Stretchable and transparent PVA/Borax organohydrogel-based triboelectric nanogenerator for self-powered wearables and human–machine interfaces

Abstract

Highly stretchable triboelectric nanogenerators (TENGs) are indispensable for conformal energy harvesting and self-powered sensing. The hydrogel-based TENGs have demonstrated encouraging performance in the fabrication of flexible and transparent devices. Here, we introduce a transparent and stretchable conductive organohydrogel which was synthesized in a water/glycerol co-solvent system via cross-linking of poly(vinyl alcohol) (PVA) and borax. The electrical conductivity of the PVA/borax organohydrogel can be tuned over a broad range simply by adjusting the borax concentration. The prepared organohydrogel can be utilized as a resistance sensor to monitor human motions. A single-electrode TENG was developed by employing the PVA/borax organohydrogel as an electrode and silicone rubber as a triboelectric layer. An optimally formulated organohydrogel-based TENG (OH-TENG) delivers a peak-to-peak voltage of approximately 500 V, a short-circuit current of 3.0 µA, and a transferred charge of 145 nC under a 3.0 Hz mechanical excitation. Demonstrations show that the device rapidly charges an electrolytic capacitor, effortlessly illuminates a string of green LEDs, and powers portable electronics. When interfaced with Darlington transistors and relay modules, the OH-TENG can reliably switch external circuits on and off. It has also been integrated with a Bluetooth oscilloscope module, enabling real-time monitoring of human movements. These results highlight its potential applications in human–machine interfaces and safety systems. This study elucidates how organohydrogel properties govern the performance of OH-TENGs and provides a general blueprint for designing next-generation, highly stretchable TENGs.

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1. Introduction

Global energy demand keeps climbing, compelling humanity to seek alternatives that are both clean and inexhaustible.^1–3 Conventional renewable routes—hydropower, wind power, and photovoltaic power generation—rely on bulky installations and expansive space.⁴ In contrast, scavenging the random, small-amplitude mechanical energy that pervades nature and everyday activities has gained momentum over the past decade.^5–8 Among the available transduction schemes, the triboelectric nanogenerator (TENG) stands out for its ability to convert virtually any mechanical excitation—be it a whisper or ocean waves—into electricity.^9–15 Various TENGs with tailored architectures have been engineered to match the disparate amplitudes, frequencies, and environmental conditions of these ubiquitous motions.^16–22 Simultaneously, the rapid rise of wearable electronics has intensified the call for mechanically compliant, optically transparent devices that can laminate onto human skin or other curved surfaces without altering their intrinsic look or feel.^23–27 Developing TENG with high flexibility and transparency is an ongoing demand for improving device adaptive ability and human comfortability when used for human motion energy harvesting.

Stretchable TENGs have lately attracted intense attention. A common route is to deposit a thin conductive film—silver nanowires, PEDOT: PSS, or graphene—onto an elastomeric triboelectric layer.^28–30 Yet the mechanical mismatch between the stiff coating and the soft substrate frequently triggers interfacial cracking or delamination, eroding the device's long-term reliability.^31–33 Ionic conductors offer an alternative paradigm. Instead of electrons, mobile ions shuttle charge through a polymer network, yielding electrodes that are inherently compliant, optically transparent, and deformable.^34–36 Among these, hydrogels stand out for their tunable chemistry, high water content, and exceptional stretchability.^37–42 Poly(vinyl alcohol) (PVA), prized for its biodegradability, compatibility, and chemical inertness, is a particularly attractive base.^43,44 Traditional PVA hydrogels are formed by physical or chemical cross-linking in water, with ionic conductivity imparted by dissolving mobile salts.^44,45 Recent literature illustrates the versatility of PVA/borax systems. Lu et al. embedded microfibrillated cellulose to create self-healing, pH-responsive hydrogels.⁴⁶ Dai group engineered a dual-physically/chemically cross-linked network for transparent, robust sensing skins.⁴⁷ Peymanfar et al. dispersed graphene oxide, graphitic carbon nitride, and carbon microspheres to craft PVA/borax matrices that transmit visible light yet absorb UV, IR, and microwave radiation.⁴⁸ Cui et al. introduced a triple-network PVA/borax hydrogel for artificial cartilage,⁴⁹ and Lee et al. exploited borax-cross-linked PVA slime as a stretchable, transparent electrode, leveraging Na⁺ and B(OH)₄⁻ ions for charge transport.⁵⁰ Despite these advances, simultaneously achieving high ionic conductivity and mechanical robustness in hydrogels remains an open challenge for TENG electrodes.

In this study, we introduce a PVA/borax hybrid organohydrogel featuring a dual cross-linked architecture and deploy it as a compliant, transparent electrode for a stretchable triboelectric nanogenerator (OH-TENG). Under optimal conditions, the OH-TENG delivered an open-circuit voltage (V_oc) of approximately 500 V, a short-circuit current (I_sc) of 3.0 µA, and a transferred charge (Q_sc) of 145 nC. The device rapidly charges electrolytic capacitors, effortlessly powers a chain of green LEDs, and drives portable electronics. When interfaced with Darlington pairs and relay modules, the OH-TENG further functions as a self-powered switch for external circuitry. It also integrated a Bluetooth oscilloscope module for real-time monitoring of human movements. These findings illuminate how organohydrogel design dictates TENG performance and lay a practical foundation for next-generation, highly deformable energy-harvesting systems.

2. Experimental methods

2.1. Materials

Poly(vinyl alcohol) (PVA, M_w 1750 ± 50), glycerol and sodium tetraborate (Na₂B₄O₇·10H₂O, 99.5%, abbreviated as borax later) were purchased from Sinopharm Chemical Reagent Co., Ltd. Silicone rubber (part A and part B) was bought from Beijing Sanjingxinde Technology Co., Ltd. All reagents were used as received. The Darlington transistor–relay modules and Bluetooth oscilloscope modules were purchased from the Taobao website (taobao.com).

