Hydrogel Electrodes Research Articles

Ultrafast reaction kinetic of polyaniline in flexible hydrogel electrodes facilitated by graphene ribbons

Abstract By loading energy storage active materials on hydrogel which is inherently flexible, the flexibility of electrode materials can be simply realized, thereby achieving the flexibility of energy storage devices. However, the polymer network that constructs the three-dimensional skeleton of the hydrogel is not conductive, which inhibits the redox ability of the active material. If a high speed conductive structure can be added to the colloidal phase, the performance of the flexible electrode material can be greatly improved. Here, we introduce redox graphene ribbons into the polyvinyl alcohol hydrogel loaded with polyaniline. The in situ three-dimensional conductive graphene network greatly enhanced the conductivity of the hydrogel electrode, thus increasing the specific capacitance to as high as 1117 F g−1 at 2 mg cm−2 mass loading, with a retention ratio of 66.96% from 0.5 A g−1 to 20 A g−1. These highlighted properties enable the PRP hydrogel as an electrode for flexible supercapacitors, which provides a promising possibility for the practical application of wearable electronics.

Super-Elastic and Temperature-Tolerant Hydrogel Electrodes for Supercapacitors via MXene Enhanced Ice-Templating Synthesis.

Developing flexible energy storage devices with good deformation resistance under extreme operating conditions is highly desirable yet remains very challenging. Super-elastic MXene-enhanced polyvinyl alcohol/polyaniline (AMPH) hydrogel electrodes are designed and synthesized through vertical gradient ice templating-induced polymerization. This approach allows for the unidirectional growth of polyaniline (PANI) and 2D MXene layers along the elongated arrayed ice crystals in a controlled manner. The resulting 3D unidirectional AMPH hydrogel exhibits inherent stretchability and electronic conductivity, with the ability to completely recover its shape even under extreme conditions, such as 500% tensile strain, 50% compressive strain. The presence of MXene in the hydrogel electrode enhances its resilience to mechanical compression and stretching, resulting in less variation in resistance. AMPH has a specific capacitance of 130.68 and 88.02mFcm-2 at a current density of 0.2 and 2mAcm-2, respectively, and retains 90% and 70% of its original capacitance at elongation of 100% and 200%, respectively. AMPH-based supercapacitors demonstrate exceptional performance in high salinity environments and wide temperature ranges (-30-80°C). The high electrochemical activity, temperature tolerance, and mechanical robustness of AMPH-based supercapacitor endow it promising as the power supply for flexible and wearable electronic devices.

Integrating conductive electrodes into hydrogel-based microfluidic chips for real-time monitoring of cell response.

The conventional real-time screening in organs-on-chips is limited to optical tracking of pre-tagged cells and biological agents. This work introduces an efficient biofabrication protocol to integrate tunable hydrogel electrodes into 3D bioprinted-on-chips. We established our method of fabricating cell-laden hydrogel-based microfluidic chips through digital light processing-based 3D bioprinting. Our conductive ink includes poly-(3,4-ethylene-dioxythiophene)-polystyrene sulfonate (PEDOT: PSS) microparticles doped in polyethylene glycol diacrylate (PEGDA). We optimized the manufacturing process of PEDOT: PSS microparticles characterized our conductive ink for different 3D bioprinting parameters, geometries, and materials conditions. While the literature is limited to 0.5% w/v for PEDOT: PSS microparticle concentration, we increased their concentration to 5% w/v with superior biological responses. We measured the conductivity in the 3-15m/m for a range of 0.5%-5% w/v microparticles, and we showed the effectiveness of 3D-printed electrodes for predicting cell responses when encapsulated in gelatin-methacryloyl (GelMA). Interestingly, a higher cellular activity was observed in the case of 5% w/v microparticles compared to 0.5% w/v microparticles. Electrochemical impedance spectroscopy measurements indicated significant differences in cell densities and spheroid sizes embedded in GelMA microtissues.

