Ionic hydrogels-based triboelectric nanogenerators for self-powered human–machine interfaces

Ionic hydrogels outperform existing rigid and bulky electronics with many remarkable advantages including great flexibility, high conductivity, exceptional biocompatibility, and transparency, making them ideal materials for wearable human–machine interfaces (HMIs). However, traditional HMIs typically rely on external power sources, which impose limitations in terms of device size and weight, thereby compromising the user experience in HMIs. The advent of triboelectric nanogenerators (TENGs) employing ionic hydrogels has introduced a sustainable energy solution for self-powered HMIs. These TENGs can harvest the electrical energy resulting from the migration of ions induced by mechanical motion, thereby offering a sustainable energy solution for applications in wearable HMIs. Hence, the development of ionic hydrogels-based TENGs holds immense potential for the advancement of self-powered HMIs. This review first introduces the latest achievements in the fabrication of ionic hydrogel-based TENGs using diverse materials, including synthetic polymers, natural polymers, and low-dimensional materials. Then different working principles and modes of the ionic hydrogel-based TENGs are elucidated. Subsequently, the applications of these TENGs in self-powered HMIs are discussed, such as robot control, medical applications, electronic device control, and other applications. Finally, the current status and future prospects of ionic hydrogel-based TENGs in self-powered HMIs are summarized. We hope that this review will provide inspiration for the future development of self-powered human–machine interfaces utilizing ionic hydrogels-based TENGs.


Introduction
Recently, the rapid development of wearable human-machine interfaces (HMIs) has been transforming our interaction mode with the digital world [1,2].From smartphones and smart homes to wearable devices and virtual reality (VR), HMIs have become an indispensable component of our daily lives, offering a wide range of possibilities for interaction between humans and machines [3][4][5].However, conventional rigid electronic devices face certain challenges in meeting the demands of modern wearable HMIs [6,7].The flexibility of rigid electronic devices is limited, making them difficult to adapt to the intricate curved surfaces of the human body, thereby constraining the comfort and stability of their wearable applications in HMIs [8][9][10][11].On the other hand, traditional rigid sensors may present issues of biocompatibility, potentially causing skin allergy reactions or discomfort in applications that involve direct contact with human skin, such as the medical field or health monitoring [12][13][14][15][16]. Additionally, conventional rigid electronics often possess relatively large weight and volume, which significantly restricts their utility in wearable and portable devices [17][18][19][20].Besides, these devices typically rely on external power sources, limiting their applications in fields of wearable technology, continuous health monitoring, and wireless sensor networks [2,[21][22][23][24][25].Therefore, it is desirable to develop novel sensor technologies with high flexibility to achieve self-powered and enhanced experiences for human-machine interaction applications.Illustration of ionic hydrogels-based triboelectric nanogenerators for self-powered human-machine interfaces.Image for 'Medical Applications': Reproduced with permission.[50] Copyright 2021, American Chemical Society.Image for 'Touch Screen Sensor': Reproduced with permission.[51] Copyright 2022, Wiley.Image for 'VR Control': Reproduced with permission.[52] Copyright 2023, Wiley.Image for 'Non-Contact Gesture Recognition': Reproduced with permission.[53] Copyright 2022, AAAS.Image for 'Electronic Device Control': Reproduced with permission.[54] Copyright 2021, Elsevier.Image for 'Robot Control': Reproduced with permission.[55] Copyright 2023, American Chemical Society.
The triboelectric nanogenerator (TENG) based on ionic hydrogels is promising to bring new opportunities to the next generation of HMI [26,27].The concept of TENG was initially developed by Prof. Zhonglin Wang's group in 2012 [28].As an emerging energy harvesting technology, the TENG generates surface charges when two materials with opposite charges come into contact, thereby creating a triboelectric potential between two electrode materials [29][30][31][32][33].An external load connected between the two electrodes allows current to flow driven by the triboelectric potential.Upon subsequent separation between the frictional materials, the current flows in the opposite direction due to changes in the electrode potential difference [34,35].By repeating this cycle continuously, a continuous alternating current output is achieved, thus enabling self-sustaining electrical energy [36][37][38][39].Ionic hydrogels can be considered an excellent component for the construction of TENG [40,41].They possess unique advantages such as excellent mechanical flexibility, high conductivity, good biocompatibility, and high water retention capacity [42][43][44][45][46].By introducing ions as charge carriers, the hydrogels are endowed with rapid internal ion transfer and separation, thereby achieving efficient triboelectric charging effects [47][48][49].Hence, the ionic hydrogel-based TENG can offer great advantages for self-powered wearable HMI applications.
In this review, we discuss the latest research advancements in self-powered HMIs by using ionic hydrogel-based TENG from three perspectives as illustrated in figure 1.In the second section, we provide an overview of the material categories used for constructing ionic hydrogel-based TENG, including synthetic polymers, natural polymers, and low-dimensional materials.In the third section, we summarize the main working principles of ionic hydrogel-based TENG, which include vertical contact-separation and single-electrode working modes.Next, the applications of self-powered HMI based on ionic hydrogel-based TENG are overviewed, such as robot control, medical applications, and electronic device control.Finally, we present a prospective outlook on future directions based on the current development status of self-powered HMI using ionic hydrogel-based TENG, aiming to achieve more efficient practical applications.We hope that this review will inspire further innovations in self-powered HMI based on ionic hydrogel-based TENG.

Materials for ionic hydrogel
Hydrogel materials are a type of highly water absorbent polymer material with a distinctive structure [56], which also known as water-aggregating gels or superabsorbent polymers.Hydrogels are typically composed of one or more type of high-molecular-weight polymers [57].These materials possess numerous characteristics and advantages, such as remarkable water-absorbency, excellent water-retention capacity, self-healing properties, biocompatibility, as well as environmental friendliness [58].
Compared to other types of hydrogels, ionic hydrogels exhibit superior water absorption and retention capabilities [59].The polymers within ionic hydrogels form a three-dimensional network structure and porous configuration through cross-linking.These micro pores and channels enable the absorption and retention of a substantial amount of water molecules [60].Additionally, the cross-linked structure of ionic hydrogels typically contains ion exchange sites, which facilitate the exchange of ions with those present in the surrounding water.This electrostatic interaction also enhances the adsorption and retention capacity of water molecules [61].These hydrogels generally exhibit remarkable stability, maintaining their structure and functionality even in lower temperature and humidity conditions [62].Due to their ionic nature, they also demonstrate enhanced conductivity, with the specific conductivity depending on the water content and ion concentration within the hydrogel.Typically, higher water content and ion concentration lead to greater conductivity [63].These unique properties endow ionic hydrogels with broader application prospects, playing a significant role in self-powered HMI such as medical care, robotics, and personal electronics.The diverse performances of various ionic hydrogel-based TENG for self-powered HMI are summarized in table 1.