2.2. Preparation of PVA/Borax organohydrogel

PVA (3.0 g) was dispersed in a solvent mixture of deionized water (12 mL) and glycerol (4 mL). The suspension was kept at 90 °C under constant stirring in a water bath for 2 h until a homogeneous, viscous solution was obtained. Separately, 0.25 g of borax was dissolved in 5 mL of deionized water with mild heating. The resulting borax solution was then added dropwise to the PVA solution under vigorous stirring; gelation occurred within minutes, yielding an organohydrogel containing 1 wt% borax. Analogous protocols, varying only the borax feed, were employed to prepare counterparts with 2, 3, and 4 wt% cross-linker. A glycerol-free hydrogel control was synthesized in the same manner for comparative purposes.

2.3. Fabrication of organohydrogel-based triboelectric nanogenerators

In a 50 mL beaker, components A and B of the silicone rubber were added in a volume ratio of 1 : 1 and stirred uniformly in a clockwise direction with a glass rod. Then the mixed silicone rubber liquid was injected into the acrylic plate mold prepared by the laser cutting machine to prepare the silicone rubber groove with an empty cavity (size: 38 mm × 58 mm; depth: 2 mm) in the middle and a silicone rubber film (0.5 mm). After curing at room temperature for 12 h, the silicone rubber is stripped from the mold. Subsequently, the non-gelatinized PVA/borax organohydrogel liquid was poured into the silicone rubber tank and leveled. One end of the PVA/borax organohydrogel was connected to a piece of conductive fabric of appropriate size to serve as an external electrode. Finally, the silicone rubber film was spread over the surface to seal the device with silicone rubber liquid.

2.4. Characterizations and measurements

Transmission spectrum of the organohydrogel was recorded using a UV-visible spectrophotometer (TU-1901, Bejing). Tensile strain of the organohydrogel was measureing using a tensile testing machine (LDW-500N, Xinhechuang, Shandong). Electrical characterization of the OH-TENG was carried out with a programmable linear motor (LinMot E1100) supplying a precisely tunable and steady mechanical input. A layer of degreased cowhide served as the positive triboelectric layer, while a silicone-rubber film acted as its negative counterpart in the contact-separation cycle. An electrometer (Keithley 6514) was used to record the values of V_oc, Q_sc, and I_sc. An acrylic mold was prepared using a CO₂ laser engraving machine (Shandong Ketai). The resistance of the conductive organohydrogel was measured with a digital multimeter, and the conductivity was calculated based on the measured resistance.

3. Results and discussion

The obtained PVA/borax organohydrogel was a physically and chemically crosslinked dual-network structure. Initially, intermolecular hydrogen bonding among PVA hydroxyl groups—together with topological chain entanglement—creates a primary physical network. Subsequently, PVA reacts with borate ions in a two-stage process, establishing reversible di-diol coordination cross-links that constitute the secondary, chemically cross-linked architecture,^46,47,49 as shown in Fig. 1a and Fig. S1. In the PVA/borax organohydrogel, PVA reacts with borate ions in a two-stage process, establishing reversible di-diol coordination cross-links. Borax rapidly hydrolyses in the glycerol/H₂O binary solvent to give [B(OH)₄]⁻, which forms reversible di-diol esters with the 1,3-diol units of the PVA backbone (Fig. S1). The resulting bis-diol borate cross-links (PVA–O–B–O–PVA) are dynamic at room temperature, endowing the network with an energy-dissipative mechanism that underpins the high stretchability. Meanwhile, borax supplies free Na⁺ and [B(OH)₄]⁻ ions suppresses water evaporation and promotes ion dissociation, while the hydroxyl-rich PVA chains provide continuous ion-transport pathways.^46,49 The prepared conductive organohydrogel was placed in a small bottle and left to stand for 0.5 h. Regardless of whether the small bottle was tilted or inverted, the organohydrogel remained stationary without any flow (Fig. S2). As shown in Fig. 1b, the prepared organohydrogel exhibits excellent light transmittance in the wavelength range of 400 to 800 nm, which endows it with potential application in visual interactive wearable strain sensors. It has been reported that the addition of glycerol reduces the crystallinity of PVA molecules, as glycerol can insert itself between the polymer chains and disrupt their regular arrangement, thereby enhancing light transmittance.⁵¹ Therefore, the relatively high transmittance observed in this study is likely attributable to the formation of novel cross-linked structures between PVA and glycerol molecules. To provide a visual demonstration of the electrical conductivity, the organohydrogel was wired into a simple circuit containing an LED indicator (Fig. 1c). When this conductive organohydrogel was connected to the circuit, it was found that the LED lamp could emit light normally. The test results showed that its electrical conductivity was excellent. Within the organohydrogel, a reversible di-diol-borax coordination bond was formed due to the reaction of the PVA chains and borax. Because the thermoreversible PVA/borax cross-links persist in dynamic equilibrium, Na⁺ and B(OH)₄⁻ ions remain unbound and highly mobile, endowing the organohydrogel with pronounced ionic conductivity. Fig. 1d shows the conductivity of the organohydrogels containing different concentrations of borax. As the concentration of borax increases, the electrical conductivity of the organohydrogel gradually increases. When the concentration of borax was 3 wt%, the conductivity reached its maximum value of 1.2 S m⁻¹. In the previous report, the conductivity of the pure PVA hydrogel was 0.6 S m⁻¹.⁵² The conductivity of the prepared PVA/borax organohydrogel is higher than that of the PVA/borax slime reported in previous work.⁵⁰ In addition, glycerol as a plasticizer can enhance the mobility of polymer chain segments, increase the proportion of the amorphous region, thereby facilitating the migration of ions and consequently increasing ionic conductivity.⁵³ However, when the concentration of borax was further increased, the conductivity of the organohydrogel actually decreased, which is probably due to the suppression of ion dissociation at high borax concentration. A similar phenomenon was observed in the PVA-based gel electrolyte.⁵⁴

Fig. 1 (a) Schematic diagram of the preparation process and internal structure of the PVA/borax organohydrogel. (b) Transmittance spectrum of the PVA/borax organohydrogel. Inset: Organohydrogel strip. (c) Circuit comprises a PVA/borax organohydrogel in series with a LED indicator. (d) The conductivity of the PVA/borax organohydrogel with varying borax concentrations. (e) Load-bearing resilience under uniaxial elongation and knotted extension. (f) Structural integrity while suspending an 850 g mass. (g) Tensile stress–strain curve of the PVA/borax organohydrogel. (h) Relative resistance variation curves of the organohydrogel at different strains.

