1 Introduction
In today's society, with the intensification of population aging and the continuous improvement of people's pursuit of a healthy quality of life, human health monitoring has become a key link in the healthcare field. Flexible electronic devices feature bendability, stretchability, lightness, and comfort, enabling them to conform well to human skin, thereby achieving real-time, continuous, non-invasive, and convenient monitoring as well as precise acquisition of human physiological signals and movement states
[1]. However, the continuous operation of flexible electronic devices relies on a stable and reliable energy supply. Traditional batteries, as an energy supply method, suffer from drawbacks such as large volume, heavy weight, poor flexibility, and the need for frequent charging or replacement, making it difficult to meet the requirements for long-term and stable operation of flexible wearable health monitoring devices. Therefore, developing a self-powered flexible energy harvesting and conversion technology is crucial for promoting the development of flexible wearable health monitoring devices.
Among numerous self-powered energy technologies, triboelectric nanogenerators (TENGs) stand out due to their unique advantages. Based on the principles of triboelectrification and electrostatic induction, TENGs can convert mechanical energy widely present in daily life, such as human motion, breathing, and heartbeat, into electrical energy. This energy conversion method features significant characteristics including a wide selection of materials, diverse and simple structures, low cost, and relatively high energy conversion efficiency, making it particularly suitable for powering flexible wearable devices. Since the concept of TENGs was first proposed by Academician Zhong Lin Wang's team in 2012, this technology has achieved rapid development in fields such as energy harvesting and self-driven sensing, providing a highly promising solution strategy for the self-powering issues of flexible wearable health monitoring devices.
[2]. However, traditional TENG materials still exhibit some deficiencies when applied to human health monitoring. For instance, commonly used metal electrode materials possess poor flexibility, making it difficult to achieve a tight and comfortable fit with human skin; they are prone to mechanical wear during human movement, which affects the stability and service life of the devices. Meanwhile, some rigid friction layer materials lack sufficient flexibility to adapt to the complex and variable forms of human motion. Therefore, developing new materials with high flexibility and tunability has become key to further promoting the application of TENG technology in the field of human health monitoring.
As a new type of functional polymer material, hydrogels are formed by the physical or chemical crosslinking of hydrophilic macromolecular polymers. The interior of hydrogels contains a large amount of water, endowing them with unique physicochemical properties
[3]. Hydrogels not only possess excellent flexibility and stretchability, enabling tight and comfortable adhesion to human skin, but also feature tunable mechanical properties that can be designed according to the requirements of different application scenarios to adapt to the movement characteristics of various body parts. Introducing hydrogels into the field of TENGs to construct hydrogel-based TENGs for human health monitoring brings new opportunities for self-powered operation and signal acquisition in flexible wearable health monitoring devices. Hydrogel-based TENGs not only inherit the self-powered advantages of traditional TENGs but also, leveraging the unique properties of hydrogels, effectively overcome the deficiencies of traditional TENG materials in terms of flexibility, conformability, and tunability, thereby better meeting the practical needs of human health monitoring.
Based on this, this article aims to systematically review the research progress of hydrogel-based TENGs for human health monitoring. First, the basic working principle of hydrogel-based TENGs is introduced; second, the roles of hydrogels as triboelectric layers and electrode layers in TENGs are analyzed; subsequently, reinforcement strategies for hydrogel materials used in TENGs regarding surface properties, mechanical performance, electrical conductivity, and environmental stability are elaborated; next, the application progress of hydrogel-based TENGs in monitoring human physiological signals and motion states is summarized; finally, the challenges and future development directions facing hydrogel-based TENGs are outlooked.
2 Working principle of hydrogel-based TENGs
The working principle of hydrogel-based TENGs is based on the triboelectric effect and the electrostatic induction effect. The triboelectric effect refers to the phenomenon where, when two different materials come into contact and then separate, electrons transfer from one material to the other due to differences in their ability to bind electrons, causing the two materials to acquire equal amounts of opposite charges. The electrostatic induction effect manifests as follows: when a charged object approaches a conductor, the charges within the conductor redistribute, inducing charges with polarity opposite to that of the charged object at the end closer to it, and inducing charges with the same polarity as the charged object at the end farther away.