Synthetic polymers
In the synthesis of hydrogels for TENG-based electronic skin employed in self-powered HMI, synthetic polymers find their extensive applications [79][80][81][82].Firstly, synthetic polymers possess high molecular weight, allowing them to form long-chain structures that provide the hydrogel with excellent mechanical strength to withstand external stress and environmental variations [83][84][85].Secondly, synthetic polymers offer readily tunability, as their molecular weight, molecular weight distribution, and chemical structure can be controlled by selecting appropriate monomers and polymerization conditions [86][87][88][89].This enables the design and synthesis of polymers with specific functionalities and properties, meeting the requirements of ionic hydrogels-based self-powered TENG [90][91][92].Moreover, they can serve as gel stabilizers by forming hydrogen bonds through interactions with water molecules, thereby enhancing the stability and self-healing capacity of the hydrogels [93,94].Additionally, synthetic polymers can act as carriers for electroactive materials, facilitating their stable encapsulation and controlled release, thus ensuring the stability and controlled release of the electroactive substances [95].Synthetic polymers play a crucial role in the ionic hydrogel by carrying and dispersing charge transfer materials, enabling energy conversion and storage functions in ionic hydrogels for self-powered TENG [96][97][98].By controlling the structure and composition of synthetic polymers, the rate and efficiency of charge transfer can be regulated, thereby influencing the performance of the self-powered hydrogel [99,100].Commonly used synthetic polymer materials for the fabrication of TENG-based electronic skin for self-powered HMI include polyvinyl alcohol (PVA), polyacrylic acid (PAA), and polyacrylamide (PAM) [65,[101][102][103][104][105].
PVA exhibits excellent water solubility, gelation ability, high tunability, and biocompatibility, making it an ideal material for ionic hydrogel fabrication [106].It plays a significant role in various applications such as biomedical, tissue engineering, and biomimetic materials [107,108].Inspired by the natural construct principles of cartilage-bone joints, Zhang et al [64] proposed a biomimetic mineralization strategy to achieve strong adhesion between PVA-based ionic hydrogels and different substrates.In figure 2(a), as the hydrogel bonds with the substrate (including glass, aluminum, and polydimethylsiloxane (PDMS)), ions gradually diffuse from the hydrogel into the interfacial region.Cations in the hydrogel react with anions in the substrate, leading to the formation of mineralized nanoparticles (NPs) at the interface.Polymer chains physically adsorb onto the mineralized NPs, creating a structure resembling the mineralized layer in cartilage-bone joints at the interface, thus achieving tight adhesion between the hydrogel and the substrate.As shown in figure 2(b), the adhesion performance at the interface is quantitatively represented.The PVA hydrogel treated with biomimetic mineralization exhibits a shear strength of 700 N m −1 , whereas the control group without interface mineralization only shows a shear strength of 14 N m −1 .The mineralized hydrogel remains adhered to the substrate even when stretched to twice its original length.It is worth noting that the ionically crosslinked hydrogel fractures when stretched to approximately 230%, while the interface of the mineralized hydrogel remains intact, maintaining its adhesion performance.Meanwhile, the non-mineralized adhesive interface detaches between the hydrogel and the substrate at approximately 10% elongation.When utilized in a single-electrode TENG, the strong adhesion between the hydrogel and the elastic substrate enables the generation and output of precise and stable electrical signals.
PAA possesses characteristics such as high water absorption, good solubility, pH responsiveness, and biocompatibility, making it an ideal material for ionic hydrogel fabrication [59,110].In the field of hydrogels, PAA ionic hydrogels find applications in areas such as water-absorbent materials, drug delivery, tissue engineering, and biosensors [111].Sheng et al [109] designed a dual-network (DN) ionic hydrogel based on PAA, which exhibits excellent stretchability (>10 000%), high transparency (over 95%), and good conductivity (0.34 S m −1 ).As shown in figure 2(c), the DN structure is formed by crosslinking the polymer PAA with sodium alginate (SA), resulting in an ionic hydrogel with outstanding stretchability.The interaction between Zn ions in the ionic hydrogel and the DN structure facilitates ion migration, contributing to its good conductivity and stretchability.Figure 2(d) demonstrates the hydrogel's excellent skin conformability and high transparency, with a transmittance exceeding 90% in the visible light range for a 2.0 mm thick ionic hydrogel.Figure 2(e) shows that as the PAA content decreases, the fracture strain and tensile strength of the hydrogel initially increase and then decrease.As a stretchable TENG sensor, the PAA-based ionic hydrogel exhibits excellent tensile strain as high as 15 000% with the optimized PAA ratio.By using the DN ionic hydrogel as an electrode, a single-electrode TENG with an area of 16 cm 2 can harvest energy from human motion, which can power up to 234 commercial LEDs.Furthermore, ionic hydrogel-based TENGs can be used to construct self-powered HMIs for detecting and analyzing human movements.
PAM is suitable for the preparation of ionic hydrogels due to its excellent water solubility, high water absorption, good water retention, and mild gelation conditions [112].These characteristics make PAM hydrogels have a wide range of potential applications in agriculture, environmental science, biomedical, and cosmetic fields [113].Pu et al [66] reported a skin-like TENG (STENG) based on PAM, which utilizes a combination of elastomers and ionic hydrogels as the friction layer and electrode, respectively.It can achieve biomechanical energy harvesting and tactile sensing.As shown in figure 2(f), the STENG is designed with a sandwich-like architecture.Commercial PDMS or very high bond (VHB) elastomers are used as the electrification layer for energy harvesting.When the elastomers are subjected to mechanical forces from other materials, static charge separation occurs, resulting in the generation of electrical outputs.This energy harvesting mechanism utilizes the energy from biomechanical motion or touch, such as human body movements or finger touches.PAM-LiCl ionic hydrogel is employed as the electrode to receive and conduct the collected electrical energy.The ion hydrogel is a material with good conductivity, allowing the transfer of charges from the electrification layer to the external circuit for power supply or storage.In figure 2(g), the average transmittance of a 2 mm thick PAM-LiCl hydrogel electrode is 98.2% in the visible light range.Figure 2(h) shows that the STENG demonstrates remarkable stretchability (1160%) as an energy harvesting device.When slightly tapped by a finger, the human body acts as a contact point for the device, establishing an electrical circuit and providing a potential reference.The finger tapping may induce charge separation or changes in potential difference, thereby activating the circuit to illuminate LEDs.In the experiment, a series of 20 LEDs could be easily lit up.Furthermore, when the STENG generates electricity, it can be stored in capacitors or batteries.Specifically, the device can charge a lithium-ion battery to 3.83 V within 4 h.

