Recent Advances in Hydrogel‐Based Self‐Powered Artificial Skins for Human–Machine Interfaces

With the rapid development of artificial intelligence, human‐machine interfaces (HMI) are continuing to affect human lifestyles. Artificial skin is a new type of HMI sensor that enables a seamless connection between human and electronic devices. Currently, artificial skin is mostly prepared from rigid materials, which lack flexibility and scalability, thus impeding the development of HMI. Hydrogel, consisting of 3D polymer network and water is similar to human tissues and is therefore an excellent candidate for artificial skin in HMI. The conventional HMI of hydrogel‐based artificial skin includes touch pad and machine control based on capacitive or resistive sensors. However, the energy supply of HMI depends on battery which requires frequent recharging and replacement. Therefore, hydrogel‐based self‐powered artificial skin is expected to become primary interaction medium for the forthcoming generation of HMI. This article reviews the development of hydrogel‐based self‐powered artificial skin for HMI. Various power supply mechanisms of hydrogel‐based artificial skin are discussed. The materials for self‐powered artificial skin are introduced, including ionic hydrogel, ionic‐liquid hydrogel, metal‐based hydrogel, carbon‐based and MXene‐based hydrogel, and conductive polymer‐based hydrogel. The application of the hydrogel‐based self‐powered artificial skin in HMI is also reviewed. Finally, the challenges and development trends in HMI are outlined.


Introduction
[3] It can be processed into different forms and applied to irregular surfaces like clothing, enabling the machine to perceive the location, orientation, and hardness of objects. [4][7] By attaching artificial skin to a robot's fingers and arms, the robot can acquire the ability to sense external touch force,similar to that of human skin. [8]oreover,connecting artificial skin to the Internet of Things allows machines to be controlled in a variety of ways.[11] Through the interaction system, humans can observe and interact with their surroundings, enabling a natural and seamless connection with the environment.This enables the integration of human intelligence and abilities into intelligent machines via the interaction system. [9,12,13]However, conventional highperformance rigid inorganic semiconductor materials used in HMI often make it challenging to achieve seamless connection with human skin, leading to a suboptimal user experience, and hindering effective communication between humans and machines.Additionally, HMI typically employs resistive and capacitive sensors, requiring frequent charging and replacement, which affects the continuity and persistence of sensing signals.Therefore, it is necessary to develop new self-powered flexible artificial skin sensors for better user experience in HMI applications.
[16][17] These sensors obtained from rigid materials are prone to damage during use, making it difficult to satisfy the flexibility and stretchability prerequisites of artificial skin, thereby hindering the development of HMI.On the other hand, hydrogels made of 3D polymer networks with a certain amount of water have more similarity to human tissues and organs, making them safe for contact with human skin.Therefore, hydrogels have attracted significant attention due to their low elastic modulus, transparency, extensibility, self-healing ability, and biocompatibility.They have been considered better candidates for application as sensors in selfpowered artificial skin, [18][19][20] which are expected to be directly employed in the field of HMI.[23] For touch panels, stretchable and biocompatible sensing materials are required for easy integration with the human body. [24,25]Sun et al. [26] demonstrated an ion touch panel by using polyacrylamide (PAAm) hydrogel that contained lithium salt.Because of the hydrogel's excellent stretchability and transparency, the ion touch panel exhibited strong tensile properties, and its visible light transmittance was as high as 98%.The touch pad could be used to draw simple strokes, transfer data to the computer screen, or play various computer games.To increase the extensibility and achieve self-heal ablility of touch panels, Wang et al. [27] developed a polyampholytic-clay nanocomposite-based hydrogel that acted as a soft, stretchable, and transparent ionic conductor, which had a transmissivity of 98.8% and a fracture strain exceeding 1500%.It could be utilized as a self-adhesive and self-healing HMI touch panel that adopted the surface capacitive sensing mechanism to perceive touch position.Then the finger position could be sensed during both pointby-point touch and continuous movement.The hydrogel-based touch pad could be adhered to both curved and flat insulators, and exhibited high-resolution and self-healing capabilities for activities such as painting, writing, and playing video games.
Gesture recognition is generally utilized for machine control.Wan et al. [28] developed a MXene-based hydrogel multifunctional skin sensor with self-healing, injectable, and antibacterial properties.This was achieved by introducing MXene nanosheets modified with Ag nanoparticles into the polymer network of guar gum and phenylboric acid grafted sodium alginate (SA) for intelligent HMI and gesture recognition.Due to the fact that most hydrogels tend to freeze in severe cold environments, accompanied by the loss of some mechanical and electrical properties, and low sensitivity, which greatly limits the application of hydrogel-based artificial skin in HMI.To address these problems, Wang et al. [29] optimized hydrogel properties including strong adhesion, electrical conductivity, and frost resistance by introducing adhesive photo-curing monomers and materials.The structural design of hydrogel was used to improve the strain sensitivity for motion signal monitoring and achieve a wide range of motion sensing.By designing 3D printed flexible hydrogel electrodes to collect the electromyogram (EMG) signals from the human body, the EMG signals could be used as the user interface to control the synchronous movement of the manipulator even under the low temperature of À80 °C.The introduction of microscale 3D printing technology enabled the rapid preparation of complex 3D structures for multifunctional flexible electronics and complex HMI.However, these hydrogel-based artificial skins rely on external power sources, which hinders further development of hydrogel-based artificial skin in the field of HMI.Self-powered HMI, which generates electricity from the surrounding environment or from the human body itself, could enable continuous monitoring and analysis of various physiological parameters without frequent battery replacements or recharging, potentially revolutionizing the way we interact with the surrounding environment.
In this article, we review the recent development of hydrogelbased self-powered artificial skin for HMI.As illustrated in Figure 1, we present the basic working mechanism of hydrogel-based artificial skin based on the triboelectric effect, piezoelectric effect, and thermoelectric effect, respectively.We then classify hydrogels into five categories, which include ionic, ionic liquid, metal-based, carbon-based, and MXene-based, conductive polymer-based hydrogels, and summarize the features and functions of hydrogel-based self-powered artificial skins.Furthermore, we discuss the diverse applications of hydrogelbased self-powered artificial skin in HMI fields including intelligent control, medical care, and touch screens.Finally, we assess the challenges facing hydrogel-based artificial skin in the field of HMI and provide our perspective on future research trends.

Self-Powered Working Mechanism of Hydrogel-Based Artificial Skins
The power consumption of an artificial skin system mainly comes from the functional devices including various sensors.These sensors are known for their high-energy consumption, making them a major obstacle in the design of artificial skin.Researchers have conducted numerous studies to reduce the power consumption of sensors, such as designing appropriate impedance and operating voltage for piezoresistive sensors, and developing thin dielectric materials with higher dielectric constant for capacitive sensors. [30,31]However, these studies have failed to fundamentally address the issue of artificial skin power consumption.The inflexibility, inelasticity, and environmental pollution of conventional batteries make them unsuitable for use in flexible artificial skin systems. [32]Although flexible and stretchable solar cells are more adaptable to electronic skin systemsin recent years, they suffer from structural complexity and require regular maintenance. [33]Currently, the most promising approach to addressing power supply problems for artificial skin is the self-powered sensing mechanism, which integrates energy harvesting technology and sensing functions.On one hand, it could collect energy from the ambient environment, especially the energy generated by human movement and thermal radiation.On the other hand, it could also act as an active sensor that responds to environmental changes in the form of electrical signals (e.g.voltage and current).[36] Currently, the main self-powered technologies used for hydrogel-based artificial skin are the triboelectric effect, piezoelectric effect, and thermoelectric effect.