Smart sensing hydrogel actuators conferred by MXene gradient arrangement

Smart sensing and excellent actuation abilities of natural organisms have driven scientists to develop bionic soft-bodied robots. However, most conventional robots suffer from poor electrical conductivity, limiting their application in real-time sensing and actuation. Here, we report a novel strategy to enhance the electrical conductivity of hydrogels that integrated actuation and strain-sensing functions for bioinspired self-sensing soft actuators. Conductive hydrogels were synthesized in situ by copolymerizing MXene nanosheets with thermosensitive N-isopropylacrylamide and acrylamide under a direct current electric field. The resulting hydrogels exhibited high electrical conductivity (2.11 mS/cm), good sensitivity with a gauge factor of 4.79 and long-term stability. The developed hydrogels demonstrated remarkable capabilities in detecting human motions at subtle strains such as facial expressions and large strains such as knee bending. Additionally, the hydrogel electrode patch was capable of monitoring physiological signals. Furthermore, the developed hydrogel showed good thermally induced actuation effects when the temperature was higher than 30 °C. Overall, this work provided new insights for the design of sensory materials with integrated self-sensing and actuation capabilities, which would pave the way for the development of high-performance conductive soft materials for intelligent soft robots and automated machinery.

Self‐healable Mn‐doped PVA‐borax hydrogel electrodes for supercapacitor applications

AbstractThis study aims to investigate the effect of Mn doping on the performance of polyvinyl alcohol/borax‐structured (PMB) supercapacitor electrodes for high‐capacity, flexible, and self‐healing supercapacitor electrodes. The PMB electrodes were characterized by x‐ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). Electrochemical characterization was conducted using cyclic voltammetry (CV), which revealed that redox reactions, which are evident at low scan rates, enhance capacitive performance. However, high scan rates introduce limitations owing to the increased vibrations of the PVA chains and incomplete electrolyte polarization. The PMB electrodes exhibited remarkable electrochemical properties and capacitive performances, which were significantly affected by the MnO₂ structure. The electrodes exhibited self‐healing properties and demonstrated flexibility to stretch by up to 40%. In addition, the results indicate that the electrode capacity of the PVA/borax system increased by 2.03 times after 1000 cycles with manganese doping. The solid electrolyte interface (SEI) layer formed on the electrode surface plays a critical role in stabilizing the capacity loss during prolonged charge–discharge cycles. The study concluded that PMB supercapacitors offer higher capacitance in 0‐2 V applications compared to other PVA/borax combination hydrogels. Moreover, the incorporation of metal‐based additives, particularly manganese, enhanced the capacitive performance, and operating voltage levels.

Ultra-stretchable triboelectric touch pad with sandpaper micro-surfaces for Transformer-assisted gesture recognition

Touch pad based on triboelectric nanogenerator are attracting attention in the field of wearable electronics and human-machine interaction. However, it is still a challenge to realize touch pads combining large-area metamorphic stretchability, high power triboelectric sensing, intelligent free-sliding recognition and availability in extreme environments. Here, we fabricate an ultra-stretchable triboelectric touch pad by integrating liquid metal and high performance hydrogel-based triboelectric sensor arrays, which achieves Transformer-assisted gesture recognition for drone flight direction control. The internal hydrogel electrode has great stretchability, electrical conductivity and anti-freeze properties. Encapsulated in Ecoflex with sandpaper micro-surfaces, it endows high triboelectric power density and pressure sensitivity. With screen-printed liquid metal, the integrated touch pad is fully ultra-stretchable and large-area strain stability. Combining Internet of Things, we build a Transformer-assisted human machine interface. Notably, slide gesture signal peak deviation is reduced by integral processing. The random free-sliding gesture signals are parsed by the Transformer algorithm with 96.83 % accuracy. Multi-threaded signal processing improves the response speed and throughput of the system. The accurate real-time gesture recognition for drone flight direction control in extreme environment is realized. This human machine interface can facilitate development of next generation intelligent remote interactive platforms.