Natural polymers
Compared to synthetic polymer materials, natural polymer materials have emerged as one of candidates for the preparation of ion hydrogels due to their unique advantages [95].Typically, natural polymer materials refer to those extracted from various organisms, including microorganisms, animals, and plants [114].For the fabrication of self-powered human-machine interactive electronic skins, natural polymer materials with properties such as biodegradability, ease of access, environmental friendliness, excellent biocompatibility, and low cost have gained increasing research attention in recent years [80,115].So far, two major classes of natural materials, namely proteins and polysaccharides, have been commonly employed in the fabrication of ionic hydrogels.Their applications in the next generation of environmentally friendly wearable electronics have demonstrated significant potential [116,117].Commonly used natural polymer materials for the preparation of hydrogel-based TENG for self-powered human-machine interactive electronic skins include SA, chitin, and gelatin [118][119][120][121].
SA, a water-soluble natural polysaccharide, serves as an ideal choice for the production of hydrogels due to its easy preparation, excellent gelation properties, good water absorption capacity, biocompatibility, and tunability [122].Li et al [67] proposed an SA-enhanced DN ion hydrogel as a TENG electrode material, which can perceive externally applied stress and generate corresponding electrical signals.Additionally, it can harvest energy from the environment, enabling the fabrication of self-powered HMI sensing systems.Figure 3(a) illustrates the schematic diagram of the SA-enhanced ionic hydrogel network.Initially, a one-pot method was employed to prepare the SA-enhanced DN hydrogel, followed by immersing the hydrogel in a water solution of metal chlorides to form the ionic hydrogel.The interaction between SA and metal ions significantly enhances the mechanical strength of the ionic hydrogel.Furthermore, the addition of the inorganic salt LiCl plays two crucial roles.Firstly, the abundant free ions impart high ionic conductivity to the hydrogel.Secondly, LiCl greatly reduces water loss and lowers the freezing point of the hydrogel, ensuring its long-term stable operation even in harsh environments.Figure 3(b) displays the linearly increasing region of the relative resistance of the ionic hydrogel with elongation.And the strain sensitivity, defined as the strain coefficient GF determined by the slope of the linear fitting curve, is calculated to be 4.56.Figure 3(c) demonstrates that the hydrogel sensor responds to external stimuli within 150 ms and requires 100 ms to return to its initial state, making it crucial for TENG electrodes to accurately and rapidly detect vocal cord vibrations.By attaching the DN ionic hydrogel sensor to different parts of the face, it can detect subtle changes in facial expressions.The sensor is also capable of detecting larger deformations caused by limb movements, thus finding applications in gesture recognition, motion monitoring, running posture analysis, VR, and other fields.
Chitin is a polymer composed of glucose molecules linked by β-1,4-glycosidic bonds, which is widely found in many animals such as arthropods as well as some fungi and microorganisms [123,124].It serves as the main component of exoskeletons in crustaceans.Nanochitin (NCT) refers to the nanoscale particles or nanofibers of chitin [125,126].As a candidate for hydrogel synthesis, NCT offers many advantages such as biodegradability, biocompatibility, and water absorption capacity, making it highly promising in fields such as medicine, bioengineering, and drug delivery [127].Jing et al [68] prepared ionic hydrogels by using PAA and NCT.These hydrogels possess a dual-crosslinking network structure comprising both physical and chemical crosslinks.This structure endows the hydrogel with high stretchability, allowing it to withstand significant strain without fracturing.Moreover, the composite ionic hydrogel exhibits good transparency, self-healing ability, and antifreeze properties, which could be utilized to construct durable wearable sensors.Figure 3(d) illustrates the schematic diagram of the PAA-NCT ionic hydrogel.Through functionalization, carboxylated NCT fibers form hydrogen bonds with PAA and form ion coordination bonds with aluminum ions (Al 3+ ), leading to the creation of a dual-crosslinked PAA/NCT composite gel.This composite gel combines various bonds and interactions, including covalent bonds, hydrogen bonds, and ion coordination bonds, resulting in an increase in mechanical strength and self-healing efficiency (97% self-healing rate).Figure 3(e) demonstrates that the transparency of the glycerol-containing chitin-based hydrogel remains almost unchanged after freezing for 2 h at −10 • C, while the transparency decreases by half after storage for 2 h at −20 • C. Nine TENG sensor units based on the NCT hydrogels are assembled into a 3 × 3 square array panel.It is capable of detecting the distribution and velocity of liquid droplets deposited on the panel surface.
Gelatin, composed of collagen proteins extracted from animal connective tissues, possesses properties of good solubility, ease of gelation, high transparency, taste neutrality, and biodegradability as a hydrogel material [128].It finds wide application and numerous advantages in various fields, including the food industry and medical field [129].Wu et al [69] prepared a gelatin-based ionic hydrogel with high ionic conductivity as the working electrode of TENG, which can be applied as a self-powered calculator with a 4 × 4 arrangement of TENG keys. Figure 3(f) depicts the preparation of gelatin-NaCl organic hydrogel (GNOH) with excellent transparency, long-term stability, and high conductivity.By immersing gelatin prehydrogel in a glycerol/water binary solvent, hydrogen bonds and ion interactions are formed between gelatin and glycerol.The GNOH ionic hydrogel exhibits excellent anti-freezing and anti-drying properties, along with remarkable mechanical performance.Figure 3(g) demonstrates the outstanding conductivity and long-term stability of GNOH.When it was stored in ambient air at 70% relative humidity for over 30 d, the conductivity gradually decreases from 1.6 S m −1 to 1.0 S m −1 while maintaining a high level.Additionally, the mass retention rate of GNOH reaches 86%, indicating minimal mass loss during storage.Finally, the GNOH TENG sensors are arranged into a 4 × 4 touch panel array, resulting in a self-powered calculator touch panel.When fingers slightly touch the self-powered touch panel worn on the back of the hand, the calculated equations can be displayed on the connected screen.