Triboelectric Effect
The triboelectric effect of hydrogel-based artificial skin is a coupled power generation through two effects of contact initiation and electrostatic induction. [37]Friction electrification is a special phenomenon associated with contact electrification.The triboelectric nanogenerator (TENG) is a special contact initiation phenomenon, where the contact or friction of different materials generates charges on the surface.The triboelectricity generated from the inner surface of the dielectric could be retained for a long time and used as an induction source during the power generation process.Driven by mechanical motion, relative positions of the frictionally charged surface in the hydrogel artificial skin would change periodically, resulting in periodic changes of the induced potential difference between different electrodes.For maintaining the electrostatic balance between different electrodes, free electrons flow back and forth between the electrodes to shield the induced potential difference, thus completing the energy conversion process from mechanical energy to electrical energy. [38]Based on different electrode configurations and friction layer movement modes, the TENG can be categorized into four basic modes, including vertical contact-separation mode, horizontal sliding mode, single-electrode mode, and independent layer mode. [39]The hydrogel-based artificial skin mainly employs the vertical contact-separation mode and single-electrode mode to achieve the triboelectric effect.
Hydrogel-based artificial skin in vertical contact-separation mode typically requires dielectric membranes with different frictional properties, where the two frictional dielectric layers come into physical contact under external pressure, generating the same quantity of opposite charges on different surfaces.As the gap between the two surfaces gradually increases perpendicular to the plane, an induced potential difference is created at the electrodes, driving electrons to flow through the external load.Once the gap is removed, the induced potential difference disappears and electrons would flow back to reach a new state of electrical equilibrium. [40]In this mode, the hydrogel can function both as a tribopositive material and an electrode material. [41]For example, Kim et al. [42] reported a nanogenerator made of hyaluronic acid (HA) hydrogel film in a vertical contact mode, where HA hydrogel film acted as a tribopositive layer.As presented in Figure 2a, the initial net charge of HA hydrogel film and PTFE was minimal, and electrons were transferred from the surface of HA hydrogel film to PTFE when the two materials came into contact under external force.The separation of the negatively charged PTFE and the positively charged HA occurred when the external force was released, causing electrons to flow through the external circuit.Finally, the direction of electron flow was reversed during the pressing process, causing the electric field at the electrode to become zero again.Zhao et al. [43] prepared a self-powered tactile sensor on the basis of a double-network ionogel with a contact-separation TENG structure.The doublenetwork ionogel acted as both the electrode and friction layer of the nanogenerator.The tactile sensor was stretchable and transparent, and able to monitor human body movements including finger flexion, respiration, and pulse.
The single electrode mode TENG is the most common selfpowered mode employed by the hydrogel-based artificial skin.[46] Wang et al. [47] prepared flexible and transparent artificial skin with polyionic hydrogel based on the triboelectric effect using a single-electrode mode.As exhibited in Figure 2b, the polyionic hydrogen-PDMS was connected to the ground through a metal lead to construct a single-electrode TENG.When the skin came in contact with the PDMS in the sensor, both became frictionally charged and produced an equal number of charges with opposite polarity on the surfaces of the dielectric film and the elastomer.When skin moved away from the device, the electrostatic charges on the surface of PDMS causedthe movement of ions in the poly vinyl alcohol (PVA)/polyethylenimine (PEI) hydrogel to balance the electrostatic charges, thus producing an excess ion layer at the interface. [48]In the meantime, the double electric layer was formed at the interface of the silver and PVA/PEI hydrogel got polarized, leading to the accumulation of positive charges within the metal wire. [49]To realize potential balance, electrons flowed from the metal wires to the ground through an external circuit until all electrostatic charges within the PDMS layer were shielded, at which point the TENG reached its maximum separation state.If the skin approached the PDMS again, the entire process was reversed, and the electrons were transferred from the ground to the silver/PVA/PEI hydrogel interface through the external load.As a result, a continuous alternating current would be generated by repeatedly performing contact-separation motions between the skin and TENG.The hydrogel device based on TENG has a simple structure, good output performance, and offers an unrestricted selection of materials, making it the most suitable choice for the preparation of self-powered artificial skin.

Piezoelectric Effect
The working mechanism of the hydrogel-based artificial skin that employs the piezoelectric effect can be elucidated as follows: the mechanical stress on the piezoelectric material gives rise to deformation of the oriented noncentrosymmetric crystal structure, resulting in the separation of electric dipole moments as well as the generation of piezoelectric voltage. [50,51]Dobashi et al. [52] investigated that hydrogel could similarly convert pressure into ionic currents, which could be used as a kind of piezoelectric ionic skin.As illustrated in Figure 2c, since anion and cation had different mobility through the hydrogel, squeezing the hydrogel produced an ionic gradient and then generated voltage.These currents could directly induce neuromodulation and muscle excitation, providing a promising pathway for the bionic sensory interface.Finally, to show the potential of self-powered piezoelectric neuromodulation, the researchers presented a few potential applications, including piezoelectric skin and peripheral nerve stimulation.Fu et al. [53] designed a highly resilient hydrogel for self-powered artificial skin, which was prepared based on a composite hydrogel consisting of ferroelectric polymer polyvinylidene fluoride (PVDF) doped with high-strength polyacrylonitrile (PAN) hydrogel.When the external circuit was connected in the positive and reverse direction, it was found that the output voltage values differed only in polarity without any difference in magnitude.This confirms that the hydrogel experienced the directional alignment of the net dipoles under an external force, ultimately resulting in the generation of electrical signals.Then this hydrogel was utilized as a sensor for detecting physiological signals of the human body.By applying the sensor to the wrist, the P1, P2, and P3 peaks for the human pulse could be detected.Further attaching the sensor to a person's vocal cords, the strength of different syllables in words could be detected, demonstrating the hydrogel sensor's excellent sensitivity.At present, research in the field of high-performance piezoelectric devices for self-powered hydrogel-based artificial skin is just beginning.

Thermoelectric Effect
The working mode of hydrogel-based artificial skin that utilizes thermoelectric effect can be categorized into two different mechanisms.One is based on the ions thermodiffusional effect, [54,55] while the other is the thermogalvanic effect. [56,57]The ions thermodiffusional effect refers to the directional migration of ions in the electrolyte resulting from the Soret effect.The different sizes of anions and cations restrict their migration rates within ion-conductive thermoelectric materials, thereby generating a concentration gradient between at the high and low temperature ends of such materials.This gradient gives rise to a substantial thermoelectric potential and enables the discharge under appropriate conditions. [58]Chen et al. [59] prepared a hydrogelbased self-powered human motion sensor with high-tensile strength by using PAAm/calcium alginate/lithium sulfate.As shown in Figure 2d, the establishment of a temperature gradient between the two sides of the conductive ionic PAAm/calcium alginate/lithium sulfate-based hydrogel caused lithium and sulfate ions to migrate from the hot end to the cold end as a result of thermal diffusion.In the hydrogel system with high cross-link density, lithium ions migrated faster because of their smaller size than that of sulfate ions.Finally, lithium ions would accumulate at the cold end while sulfate ions remained at the hot end.This concentration difference of internal ions would generate a thermal potential.During the discharging process, the two sides of the ionic conductive hydrogel were connected in series through an external circuit, and the electrons would flow from the hot end to the cold end.This net charge flow would cause the potential in the hydrogel to decrease, resulting in a voltage drop.When the external circuit was disrupted, the ion concentration difference was reestablished due to the presence of temperature gradient, and the voltage would recover within one cycle of time.The hydrogel strain sensors could withstand stretching up to 2800% and exhibit excellent sensitivity to detect human motion and sound.
The ionothermal redox reaction is a galvanic cell reaction whereby reductants lose electrons at the negative electrodes in the oxidation reaction, and the electrons are then transferred to the positive electrode through the external circuit.The oxidants get electrons at the positive electrode in the reduction reaction, thus completing the transfer of electrons between reductant and oxidant.The movement of the ions in solution between the two electrodes and the directional flow of the electrons in the external circuit constitute a closed loop under a temperature gradient field, which enables the two electrode reactions to proceed continuously, allowing for an orderly electron transfer process and resulting in the conversion of chemical energy to electricity. [60,61]Lei et al. [62] prepared a stretchable and highstrength thermal cell based on a double chemical cross-linked network hydrogel.Under a temperature gradient field, the thermal cell would undergo an invertible redox reaction, as exhibited in Figure 2e.During thermal charging, the output voltage rose and then stabilized as the temperature gradient reached a constant state.This work will be beneficial to the rapid development of self-driven wearable electronic devices in the Internet of Things era.To address the fragility and complex fabrication process of conventional thermoelectric materials, Bai et al. [63] prepared a gel electrolyte-based thermoelectric device that uses Fe 3þ /Fe 2þ as the redox pair.The gel patch could be applied to the forehead to create a self-powered body temperature sensing device due to its excellent temperature sensitivity, which may help reduce fever in patients.To effectively overcome the limitations that the thermoelectric hydrogel freezes at low temperatures and dehydrates severely at high temperatures, the research team [64] developed a gel electrolyte thermocouple device with I À /I 3À as the redox pair.The device exhibited good temperature resistance (À20-80 °C) and good resistance to water loss.Considering its good thermoelectric performance, a gel-based artificial skin was developed for human temperature monitoring.In addition, an artificial skin-based smart window system has been established for self-powered indoor and outdoor temperature monitoring, which can work normally even in extreme weather conditions.