The freeze–thaw cycling can further enhance the stability, toughness and tensile of the hydrogels.⁴⁷ After undergoing three freeze–thaw cycles, the mechanical strength of the PVA/borax organohydrogel was tested. The length of the organohydrogel can be stretched from the initial 8 cm to 15 cm, with an elongation of 82.5%. Moreover, when we made a knot in the middle part of the organohydrogel, it could still be stretched to a length of 13 cm, with an elongation of 62.5% (Fig. 1e). The organohydrogel strip can lift a weight of 850 g (Fig. 1f). By conducting tensile tests, the mechanical properties of the organohydrogel were further evaluated. Fig. 1g presents the typical tensile stress curve of the organohydrogel, with the fracture strain and fracture tensile strength being 419% and 0.45 MPa, respectively (Fig. 1g). These results indicate that the PVA/borax conductive organohydrogels exhibit excellent tensile properties.

To further verify the organohydrogel's sensing performance, dynamic tensile tests were carried out, as shown in Fig. 1h. The results demonstrate that the organohydrogel exhibits a rapid and reliable response to various strain changes at room temperature. Both the response and recovery times of the organohydrogel sensor depend on the stretching frequency. At 100% tensile strain, the organohydrogel sensor exhibits a response time of 466 ms and a recovery time of 344 ms (Fig. S3). In the recent report, the response time and recovery time of the TCPA hydrogel strain sensor during low-strain (25%) loading and unloading were 134 ms and 162 ms respectively.⁵⁵ The relationship between tensile strain and resistance change can be clearly observed during stretching. Fig. S4 shows the relative resistance change variation as a function of tensile strain for the PVA/Borax organohydrogel. The slope of the linear fit yields a gauge factor (GF) of 3.31 (R² = 0.976) in the 33–133% strain range, which is higher than that of gelatin/NaCl organohydrogel.⁵⁶ The resistance of the organohydrogel increases with elongation due to the decrease in conductive ion concentration and the extension of ion transport pathways. These results indicate that the as-prepared PVA/borax organohydrogel holds promising potential for application in resistive sensors.

The freshly prepared PVA/borax conductive organohydrogel exhibits favorable adhesive properties at room temperature. It can firmly adhere to tiny objects of different materials and weights (Fig. 2a). Utilizing the PVA/borax conductive organohydrogel as the electrode and a silicone rubber sheet as the encapsulation layer and the triboelectric layer, a single-electrode TENG was successfully constructed. The fabricated OH-TENG possesses transparency and exhibits good flexibility and shape adaptability (Fig. 2b). In addition, silicone rubber sheets can effectively enhance the water retention of hydrogels and prevent water evaporation. As shown in Fig. 2c, the dehydration performance of the hydrogel without glycerol and the organohydrogel with glycerol was tested at room temperature. With the increase of time, the mass of both hydrogels decreased. The hydrogel without glycerol stabilized at 40% of its initial mass after about 7 days, while the organohydrogel with glycerol stabilized at 60% of its initial mass. Glycerol can form a stable hydrogen bond network with water, thereby inhibiting water evaporation. This experimental result indicates that the addition of glycerol effectively slowed down the evaporation of water molecules and significantly enhanced the water retention capacity of the hydrogel. The organohydrogel was sealed in the TENG silicone groove as a control experiment. It was found that under the same conditions, the mass reduction rate of the sealed organohydrogel tended to be stable and changed approximately linearly. Within 14 days, the organohydrogel could maintain 92.4% of its initial weight. This demonstrates that the sealed structure of the TENG silicone groove can effectively isolate the external environment from the organohydrogel and work in concert with glycerol to provide a favorable moisture-retaining environment for the organohydrogel.

Fig. 2 (a) Adhesion demonstration of the PVA/borax conductive organohydrogel on different substrates. (b) Photographs of the prepared TENG in the flat and curled states. (c) Weight changes of PVA/borax hydrogel and organohydrogel in air and PVA/borax organohydrogel in TENG as a function of storage time for 14 days. (d) The working mechanism of the prepared TENG.

Fig. 2d schematically illustrates the energy-harvesting cycle of the OH-TENG, which relies on the synergistic action of contact electrification and electrostatic induction.^57–59 In the triboelectric sequence, human skin is strongly electropositive, whereas silicone rubber is markedly electronegative.⁶⁰ At rest, the two surfaces are separated and electrically neutral; Na⁺ and B(OH)₄⁻ ions are homogeneously dispersed in the PVA matrix. When skin contacts the silicone layer, electrons migrate from skin to rubber, leaving the silicone negatively charged and the skin positively charged (Fig. 2d-I).⁶¹ Upon release, the opposite charges separate, establishing an electric field. This field drives Na⁺ ions toward the silicone–organohydrogel interface and B(OH)₄⁻ ions toward the organohydrogel–fabric interface, forming an electric double layer (Fig. 2d-II).^50,62 Consequently, a transient current flows from the fabric electrode to ground, producing a positive voltage pulse. At maximum separation, electrostatic equilibrium is reached (Fig. 2d-III). Re-pressing the surfaces collapses the electric field, triggering a reverse ionic current and a negative voltage pulse (Fig. 2d-IV). Continuous mechanical cycling therefore yields an alternating electrical output.

According to previous reports, degreased cowhide is a well-performing positive triboelectric material.⁶³ Therefore, in this study, defatted cowhide was selected as the counterpart of silicone rubber. We evaluated how the gap between the silicone-rubber film and the defatted-cowhide layer influences the OH-TENG output. Fig. 3a–c presents the V_oc, Q_sc, and I_sc recorded at a fixed load of 16 N and 3.0 Hz contact frequency while the spacer distance was varied from 20 to 100 mm. V_oc climbs steadily from 265 V to 508 V, Q_sc rises from 83 nC to 145 nC, and I_sc increases from 1.7 µA to 3.0 µA over the same range. The progressive enhancement originates from two effects: (i) the larger separation enlarges the electric potential difference across the gap, and (ii) the increased relative impact velocity under a constant frequency accelerates charge transfer, yielding higher instantaneous current and charge.⁶⁴

Fig. 3 (a) V_oc, (b) Q_sc, and (c) I_sc of the OH-TENG at an operating frequency of 3.0 Hz as the spacer distance was increased from 20 to 100 mm. (d) V_oc and (e) Q_sc of OH-TENG under different impact forces. (f) The sensitivity curves of the OH-TENG. (g) V_oc, (h) Q_sc, and (i) I_sc of the OH-TENG at a spacer distance of 100 mm as the operating frequency was increased from 0.5 to 3.0 Hz.