Highly Conductive and Stretchable Hydrogel Nanocomposite Using Whiskered Gold Nanosheets for Soft Bioelectronics.

The low electrical conductivity of conductive hydrogels limits their applications as soft conductors in bioelectronics. This low conductivity originates from the high water content of hydrogels, which impedes facile carrier transport between conductive fillers. This study presents a highly conductive and stretchable hydrogel nanocomposite comprising whiskered gold nanosheets. A dry network of whiskered gold nanosheets is fabricated and then incorporated into the wet hydrogel matrices. The whiskered gold nanosheets preserve their tight interconnection in hydrogels despite the high water content, providing a high-quality percolation network even under stretched states. Regardless of the type of hydrogel matrix, the gold-hydrogel nanocomposites exhibit a conductivity of ≈520Scm-1 and a stretchability of ≈300% without requiring a dehydration process. The conductivity reaches a maximum of ≈3304Scm-1 when the density of the dry gold network is controlled. A gold-adhesive hydrogel nanocomposite, which can achieve conformal adhesion to moving organ surfaces, is fabricated for bioelectronics demonstrations. The adhesive hydrogel electrode outperforms elastomer-based electrodes in in vivo epicardial electrogram recording, epicardial pacing, and sciatic nerve stimulation.

Fabric and Fiber-Integrated, Washable Hydrogel Electrodes for Clinically Accurate Sleep Monitoring in-Home

Longitudinal tracking of sleep metrics is important for detecting and managing various diseases, spanning cardiorespiratory disorders to dementia. However, at present, sleep monitoring primarily occurs in specialized medical facilities that are not conducive to longterm studies. In-home solutions either compromise user comfort or signal accuracy in tracking sleep variables and have not yet provided reliable longitudinal data. We believe that human-centered design of multimodal, low-form-factor, comfortable sensing systems is needed for this increasingly-important area of preventative health monitoring. Our lab developed a suite of fabric-based and/or garment-integrated sensors and sensing modalities that accurately and reliably extract important sleep metrics, such as posture, cardiorespiratory signals, eye movement and brain activity, over the course of an eight-hour sleep session. Our garment-integrated sensing arrays do not require a medical professional for on-body positioning, maintain their function independent of sizing and fit on individual users, and can be laundered or cleaned with antibacterial wipes without compromising its varied sensing elements. Here, we describe one of these two key sensors, a fabric or fiber-based electrochemical hydrogel electrode that enables wearable biopotential monitoring, including electrocardiography (ECG), eletrooculography (EOG) and electroencephalography (EEG), from which the aforementioned sleep metrics can be recorded and quantified with clinical precision and accuracy. The hydrogel is fabricated as a coating on either commodity fabrics or threads using a low-temperature reactive deposition process, termed photoinitiated chemical vapor deposition (piCVD), during which hydrogel monomers react with a photoinitator in the vapor phase to create a polymeric hydrogel coating directly on the surface of fabrics or threads that have been temporarily surface modified with a skin-safe electrolyte gel. The resulting composite hydrogel coating isolated from this operation is ionically conductive, can be reversibly swelled in water and dried without losing ionic conductivity, and transduces the ionic signals (fluxes) experienced by the body into an electrical signal that can be captured by any commercial ECG, EOG or EEG monitoring instrument. An array of these fabric or thread-based hydrogel electrodes can be integrated into readily-available garments using simple cut-sew techniques and these garments can then be worn throughout the night to effect continuous ECG, EOG and EEG monitoring at home. Design elements of a custom-made circuit board that allows simultaneous capture of these signals will be described. Results from an ongoing user study of >50 adults will be presented to validate the clinical precision and accuracy of these garment sensor for sleep quality monitoring.