The Classification of Hydrogels for Self-Powered Artificial Skins
The cross-linking mechanisms of hydrogels used to prepare self-powered artificial skin can be classified as physical, chemical, and hybrid cross-linking. [65][68] The properties of physically interacting cross-linked hydrogels depend mainly on the inherent properties of the polymer, which limits the diversity of hydrogel functions.However, the gelation process of such hydrogels is simpler and does not require modification of the polymer chains.It is also typically reversible andin a continuous cycle of generation and dissolution, which confers a certain self-healing capability to most physically cross-linked hydrogels.Chemically cross-linked hydrogels are formed by chemically cross-linking agents that weave polymer networks together by covalent bonding to form a 3D structured hydrogel network. [69]They have the advantages of good stability and excellent mechanical properties.[75] Based on the type of hydrogel used for the preparation of self-powered artificial skin, it can be divided into ionic hydrogels, ionic-liquided hydrogels, metal-based hydrogels, carbon-based and MXene-based hydrogels,and conductive polymer-based hydrogels.The performances of these six types of hydrogels as self-powered artificial skins are summarized in Table 1.

Ionic Hydrogels
The ionic hydrogels in self-powered artificial skins are typically prepared by adding electrolytes, including inorganic salt and acid solutions and amphoteric ions, into the hydrogel for better conductivity.Due to the abundance of water in the hydrogels, the electrolytes can be dissolved in the hydrogel and provide large amounts of free ions that can move directionally and conduct electric currents under certain electrical potential difference.[78][79][80] Chen et al. [81] first investigated a practical ultraflexible PAM-LiCl-based 3D TENG that could collect energy from low-frequency biological motion and convert it into electricity to drive or charge electronic devices.More importantly, power generation devices were realized through distinctive additive fabrication technology-mixed 3D printing.Unlike conventional TENG based on dielectric film as triboelectric materials, this ultraflexible 3D-TENG utilized printed 3D composite resin structures for frictional charged layers and ionic hydrogels for electrodes, which could achieve ultraflexible 3D power generation structures and high-density integration of TENGs.The practicality, creativity, and novelty were successfully demonstrated through self-assembled self-powered wearable devices.This included self-powered LED flickering, buzzer distress signaling system,and self-powered smart LED lighting shoes system.Although self-powered artificial skins based on ionic hydrogels offer several advantages, such as high sensitivity, self-powered capability, biocompatibility, and stretchability, they also suffer from the problem of leaching, which means that hydrogel could lose the ionic counterpart over time during solution-gelation cycles. [20,78]Therefore, it is necessary to address this problem during the fabrication process for better practicality of ionic hydrogels-based self-powered artificial skins.

Transparency
To achieve transparency of artificial skins and reduce hysteresis in hydrogel elastomer composites, Xing et al. [82] reported a type of high-transparency egg white hydrogels (EWH) that underwent a sol-gel-sol phase transition process.As shown in Figure 3a, the alkaline environment not only reconfigured physical equilibrium to form a solid EWH, but also synchronously initiated self-liquidation to EWL.During the self-clearance process, the light transmission increased further due to the increasingly shorter length of peptide chain residues after hydrolysis, resulting in a dramatic reduction of insoluble aggregates in the liquid.EWL prepared after phase transition showed high transparency (99.8%), high sensitivity, fine durability, low hysteresis, and high-ionic conductivity.It remained the complete 3D structure of EWH, which retained direct 3D printability.Hybrid voltage generation and pressure sensing were also demonstrated by converting mechanical motion into electricity.This material has a wide range of applications in durable health monitoring, HMI, the Internet of Things, clean energy, gesture-controlled platform, and TENG.

Stretchability
To enhance the stretchability of transparent artificial skins devices, Jing et al. [83] synthesized a double network hydrogel from PVA and SA and applied it as an ion electrode for hydrogel-based TENG (H-TENG).By changing the concentration of SA to control the elasticity of the hydrogels, the significant effect of hydrogels' viscoelastic propertyon the performance of H-TENG was verified for the first time.The optimal H-TENG was prepared by adjusting the conductivity and viscoelasticity of the PVA/SA hydrogel, which has high transparency (more than 90%) and high stretchability (more than 250%).Pu et al. [84] developed a super-stretchable and transparent flexible skin-like TENG that utilized the hybridization of elastomers and ionic hydrogels as electrification layers and electrodes, enabling mechanical energy harvesting and tactile monitoring.As exhibited in Figure 3b, the TENG used a sandwich-like structure where ionic hydrogels (PAAm-LiCl) were sealed between two elastomer films of PDMS and VHB.The TENG was high transparency (96.2%).Since the elastomer and PAAm-LiCl hydrogels involved were extensible, this skin-like TENG was also super stretchable (1160%), which would be promising for intelligent artificial skins, soft robotics, functional displays, and flexible electronics.Sheng et al. [85] reported a TENG constructed from SA/zinc sulfate/PAAm (SA--ZnSO 4 -PAAm) double network polymer ionic hydrogel, in which the hydrogel had great stretchability, excellent transparency, and high conductivity.This double network structure and dynamic interactions built from two macromolecular monomers (SA and PAAm) greatly enhance the hydrogels' tensile performance.Moreover, Zn ions could bind to double network structure on polymer chains for controlling ions migration, thus improving the conductivity and hydrogels' tensile properties.The double-network ionic hydrogel TENG could harvest energy from human motion, including bending, stretching, Reproduced with permission. [82]Copyright 2020, Wiley.b) Ultrastretchable, transparent electronic skin.Reproduced with permission. [84]Copyright 2017, American Association for the Advancement of Science.c) Transparent, stretchable and high-performance TENG.Reproduced with permission. [86]Copyright 2021, Elsevier.d) High-transparency, stretchable, and self-healing ionic skin.Reproduced with permission. [92]Copyright 2017, Wiley.e) Multifunctional self-powered electronic skin.Reproduced with permission. [94]Copyright 2022, Wiley.f,g) Antifreezing stretchable TENG.Reproduced with permission. [101]Copyright 2020, Royal Society of Chemistry.
and twisting.Thus it could be applied to fabricate self-powered intelligent elastic belt sensors for detecting arm movements.