The performance correlation between the OH-TENG and impact force was systematically investigated. As depicted in Fig. 3d and e, both the V_oc and Q_sc were recorded while the contact frequency and separation distance were fixed at 3.0 Hz and 100 mm, respectively. With the impact force increasing from 2 N to 10 N, V_oc rises progressively from 319 V to 460 V, and Q_sc increases from 94 nC to 140 nC. This enhancement is attributed to the larger contact area achieved between the degreased cowhide and the device at bigger forces. Notably, the relationship between this electrical output and pressure exhibits excellent linear characteristics. Within the pressure testing range, the V_oc and Q_sc have sensitivity of 17 V N⁻¹ and 5.85 nC N⁻¹ respectively (Fig. 3f).

The frequency response of the device was evaluated with a linear motor whose arm carried the OH-TENG, enabling repeatable contact–separation cycles with defatted cowhide. Fig. 3g–i summarizes the electrical output recorded at 16 N contact force and 100 mm separation while the frequency was varied from 0.5 to 3.0 Hz. Owing to the constant surface-charge density generated per cycle, the open-circuit voltage (V_oc ≈ 500 V) and transferred charge (Q_sc ≈ 145 nC) remained essentially flat across the entire frequency range. By contrast, the I_sc rose monotonically—from 0.46 µA at 0.5 Hz to 3.0 µA at 3.0 Hz—because the shortened contact interval at higher frequencies accelerates the charge-transfer process. In effect, the faster contact/separation velocity raises the rate of electrostatic induction without altering the total amount of charge, yielding a proportionally larger instantaneous current while V_oc and Q_sc stay unchanged. Due to the differences in preparation methods, device structures, and material selections, as well as variations in testing conditions, the electrical output performance of the TENGs based on PVA-related hydrogels reported in the literature is difficult to be directly compared. Table S1 provides a comparison of the output performance of PVA-related hydrogel based TENGs reported in recent literature. The output performance of TENG based on the PVA/Borax organohydrogel is comparable to that of the previously reported TENGs.

The electrical properties of the OH-TENG are strongly governed by the external load. Fig. 4a illustrates how instantaneous power evolves with load resistance at different actuation frequencies. In a purely resistive circuit, the output power is P = U²/R, so the recorded load voltage of the OH-TENG was evaluated first (Fig. S5). Throughout the entire range of resistance changes, the voltage rises slowly at low resistances, rises sharply at medium resistances, and remains stable at high resistances, thus forming a typical bell-shaped power curve. Maximum output power is achieved when the external resistor matches the device's internal impedance. Raising the actuation frequency from 0.5 Hz to 3.0 Hz shifts the peak power from 201 µW to 1089 µW because the higher sliding velocity increases the charge-transfer rate per unit time and reduces the parasitic charge-decay interval. The higher the frequency, the lower the resistance of the external load corresponding to the optimal output power of the TENG. This means that the higher the working frequency of the TENG, the smaller its internal resistance will be. This is related to the capacitance model of the TENG,⁶⁵ the effective internal resistance falls from 500 MΩ at 0.5 Hz to 100 MΩ at 3.0 Hz. Consequently, dynamic impedance matching provides a straightforward route to tune the OH-TENG's output power for diverse practical scenarios.

Fig. 4 (a) Output power versus load resistance for the OH-TENG at operating frequencies from 0.5 to 3.0 Hz. (b) The voltage output retention rate of the device over a seven-day period. (c) The voltage output of the device after storage for 1 day, 31 days, and rehydration treatment. (d) Rectifier equivalent circuit for charge and discharge of small electronic equipment. (e) Voltage–time profiles when a 2.2 µF capacitor is charged by the OH-TENG at different contact frequencies (0.5–3.0 Hz). (f) Charging behaviour of the OH-TENG when driving capacitors ranging from 1.0 to 4.7 µF at a 3.0 Hz frequency. (g) Demonstration of 300 commercial LEDs simultaneously illuminated by the OH-TENG at 3.0 Hz. (h) Charge–discharge curve of a 10 µF capacitor powering a pocket calculator via the OH-TENG; inset: photograph of the working calculator. (i) Charge–discharge curve of a 10 µF capacitor powering a digital wristwatch via the OH-TENG; inset: photograph of the operating watch.

Considering the high electrical output of the fabricated OH-TENG, we assessed its viability as a portable power source. A 30 min continuous contact-separation test at 1.0 Hz revealed a virtually unchanged V_oc (Fig. S6), underscoring the device's mechanical stability and suitability for sustained, real-world operation. To evaluate the long-term robustness of OH-TENG, its output characteristics were monitored through daily assessments over a week-long period. As depicted in Fig. 4b, OH-TENG exhibited consistent performance throughout the test duration, with only a modest 2.94% reduction in output voltage. This behavior aligns with findings from earlier studies on NaCl/PVA hydrogel-based triboelectric sensors.³⁹ Following 31 days of storage, the PVA/borax organohydrogel electrode retained its original dimensions, though a minor loss of pliability was observed. Despite this, the device maintained structural integrity, delivering an output voltage that reached roughly 82.5% of the original value. Significantly, the flexibility of the PVA/borax organohydrogel could be revived by syringe-injecting an appropriate amount of deionized water into the electrode and subsequently heating the device at 90 °C for about 10 min. After rehydration, the device's voltage output recovered to approximately 90.3% of its initial level (Fig. 4c). A comparable restoration of electrical properties through rehydration was previously documented for TENGs constructed with PAAm-βCD-NaCl hydrogels.⁶⁶ Collectively, these findings underscore the remarkable stability and durability of OH-TENG, substantiating its potential for real-world employment.