The working modes of hydrogel-based TENGs can be classified into four types: vertical contact-separation mode, single-electrode mode, lateral sliding mode, and freestanding mode.
[4], as
shown in Figure 1.
The vertical contact-separation mode is the earliest proposed working mode and is currently the most typical working mode
[5]. In this mode, hydrogel-based TENGs consist of two materials with different electronegativities serving as triboelectric layers, separated by a certain distance, with metal electrodes added to the upper and lower surfaces of the triboelectric layers respectively. Under external force, the surfaces of the two triboelectric layers come into contact with each other, generating opposite charges. When the two layers separate due to the release of the external force, the triboelectric layer surfaces retain the charges and induce a potential difference between the two metal electrodes. These charges flow freely between the metal electrodes under the action of an electrostatic field. Through continuous contact and separation, a potential difference is generated, causing current to flow through the circuit, thereby producing an alternating current power source. Among the four fundamental working modes of hydrogel-based TENGs, the vertical contact-separation mode demonstrates significant advantages in energy conversion efficiency and the expansion of multifunctional applications, owing to its unique interfacial charge regulation mechanism
[6].
Hydrogel-based TENGs in single-electrode mode have the simplest structure, where the only electrode is grounded. Contact-separation or sliding between human skin or other conductive materials and the triboelectric layer establishes current flow between the ground and the electrode.
[7]. When the triboelectric layer is in full contact with the electrode, surface charge transfer occurs, inducing opposite charges on both surfaces without any electron flow. After separation, the induced charge on the single electrode surface decreases, balancing the potential by exchanging charge with the ground. Once they approach and contact again, electrons flow from the ground to the electrode to establish equilibrium until the two surfaces are in full contact.
Hydrogel-based TENGs in lateral sliding mode utilize two different triboelectric materials, enabling them to rub against each other laterally under varying effective contact areas. In this mode, the two triboelectric layers generate triboelectric charges through horizontal sliding, thereby creating a potential difference between the two electrodes. This potential difference varies periodically with changes in the effective contact area, generating triboelectric power. Such a potential difference drives current to flow from top to bottom, counteracting the reduction in induced potential caused by friction. Compared to the vertical contact-separation mode, the lateral sliding mode generates more contact charges through friction, thus offering higher conversion efficiency.
[8].
Hydrogel-based TENGs in independent mode include a moving triboelectric layer with two fixed electrodes positioned beneath it, interconnected via an external load. When the triboelectric layer slides in contact with the electrodes, electrons flow between the electrode layers due to the asymmetric charge distribution generated by the motion of the movable triboelectric layer, thereby reducing the potential difference. Consequently, hydrogel-based TENGs based on this mode exhibit high conversion efficiency. However, their durability is somewhat limited due to excessive wear of the electrodes.
3 Performance Optimization of Hydrogel-Based TENGs
As a material combining flexibility and tunable functionality, hydrogels demonstrate unique application potential in the field of TENGs. They can serve as either triboelectric layers or electrode layers, providing insights for the construction of high-performance TENGs.
The triboelectric layer refers to the functional layer in TENGs where contact electrification directly occurs; its core role is to generate and maintain surface electrostatic charges. During the contact-separation process, differences in electron affinity between different materials lead to electron transfer, resulting in surface electrostatic charging. This layer typically requires good dielectric properties (such as high surface charge density and high resistance) to prevent charge recombination. When serving as a triboelectric layer material, hydrogels primarily function in charge generation and storage; their surfaces directly contact another material, undergoing electron transfer to generate and trap surface electrostatic charges. In this case, the surface characteristics of the hydrogel (such as functional groups and roughness) directly participate in and dominate charge generation. Furthermore, the unique flexibility and tunability of hydrogels enable them to seamlessly adapt to various complex shapes and surface structures, thereby meeting the demands of diverse application scenarios. Meanwhile, by optimizing the surface characteristics and enhancing the mechanical properties of hydrogels, the triboelectric output performance and charge retention capability of TENGs can be significantly improved, subsequently achieving higher energy conversion efficiency.