A robust polyaniline hydrogel electrode enables superior rate capability at ultrahigh mass loadings

Simultaneously achieving high mass loading and superior rate capability in electrodes is challenging due to their often mutually constrained nature, especially for pseudocapacitors for high-power density applications. Here, we report a robust porous polyaniline hydrogel (PPH) prepared using a facile ice-templated in situ polymerization approach. Owing to the conductive, robust, and porous nanostructures suitable for ultrafast electron and ion transport, the self-supporting pure polyaniline hydrogel electrode exhibits superior areal capacitance without sacrificing rate capability and gravimetric capacitance at an ultrahigh mass loading and notable current density. It achieves a high areal capacitance (15.2 F·cm−2 at 500 mA·cm−2) and excellent rate capability (~92.7% retention from 20 to 500 mA·cm−2) with an ultrahigh mass loading of 43.2 mg cm−2. Our polyaniline hydrogel highlights the potential of designing porous nanostructures to boost the performance of electrode materials and inspires the development of other ultrafast pseudocapacitive electrodes with ultrahigh loadings and fast charge/discharge capabilities.

Ultrasound-Compatible Electrode for Functional Electrical Stimulation.

Functional electrical stimulation (FES) is a vital method in neurorehabilitation used to reanimate paralyzed muscles, enhance the size and strength of atrophied muscles, and reduce spasticity. FES often leads to increased muscle fatigue, necessitating careful monitoring of the patient's response. Ultrasound (US) imaging has been utilized to provide valuable insights into FES-induced fatigue by assessing changes in muscle thickness, stiffness, and strain. Current commercial FES electrodes lack sufficient US transparency, hindering the observation of muscle activity beneath the skin where the electrodes are placed. US-compatible electrodes are essential for accurate imaging and optimal FES performance, especially given the spatial constraints of conventional US probes and the need to monitor muscle areas directly beneath the electrodes. This study introduces specially designed body-conforming US-compatible FES (US-FES) electrodes constructed with a silver nanowire/polydimethylsiloxane (AgNW/PDMS) composite. We compared the performance of our body-conforming US-FES electrode with a commercial hydrogel electrode. The findings revealed that our US-FES electrode exhibited comparable conductivity and performance to the commercial one. Furthermore, US compatibility was investigated through phantom and in vivo tests, showing significant compatibility even during FES, unlike the commercial electrode. The results indicated that US-FES electrodes hold significant promise for the real-time monitoring of muscle activity during FES in clinical rehabilitative applications.

Starfish tube feet inspired hydrogel electrode for durable underwater sEMG acquisition

Underwater surface electromyography (UW-sEMG) acquisition with long-term high quality is crucial to underwater activities, including rehabilitation, fatigue measurement, and sports monitoring. However, most UW-sEMG electrodes suffer significant performance decline, especially for hydrogel electrodes, due to the swelling-induced mechanical, electrical, and adhesive degradation. Maintaining high performance underwater over extended periods is challenging. The controlled underwater adhesion-detachment behavior of starfish is inspirative. Here, a starfish tube feet-inspired hydrogel (STFH) electrode is developed for UW-sEMG recording, including a micro-swelling hydrogel pillar, a biomimetic silicone sucker, and an Ag/AgCl disc electrode. By soaking freeze-dried hydrophilic hydrogel in tannic acid (TA) solution, the hydrogel exhibits skin-like modulus of 22.4 kPa, low skin impedance of 18.3 kΩ (10 Hz) and low equilibrium swelling ratio. Unlike traditional strategies for improving anti-swelling properties, this work leverages the interaction between the sucker and the swollen hydrogel to promote conformal contact. Consequently, the sEMG signal exhibits SNR of 26.7 dB and baseline noise of 12.8 μV even after 60 min of underwater working. The STFH electrode also shows anti-noise capability to resist skin vibration and wave oscillation. This STFH electrode enables durable UW-sEMG acquisition, which could further extend from the arm to the wrist to adapt wearing habits.