High-performance TENGs can serve as power sources for portable electronic products. Currently, the common approach is to first convert the alternating current (AC) generated by the TENG into direct current (DC) through a rectifier and then charge a capacitor. The electrical energy stored in the capacitor can power portable electronic devices. A rectifier equivalent circuit for the charge and discharge of portable electronic devices is shown in Fig. 4d. Fig. 4e shows the influence of contact frequency on the charging kinetics of a capacitor. When the OH-TENG is connected to a full-wave rectifier followed by a 2.2 µF reservoir capacitor, the capacitor voltage rises monotonically with time. Within 60 s, the terminal voltage reaches 1.6, 3.7, 5.8, 7.0 and 10.3 V at 0.5, 1.0, 1.5, 2.0 and 3.0 Hz, respectively. The approximate linearity of the V–t traces confirms that the rectified current remains approximately constant during the charging stage. Capacitance-dependent charging behaviour at 3.0 Hz is shown in Fig. 4f. With the frequency fixed at 3.0 Hz, increasing the storage capacitance from 1.0 µF to 4.7 µF progressively retards the voltage build-up. The 30 s terminal voltages are 6.5 V (1.0 µF), 4.8 V (2.2 µF), 3.5 V (3.3 µF) and 2.2 V (4.7 µF). By cascading the OH-TENG (3.0 Hz) with a rectifier and a parallel array of 300 commercial green LEDs, simultaneous illumination is achieved (Fig. 4g and Video S1). The instantaneous output power is sufficient to overcome the combined threshold of the LEDs, yielding uniform brightness without external energy storage. After rectification, the OH-TENG charges a 10 µF capacitor to 5 V in 360 s (Fig. 4h and i). Disconnecting the OH-TENG and discharging the capacitor into a pocket calculator and an electronic wristwatch result in sustained operation for 20 s and 126 s, respectively (Videos S2 and S3). The discharge curves exhibit exponential decay, confirming the energy autonomy of these devices under intermittent human-motion frequencies. The insets demonstrate real-time numerical display and time-keeping functions, underscoring the viability of the OH-TENG as a self-power source for wearable and portable microelectronics.

Due to the excellent stretchability of silicone rubber and the shape adaptability of PVA/borax organohydrogel, the fabricated TENG possesses stretchability. Fig. 5 displays the OH-TENG's electrical output as a function of applied tensile strain. A nitrile film serves as the positive triboelectric layer. Progressive stretching monotonically boosts the output, an enhancement that originates from two cooperating mechanisms: (i) the larger effective contact area generated during elongation and (ii) the concomitant reduction in silicone-rubber thickness.^67,68 The expanded interface raises charge generation, while the thinner elastomer brings the negative tribo-charges closer to the underlying PVA/borax organohydrogel. This reduced separation intensifies electrostatic induction, yielding a pronounced rise in electrical output with increasing strain. When human skin undergoes extreme deformation, its elongation rate usually does not exceed 60%.⁶⁹ In previous reports, the maximum tensile strain that human arm and forehead skin can withstand are 27% and 57% respectively.^70,71 Therefore, the OH-TENG can operate normally under the common physical conditions of the epidermis. In previous work, for the same reason, the tensile strain range tested on the devices was also 0%–60%.⁶⁹

Fig. 5 (a) V_oc, (b) I_sc and (c) Q_sc of the OH-TENG under stretching strain from 0% to 60%.

The as-prepared OH-TENG combines flexibility, stretchability and conformability, enabling intimate attachment to diverse human articulations for biomechanical energy harvesting. To highlight the PVA/borax organohydrogel's suitability as a compliant electrode, gentle finger tapping at multiple positions produced a V_oc of 10–15 V (Fig. 6a). Evidently, the PVA/borax network serves as the primary conduit for charge transport. Previous studies suggest that mobile cations/anions within hydrogel electrodes introduced an additional triboelectric layer at the hydrogel/silicone interface.⁷² Periodic stretching tests corroborate this: stretching the OH-TENG yielded a stable V_oc of approximately 30 V (Fig. 6b), confirming triboelectrification between the organohydrogel and silicone rubber. A comparable triboelectric dual-layer mechanism has been reported for TENGs employing SA–Zn hydrogel electrodes, where a minute air gap between hydrogel and encapsulant promotes the effect.⁷³ When the device was worn on a volunteer's wrist, the V_oc generated by flexion–extension motions was about 15 V (Fig. 6c). The gentle tapping on the surface of the device produced a V_oc of approximately 200 V (Fig. 6d). The device was grasped by hand, resulting in a V_oc of approximately 220 V (Fig. 6e). Positioning the OH-TENG at the inner elbow captured joint movement energy, which can generate a V_oc of pproximately 28 V (Fig. 6f). These demonstrations underscore the OH-TENG's capability to harvest low-frequency biomechanical energy from ubiquitous human motions while simultaneously acting as a self-powered physiological monitor.

Fig. 6 Demonstrations of OH-TENG harvesting energy from human motion. (a) Gentle finger press. (b) Cyclic tensile deformation. (c) Wrist flexion–extension. (d) Light hand tapping. (e) Firm handgrip. (f) Elbow movement.

To extend the OH-TENG beyond energy harvesting, a self-powered electronic switch by coupling the device to a high-gain Darlington transistor–relay module was developed (Fig. 7a). The Darlington transistor (two cascaded NPN transistors, J3Y) multiplies the β-values of the individual transistors. As Fig. 7b and c illustrates, a base current (I_B) of only 10 µA is amplified to an emitter current (I_E) of 12 mA, more than sufficient to toggle a relay. In operation, each finger tap on the flexible OH-TENG injects a brief I_sc pulse into the Darlington base. After amplification, this pulse energises the relay and latches the external load (Fig. 7a). Tapping again generates a second pulse that de-energises the relay and turns the load off (Videos S4–S6). Fig. 7d–f demonstrate the concept with three representative loads: a miniature DC motor, a temperature-humidity sensor, and a high-brightness LED array—all switched reliably with a single finger touch. To further demonstrate the OH-TENG's potential in human–machine interfaces, it was integrated with a Bluetooth oscilloscope module for real-time monitoring of human activities. The workflow is illustrated in Fig. 7g. When the surface of the device was gently tapped, the generated signals were acquired by the Bluetooth module and instantly displayed on a mobile phone after wireless transmission (Fig. 7h and Video S7). The corresponding real-time motion data are presented in Fig. 7i. These results position the TENG system as a promising potential for human–machine interfaces and smart-home controls. In future work, the OH-TENG arrays coupled with multi-channel data acquisition will enable multifunctional human–machine interfaces such as prosthetic-hand control, offering a new technical route toward portable triboelectric sensors and advancing next-generation human–machine interface technologies.^6,74