Low-dimensional materials
Low-dimensional materials possess numerous characteristics and advantages for the preparation of ionic hydrogels [130].They exhibit higher specific surface area, enhanced conductivity and ion transport, unique mechanical properties, ease of modification, and controllable porous structures [131].These features have drawn significant attention for low-dimensional materials in hydrogel applications related to energy storage, sensors, catalysts, and biomedicine [132,133].Common examples of such materials include carbon nanotubes, graphene, metal nanowires, and MXene materials.Among them, nanowires and MXene materials have found wide applications in the preparation of ionic hydrogels [134][135][136][137].
MXene, with its two-dimensional structure and the presence of both metallic and non-metallic elements such as carbon, exhibits high conductivity, significantly large specific surface area, excellent mechanical performance, and good biocompatibility as a synthetic material for hydrogels [138].Benefitting from their biocompatible nature, MXene-based hydrogels can be utilized in tissue engineering, cell culture, drug delivery, and other medical applications [139].In general, these advantages position MXene as a promising synthetic material for ionic hydrogels in fields like energy, environment, biomedical science, and electronic devices [140,141].Wang et al [70] developed a MXene-based TENG for constructing self-powered sensing systems to monitor marine environments.Figure 4(a) illustrates the MXene-based ionic hydrogel as an electrode material for the TENG employed in the self-powered HMI system.The sensor signals from this system are transmitted in real-time to the signal processing unit via Bluetooth, allowing real-time acquisition of marine information such as ocean temperature, SO 2 concentration, and water quality.demonstrates the fast response/recovery time (43/62 s) of the prepared TENG sensor, which could enable real-time and quick monitoring of different gas concentrations in seawater.
Nanowires exhibit excellent conductivity and thermal properties, making them widely applicable in the fields of electronics and optoelectronic devices [142].They can also be used for manufacturing flexible electronic devices, transparent conductive films, flexible touch screens, and flexible displays [143].Among various nanowires, silver nanowires (AgNWs) can serve as conductive inks for printing flexible circuits and sensors [144].Furthermore, AgNWs possess antibacterial properties, effectively inhibiting the growth of bacteria and other microorganisms [145].This potential makes AgNWs valuable in medical applications and disinfection products, such as manufacturing antibacterial dressings, coatings, and disinfectants [146,147].Wang and Daoud [71] developed an ion-conductive hydrogel based on AgNWs cross-linked with chitosan (CS), serving as an electrode material for a single-electrode TENG.The AgNWs hydrogel-based TENG for energy harvesting exhibited transparency as high as 95%.After washing with deionized water, the TENG maintained constant output voltage and current density, and its performance could be restored by direct washing after dehydration.Figure 4(d) presents the synthesis process of the hydrogel network and photographs of the hydrogel.AgNWs-CS was coated on a glass slide with a concentration gradient, then AgNO 3 or CuSO 4 solution was added dropwise.The metal ions underwent coordination reactions with chitosan functional groups, resulting in a freestanding, flexible, and transparent hydrogel.Figure 4(e) depicts the construction of a sandwich-structured TENG device, where the contact-separation process between human skin and the PDMS layer generates static charge movements.The AgNWs-CS ionic hydrogel acts as an electrode material in the single-electrode working mode of the TENG.The AgNWs-CS ionic hydrogel-based TENG can effectively harvest mechanical energy from human body movements.By consistently tapping the TENG with a force of approximately 7 N, it can generate a maximum output voltage of 175 V.
Cellulose nanocrystals (CNCs), composed of nanoscale cellulose molecules arranged in a crystalline structure, exhibit highly ordered and periodic arrangements with long-range parallel alignment [148].These properties make CNC a highly appealing nanomaterial widely used in nanocomposites, nanocoatings, nanoelectronic devices, and biomedical materials [149].As a synthesis material for hydrogels, CNC also possesses several advantages, including high water absorption capacity, excellent gel mechanical properties, biodegradability, and biocompatibility [150].To enhance the robustness and stretchability of ionic hydrogels, Wang et al [72] proposed a CNC-based composite hydrogel with improved conductivity, mechanical performance, and frost resistance.Figure 4(f) demonstrates the schematic illustration of the preparation for CNC-based ionic hydrogel, where salt-permeated nanocellulose composite hydrogel was synthesized through free radical polymerization, utilizing the electrostatic interaction between salt ions and CNC as well as PAA. Figure 4(g) exhibits the good stretchability of CNC-based ionic hydrogel (2600%).Figure 4(h) reveals that the mechanical properties of the hydrogel remain nearly unchanged even after multiple stretching cycles.As an electrode material for the construction of TENG, the obtained self-powered sensor can be used to differentiate subtle changes occurring in vocal fold vibrations, as well as for detecting electromyography and electrocardiography signals.