Scaled Elastic Hydrogel Interfaces for Brain Electrophysiology

AbstractThe advancement in brain–computer interface technology has facilitated real‐time communication between the brain and external devices, rapidly expanding invasive brain–computer interfaces. Nevertheless, little progress has been made in improving the one‐way, noninvasive brain–computer interface technology using conductive methods, necessitating enhancements to the current paste contact electrodes. In this paper, a captivating spherical electrode is fabricated that exhibits noncytotoxic properties, combining alginate hydrogel with indium tin oxide conductor components. The electrode's remarkable and consistent electrical conductivity enables it to generate stable adaptive deformations that conform to diverse scalp topographies. Most impedance values fall below 10 kΩ, indicating heightened hydrophilicity compared to traditional conductive pastes, as evidenced by contact angle measurements. An empirical electroencephalography data collection study confirms its conductivity and signal transmission reliability. The acquired electroencephalography signals are utilized for emotion recognition utilizing band energy ratio analysis and support vector machine (SVM) algorithms. The operation of this system is both straightforward and convenient, requiring minimal preparation before and after the collection of electroencephalography signals. Utilizing an extensive range of raw materials and straightforward preparatory procedures enhances the potential industrial application of hydrogel electrodes, thereby significantly contributing to advancing civilian electroencephalography equipment and investigating depression‐related pathologies.

Thixotropic Composite Hydrogel Electrode Composed of a Polymer Hydrogelator and Water-Dispersive Tungsten Oxide Flat Microparticles.

Here, we introduce an organic/inorganic composite hydrogel as a versatile gel electrode material. This composite hydrogel was formed by simply mixing an aqueous solution of flat microparticles of tungsten oxide, exhibiting superior water dispersibility, with a hydrogel composed of a water-soluble polyaramide-based polymer hydrogelator. The resulting composite hydrogel exhibited uniform dispersion of tungsten oxide flat particles throughout the hydrogel matrix, supplementing the structure formed by the polymer hydrogelator. It maintained the gel-forming capability and thixotropic behavior inherent to the polymer hydrogelator while showcasing the electrochemical characteristics of tungsten oxide. With its spreadability and applicability to various electrode shapes, a composite hydrogel is presented as a potential spreadable gel electrode material.

Highly deformable bi-continuous conducting polymer hydrogels for electrochemical energy storage

Conducting polymer hydrogels with inherent flexibility, ionic conductivity and environment friendliness are promising materials in the fields of energy storage. However, a trade-off between mechanical and electrochemical properties has limited the development of flexible/stretchable conducting polymer hydrogel electrodes, owing to the intrinsic conflict among mechanical and electrical phases. Here, we report a reliable design to enable conducting polymer with both exceptional mechanical and electrical/electrochemical performance through the construction of bi-continuous conducting polymer crosslinked network. The resultant bi-continuous conducting polymer hydrogels (BCPH) demonstrate significantly improved mechanical and electrochemical properties compared to the conventional conducting polymer hydrogel (CPH) electrode. BCPH presents a high specific capacitance of 715 F g−1 at 0.5 A/g, a high mechanical strength (∼1 MPa) and a large stretchability (∼300%). Enabled by such intrinsically deformability and electrochemical properties, we further demonstrate its utility in flexible solid-state supercapacitor (FSSC), which exhibits an outstanding specific capacitance of 760 mF cm−2 at 2 mA cm−2, excellent electrochemical stability with 81% capacitance retention after 5000 charge/discharge cycles, and superior bending cycle stability. This simple and scalable strategy provides a platform for the fabrication of high-performance conducting hydrogel electrodes for various wearable electronic equipment.

Conductive Bio‐based Hydrogel for Wearable Electrodes via Direct Ink Writing on Skin