Fig. 7 (a) Circuit schematic of the self-powered electronic switch. (b) Base current (I_B) supplied to the Darlington pair. (c) Corresponding amplified emitter current (I_E). Demonstrations of relay actuation driving (d) a miniature DC motor, (e) a temperature-humidity sensor, and (f) a high-brightness LED array. (g) Schematic diagram of the working process of the TENG-Bluetooth sensor. (h) Mobile phone monitors human movement. (i) Visualization of motion signals.

The prepared PVA/borax conductive organohydrogel possesses excellent elasticity, extensibility and morphological adaptability, enabling it to closely adhere to different movement areas of the human body and achieving the goal of human activity sensing. After connecting wires to both ends of the PVA/borax conductive organohydrogel, a wearable resistance sensor was constructed. As shown in Fig. 8, different-sized wearable sensors attached to different joints of the human body can detect complex movements of the human body, including finger bending (Fig. 8a), wrist bending (Fig. 8b), and elbow bending (Fig. 8c). During the periodic bending and straightening of the finger, the organohydrogel undergoes stretching and rebounding accordingly, and its resistance value also shows periodic changes within a range of 0% to 60%. The organohydrogel sensor was then fixed at the wrist joint, and periodic bending and stretching motions were repeatedly performed. During this process, the resistance of the organohydrogel also exhibited periodic changes, with a fluctuation range of approximately 0% to 40% (Fig. 8b). Subsequently, the organohydrogel sensor was fixed at the elbow joint to perform periodic bending and stretching motions, and the resistance again showed periodic changes, with a fluctuation range as high as 80% (Fig. 8c). In addition, other motion-sensing experiments were performed. As shown in Fig. 8d, when the organohydrogel sensor was placed on the human laryngeal prominence, it could measure the resistance changes during swallowing motions. Fig. 8e and f respectively illustrate the resistance changes when the hydrogel sensor was placed on one side of the human cheek to measure the changes during cheek puffing and smiling. These experimental results further validate the excellent performance of the PVA/borax conductive organohydrogel as a resistive sensor.

Fig. 8 Dynamic tracking of physiological motions with the organohydrogel sensor: relative resistance responses to (a) finger flexion, (b) wrist flexion, (c) elbow flexion, (d) swallowing, (e) pouting, and (f) smiling.

4. Conclusion

In summary, a PVA/borax organohydrogel with high flexibility, transparency, and excellent electrical conductivity has been developed in a glycerol/water binary solvent. Based on the PVA/borax conductive organohydrogel, a wearable resistance sensor was constructed, which can be used to monitor human motions. Utilizing the PVA/borax organohydrogel as a flexible electrode and silicone rubber film as the triboelectric layer and protective layer, a single-electrode OH-TENG was fabricated. The effects of contact frequency, applied force, separation distance, and strain level on the electrical outputs of the OH-TENG, as well as the influence of external load resistance on its output power, were investigated. The fabricated OH-TENG exhibits good signal output stability, can charge various capacitors, and can generate electrical energy to power portable electronic devices. The OH-TENG shows high sensitivity, allowing it to gather biomechanical energy from human motion and detect physiological signals. When combined with a Darlington transistor-relay module, it can be used as an electronic switch to control various electrical devices. By integrating the OH-TENG with the Bluetooth oscilloscope module and processing system, the OH-TENG is capable of generating significant voltage signals when monitoring human movements. These results show great potential in human–machine interaction. Therefore, the OH-TENG is expected to be widely applied in future wearable electronics, human–machine interaction, and biomedical monitoring.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available on request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5nr03778f.

Acknowledgements

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 11204122), Natural Science Foundation of Henan (No. 252300420058, 252300423328).