The electrode layer refers to the conductive layer in TENGs that couples with the triboelectric layer to sense, collect, and export charges to the external circuit. Its core function is to provide a channel for rapid charge transport, achieving efficient conversion of the potential difference generated by electrostatic induction into usable current. Electrode materials must possess good electrical conductivity. When serving as an electrode layer, hydrogels primarily function in charge collection and transmission. The ions or electrons within them act as charge carriers, responding to the electrostatic induction from surface charges on the triboelectric layer to form displacement or conduction currents, which are then delivered to the external circuit; the hydrogel itself does not directly participate in contact electrification. In this context, key parameters such as mechanical and electrical properties must be comprehensively considered to optimize the triboelectric output characteristics of TENGs.
In summary, the comprehensive properties of hydrogels, including surface characteristics, mechanical performance, and environmental stability, are closely related to their functional realization and application efficiency in TENGs. Therefore, systematically enhancing the comprehensive performance of hydrogels is key to promoting their application in the field of TENGs.
3.1 Surface characteristics
The surface properties of hydrogels, such as surface energy, roughness, and functional group distribution, directly influence the charge generation and transfer processes in the fabricated TENGs. Compared to other properties like dielectric performance, surface characteristics play a more direct and dominant role in the contact electrification process and interfacial charge behavior of hydrogel-based TENGs. This is because the essence of contact electrification is electron transfer caused by differences in electron affinity between the surfaces of two different materials during contact and separation. This process is closely related to surface morphology and chemical state; surface roughness and microstructure directly affect the real contact area, while the type and distribution of surface functional groups determine the local electronic state density and work function, thereby regulating the probability and direction of electron tunneling.
To improve the performance of hydrogel-based TENGs, constructing microstructures on the hydrogel surface is an effective method. This approach increases the specific surface area of the hydrogel and enhances charge transfer efficiency, thereby boosting the output performance of TENGs. A common method for constructing microstructures on hydrogel surfaces is the template method
[9], which utilizes molds with different microstructures to construct various microstructures on their surfaces, allowing precise control over their shape, size, and arrangement. Common microstructures include pyramid structures
[10], hemispherical structures
[11], etc. Shi et al.
[12], inspired by the microstructures on natural leaf surfaces, fabricated gelatin-based hydrogels with pyramid microstructures on their surfaces. Using these as the positive triboelectric layer and PDMS as the negative triboelectric layer, they constructed a vertical contact-mode gelatin-based TENG (LMFG-TENG), as
shown in Figure 2. The results indicate that the maximum open-circuit voltage and short-circuit current of the LMFG-TENG are 320 V and 0.80 μA, respectively, representing 5.8-fold and 3.8-fold improvements compared to TENGs without surface structuring. This is primarily attributed to multiple physical mechanisms introduced by the surface pyramid microstructures. First is the geometric enhancement effect. According to contact mechanics, under the same normal load, the pyramid microstructures generate a larger real contact area between the gelatin-based hydrogel and PDMS. This enables more molecular or atomic pairs to participate in electron transfer. Based on the quantum mechanical model of triboelectrification, the generated surface charge density is directly proportional to the effective contact area, thereby linearly increasing the initial charge intensity. Second is the electric field concentration effect. The sharp vertices of the pyramid structure follow the "tip effect" of electrostatic fields. According to Laplace's equation, regions with smaller radii of curvature exhibit more concentrated electric field strength. This enhanced local electric field produces two key effects at the moment of contact-separation: firstly, it significantly lowers the potential barrier for charge backflow, suppressing interfacial charge recombination; secondly, acting as a powerful driving force, it accelerates the directional separation and extraction efficiency of charge carriers, thereby improving charge transfer efficiency. Third is the directional transport effect. The periodic pyramid microstructures constitute physical channels for charge transport. The regular geometric edges and vertices provide low-resistance priority paths for the diffusion of surface charges and the collection of induced electrode charges, reducing scattering and relaxation losses of charges on disordered surfaces, and optimizing the overall kinetic process of charges from generation points to collection points. Ultimately, the synergy of these three factors enhances the charge transfer efficiency of the LMFG-TENG, manifested as a significant increase in maximum open-circuit voltage and short-circuit current. Therefore, surface microstructures, by coupling geometric effects, field effects, and transport effects, physically reconstruct the interfacial environment of contact electrification, representing one of the fundamental strategies for enhancing TENG performance.