AbstractNon‐invasive electrodes for recording and delivering electric signals to the human body are crucial for health monitoring and rehabilitation applications. However, high‐fidelity signal recording or delivery with epidermal electrodes remains a challenge due to the need for shape customization, to account for the variance of body morphology among individuals, and the need for conformal contact, to accommodate creviced skin surfaces, intricate curves, and moving bodies. In this study, a conductive and self‐adhesive hydrogel for direct ink writing of wearable electrodes on the skin is presented, utilizing physical cross‐linking mechanisms between bio‐based polymers. With a fast gelation time and a facile fabrication method, the printed hydrogel achieves a 0.40 mm resolution via handheld 3D printers. Compared with silver/silver chloride (Ag/AgCl) coated gel electrode standards, the hydrogel electrode formed in situ achieves a higher signal‐to‐noise ratio by 88%, for the monitoring of forearm muscle biopotential and decreases the required current from 3.5 to 2.25 mA, for the functional electrical stimulation for eye closure. The lowered contact impedance of the hydrogel electrode is attributed to its sol–gel transition in situ on the skin, demonstrating its potential to enable future healthcare applications with improved personalization, efficiency, and comfort.

Low Driving Voltage Electroluminescence Device for Integrated Visual Strain Sensing.

As a pivotal component in human-machine interactions, display devices have undergone rapid development in modern life. Displays such as alternative current electroluminescence|alternative current electroluminescent (ACEL) devices with high flexibility and long operational lifetimes are essential for wearable electronics. However, ACEL devices are constrained by their inherent high driving voltage and complex fabrication processes. Our work presents an easy blade-coating method for fabricating flexible ACEL display devices based on an all-solution process. By dispersing BaTiO3 and ZnS/Cu powder into waterborne polyurethane, we successfully combined dielectric and fluorescence functionalities within a single layer, significantly reducing the device's driving voltage. Additionally, the ionic conducting hydrogel was chosen as a transparent electrode to achieve good electrical contact and strong interfacial adhesion through in situ polymerization. Owing to the unique method, our ACEL device exhibits high flexibility, low driving voltage (20-100 V), high brightness (300+ cd/m2 at 60 V), and environmental friendliness. Furthermore, by repurposing the hydrogel electrode, we integrated strain visualization capabilities within a single device, highlighting its potential for applications such as wearable healthcare monitoring.

A novel functional electrical stimulation sleeve based on textile-embedded dry electrodes

BackgroundFunctional electrical stimulation (FES) is a rehabilitation technique that enables functional improvements in patients with motor control impairments. This study presents an original design and prototyping method for a smart sleeve for FES applications. The article explains how to integrate a carbon-based dry electrode into a textile structure and ensure an electrical connection between the electrodes and the stimulator for effective delivery of the FES. It also describes the materials and the step-by-step manufacturing processes.ResultsThe carbon-based dry electrode is integrated into the textile substrate by a thermal compression molding process on an embroidered conductive matrix. This matrix is composed of textile silver-plated conductive yarns and is linked to the stimulator. Besides ensuring the electrical connection, the matrix improves the fixation between the textile substrate and the electrode. The stimulation intensity, the perceived comfort and the muscle torque generated by the smart FES sleeve were compared to hydrogel electrodes. The results show a better average comfort and a higher average stimulation intensity with the smart FES sleeve, while there were no significant differences for the muscle torque generated.ConclusionsThe integration of the proposed dry electrodes into a textile is a viable solution. The wearable FES system does not negatively impact the electrodes’ performance, and tends to improve it. Additionally, the proposed prototyping method is applicable to an entire garment in order to target all muscles. Moreover, the process is feasible for industrial production and commercialization since all materials and processes used are already available on the market.

Ink-based textile electrodes for wearable functional electrical stimulation: A proof-of-concept study to evaluate comfort and efficacy.