High Self-Powered Performance
In order to improve the self-powered performance of stretchable and highly transparent artificial skins, Li et al. [86] first proposed a novel TENG with high open-circuit voltage and exceptional stretchability using polymer electrolytes (SPE) as stretchable and transparent electrodes (Figure 3c).The combination of phosphoric acid (H 3 PO 4 ) with biocompatible PVA resulted in a high-SPE stretchability of 1058% and an ionic conductivity of 1.39 S m À1 due to the dual role of H 3 PO 4 as a plasticizer and ionic conductive carriers.The electrical double layers that were easily formed between ion-conductive SPE and conductive ribbon significantly increase the output current, leading to an ultrahigh open-circuit voltage (992 V).This wearable SPE-TENG exhibited good energy harvesting capacity, which could overcome the performance degradation caused by liquid evaporation in ionic hydrogels.This work holds promising practical application prospects in wearable energy harvesters and self-powered flexible sensors.

Self-Healability
The long-time application of artificial skin to the human body requires certain characteristics.First, the hydrogel components of artificial skins need to possess the ability to recover to their original forms after elastic deformation.[89][90] Han et al. [91] fabricated a soft, self-healing TENG as an artificial skin by incorporating gelatin into polyacrylic acid (PAA) and adding NaCl solution for conductive components.This artificial skin with good sensitivity can reach 800% elongation and self-heal within 2.5 min, enabling touch and pressure sensing.While these efforts succeed in conferring stretchability and self-healing properties to artificial skins, they all sacrifice their transparency.
To simultaneously achieve stretchable, transparent, and self-healing properties of artificial skins, Parida' team [92] reported a novel type of stretchable, highly transparent, and self-healing TENG (IS-TENG) ionic skin using mucus-based ionic conductors as current collectors.This IS-TENG had 92% of light transmission and could withstand uniaxial strain of up to 700%.As illustrated in Figure 3d, the prepared IS-TENG could complete self-healing without the need for external stimuli after being completely fractured.Furthermore, it could recover its energy harvesting performance even after 300 repetitive mechanical damages.

Multifunction
Artificial skins serve a multiple purpose, not only enabling the perception of tactile sensations, but more significantly possessing the ability to differentiate multiple stimuli simultaneously. [93]u' team [94] prepared a flexible multifunctional bionic skin using a highly stretchable hydrogel formed through the chemical crosslinking of PAAm and lithium magnesium silicate modified with carbon quantum dots (Figure 3e).The lithium magnesium silicate offered abundant covalent bonds to enhance the mechanical performance of hydrogels.This hydrogel-based strain sensor showed high sensitivity, a wide sensing response range, and excellent cyclic stability.Liang et al. [95] reported an all-polymer, multifunctional wearable intelligent skin capable of energy harvesting, touch sensing, and external sensory visualization.The stretchable hydrogel (PAAm-NaCl) was sandwiched between the triboelectric layer and the ZnS phosphorus-mechanoluminescent layer.This smart skin could act as a TENG with an output voltageof 180 V.Moreover, the skin can detect pressure as low as 0.58 kPa with a sensitivity of 0.23 kPa À1 , thereby position detection can be achieved by applying pixel configuration to the device and the location of the applied force could be visualized through the exciting luminescent response.Wang et al. [96] prepared highly tensile and transparent ionic conductive cellulose/polyvinyl alcohol hydrogels (CPH) by a simple, environmentally friendly method.The CPH-based sensor exhibited remarkable sensitivity and instantaneous response in detecting temperature, pressure, and strain.In the meantime, the CPH-based TENG was resistant to temperature changes and sustains a great number of operating cycles.

Frost Resistance
[99] Liu et al. [100] reported a skin-like TENG made from hydrogel-elastomer composites.The TENG was applied to human skin and adapted to different movements for collecting mechanical energy to convert it into electricity for powering flexible electronic devices.The elastomer Ecoflex or PDMS, and the PAAm-SA hydrogel were chosen as the basic materials in this work.Since elastomers and hydrogels were hydrophobic and hydrophilic, respectively, the interface between the two materials was inherently more vulnerable.To solve the problem, the authors first modified the elastomer surface with benzophenone in ethanol, then added the solution of hydrogel on the top layer, and finally formed a composite layer by the application of ultraviolet radiation.Moreover, hydrogel materials face a significant challenge with regards to water loss, which could be effectively alleviated by introducing an elastomer thin layer.The stretchability and electrical output properties of conventional stretchable self-powered artificial skins, primarily consisting of low Young's modulus elastomer, and gel-like electrode materials, experience considerable degradation under subzero temperature conditions, posing a great challenge for the application of artificial skins in low-temperature environments.Thereby, it is urgent to investigate stretchable self-powered artificial skins that are resistant to low temperatures to efficiently harvest low-grade mechanical energy.Bao' team [101] reported a flexible stretchable artificial-skin TENG based on antifreeze hydrogel for efficiently harvesting mechanical energy from human motion at subzero conditions.The glass transition temperature of hydrogel could be decreased by regulating the content of LiCl in the hydrogel (Figure 3f-g), thus achieving the exceptional antifreezing performance of the hydrogel material.For obtaining an artificial skin with high stretchability, good transparency, self-healability, and resistance to freezing, Jing et al. [102] fabricated PAA/nanochitin composite hydrogels with a double cross-linked network by an environmental friendly method.The formation of dynamic metal coordination bonds and the hydrogen bonds between Al 3þ and carboxyl groups endowed the hydrogel with great self-healing capacity.The hydrogel-based sensors were capable of monitoring multiple external stimuli with an ultrahigh measurement coefficient of 2.36.Notably, the composite hydrogels exhibited ultrasensitive thermal responses during the cooling process with a sensitivity of 252%/°C.

Ionic-Liquided Hydrogels
Ionic-liquided hydrogels in self-powered artificial skins are prepared by adding ionic-liquid salts, which consist entirely of anions and cations and remain liquid at room temperature, into the hydrogels for better performances. [43]Ion liquids own many distinctive physicochemical properties, including high-ionic conductivity, flame retardancy, thermal stability, and chemical stability. [103]Sun et al. [104] proposed a highly stretchable transparent ionic gel-based TENG with a wide temperature tolerance range (Figure 4a  Reproduced with permission. [104]Copyright 2019, Elsevier.c-e) Hydrophobic ionic liquid gel-based TENG.Reproduced with permission. [105]Copyright 2020, American Chemical Society.f,g) Ionic liquid-containing nanocomposite hydrogel strain sensors.Reproduced with permission. [106]Copyright 2021, American Chemical Society.h,i) Self-healing ionogel.Reproduced with permission. [107]Copyright 2021, Elsevier.
gel-based TENG could maintain stable electrical output over a large temperature range (À20-100 °C).Yang et al. [105] prepared hightransparent stretchable hydrophobic ionic liquid gels with excellent reliability for TENG applications, which could remain stable under different weather conditions for up to 3 months (Figure 4c-e).This TENG can be utilized to drive various kinds of wearable electronic devices, includingLEDs, strain sensors, and transparent keyboards.These hydrogel-based self-powered artificial skins possessed multifunctional characteristics and exceptional reliability,rendering them increasingly popular in the field.
In addition to the multifunctional features, it is essential to enhance the freeze and dehydration resistance of hydrogels to prevent property degradation in harsh environments.He et al. [106] prepared stable nanocomposite ionic gels using [EMIM][Cl] ionic liquids as conductive materials and incorporating clay nanosheets as physical cross-linkers.The nanocomposite ionic liquid hydrogel exhibited great mechanical performance, repetitive self-adhesion to various substrates, frost resistance, and antidrying capability.Due to the presence of the ionic liquid, the nanocomposite ionic gel displayed high-ionic conductivity and temperature tolerance (Figure 4f,g).In addition, the sensor possessed dual sensing capabilities for both pressure and strain with high sensitivity.This selfpowered device can realize an electrical output up to 2.8 V by assembling four ionized gel units together, achieving energy supply in harsh environments.Since most self-healing ionic liquid hydrogels have limited mechanical strength and compressive resistance, it is significant to develop a self-healing hydrogel with highthermal stability and mechanical reliability.Li et al. [107] proposed high-transparency, self-healing ionogel-based TENG with great tensile property, compressive resistance, and environmental stability.These ionic gels were prepared by in situ polymerization of acrylic acid in 1-vinyl-3-ethylimidazolium dicyandiamide ([Emim][OAc]) in the presence of zinc acetate dihydrate and zinc oxide nanoparticles (ZnO NPs).The gel networks were form through coordination bonds between the carbonyl group and Zn ion, as well as entangled cross-linking between PAA and ZnO NPs.Thanks to the presence of ZnO NPs and their good compatibility with PAA and [Emim][OAc], this ionic gel exhibited outstanding mechanical stability, compression resistance, high transparency, and excellent ionic conductivity.The self-healing ionogel's good mechanical robustness and environmental stability enabled the TENG to deliver reliable electrical output performance across a temperature range of À30-80 °C, even after being subjected to folding, twisting, stretching, and trampling.As a result, the TENG can harvest energy from human motions such as tapping, walking, and running (Figure 4h,i).After mechanical damage, the TENG can be self-healed at room temperature, fully restoring its electrical output performance,and ensuring its long service life and reliability.While ionic-liquid hydrogels-based self-powered artificial skins offer numerous advantages, including stability, sensitivity, self-powering capability, and biocompatibility, they also possess certain limitations, such as manufacturing complexity, limited conductivity, limited availability, and potential toxicity.