References

1. Z. L. Wang, MRS Bull., 2023, 48, 1014–1025.
2. S. M. S. Rana and Z. L. Wang, Coord. Chem. Rev., 2025, 543, 216914.
3. S. M. S. Rana, O. Faruk, M. R. Islam, T. Yasmin, K. Zaman and Z. L. Wang, Coord. Chem. Rev., 2024, 507, 215741.
4. D. Choi, Y. Lee and Z. H. Lin, et al. , ACS Nano, 2023, 17, 11087–11219.
5. S. Y. Cho, Y. Xiong, H. Jiao, D. H. Ho, J. Yang, C. Liu, S. Kim, L. Wei, Z. L. Wang, Q. Sun and J. H. Cho, Adv. Funct. Mater., 2025, 35, e11492.
6. R. Gu, L. Wei, N. Xu, Y. Xiong, Q. Sun and Z. L. Wang, Adv. Funct. Mater., 2025, 35, 2505719.
7. A. Babu, N. Madathil, R. K. Rajaboina, H. Borkar, K. C. Devarayapalli, Y. K. Mishra, S. Hajra, H. J. Kim, U. K. Khanapuram and D. S. Lee, Mater. Adv., 2025, 6, 4725–4737.
8. M. Velpula, N. Madathil, S. Potu, L. Bochu, K. Chithari, J. Vallamkondu, P. Kodali, U. K. Khanapuram and R. K. Rajaboina, ACS Appl. Electron. Mater., 2024, 6, 7890–7897.
9. D. Liu, J. Luo, L. Huang, M. Chen, M. Ji, Z. L. Wang and J. Kang, Nano Energy, 2025, 136, 110767.
10. C. Zhang, Y. Hao, X. Lu, W. Su, H. Zhang, Z. L. Wang and X. Li, Nano-Micro Lett., 2025, 17, 124.
11. S. Sharma, V. K. Singh and M. K. Gupta, Nanoscale, 2025, 17, 17830–17845.
12. H. Li, M. Li, J. Wang, H. Han, J. Liu, W. Wang and Y. Chen, Nanoscale, 2025, 17, 12396–12405.
13. M. H. Albuqami, E. Kovalska, K. S. Sadanandan, M. S. Alghamdi, A. I. S. Neves, S. Russo and M. F. Craciun, Nanoscale, 2025, 17, 12080–12086.
14. S. Potu, M. Navaneeth, A. Bhadoriya, A. Bora, Y. Sivalingam, A. Babu, M. Velpula, B. Gollapelli, R. K. Rajaboina, U. K. Khanapuram, H. Divi, P. Kodali and L. Bochu, ACS Appl. Nano Mater., 2023, 6, 22701–22710.
15. A. Babu, S. Gupta, R. Katru, N. Madathil, A. Kulandaivel, P. Kodali, H. Divi, H. Borkar, U. K. Khanapuram and R. K. Rajaboina, Energy Technol., 2024, 12, 2400796.
16. H. Zhou, Z. Cao, Z. L. Wang and Z. Wu, Nano Today, 2025, 61, 102628.
17. D. Xu, Z. Jing, H. Wang, W. Yang, P. Xu, D. Niu and P. Ma, Nanoscale, 2025, 17, 7713–7737.
18. S. Fang, X. Xu, Y. Wei, F. Qiu, W. Huang, H. Jiang, N. Zhang, Y. Song, M. Gao, H. Liu, Y. Liu and B. Cheng, Nanoscale, 2025, 17, 8047–8056.
19. G. Khandelwal, D. A. John, V. Vivekananthan, N. Gadegaard, D. M. Mulvihill and S. J. Kim, Nanoscale, 2025, 17, 3211–3220.
20. N. Madathil, A. Babu, M. Velupula, A. Kulandaivel, R. K. Rajaboina, U. K. Khanapuram, K. C. Devarayapalli and D. S. Lee, Sustain. Energy Fuels, 2025, 9, 4364–4374.
21. S. Potu, N. Madathil, S. Mishra, A. Bora, Y. Sivalingam, A. Babu, M. Velpula, L. Bochu, B. Ketharachapalli, A. Kulandaivel, R. K. Rajaboina, U. K. Khanapuram, H. Divi, P. Kodali, B. Murali and R. Ketavath, ACS Appl. Eng. Mater., 2023, 1, 2663–2675.
22. A. Babu, K. Ruthvik, P. Supraja, M. Navaneeth, K. U. Kumar, R. R. Kumar, K. Prakash and N. Raju, J. Mater. Sci: Mater. Electron., 2023, 34, 2195.
23. J. Yang, K. Hong, Y. Hao, X. Zhu, Y. Qin, W. Su, H. Zhang, C. Zhang, Z. L. Wang and X. Li, Adv. Mater. Technol., 2024, 10, 2400554.
24. K. Parida, G. Thangavel, G. Cai, X. Zhou, S. Park, J. Xiong and P. S. Lee, Nat. Commun., 2019, 10, 2158.
25. N. Xu, J. Han, J. Yang, Y. Wang, Y. Xiong, Z. L. Wang and Q. Sun, Nano Energy, 2025, 142, 111153.
26. S. M. S. Rana, M. A. Zahed, M. R. Islam, O. Faruk, H. S. Song, S. H. Jeong and J. Y. Park, Chem. Eng. J., 2023, 473, 144989.
27. S. Potu, M. Navaneeth, R. K. Rajaboina, B. Gollapelli, J. Vallamkondu, S. Mishra, H. Divi, A. Babu, K. Uday Kumar and P. Kodali, ACS Appl. Energy Mater., 2022, 5, 13702–13713.
28. H. Fang, X. Wang, Q. Li, D. Peng, Q. Yan and C. Pan, Adv. Energy Mater., 2016, 6, 160829.
29. X. Liang, T. Zhao, W. Jiang, X. Yu, Y. Hu, P. Zhu, H. Zheng, R. Sun and C. P. Wong, Nano Energy, 2019, 59, 508–516.
30. J. Shi, X. Chen, G. Li, N. Sun, H. Jiang, D. Bao, L. Xie, M. Peng, Y. Liu, Z. Wen and X. Sun, Nanoscale, 2019, 11, 7513–7519.
31. J. Lee, P. Lee, H. Lee, D. Lee, S. S. Lee and S. H. Ko, Nanoscale, 2012, 4, 6408–6414.
32. G. Zhao, Y. Zhang, N. Shi, Z. Liu, X. Zhang, M. Wu, C. Pan, H. Liu, L. Li and Z. L. Wang, Nano Energy, 2019, 59, 302–310.
33. H. Chen, Y. Xu, J. Zhang, W. Wu and G. Song, Nano Energy, 2019, 58, 304–311.
34. K. Parida, J. Xiong, X. Zhou and P. S. Lee, Nano Energy, 2019, 59, 237–257.
35. L. Sun, S. Chen, Y. Guo, J. Song, L. Zhang, L. Xiao, Q. Guan and Z. You, Nano Energy, 2019, 63, 103847.