Working principles of ionic hydrogel-based TENG
TENG is a crucial device for converting mechanical energy into electrical energy [151].The electrode material used in TENG plays a pivotal role in achieving enhanced energy conversion efficiency, stability in diverse environments, prolonged lifespan, and reduced fabrication costs [152].Therefore, the electrode material should possess the following characteristics: exceptional triboelectric properties, excellent conductivity, wear resistance, durability, mechanical flexibility, stability, and ease of preparation [153].Commonly used TENG electrode materials include, but are not limited to, metals such as aluminum and copper which exhibit excellent conductivity.Additionally, conductive polymers like polyaniline and PEDOT:PSS, as well as composite materials incorporating Ag NPs, graphene, and carbon nanotubes are employed as electrode materials [154].They not only possess favorable conductivity but also exhibit a certain level of mechanical flexibility [155].
Hydrogel, as an electrode material in TENG, possesses several unique advantages, making it an ideal choice for certain wearable applications [156].Hydrogels exhibit excellent tensile strength and deformability, allowing them to adapt to complex friction interfaces and generate displacement and deformation with minimal force.The properties of hydrogels can be tailored by adjusting their composition and structure to meet specific application demands.Doping conductive materials such as carbon nanotubes or metal NPs can enhance their conductivity, effectively collecting and conducting the charges generated through friction.Hydrogels can form porous structures, increasing their surface area, which improves the contact area with the friction material and consequently enhances energy harvesting efficiency.Typically composed of water and high-molecular-weight polymers, hydrogels are free from toxic substances, displaying excellent biocompatibility.This eco-friendliness makes hydrogel an attractive electrode material, finding extensive applications in biomedical fields and beyond.Furthermore, the simple preparation process and easy availability of raw materials enable hydrogel as an economically practical electrode in self-powered TENGs.
The TENG is fabricated by stacking two polymer films with different frictional electrical properties [157].Metal films are deposited on the top and bottom of the assembled structure to act as electrodes.Upon mechanical deformation, due to nanoscale surface roughness and different ability to attract electrons between each polymer, friction between the two thin polymer films generates equal but opposite charges on both sides.Thus alternating currents can be generated and conducted by electrodes to external circuits.TENG is a simple, cost-effective, and easily scalable manufacturing device that can convert random mechanical energy from our living environment into electricity using conventional flexible polymer materials [158].This technology holds enormous potential for driving environmental monitoring systems, personal healthcare networks, electronic emergency devices, and other self-powered systems used in mobile and personal electronics [159].Based on electrode configuration and frictional layer motion patterns, the TENG can be classified into four fundamental modes: vertical contact-separation mode, horizontal sliding mode, single-electrode mode, and independent-layer mode [32].The commonly used working modes for ionic hydrogel-based TENG mainly are usually the vertical contact-separation mode and single-electrode mode.

Vertical contact-separation mode
The vertical contact-separation mode of TENG operates based on the vertical contact and separation motion of the frictional layers and back electrodes.In the vertical contact-separation mode, the frictional layers and back electrodes are typically designed as parallel plate structures that undergo relative movements [160].During the contact process, the materials of the frictional layer and back electrode come into contact, resulting in the phenomenon of electrostatic charge separation.As the frictional layer and back electrode separate, charges are separated, creating a potential difference between them [161].During the separation process, charges move between the frictional layer and the back electrode due to the potential difference caused by electrostatic charge separation, generating current through an external circuit.This process can be repeated through the periodic contact and separation motion of the frictional layer and back electrode, resulting in the output of alternating current [162].Using PAM-SA (PAAm-alginate) ionic hydrogel as the electrode material, Liu et al [163] developed a double-electrode TENG based on the contact-separation mode.Figure 5(a) illustrates the structure of a TENG working in the contact-separation mode.While figure 5(b) demonstrates the high flexibility of TENG, enabling it to conform to curved surfaces such as the skin, making it a promising candidate for wearable HMI sensors.By pressing the double-electrode TENG with fingers, the PDMS and Ecoflex in the composite structure acquire negative and positive charges, respectively, generating a potential difference.When the external force is removed, the object that undergoes elastic deformation returns to its original shape, causing the two charged surfaces to separate.Simultaneously, due to the presence of the potential difference, induced electrons move from the ionic hydrogel electrode to flow through the external circuit.The vertical contact-separation mode offers advantages such as ease of manufacturing and adjustment, as well as the ability to achieve high-frequency contact and separation motion.TENG based on ionic hydrogel can convert mechanical energy into electricity and store it in batteries to power electronic devices.Pressing the ionic hydrogel-based TENG at a frequency of 1 Hz allows for charging capacitors, with the capacitor voltage increasing to 5 V within 150 s, providing short-term power supply for a wristwatch.

Single-electrode mode
Different from the double-electrode TENG of the vertical contact-separation mode, the single-electrode mode operates with only one electrode [165].In the single-electrode mode of TENG, the frictional layer is usually designed as a continuous structure divided into two parts: the friction region and the collection region.The friction region is responsible for friction or mechanical deformation with external objects, while the collection region is used to collect the charges generated during the friction or deformation process [166].During friction or mechanical deformation, the material in the friction region undergoes charge transfer, resulting in a potential difference within the frictional layer.This potential difference drives the charges to move from the frictional layer, ultimately being collected by the electrode in the collection region [167].As there is only one electrode, these charges undergo periodic movement and accumulation between the frictional layer and the ground, generating alternating current in the external circuits.The single-electrode mode TENG features a simple structure and operation mechanism, reducing the complexity of materials and circuits.Chen et al [73] first developed a single-electrode TENG using a hybrid ultraviolet 3D printing technique.Figure 5(d) schematically illustrates the structure of the hybrid 3D printing system.In addition to the utilization of photocurable resin and PAAm-LiCl ionic hydrogel, other types of materials such as metal particles, ceramic powders, or fiber-reinforced materials were incorporated.Figure 5(e) shows a typical working state of the single-electrode mode TENG, with the resin component serving as the frictional charging layer and the ionic hydrogel as the electrode for the charge collection layer.Based on the 3D-printed single-electrode TENG, a self-powered LED lighting shoe has been developed to realize illumination upon running or walking for wearers.In order to harvest human motion energy even at extremely low temperatures, Liu's group [164] has developed a single-electrode TENG by using an ionic hydrogel doped with glycerol and SA as the electrode material.Figure 5(f) presents a schematic diagram of the ionic hydrogel-based TENG (iTENG) that possesses a sandwich structure.An Ecoflex film serves as the frictional charging layer, while the antifreeze ionic hydrogel acts as the electrode material sandwiched between the Ecoflex layers.The iTENG demonstrates excellent antifreeze properties that it is capable of harvesting biomechanical energy even at −53 • C. The operational principle of the single-electrode iTENG is depicted in figure 5(g).The iTENG exhibits good conductivity both at room temperature and under extremely low temperature conditions.At room temperature, a gentle tap on the iTENG can easily illuminate 30 series-connected LED lights.While at −20 • C, a gentle tap on the iTENG can linearly charge a capacitor to 3 V within 204 s, thus providing a continuous power source for a timepiece.