3.2 Mechanical Properties
Hydrogel-based TENGs can convert mechanical energy generated by human activities such as walking, speaking, pulse vibrations, and typing into electrical signals. However, when subjected to external mechanical forces, hydrogel-based TENGs may suffer damage, such as cracking or fracture, which adversely affects their overall performance.
[13]. Therefore, improving the mechanical properties of hydrogels has become a key focus of current research. Currently, the enhancement of mechanical properties in hydrogel-based TENGs is primarily achieved through three approaches: introducing chemical bonds, constructing multiple networks, and adding nanofillers.
3.2.1 Introduce chemical bonds
Chemical bonds can be classified into two major categories: covalent bonds (such as C—C and C—O bonds) and non-covalent bonds (such as hydrogen bonds, ionic bonds, and hydrophobic interactions). Hydrogels with network structures built on covalent bonds possess high mechanical strength and stability, providing a rigid skeleton for the hydrogel. However, such hydrogels are typically brittle; once fractured, cracks propagate rapidly. In contrast, hydrogels with network structures formed by non-covalent bonds have relatively lower bond strengths. Under stress, non-covalent bonds can dissipate energy through reversible breaking and reformation processes, thereby enhancing the toughness and recoverability of the hydrogel.
Sun et al.
[14]Through a simple esterification reaction, hydroxyethyl cellulose containing both acrylate and hydroxyl groups (HECM) was successfully synthesized. It was used as a macromolecular crosslinker to polymerize with acrylic acid (AA), preparing a double-crosslinked cellulose composite hydrogel, as
Figure 3shows. Testing revealed that the tensile strength of this hydrogel can reach 110.0 kPa, with an elongation at break of 316.2%. This is because HECM and AA not only formed covalent bonds such as C—C and C—O through free radical polymerization but also formed hydrogen bonds between hydroxyl and carboxyl groups. The synergistic effect between them achieved a significant improvement in the mechanical properties of the hydrogel. Furthermore, using PDMS as the friction layer and the cellulose composite hydrogel as the electrode, a single-electrode TENG (H-TENG) was fabricated. During over 1000 cycle tests, its output voltage remained stable at 6.4 V. Feng et al.
[15]prepared a double-crosslinked network composite hydrogel based on multi-walled carbon nanotubes (MWCNTs), cellulose nanowhiskers (CNWs), polyacrylamide (PAM), and sodium alginate (SA). Among them, a double-crosslinked network structure was formed between the SA and PAM molecular chains. Meanwhile, the —COOH and —OH groups on the SA molecular chains and the —NH groups on the PAM molecular chains
2formed physical hydrogen bonds between them. This interaction effectively enhanced the elongation at break of the hydrogel. When the hydrogel is subjected to external force, the hydrogen bonds inside it undergo dynamic breaking and reformation. This characteristic endows the hydrogel with certain tensile strength. Based on the above performance, this hydrogel can serve as an electrode material for assembling a single-electrode TENG, powering small electronic devices by generating alternating current voltage.