Metal-Based Hydrogels
Metal-based hydrogels are prepared by introducing metal-based nanomaterials that have excellent metallic conductivity and nanomaterial properties, into hydrogel matrix for the development of self-powered artificial skins.[110] Metalbased nanomaterials are ideal fillers for preparing conductive gels because of their superior conductivity. [111,112]To improve electrical conductivity, Wang et al. [113] fabricated ion/electronic conductive hybrid hydrogel network by cross-linking chitosan with metal ions (Ag þ /Cu 2þ ) and silver nanowires (AgNWs).This hybrid hydrogel network was effectively employed as an efficient current collector of TENG for mechanical energy harvesting from human motions (Figure 5a-c).It was observed that the concentration of AgNWs and the type of complexed metal ions had important influences on the power output of hydrogel-based motion energy collector.Meantime, the hybrid hydrogel-based motion energy collector exhibited excellent washability, and could restore its output through hydration even in the dehydrated state.Moreover, the hybrid hydrogel was utilized to fabricate a self-powered sensor for dual temperature-pressure monitoring, demonstrating great linear correlation, and high sensitivity.These findings offer valuable insights for the reasonable design of hybrid hydrogel networks and their corresponding devices with advantages of hybrid conductivity.
While significant progress has been made in the development of artificial skins with high sensitivity and stability, there is a lack of research focusing on their wearing comfort, environmental friendliness, and antibacterial property.Peng team [2] developed permeable, degradable, and antibacterial artificial skins by sandwiching AgNWs between PLGA and PVA (Figure 5d,e).This design not only provided a large surface area for contact charging and stress response, but also ensured the thermal and moisture balance of the microenvironment on the skin surface and provided wearing comfort.By adjusting the concentration of AgNW, PVA, and PLGA, the antibacterial and biodegradable properties of artificial skin can be regulated, respectively.This artificial skin could enable real-time automatic monitoring of human physiological signals and joint movements.Zhang et al. [114] developed a type of organic hydrogel-based TENG (O-TENG) that possessed antibacterial, antifreeze, flexible, and self-healing properties.By incorporating Ag nanoparticles onto reduced graphene oxide flakes (Ag@rGO), and integrating them with PVA-PAAm double network organo-hydrogel (Ag@rGO/PVA-PAAm) with dynamic borate bonds, the resulting organo-hydrogels had excellent electrical conductivity, great stretchability, and antifreeze self-healing characteristics.As exhibited in Figure 5f,g, when Ag@rGO/PVA-PAAm was used as the electrode layers for the O-TENG, it could efficiently and easily suppress Gram-negative bacteria such as Escherichia Colis as well as Gram-positive bacteria such as Staphylococcus aureus, while maintaining good cell compatibility.The self-healing and antifreeze properties of these O-TENGs ensured stable output performance even when damaged, and they exhibited self-healing capabilities at room temperature or even as low as À30 °C.Furthermore, these O-TENGs were also developed as self-powered sensors to recognize handwriting and wrist motion states which opens up novel avenues for multifunctional flexible wearable devices.While metal-based hydrogels offer several advantages for self-powered artificial skins, such as high conductivity, biocompatibility, and durability, they also have limitations, including limited stretchability, susceptibility to corrosion, and potential toxicity.

Carbon-Based and MXene-Based Hydrogels
Carbon-based hydrogels are fabricated by adding carbon nanomaterials, specifically carbon nanotubes (CNTs) and graphene [115] into hydrogels for self-powered artificial skin.Carbon nanomaterials possess exceptional physical and chemical structures, high conductivity, and large specific surface areas.Moreover, they can be easily functionalized, making them suitable for a wide range of applications including biosensing and lithium-ion batteries.118] CNTs have a high-aspect ratio and can conduct charges stably even in wet environments, making them ideal for conductive hydrogel filler.However, unmodified CNTs have poor dispersion in water due to the lack of surface functional groups.Therefore, modifications such as carboxyl or amino functionalization, as well as organic molecular modifications, are commonly employed to successfully incorporate CNTs into hydrogels as conductive media.Guan et al. [119] designed and prepared a self-healing TENG (HS-TENG) based on PVA/agarose hydrogels.It possessed high deformability, as well asboth mechanical and electrical self-healing capability.Combining photothermal conversion active polydopamine particles with multiwalled CNTs, HS-TENG can achieve physical self-healing under near-infrared (NIR) light irradiation.As illustrated in Figure 6a,b, the stretchable and dual self-healing capabilities of HS-TENG enabled effective energy harvesting from human movement.The rectified electrical energy can be utilized as a renewable energy source to charge LED light under various complex deformations.It was noteworthy that the electrical output of HS-TENG remained steady at 200% strain due to the uniform dispersion of multiwalled CNTs as conductive filler in the matrix.The self-healing flexible conductive layer was able to withstand a certain degree of deformation and self-heal from damage, gradually becoming an  [113] Copyright 2019, Elsevier.d,e) All nanofiber AgNWs hydrogel-based TENG.Reproduced with permission. [2]Copyright 2020, American Association for the Advancement of Science.f,g) Ag hydrogel-based TENG.Reproduced with permission. [114]Copyright 2022, Wiley.
indispensable application guarantee in the field of flexible electronics.To address the freezing and drying concerns in hydrogel-based power sources, Sun et al. [120] successfully prepared an environmentally tolerant, highly stretchable organic conductive hydrogel by replacing part of the water in the PAAm/montmorillonite/CNT hydrogels with glycerol.This organic hydrogel could be assembled into a single-electrode TENG to power wearable electronic devices even under cold conditions.It offers a new method for the preparation of multifunctional organic hydrogels, opening up possibilities for flexible self-powered wearable devices in extreme environments.To develop highly elastic and biocompatible devices, Yang et al. [121] reported a multifunctional TENG (MF-TENG) with rapid self-healability and photothermal performance (Figure 6c).These self-healing hydrogels were prepared by adding sodium borate and conductive poly(dopamine)-CNTs (PDA-CNTs) to the PVA gel matrix.The CNTs can sense the relative-resistance change and provide the photothermal effect, which could especially assist in the recovery of human joint motions under NIR laser irradiation.
124] Graphene oxide (GO), an oxide derivative of graphene, possesses improved reactivity compared to graphene due to the introduction of numerous hydroxyl, epoxy, and carboxyl groups through the process of oxidation.Furthermore, the intrinsic properties of GO can be further enhanced through the chemical reaction of  [119] Copyright 2019, Royal Society of Chemistry.c) CNTs hydrogel-based TENGs.Reproduced with permission. [121]Copyright 2021, American Chemical Society.d,e) GO hydrogel-based self-powered sensing.Reproduced with permission. [126]Copyright 2021, Royal Society of Chemistry.f,g) GO hydrogel-based wearable strain/pressure sensors.Reproduced with permission. [127]Copyright 2020, American Chemical Society.h-k) MXene hydrogel-based multifunctional TENG.Reproduced with permission. [129]Copyright 2021, Wiley.
oxygen-containing functional groups. [125]Li et al. [126] prepared a super-stretchable and healable hydrogel-based TENG using GO and Laponite as raw materials.The TENG exhibited high-tensile strain (1356%) and self-healing ability.When employed as electrodes, the TENG can work properly under a tensile strain of 900%, and its output current can be fully restored to the initial value after healing from the damage.As exhibited in Figure 6d,e, the hydrogel TENG can be applied as a self-powered sensor for pressure sensing.His team [127] then designed a flexible conductive hydrogel by mixing silk, PAAm, GO, and poly (3,4ethylenedioxythiophene): polystyrene sulfonate in a certain proportion.As exhibited in Figure 6f,g, the synthetic hydrogels had good tensile and compressive properties, which could be integrated into strain/pressure sensors owing to the large response ranges and good reliability.The corresponding sensors can detect various physiological signals from the human body, such as joint movements, facial expressions, pulse, breathing, etc.The GO-based hydrogel emerges as a multifunctional wearable electronic material with a large scope of applications in health monitoring, soft robotics, and wearable power sources.While carbon-based hydrogels for self-powered artificial skins possess high conductivity, biocompatibility, and flexibility, they are accompanied by certain limitations such as low durability, limited sensing capabilities, and fabrication complexity.
MXene-based hydrogels are prepared by introducing MXene, a 2D transition metal carbide nanomaterial, into hydrogels which represents a novel approach for the developmentof self-powered artificial skins. [28]Compared with graphene, MXene has unique metallic conductivity, hydrophilicity, ease of processing, large specific surface area, and good mechanical properties. [128]herefore, the introduction of MXene into hydrogel not only improves their conductivity and overall mechanical robustness but also imparts new properties to the hydrogels, leading to multifunctional Mxene-based hydrogel materials.Wang et al. [129] reported a novel MXene/PVA hydrogel-based TENG (MH-TENG).Because of doping of MXene nanosheets, it not only enhanced the stretchability of the TENG and conductivity of the hydrogel electrodes, but also generated additional triboelectric output through the flow vibration potential mechanism.The MH-TENG exhibited excellent stretchability and ultrahigh sensitivity, which could be employed in human motion monitoringand high-precision stroke recognition (Figure 6h-k).While MXene-based hydrogels for self-powered artificial skins own high conductivity and substantial surface area, they are usually suffered from instability and lack of transparency.