Robot control
The ionic hydrogel-based self-powered HMI based on the TENG effect serves as an effective means of information exchange between humans and computers [168].It acquires signals of human body movements from the user, which can be electromyographic signals generated by muscle movements or other signals related to body actions, such as pressure and deformation [169].By integrating the ionic hydrogel-based TENG with corresponding signal processing circuits and communication interfaces, it enables the transmission and interaction of signals from the human body to the machine, converting them into recognizable and responsive commands for the machine.This interactive approach is more intuitive, natural, and innovative, facilitating convenient and efficient communication between humans and machines [170].
In order to achieve efficient energy harvesting, enable multi-channel signal acquisition, and facilitate robotic arm control, Zhang et al [55] reported an ionic hydrogel (named PTSM hydrogel) prepared from PAM, tannic acid (TA), SA, and MXene, which was used to construct a machine learning-assisted HMI system.Figure 6(a) depicts the PTSM hydrogel fabricated through a one-pot synthesis of chemically crosslinked PAM/tannic acid/SA/MXene double network (DN) hydrogel.Based on the PTSM ionic hydrogel, a PTSM-TENG was constructed and further employed in an HMI gesture recognition system which could display hand postures on the monitor, as shown in figure 6(b).Figure 6(c) shows the optical images of the HMI system which are connected to a robotic hand via Bluetooth.The PTSM-TENGs sensors attached to human fingers can detect and transmit hand motion information into electrical signals, and the machine learning-assisted HMI system processed the signals and recognizes gestures, then action commands was sent to the robotic hand to perform corresponding identical gestures.Due to the complexity of underwater environments, direct manipulation of objects by conventional robots is challenging.
To develop TENG sensors capable of interaction for underwater soft robot manipulation, Qu et al [74] developed a TENG sensor capable of facilitating underwater soft robot operations.They employed a one-pot synthesis method to fabricate an ionic hydrogel named PXGN, composed mainly of PVA, xanthan gum (XG), glycerol (GL), and NaCl, which exhibited high fracture toughness (146.5 kJ m −3 ).This single-electrode TENG sensor can monitor human body motion and realize control of underwater soft robotic hands based on human motions.Figure 6(d) demonstrates the real-time grasping detection system of the developed underwater soft robotic hand, which is capable of underwater intelligent perception and real-time control.This ability is essential for safe and stable grasping during human-assisted robotic operations in underwater environments.Figure 6(e) shows that when the soft robotic hand grips an object, the feedback strain data can help determine the size and grasping status of the object.In practical underwater object manipulation processes, unexpected interference, slippage, or collision may cause grasping failure or damage to the gripper.Therefore, three TENG sensors based on ionic hydrogels are integrated as an array into the concave grooves on the inner surface of the gripper, which could ensure safe and stable grasping.
To fabricate a touch hydrogel sensor (THS) capable of detecting subtle human body movements and exhibiting long-term working capability and dehydration resistance in harsh environments from −20 • C to 60 • C, Tao et al [75] reported a DN PAM-KCl ionic hydrogel.The self-powered THS exhibits remarkable flexibility, good transparency, and excellent sensing performance even under extreme conditions.The one-pot preparation method for the DN PAM-KCl ionic hydrogel is presented in figure 6(f).Overall, the TENGs fabricated using ionic hydrogels hold immense potential and value in the application of self-powered HMI systems, particularly in relation to robotic arm control.This technology allows users to directly interact with devices through gestures or subtle movements, eliminating the need for traditional buttons or touchscreens.It enhances the convenience of user experience while improving the accuracy of operations and the efficiency of human-robot collaboration.The introduction of this technology opens up new opportunities for the development of robotic arm control systems and contributes to the advancement of autonomous robotics and industrial automation.

Medical applications
In addition to robotic control, the ionic hydrogel-based self-powered HMI also finds extensive applications in the medical field [171].In comparison to traditional complex medical instruments, the ionic hydrogel-based TENG for self-powered HMI offers greater portability and convenience for monitoring patients' vital signs such as heart rate, blood pressure, and body temperature [172].These self-powered HMIs can be integrated with medical sensors and devices, eliminating the need for external power sources and enabling real-time health monitoring and data transmission [173][174][175].Ionic hydrogel-based TENG for self-powered HMIs can be used in medical imaging devices such as x-ray machines and ultrasound equipment.They provide convenient operation and observation by incorporating touchscreens or gesture controls and displaying medical images in real time.Additionally, ionic hydrogel-based TENG for self-powered HMIs can be employed in rehabilitation therapy devices, such as electronic prosthetics, electric wheelchairs, and control panels for medical instruments like surgical tools and drug delivery devices [176,177].These application examples demonstrate the great potential of ionic hydrogel-based self-powered HMIs in the medical field [178].This technology provides medical devices with self-powering capabilities, reducing reliance on external power sources and enhancing device portability and flexibility.It opens up new possibilities for medical monitoring, treatment, and operations [179,180].
In order to meet the ever-growing demands for elderly care and enhance healthcare efficiency, Yang et al [76] developed a PAAm-Cu ionic hydrogel-based TENG with high mechanical strength, conductivity, and transparency.On the basis of the TENG, they designed an HMI system for smart aging.Figures 7(a) and (b) illustrate the ionic hydrogel-based TENG used to recognize finger movements, where greater bending angles of the fingers result in larger output current values from the ionic hydrogel-based TENG sensors.When the fingers bend at angles of 30 • , 60 • , and 90 • , the corresponding output voltages are approximately 2.5 V, 4.6 V, and 6.8 V, respectively.The detailed application of the HMI smart aging system is presented in figure 7(c), it can be seen that with the aid of the HMI system, elderly individuals can communicate their needs in real time by bending specific fingers at designated angles.To treat skin diseases, achieve self-powered electrical stimulation for drug delivery, and report real-time drug release state, Wang et al [181] developed a multifunctional triboelectric micro-needle (MN) hydrogel patch with functionalities required for skin disease treatment.Figure 7(d) schematically presents the triboelectric MN ionic hydrogel used for skin disease treatment.The MN efficiently, minimally, and invasively administers drugs to the skin, including organic compounds and drug-protein NP hybrids.Figure 7(e) demonstrates the drug encapsulation at the tip of the MN, allowing uniform application and quantitative control of drugs for skin disease treatment through the triboelectric effect.Figure 7(f) illustrates the working process of the MN patch, demonstrating the exceptional mechanical strength of the nanoneedle tips, which remain unbent even after multiple pressing actions.Moreover, friction and pressure promote blood vessel regeneration at the site of skin lesions, positively benefiting the treatment of skin diseases.Assessment of muscle functions in the elderly plays a crucial role in understanding and evaluating their muscle status, functional level, and age-related muscle degradation.In order to provide essential information for evaluating muscle conditions, assessing physical abilities, and preventing falls, Wang et al [50] reported a scalable, self-healing, and skin-adhesive (Triple S) active sensor (TSAS).It is made of a PVA-LiCl ionic hydrogel-based TENG sensor capable of evaluating muscle function and joint flexibility in the human body.The seven-layer composite structure of the TSAS is shown in figure 7(g), wherein PVA-LiCl ionic hydrogel serves as the electrode, silicon rubber and polystyrene-c act as the triboelectric friction layers.Figure 7(h) shows in detail how the TSAS converts mechanical signals from arm muscle movements into electrical signals.These processes enable an understanding of muscle decline or recovery status and can provide objective support for clinical treatments.
In general, TENGs fabricated from ionic hydrogels have significant importance and potential in self-powered HMI systems specifically related to medical applications.This innovative technology harvests and converts mechanical energy from human activities to provide an autonomous power supply for health monitoring devices.Moreover, it offers more convenient and flexible ways for medical HMI systems, enhancing the interactive experience between patients and medical devices.The successful application of this technology will drive the development of healthcare, health monitoring, and personal care domains, providing reliable and convenient solutions for real-time health monitoring and remote healthcare.