36. T. Liu, M. Liu, S. Dou, J. Sun, Z. Cong, C. Jiang, C. Du, X. Pu, W. Hu and Z. L. Wang, ACS Nano, 2018, 12, 2818–2826.
37. Z. Wang, Q. Hu, S. Yao, S. Wang, X. Liu, C. Zhang, Z. L. Wang and L. Li, Small, 2025, 21, 2409756.
38. M. Yu, H. Zhang, Q. Yang, N. Li, J. Du, L. Xu and J. Xu, Chem. Eng. J., 2025, 505, 159105.
39. F. Luo, B. Chen, X. Ran, W. Ouyang, Y. Yao and L. Shang, Nano Energy, 2023, 118, 109035.
40. R. Li, Z. Xu, L. Li, J. Wei, W. Wang, Z. Yan and T. Chen, Chem. Eng. J., 2023, 454, 140261.
41. Y. Luo, M. Yu, Y. Zhang, Y. Wang, L. Long, H. Tan, N. Li, L. Xu and J. Xu, Nano Energy, 2022, 104, 107955.
42. B. Jiang, Y. Long, X. Pu, W. Hu and Z. L. Wang, Nano Energy, 2021, 86, 106086.
43. X. Qi, X. Hu, W. Wei, H. Yu, J. Li, J. Zhang and W. Dong, Carbohydr. Polym., 2015, 118, 60–69.
44. W. Xu, L. B. Huang, M. C. Wong, L. Chen, G. Bai and J. Hao, Adv. Energy Mater., 2016, 7, 1601529.
45. X. Jing, H. Li, H. Y. Mi, P. Y. Feng, X. Tao, Y. Liu, C. Liu and C. Shen, ACS Appl. Mater. Interfaces, 2020, 12, 23474–23483.
46. B. Lu, F. Lin, X. Jiang, J. Cheng, Q. Lu, J. Song, C. Chen and B. Huang, ACS Sustainable Chem. Eng., 2017, 5, 948–956.
47. S. Dai, S. Wang, X. Dong, X. Xu, X. Cao, Y. Chen, X. Zhou, J. Ding and N. Yuan, J. Mater. Chem. C, 2019, 7, 14581–14587.
48. R. Peymanfar, E. Selseleh-Zakerin, A. Ahmadi, A. Saeidi and S. H. Tavassoli, Sci. Rep., 2021, 11, 16161.
49. L. Cui, J. Chen, C. Yan and D. Xiong, J. Bionic Eng., 2023, 20, 1072–1082.
50. K. Parida, V. Kumar, W. Jiangxin, V. Bhavanasi, R. Bendi and P. S. Lee, Adv. Mater., 2017, 29, 1702181.
51. Y. Cai, J. Che, M. Yuan, X. Shi, W. Chen and W.-E. Yuan, Exp. Ther. Med., 2016, 12, 2039–2044.
52. R. Liu, K. Chen, H. Liu, Y. Liu, R. Cong, J. Guo and Y. Tian, ACS Appl. Mater. Interfaces, 2022, 14, 51341–51350.
53. N. M. Ali and A. A. Kareem, Chalcogenide Lett., 2022, 19, 217–225.
54. S. Alipoori, M. M. Torkzadeh, S. Mazinani, S. H. Aboutalebi and F. Sharif, SN Appl. Sci., 2021, 3, 310.
55. Y. Mi, T. Wu, Y. Lu, Y. Wang, Z. Ma, X. Cao and N. Wang, Chem. Eng. J., 2025, 518, 164460.
56. M. Wu, X. Wang, Y. Xia, Y. Zhu, S. Zhu, C. Jia, W. Guo, Q. Li and Z. Yan, Nano Energy, 2022, 95, 106967.
57. P. Chen, X. Zhu, G. Tian and X. Feng, Nano Energy, 2025, 142, 111212.
58. J. C. Ye, C. S. He, X. R. Gong, H. H. Zhang and X. Li, J. Alloys Compd., 2025, 1014, 178710.
59. Y. Wang, H. Mu, S. He, H. Yao, S. Wang, Y. Yang and G. Zhang, Chem. Eng. J., 2025, 512, 162397.
60. H. Zou, Y. Zhang, L. Guo, P. Wang, X. He, G. Dai, H. Zheng, C. Chen, A. C. Wang, C. Xu and Z. L. Wang, Nat. Commun., 2019, 10, 1427.
61. P. Manchi, M. V. Paranjape, A. Kurakula, V. S. Kavarthapu, C. W. Kim and J. S. Yu, Nano Energy, 2025, 142, 111184.
62. B. U. Ye, B. J. Kim, J. Ryu, J. Y. Lee, J. M. Baik and K. Hong, Nanoscale, 2015, 7, 16189–16194.
63. J. Zhao, Y. Wang, X. Song, A. Zhou, Y. Ma and X. Wang, Nanoscale, 2021, 13, 18363–18373.
64. L. Shi, S. Dong, H. Xu, S. Huang, Q. Ye, S. Liu, T. Wu, J. Chen, S. Zhang, S. Li, X. Wang, H. Jin, J. M. Kim and J. Luo, Nano Energy, 2019, 64, 103960.
65. J. Shao, T. Jiang and Z. Wang, Sci. China: Technol. Sci., 2020, 63, 1087–1109.
66. L. N. Zhou, W. Z. Song, D. J. Sun, B. Y. Yan, T. Chen, T. Li, J. Zhang, G. F. Yu, S. Ramakrishna and Y. Z. Long, ACS Appl. Electron. Mater., 2022, 5, 216–226.
67. F. Yi, X. Wang, S. Niu, S. Li, Y. Yin, K. Dai, G. Zhang, L. Lin, Z. Wen, H. Guo, J. Wang, M. H. Yeh, Y. Zi, Q. Liao, Z. You, Y. Zhang and Z. L. Wang, Sci. Adv., 2016, 2, e1501624.
68. X. Wang, Y. Yin, F. Yi, K. Dai, S. Niu, Y. Han, Y. Zhang and Z. You, Nano Energy, 2017, 39, 429–436.
69. W. Zhang, Q. Liu, S. Chao, R. Liu, X. Cui, Y. Sun, H. Ouyang and Z. Li, ACS Appl. Mater. Interfaces, 2021, 13, 42966–42976.
70. K. Dong, Z. Wu, J. Deng, A. C. Wang, H. Zou, C. Chen, D. Hu, B. Gu, B. Sun and Z. L. Wang, Adv. Mater., 2018, 30, 1804944.
71. C. Jacquemoud, K. Bruyere-Garnier and M. Coret, J. Biomech., 2007, 40, 468–475.
72. J. Zhao, Y. Wang, B. Wang, Y. Sun, H. Lv, Z. Wang, W. Zhang and Y. Jiang, Nanoscale, 2023, 15, 6812–6821.
73. F. Sheng, J. Yi, S. Shen, R. Cheng, C. Ning, L. Ma, X. Peng, W. Deng, K. Dong and Z. L. Wang, ACS Appl. Mater. Interfaces, 2021, 13, 44868–44877.
74. B. Yang, J. Cheng, X. Qu, Y. Song, L. Yang, J. Shen, Z. Bai and L. Ji, Adv. Sci., 2024, 12, 2408689.

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