Conductive Polymer-Based Hydrogels
Conductive polymer-based hydrogels involve the incorporation of conductive polymers, which mainly consist of long-chain polymers with a conjugated electron system that allows for the conduction of electrons along the molecular chain, into hydrogels. [130]Conductive polymers offer distinct advantages compared to other conductors.First, they can be polymerized in situ withina solution, enabling more uniform distribution throughout the system.Second, conductive polymers exhibit good flexibility, which is suitable for the desired mechanical properties of hydrogels.Third, conductive polymers have a higher electrical conductivity than ionic conductors.The common conductive polymers include poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS), polyaniline (PANi), and polypyrrole (Ppy).
PEDOT is a typical conductive polymer that possesses excellent electrical conductivity but poor water solubility.Mixing PEDOT and PSS could effectively enhance the water solubility of PEDOT.In colloidal water dispersions, PEDOT/PSS tends to form micellar microstructures, which consist of a core of hydrophobic PEDOT and a shell of hydrophilic PSS.The PEDOT/PSS has three main molecular interactions, including electrostatic attraction between the π-π conjugated PEDOT chain and the negatively charged PSS chain, the π-π stacking of adjacent PEDOT chains, and interchain entanglement of long chains of PSS. [131,132]Sun et al. [133] prepared stretchable polyacrylamide (PAM)/Gelatin/PEDOT/PSS (MGP) conductive hydrogels by combining stiff physically cross-linked gelatin, ductile chemically cross-linked PAM, as well as conductive PEDOT/PSS.The PEDOT/PSS served as the physical cross-linker through noncovalent interactions between PAM chains and gelatin.And the cross-linker can improve the self-healing property of MGP hydrogels (see Figure 7a,b).Well-connected conductive paths were created due to the uniform dispersion of PEDOT/PSS, which endowed the obtained hydrogels with excellent electrical conductivity.The MGP hydrogel was encapsulated in two layers of commercial polyurethane tape to create a transparent and flexible strain sensor with a sandwiched structure.This sensor exhibited high sensitivity (GF = 1.58), a broad sensing range (0-2850%), a very quick response time (200 ms), as well as excellent durability and stability (1200 cycles) when detecting complex human motions.Additionally, the sensor could also be used for effective energy harvesting when employed as a stretchable TENG, generating a short-circuit current of 26.9 μA, an open-circuit voltage of 383.8 V, as well as a short-circuit transfer charge of 92 nC.These combined capabilities for strain sensing and energy harvesting render the MGP hydrogel an ideal candidate for high-performance self-powered artificial skin as well as stretchable power sources.
PANi is a conjugated molecular chain formed by the reduced benzene ring unit and the oxidized oxime unit in the molecular chain after oxidation of the aniline molecule.The intrinsic PANi has poor electrical conductivity and requires proton doping to enhance the electrical conductivity.Qin et al. [134] reported a conductive hydrogel electrode that possesses rapid self-healing ability and high stretchability via in situ polymerization of PANi on 2,2,6,6-tetramethylpiperidine-1-oxyl radical oxidized cellulose nanofibrils and PVA/borax, which is also rich in hydroxyl groups on the molecular chain.As shown in Figure 7c-e, a novel flexible self-powered sweat sensor was prepared by using PANi-cellulose-PVA/borax hydrogel (CPPH) as an electrode material.The CPPH hydrogel with rapid self-healing properties enabled the sensor to detect and measure concentrations of sodium, potassium, and calcium ion concentrations in sweat through the periodic biomechanical vibrations generated during human movement and the selective transmission of specific ions by ion-selective membranes, respectively.The sensing signal was transmitted wirelessly to the cell phone, allowing real-time monitoring of the wearer's health condition during exercise and providing timely warning.To achieve stable and accurate readings of multiple biophysical signals from a specific area of the human body, Chun et al. [135] fabricated a flexible and all-gel multimodal artificial skin sensor by using PANi-PVC ionic gel electrodes to measure biophysical signals (Figure 7f,g).The sensor could simultaneously measure four biophysical signals, including blood pressure (BP), electrocardiogram (ECG), EMG, and mechanomyogram (MMG) on the wrist.PANi-PVC ionomer gel was employed tomeasure BP and ECG, while PVDF-trifluoroethylene Reproduced with permission. [133]Copyright 2020, Elsevier.c-e) PANI hydrogel-based self-powered sweat sensor.Reproduced with permission. [134]Copyright 2022, Wiley.f,g) PANI hydrogel-based multimodal cutaneous sensor.Reproduced with permission. [135]Copyright 2022, Wiley.h,i) Ppy hydrogel-based electronic skin patches.Reproduced with permission. [136]Copyright 2022, Elsevier.
(PVDF-TrFe) gel was chosen as a strategic material for MMG response performance.Additionally, PANi-PVC ionic gels were also utilized as electrodes for measuring ECG signals.
Ppy is a type of conductive polymerprepared by chemical or electrochemical oxidation of penternary heterocyclic pyrroles.Zhu' group [136] prepared a TENG-based single-electrode artificial skin patch made by conductive and photothermal Ppy/Pluronic F127 hydrogels as electrolytes.It could synergistically utilize electrical stimulation and photothermal heating capabilities for both motion sensing and wound healing (Figure 7h,i).The artificial skin patch was biocompatible, stretchable, and shape-adaptive.Within the electrolytes, Ppy had good electrical conductivity and excellent photothermal conversion properties, while F127 possessed good flexibility as well as phase-changing ability.Through integrating photothermal heating and electrical stimulation, this artificial skin patch could effectively promote angiogenesis, collagen deposition and reepithelialization, resulting in the acceleration of tissue regeneration and wound healing within a relatively short period of %9 d.In addition, the patches demonstrated the ability to perceive human activity, which could compensate for a partial loss of sensation for human body.Although conductive polymer-based hydrogels in self-powered artificial skins are flexible, tunable, and biocompatible, they often encounter challenges related to durability, conductivity, and scalability.