Electronic device control
Based on the triboelectric effect of the TENG, ionic hydrogel-based self-powered HMIs can also find extensive applications in electronic device control.There are some potential examples: gesture control, pressure detection, biometrics, and energy harvesting [182].These applications exemplify the utilization of the ionic hydrogel-based self-powered HMI in electronic device control.With further advancements in this technology, we can anticipate a multitude of innovative applications across various fields.
To minimize the damage to human skin caused by epidermal electronics and enable the on-demand detachment and repeated use of ionic hydrogel-based TENG sensors, Gao et al [54] fabricated an optically transparent and mechanically compliant debonding-on-demand TENG (DoD-TENG) based on PAAm-LiCl hydrogel.They developed a HMI for a DoD-TENG unmanned aerial vehicle (UAV) system, enabling finger gestures to control the UAV's motion state.Figure 8(a) presents a wearable drone navigator integrated with four DoD-TENG devices, providing navigation commands for commercial drones.Figure 8(b) demonstrates that each DoD-TENG generates distinct electrical signals when performing arbitrary gestures, allowing real-time transmission of control signals to the UAV, and instructing its flight actions.To enhance the transparency of the ionic hydrogel and enable rapid self-healing under ambient conditions, Lai et al [77] demonstrated, for the first time, the intrinsic autonomous and self-healing properties of the frictional charging layer and electrode of the TENG.They also showed the functionality of the healed electronic skin as a directional control touchpad for playing video games.Figure 8(c) displays the self-powered sensor prepared using EHTS-TENG.By attaching the EHTS-TENG-based sensor to the curved surface of the skin, a simple HMI was employed to play computer games.Figure 8(d) presents the voltage output under different contact pressures after stretching the EHTS-TENG by 25% and subsequent self-healing, showing a negligible impact on the sensing performance compared to the original EHTS-TENG.Given the limited lifetime, high replacement costs, and environmental pollution associated with traditional battery power sources, there is a need to address the issues for continuously powering wearable intelligent sensors.Zhang et al [78] prepared an ionic hydrogel composed mainly of SA, acrylamide (AAm), and NaCl.This ionic hydrogel served as the electrode material for a stretchable fiber-based TENG (SF-TENG).Based on the SF-TENG, they designed a remote HMI system for controlling a model car.Figure 8(e) illustrates a conceptual diagram of the self-powered HMI system based on SF-TENG.Different gestures resulted in varying output voltages from the SF-TENG sensor array, which were received by the control board.These electrical signals could be logically mapped to remotely control some electronic devices.As depicted in figure 8(f), five SF-TENG sensors were integrated into a smart glove to monitor gesture changes.When predefined gestures, such as forward and left turn were performed, the model car could move in accordance with hand gestures.
The utilization of TENGs fabricated from ionic hydrogels holds significant importance in self-powered HMI systems for electronic device control.The facile preparation and high energy conversion efficiency of ionic hydrogel-based TENGs enable seamless integration with various electronic devices, facilitating wireless and autonomous human-machine interaction.Furthermore, the application of hydrogel-based TENGs in device-controlled HMIs offers flexibility and scalability, allowing adaptation to diverse application scenarios.The successful implementation of TENGs based on ionic hydrogels opens up novel possibilities for future smart living and industrial automation, presenting unprecedented opportunities in the field of self-powered human-machine interactions.

