y. Su, H. Xue, Y. Fu, S. Chen, Z. Li, L. Li, A. Knoks, O. Bogdanova, P. Lesničenoks and R. Palmbahs, et al., Monolithic Fabrication of Metal-Free On-Paper Self-Charging Power Systems, Adv. Funct. Mater., 2024, 34(24), 2313506,  DOI:10.1002/adfm.202313506.
28. Q. Zhou, A. Griffin, J. Qian, Z. Qiang, B. Sun, C. Ye and M. Zhu, Mechanically Strong and Tough Organohydrogels for Wide Temperature Tolerant, Flexible Solid-State Supercapacitors, Adv. Funct. Mater., 2024, 2405962,  DOI:10.1002/adfm.202405962.
29. L. Wang and W. A. Daoud, Hybrid conductive hydrogels for washable human motion energy harvester and self-powered temperature-stress dual sensor, Nano Energy, 2019, 66, 104080,  DOI:10.1016/j.nanoen.2019.104080.
30. H. Li, T. Lv, H. Sun, G. Qian, N. Li, Y. Yao and T. Chen, Ultrastretchable and superior healable supercapacitors based on a double cross-linked hydrogel electrolyte, Nat. Commun., 2019, 10, 536,  DOI:10.1038/s41467-019-08320-z.
31. X. Jin, G. Sun, G. Zhang, H. Yang, Y. Xiao, J. Gao, Z. Zhang and L. Qu, A cross-linked polyacrylamide electrolyte with high ionic conductivity for compressible supercapacitors with wide temperature tolerance, Nano Res., 2019, 12, 1199,  DOI:10.1007/s12274-019-2382-z.
32. C. Lu and X. Chen, All-Temperature Flexible Supercapacitors Enabled by Antifreezing and Thermally Stable Hydrogel Electrolyte, Nano Lett., 2020, 20, 1907,  DOI:10.1021/acs.nanolett.9b05148.
33. X. Jin, L. Song, H. Yang, C. Dai, Y. Xiao, X. Zhang, Y. Han, C. Bai, B. Lu and Q. Liu, et al., Stretchable supercapacitor at −30 °C, Energy Environ. Sci., 2021, 14, 3075,  10.1039/d0ee04066e.
34. J. Zou, X. Jing, Z. Chen, S. J. Wang, X. S. Hu, P. Y. Feng and Y. J. Liu, Multifunctional Organohydrogel with Ultralow-Hysteresis, Ultrafast-Response, and Whole-Strain-Range Linearity for Self-Powered Sensors, Adv. Funct. Mater., 2023, 33, 2113895,  DOI:10.1002/adfm.202213895.
35. J. Kim, J. W. Kim, K. Keum, H. Lee, G. Jung, M. Park, Y. H. Lee, S. Kim and J. S. Ha, A multi-responsive self-healing and air-stable ionogel for a vertically integrated device comprised of flexible supercapacitor and strain sensor, Chem. Eng. J., 2023, 457, 141278,  DOI:10.1016/j.cej.2023.141278.
36. H. Zhou, J. Lai, X. Jin, H. Liu, X. Li, W. Chen, A. Ma and X. Zhou, Intrinsically adhesive, highly sensitive and temperature tolerant flexible sensors based on double network organohydrogels, Chem. Eng. J., 2021, 413, 127544,  DOI:10.1016/j.cej.2020.127544.
37. S. Xia, Q. Zhang, S. Song, L. Duan and G. Gao, Bioinspired Dynamic Cross-Linking Hydrogel Sensors with Skin-like Strain and Pressure Sensing Behaviors, Chem. Mater., 2019, 31, 9522,  DOI:10.1021/acs.chemmater.9b03919.
38. Z. Y. Sun, Q. D. Ou, C. Dong, J. S. Zhou, H. Y. Hu, C. Li and Z. D. Huang, Conducting polymer hydrogels based on supramolecular strategies for wearable sensors, Exploration, 2024, 4, 20220167,  DOI:10.1002/EXP.20220167.
39. Z. Y. Dai, M. Lei, S. Ding, Q. Zhou, B. Ji, M. R. Wang and B. P. Zhou, Durable superhydrophobic surface in wearable sensors: from nature to application, Exploration, 2024, 4, 20230046,  DOI:10.1002/EXP.20230046.
40. C.-Y. Lo, Y. Zhao, C. Kim, Y. Alsaid, R. Khodambashi, M. Peet, R. Fisher, H. Marvi, S. Berman and D. Aukes, et al., Highly stretchable self-sensing actuator based on conductive photothermally-responsive hydrogel, Mater. Today, 2021,(05), 35,  DOI:10.1016/j.mattod.2021.05.008.
41. Y. S. Lv, C. C. Li, Z. Q. Yang, M. X. Gan, Y. Y. Wang, M. X. Lu, X. X. Zhang and Li. Min, Monomer Trapping Synthesis Toward Dynamic Nanoconfinement Self-healing Eutectogels for Strain Sensing, Adv. Sci., 2024, 11, 2410446,  DOI:10.1002/advs.202410446.
42. C. C. Li, J. Z. Liu, X. Y. Qiu, X. Yang, Huang and X. X. Zhang, Photoswitchable and Reversible Fluorescent Eutectogels for Conformal Information Encryption, Angew. Chem., Int. Ed., 2023, 62, e202313971,  DOI:10.1002/anie.202313971.
43. Y. Li, D. Yang, Z. Wu, F.-L. Gao, X.-Z. Gao, H.-Y. Zhao, X. Li and Z.-Z. Yu, Self-adhesive, self-healing, biocompatible and conductive polyacrylamide nanocomposite hydrogels for reliable strain and pressure sensors, Nano Energy, 2023, 109, 108324,  DOI:10.1016/j.nanoen.2023.108324.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06337f

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