Functional Electrical Stimulation (FES) represents a promising technique for promoting functional recovery in individuals with neuromuscular diseases. Traditionally, current pulses are delivered through self-adhesive hydrogel Ag/AgCl electrodes, which allow a good contact with the skin, are easy-to-use and have a moderate cost. However, skin adherence decreases after a few uses and skin irritations can originate. Recently, textile electrodes have become an attractive alternative as they assure increased durability, easy integration into clothes and can be conveniently cleaned, improving the wearability of FES. However, as various manufacture processes were attempted, their clear validation is lacking. This proof-of-concept study proposes a novel set of ink-based printed textile electrodes and compares them to adhesive hydrogel electrodes in terms of impedance, stimulation performance and perceived comfort. The skin-electrode impedance was evaluated for both types of electrodes under different conditions. These electrodes were then used to deliver FES to the Rectus Femoris of 14 healthy subjects to induce its contraction in both isometric and dynamic conditions. This allowed to compare the two types of electrodes in terms of sensory, motor, maximum and pain thresholds, FES-induced range of motion during dynamic tests, FES-induced torque during isometric tests and perceived stimulation comfort. No statistically significant differences were found both in terms of stimulation performance (Wilcoxon test) and comfort (Generalized Linear Mixed Model). The results showed that the proposed ink-based printed textile electrodes can be effectively used as alternative to hydrogel ones. Further experiments are needed to evaluate their durability and their response to sterilizability and stretching tests.

Synergy between gelatin hydrogels and electrodes for adjustable and thermally stable ionic thermoelectric supercapacitors

Gelatin-based ionic hydrogels are flexible, bio-compatible, and abundant materials, showing immense potential as quasi-solid-state electrolytes in energy conversion and storage applications. Recent studies highlighted the critical synergy between hydrogel electrolytes and electrodes on the thermopower and energy density in emerging ionic thermoelectric (i-TE) devices. In this work, the significant impact from electrodes to the thermoelectric response of gelatin hydrogels is explored and explained at the molecular level. A universal negative offset in the thermopower is observed when substituting the conventional Cu foil by carbon-based paper (CBP) electrodes. This is attributed to the electrolyte infiltration and strong cation restriction on the CBP electrodes, which facilitates anion thermodiffusion. The thermopowers of Gelatin-CsCl change from p- (0.1–0.5 mV K−1) to n-type (up to −2.89 ± 0.34 mV K−1) and remain thermally stable at a large temperature difference of −36.8 K. The power factor and average power density in the ionic thermoelectric supercapacitors (ITESCs) made from Gelatin-0.02 M CsCl and CBP electrodes reach 2.13 ± 0.50 μW m−1 K−2 and 145.37 μW m−2 at a small temperature difference of −7.7 K. Furthermore, the highest normalized instantaneous power density achieved during the quasi-continuous charge–discharge cycles is 39.42 μW m−2 K−2, among the best ITESCs. This work provides fundamental insights into the previously overlooked synergy between gelatin hydrogels and electrodes for adjustable and thermally stable i-TE conversion and energy storage devices.

Stretchable Unsymmetrical Piezoelectric BiO2-x Deposited-Hydrogel as Multimodal Triboelectric Nanogenerators for Biomechanical Motion Harvesting.

Enhancing the output performance of triboelectric nanogenerators (TENGs) is essential for increasing their application in smart devices. Oxygen-vacancy-rich BiO2-x nanosheets (BiO2-x NSs) are advanced-engineered nanomaterials with excellent piezoelectric properties. Herein, a stretchable unsymmetrical BiO2-x NSs deposited-hydrogel made of polyacrylamide (PAM) as a multimodal TENG is rationally fabricated, and the performance of TENG can be tailored by controlling the BiO2-x NSs deposition amount and spatial distribution. The alteration of resistance caused by the Poisson effect of PAM/BiO2-x composite hydrogel (H-BiO2-x) can be used as a piezoresistive sensor, and the piezoelectricity of BiO2-x NSs can effectively enhance the density of transfer charge, thus improving the output performance of the H-BiO2-x-based TENG. In addition, the chemical cross-linking between the BiO2-x NSs and the PAM polymer chain allows the hydrogel electrode to have a higher tensile capacity (867%). Used for biomechanical motion signal detection, the sensors made of H-BiO2-x have high sensitivity (gauge factor = 6.93) and can discriminate a range of forces (0.1-5.0 N) at low frequencies (0.5-2.0Hz). Finally, the prepared TENG can collect biological energy and convert it into electricity. Consequently, the improved TENG shows a good application prospect as multimodal biomechanical sensors by combining piezoresistive, piezoelectric, and triboelectric effects.