Applications in Human-Machine Interfaces
The artificial skins are applied on the surface of the robot, which immediately generates a signal to the controller upon touch, thereby achieving the robot's intelligent perception.Integrating such an HMI into smart appliances enhancesthe interaction between human actions and the sensor-mounted device,imparting a more realistic sense of touch to HMI devices. [137,138]Notably, hydrogel-based self-powered artificial skin is well-suited for the development of wearable HMI because of its sensitivity, biocompatibility as well as simple working mechanism. [139,140]The wearable HMI utilizing hydrogel-based self-powered artificial skin can be categorized into three key areas: intelligent control, medical care, and touch panel.

Intelligent Control
The hydrogel-based self-powered HMI serves as a direct communication path between human and machine, allowing the acquisition of electrophysiological signals from users and driving machines to perform specific functions.For example, controlling a robotic arm in real time can be achieved by capturing and interpreting human movements and gestures, which are considered intuitive and natural means of interaction between human and machines. [141]Tao' group [142] prepared a tactile hydrogel sensor (THS) on the basis of a micropyramid-structured double network ionic organic hydrogel.The THS can detect subtle pressure changes by measuring triboelectric output signals without the need for an external power source.Through the fabrication strategy of the pyramid-structured hydrogel and laminated PDMS encapsulation method, the self-powered THS exhibited great flexibility, good transparency, and superior sensing performance, including exceptional sensitivity, fast response, low detection limit, and good stability.As exhibited in Figure 8a,b, the THS with good self-powered sensing capability can be used as a switch button for controlling the robot hand by simulating human finger gestures to grasp a mango and a coil.It offers great potential for wearable and multifunctional electronic applications.Incorporating hydrogel-based self-powered artificial skin into the Internet of Things (IoT) enables remote control between humans and machines.Mao et al. [143] reported a smart robotic control system and a Bluetooth cell phone control system by using a protein-based bioprotonic hydrogel inspired by the sebum membrane of human skin (Figure 8c,d).The hydrogel, with excellent water retention, freeze resistance, stretchability, transparency, and biodegradability, served as a bioprotonic skin for prolonged measurement of human electrophysiological signals.This innovation shows great promise for the development of the new generation of HMI.Moreover, Sun et al. [144] proposed an integrated self-powered HMI system based on TENG, enabling wireless remote control of smart cars (Figure 8e,f ).Double network cross-linked hydrogels were synthesized and wrapped with functional layers to create stretchable fiber TENG (SF-TENG) and super capacitors (SF-SC).These components, along with power management circuits formed a self-charging power unit that harnessed mechanical energy from body movements to continuously drive the entire system.A smart glove equipped with five SF-TENG on the back side of corresponding fingers served as gesture sensors.The generated gesture signals were first processed via a microprocessor followed by transmission wirelessly to a smart car for remote control.This advancement holds great promise for wearable devices in self-powered HMI systems.
In addition to gesture recognition, finger sliding for controlling the motion trajectory of a machine is also an important application of self-powered hydrogel-based HMI. [145]Based on the principle of TENG, Chen et al. [146] reported a flexible sensor that could be applied as an HMI interface for smart and interactive products.The device integrated two sets of sensor patches for detecting finger sliding trajectory and acquiring operation command information.Then it can be applied to the 3D motion control of a robot to achieve real-time trajectory control at the end of the robot (Figure 8g).With a simple and cost-effective design, the sensor holds promise in the fields of robot control, touch screen, and electronic skin.Xu et al. [147] prepared a stable interactive patch with sensing and robot control capabilities based on TENG as shown in Figure 8h.The robust patch consisted of a few flexible materials, including polytetrafluoroethylene, nylon, hydrogel electrodes as well as silicone rubber substrates.The sensor signals of the patch were processed to be more stable by using a signal processing circuit, thus providing an efficient way for wireless sensing and robot control.The patch used a specific algorithm to transform a 1D sequence number into a 2D coordinate system and convert finger clicks into robot trajectories wirelessly.With its simple design, the HMI device exhibits promising application potential in contact perception, 2D control, robot technology, and wearable electronics.

Medical Care
The application of artificial skin in biomedicine is a bold attempt, which would kick off a revolution in biomedicine and may even be used in the future to treat patients with Parkinson's disease and epilepsy.Compared to traditional wired instruments with complex detectionprocedures, artificial skin is more comfortable for the patient and does not cause restrictions to the user's body movement.By applying the artificial skin to the test area, signs of inflammation and wound infection could be detected in advance.
Moreover, endowed with memory capabilities, the artificial skin could also store the patient's diagnostic data,enabling the provision of appropriate medical treatment and fostering interactive engagement among the patients, the health care workers and the medical devices.Sun et al. [148] (Figure 9a-c) reported a flexible self-powered sensor through the fabrication of a transparent, The robot hand controlled by micropyramid patterned hydrogel tactile sensor.Reproduced with permission. [142]Copyright 2022, Wiley.c,d) The noncontact gesture recognition system to manipulate the robot for pharyngeal swab sampling.Reproduced with permission. [143]Copyright 2023, Wiley.e,f ) Wireless remote telemetry and control of smart car by using the all-in-one self-powered HMI systems.Reproduced with permission. [144]Copyright 2021, MDPI.g) The 3D motion control of a robot.Reproduced with permission. [146]Copyright 2018, American Chemical Society.h) A stable tactile patch to control the generation of robot trajectories wirelessly.Reproduced with permission. [147]opyright 2021, MDPI.
stretchable, freeze-resistant, and self-healing hydrogel based on TENG.The self-powered sensor enabled real-time monitoring of diverse human body movements, thus providing a promising platform for TENGs-based hydrogels that exhibited stable output performances and self-healing capabilities in varying environmental conditions.To achieve interaction between the elderly and medical staff for improving the efficiency of elderly care.Hua et al. [149] developed an intelligent self-powered elderly care system using ionic hydrogel that possessed good mechanical robustness, excellent conductivity, and high transparency, as shown in Figure 9d-f.The self-powered human-machine interaction system was attached to the fingers of an elderly person to process and encode the acquired triboelectric signal.This research shows substantial potential of self-powered sensors in intelligent elderly care systems.In addition, artificial skin sensors could also proficiently monitor weak physiological signals from human body, including breathing, pulse, etc. Wang et al. [150] demonstrated a self-powered artificial skin on the basis of a gradient polyelectrolyte membrane that enabled direct and precise perception of human breathing, pulse, and body temperature (Figure 9g-i).The coupling of the inherent electromechanical and thermoelectric effects of the self-powered Figure 9. HMI applications of medical care.a-c) Self-powered sensor to monitor body movements.Reproduced with permission. [148]Copyright 2022, Elsevier.d-f ) Demonstrations of hydrogel-based TENG for assisting the elderly to express requests for help.Reproduced with permission. [149]Copyright 2023, American Chemical Society.g-i) Self-powered hydrogel-based AS for health monitoring.Reproduced with permission. [150]Copyright 2023, American Chemical Society.j,k) Body area sensor network system that used in infant motion monitoring.Reproduced with permission. [151]opyright 2022, Wiley.
multifunctional ionic skins offers a general pathway to developing novel self-powered ion sensing systems.
Infants are physically vulnerable without the ability to express their feelings.Therefore, continuous monitoring of biomechanical stress on body of the infants remains critical in order to avoid injury and illness.Figure 9j,k show that Guo et al. [151] reported a deep learning-assisted hydrogel sensor based on TENG for omnidirectional infant care.The TENG-based hydrogel sensor had a sandwich structure that was made of three layers including gelatin, agar hydrogel as well as algae.The outermost gelatin layer can be used as an excellent alternative to traditional soft friction materials due to its abundance of polyhydroxy structures and its high-negative electron affinity.Meanwhile, owing to its strong adhesion when exposed to water, it can easily adhere to human skin.The intermediate agar hydrogel served as the electrode, while the seaweed was sandwiched between the gelatin and the agar hydrogel layers for protecting the gelatin from water erosion.To achieve accurate pressure monitoring as well as early warning interaction, the authors also developed an infant care system based on the TENG-based hydrogel sensor network, including signal processing, deep learning algorithm, and APP display terminal.By connecting 11 TENG-based hydrogel sensors to various locations on the infant's body, including chest, hands, knees, feet, neck, back, wrists, as well as hips, a network of body domain sensors was established.The deep learningassisted self-powered sensor network enabled accurate and fast acquisition of mechanical pressure, as well as the recognition of various types of infant movements.After 200 training sessions, the algorithm achieved high classification accuracy and robustness, with prediction accuracy up to 100% for distinguishing different motion signals.Additionally, the customized APP program could wirelessly transmit motion signals to cell phones, enabling convenient motion monitoring as well as real-time warning.This work demonstrates the practical significance ofthe deep learning-assisted intelligence system for infant care and management in the Internet of Things era.