Other applications
Based on the TENG principle, the ionic hydrogel-based self-powered human-machine interactions also have many other application scenarios [183].They can be employed as touch screen sensor for interacting with computers, mobile communication devices, and other digital equipment.When combined with VR and augmented reality (AR) technologies, the self-powered HMI can capture body movements and facial expressions, enabling the transmission of information from the human body to the machine to facilitate an immersive interaction within virtual environments [184].Moreover, they can be used for non-contact gesture recognition, allowing users to perform gestures in the air without the need for touching screens or physical objects.These other applications including VR control, non-contact gesture recognition, and touch screen sensor further enhance the freedom and flexibility of user experience for self-powered HMIs.
In the pursuit of developing a highly stretchable and durable wearable ionic hydrogel-based TENG, Kim's group [52] successfully prepared a resilient, highly stretchable, and flexible ionic hydrogel based on PAAm-LiCl.This wearable hydrogel-based TENG sensor exhibits remarkable sensitivity in recognizing human body movements.Based on the PAAm-LiCl TENG, they further designed a self-powered wearable HMI system for controlling VR games.Figure 9(a) illustrates the schematic of VR operation.The TENG keyboard sensor first generates corresponding electrical signals.These signals are further processed by a well-designed circuit followed by transmitting through serial communication for response.On a desktop computer, custom Python scripts are employed to read the serial input and interact with the desktop operating system, enabling real-time transmission of commands to the computer for VR interaction.Figure 9(b) shows the successful implementation of VR game control using this self-powered HMI system.This breakthrough in developing a high-performance wearable ionic hydrogel-based TENG system opens up new possibilities for intuitive and immersive interactions in VR environments.
Inspired by the ability of sharks to use electroreceptive sensing systems for remote environment perception, Zhonglin Guo et al [53] developed a non-contact gesture sensor based on PAAm-LiCl ionic hydrogel TENG to create a non-contact sensing system for gesture and object surface 3D contour recognition.With the assistance of machine learning algorithms, they designed an HMI system utilizing this non-contact inductive sensor for non-contact gesture recognition and object surface 3D contour recognition.Figure 9(c) illustrates the fabrication of the non-contact gesture sensor by using a single-electrode TENG mode.PAAm-LiCl ionic hydrogel serves as the electrode material, while PDMS acts as the encapsulation substrate.Figure 9(d) demonstrates the non-contact sensor being used to play the game 'Super Mario' .Users can control the character's movements by sliding or tapping above the non-contact gesture sensor without direct physical contact.Figure 9(e) shows the implementation of non-contact 3D contour recognition using deep learning convolutional neural network models.By increasing the number of non-contact sensor units to 441, a non-contact object recognition array was designed.The system was then employed to perform three-dimensional recognition tasks for geometric configurations like spheres, cones, ellipsoids, and human faces.
To address the fragility and vulnerability of traditional rigid touchpads, Guo et al [51] developed an anti-freeze and degradable TENG (AD-TENG) based on PAA-LiCl ionic hydrogel.This novel AD-TENG exhibits high sensitivity, stretchability, self-healing capability, low-temperature resistance, and excellent biocompatibility.Based on the AD-TENG, they designed an HMI system for an ionic hydrogel-based touch screen sensor.This flexible touch screen sensor could adhere to any complex surface and enable various operations such as writing and gaming.Figure 9(f) demonstrates the schematic operation process of the AD-TENG touch screen sensor.The transparent touch sensor was adhered to a user's arm, allowing continuous finger movements on the touch screen sensor to control the computer for various operations like playing chess game. Figure 9(g) presents the principle diagram of the touch screen sensor.When a finger touches the touchpad, a potential difference is generated between the electrode and the touch position.The touch position can be determined by measuring the magnitude of the potential difference along the edge of the touchpad.Figure 9(h) presents the image of the touchpad controlling an international chess game.The development of the AD-TENG touch screen sensor represents a significant advancement in flexible, sensitive, and user-friendly wearable HMI.
In general, ionic hydrogel-based self-powered TENGs have brought about many more advanced, convenient, and energy-efficient solutions for HMI applications such as touchscreen sensors, VR and AR, and non-contact gesture recognition.The introduction of self-powered ionic hydrogel-based TENGs allows users to achieve on-skin seamless interaction with the device through light taps, swipes, or other gestures, providing a whole new touch experience.For VR, capturing users' movements and gestures is essential to achieve immersive interaction within virtual environments.The high sensitivity of self-powered TENGs enable users to realize non-contact gesture recognition and control smart electronic devices or browse web information through hand gestures in the air, eliminating the need to touch physical objects and significantly increasing the freedom of user experience.The introduction of self-powered TENG sensors has brought greater convenience, flexibility, and interactivity to users in diverse HMI applications.

Conclusions and future perspectives
This paper systematically reviews the research progress of the ionic hydrogel-based TENG in the field of self-powered HMIs.By employing ionic hydrogels as key materials, this TENG not only autonomously collects and supplies energy, providing a persistent and stable energy source for HMI devices, but also serves as an interface for human-machine interaction, transforming human behavior, intentions, and demands into machine-understandable electrical signals, facilitating effective communication and control between humans and machines.In this paper, we first introduce the types of hydrogel materials, including synthetic polymers, natural polymers, as well as low-dimensional materials.And then the working mechanisms of the ionic hydrogel-based TENG are thoroughly discussed.Furthermore, we review practical application cases of ionic hydrogel-based TENG in the field of self-powered HMIs, encompassing robot control, medical applications, and electronic device control.Our review demonstrates the tremendous potential of TENGs based on ionic hydrogels in energy harvesting and HMIs.
Despite the encouraging progress achieved by TENGs using ionic hydrogels, several challenges and future research directions remain to be solved.Firstly, it is imperative to further enhance the performance and stability of ionic hydrogel materials.Improving the stability of hydrogels primarily involves enhancing their water-retaining capacity for maintaining the hydrogels' long-term conductivity in ambient environment, and elevating their performance in terms of conductivity and mechanical strength.Additionally, in-depth research is required concerning the interaction and biocompatibility between the nanogenerator and the human body, ensuring reliability and safety in HMI applications.In future studies, we can also explore broader application domains for the ionic hydrogel-based TENGs.For example, they can play a significant role in brain-machine interfaces, biosensing, and environmental monitoring.Furthermore, we can optimize the design and structure of the nanogenerator to enhance its energy conversion efficiency and output power.
Overall, ionic hydrogel-based TENGs hold immense potential in self-powered HMI technology.Through continuous research and innovation, we hope to achieve more efficient, stable, and sustainable energy harvesting, thus opening up new prospects for the development of HMI technology.
Figure 4(b) demonstrates that the mass and conductivity of the MXene-based ionic hydrogel TENG remain almost unchanged within 21 d, indicating excellent water retention capability of the device.Figure 4(c)

Figure 5 .
Figure 5.The working mechanisms of ionic hydrogel-based TENG for self-powered HMI.(a)-(c) Vertical contact-separation mode of triboelectric effect [163].(d), (e) Schematic preparation diagram and working principle of the 3D printing single-electrode TENG [73].(f)-(g) The schematic illustration and working mechanism of the single-electrode iTENG with excellent frost resistance ability [164].
Figure 6(h) displays the current signals obtained from the THS sensors when different fingers are flexed, indicating the outstanding sensitivity of the THS.And figure 6(g) demonstrates the different human hand gestures and the motion response of the robotic hand in accordance with human gestures.

Table 1 .
Performances of various ionic hydrogel-based TENG for self-powered HMI.