Touch Panel
[154][155] These properties can be effectively achieved through the utilization of flexible and transparent hydrogel-based artificial skin.Lai et al. [156] prepared a self-powered self-healing touch-sensitive artificial skin for mobile phone touch panel(Figure 10a,b).This innovative panel exhibited excellent transparency and ultrahigh tensile property by using HTS-c-hydrogel as the TENG electrode.Notably, this phone touch panel can still perform dialing functions even after being cut and subsequently self-healed, thus opening up new possibilities for the advancement of the next generation of smart robots and flexible HMI.However, it is worth noting that the currently reported hydrogels have some shortcomings, such as low transparency, dehydration, and ease of freezing at low temperatures, leading to a loss of conductivity for applications of wearable hydrogel sensors in HMI.To address these concerns, Wu et al. [157] developed an organic hydrogel-based freezeresistant pressure sensor for application as a self-powered calculator (Figure 10c,d).This device, equipped with a touch panel and a microcontroller unit, utilized transparent, stretchable, stable, and conductive gelatin/NaCl organic hydrogel as working electrode, which could meet a wide range of application requirements from wearable and transparent electronic devices to artificial skins for HMI.
For stretchable and light-emitting displays, ion-conductive hydrogels are typically used as ionic conductors to sandwich light-emitting material-contained elastomeric layers.When the voltage is applied, the electric signal is transmitted through two ionic hydrogel conductors and stimulates the electroluminescent layer of the polymer.And the color emitted by the electroluminescent layer varies depending on the different luminescent materials incorporated within the elastomer.As shown in Figure 10e,f, Kim et al. [158] introduced a self-powered movement sensing display that was able to simultaneously detect and visualize finger movements.The self-powered finger gesture sensing display was realized by the humidity response, friction electrification, and structural color of the photonic crystal.It can accurately perceive various gestures of the fingers, including vertical and sliding movements, under natural humidity conditions.Moreover, the movements were simultaneously displayed in both contact and noncontact modes, providing a comprehensive representation of the finger gestures.

Conclusion and Perspectives
Artificial skin could be seamlessly integrated with curved surfaces such as biological tissues, offering great potential applications in fields of health monitoring, intelligent robots, artificial limbs, and wearable electronic devices.Artificial skin used in HMI needs to be self-powered, flexible, stretchable, self-healing, transparent, biocompatible, and reliable.Hydrogels have similarities with human tissues and organs, rendering them exceptionally suitable for implementation in artificial skin systems employed in HMI.In this article, the self-powered working mechanism of artificial skin is reviewed.The characteristics and advancement of various types of hydrogel-based self-powered artificial skin are outlined, with special emphasis on their reliability, transparency, stretchability, and self-healing capacities.Furthermore, the applications of self-powered artificial skin within HMI including intelligent control, medical care, and touch panel are presented.
Despite the extensive research in this field, the development of hydrogel-based self-powered artificial skin for HMI is still in the early stages.At present, the existing artificial skin still has a huge gap with comprehensive sensory capabilities of human skin.First, Human skin is capable of distinguishing slight touches with high precision, enabling precise manipulation and grasping, while also possessing the ability to perceive variations in temperature and humidity.However, the number of receptors in the current artificial skin is two to three orders of magnitude behind, so how to arrange such high-density sensors in the hydrogel-based self-powered artificial skin remains a challenge.Second, artificial skin needs to completely cover the surface of robots or a prosthesis like human skin, necessitating the development of adaptable, nonplanar hydrogel-based self-powered artificial skin that can effortlessly conform to mechanical motion.Third, the current HMI of self-powered hydrogel-based artificial skin could merely achieve one or two basic perceptual functions of human skin, which requires further advancements in comprehensive HMI with multiple perception capabilities.Fourth, hydrogels can degrade over time, which can limit the long-term stability and durability of artificial skins.Finally, the energy conversion efficiency of the self-powered artificial skins is crucial for their practical applications, while current hydrogel-based artificial skins usually exhibit limited power efficiency, thereby impeding their practical self-powered performance.
of HMI devices will need to overcome these challenges.With the booming development of self-powered hydrogel-based artificial skin, we hold the conviction that these kinds of electronic skins will experience expeditious growth in the near future.Consequently, the hydrogel-based self-powered artificial skin in HMI will make human life more convenient and colorful.
,b).Ionic gels consisting of the ionic liquid [1-ethyl-3-methylimidazolium dicyanamide ([EMI] [DCA]) were fabricated by UV photopolymerization of 3-methyl (methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) and acrylic acid.The ionic gel network structure was composed of dipoledipole interactions between the side chain amphiphilic functional groups in the DMAPS, as well as ion-dipole interactions between ionic liquid ions and ionized groups.The ionic gel-based TENG could harvest mechanical energy from various human motions and acted as a self-powered artificial skin.Furthermore, this ionic

Figure 8 .
Figure 8. HMI applications of mechanical control.a,b)The robot hand controlled by micropyramid patterned hydrogel tactile sensor.Reproduced with permission.[142]Copyright 2022, Wiley.c,d) The noncontact gesture recognition system to manipulate the robot for pharyngeal swab sampling.Reproduced with permission.[143]Copyright 2023, Wiley.e,f ) Wireless remote telemetry and control of smart car by using the all-in-one self-powered HMI systems.Reproduced with permission.[144]Copyright 2021, MDPI.g) The 3D motion control of a robot.Reproduced with permission.[146]Copyright 2018, American Chemical Society.h) A stable tactile patch to control the generation of robot trajectories wirelessly.Reproduced with permission.[147]Copyright 2021, MDPI.

Table 1 .
Performances of six types of hydrogels as self-powered artificial skins.