3.2.2 Constructing Multiple Networks
Compared to single-network and double-network structures, multi-network hydrogels have a more complex structure. Multi-network hydrogels are formed by the interweaving of identical or different polymers through chemical or physical cross-linking, with main types including interpenetrating networks, semi-interpenetrating networks, etc.
[16]. During the preparation of hydrogels, constructing multiple networks can increase the number of internal binding sites, thereby effectively enhancing the mechanical strength of the hydrogels.
[17].
Wang et al.
[18]employed a ternary system consisting of tannic acid (TA)@titanium carbide (MXene), bleached softwood pulp dissolved in ZnCl
2 solution-prepared cellulose (ZnCl
2-cellulose), and malic acid to fabricate a multi-network conductive hydrogel (TCM hydrogel) with excellent mechanical properties, good environmental adaptability, and superior electrochemical performance, as
shown in Figure 4. In this hydrogel, TA@MXene nanosheets immobilize the interpenetrating polymer segments through strong hydrogen bonding interactions; meanwhile, the ZnCl
2-cellulose skeleton, rich in hydroxyl groups and metal coordination bonds, forms crosslinks via hydrogen and ionic bonds, thereby significantly enhancing the mechanical properties of the hydrogel. Subsequently, using this hydrogel as the electrode layer and Ecoflex as the triboelectric layer, the fabricated single-electrode TENG exhibits excellent flexibility and can withstand various deformations including stretching, twisting, and folding. Furthermore, Wang et al.
[19]prepared a conductive hydrogel (CCH) based on carboxymethyl chitosan (CA) and allyl glycidyl ether (AGE) via a one-pot method. Specifically, the amino and hydroxyl groups inherent in CA form numerous hydrogen bonds; on this basis, AGE is grafted onto the CA molecular chains, and through addition reactions between the double bonds on AGE, covalent crosslinking between CA chains is achieved. The construction of this multi-crosslinked network significantly improves the mechanical strength of CCH, which also possesses excellent ionic conductivity and anti-freezing performance at low temperatures of -40 °C. Finally, using CCH as the electrode layer and VHB as the triboelectric layer, a single-electrode TENG (CCH-TENG) was successfully fabricated. Results indicate that the CCH-TENG demonstrates good stability under various complex conditions, including temperature variations, repeated stretching, long-term storage, and multiple contact-separation motion cycles.
3.2.3 Add nano-fillers
Adding nanofillers to hydrogels can significantly enhance their strength, modulus, and toughness through mechanisms such as load transfer, increased physical crosslinking/restricted chain motion, introduction of energy dissipation mechanisms, and improved effective crosslinking density. Common nanofillers include cellulose nanofibers (CNF)
[20], ZIF-8 nanocrystals
[21], graphene oxide
[22], etc.
Zhang et al.
[23]Using MXene as a crosslinker and cellulose nanofibers (CNF) as nanofillers, through the polymerization of AA, acrylic acid-
N-hydroxysuccinimide ester (AA-NHS) and sulfobetaine methacrylate (SBMA), a PAS/CNF/MXene hydrogel with conformal adhesion properties was successfully prepared, as
shown in Figure 5. AA and SBMA copolymerize to form the main network structure of the hydrogel, while the introduced CNF plays a key role, significantly increasing the fracture elongation of the hydrogel from 1737% to 2240%. Subsequently, using it as an electrode, a hydrogel-based TENG was constructed for self-powered sensing. The results indicate that this TENG exhibits good electrical output performance and can utilize mechanical energy collected during human walking to charge commercial capacitors. Qiu et al.
[24]employed 1D polymer chains, namely PAM and polydopamine (PDA), along with 2D nanosheets, including MXene and graphene oxide, jointly serving as quadruple crosslinkers to prepare a multifunctional hybrid hydrogel (H-hydrogel). In this system, the introduced MXene and graphene oxide are tightly bound to the PAM and PDA polymer chains via hydrogen bonding, thereby constructing a unique quadruple crosslinked structure. This structure endows the H-hydrogel with multiple excellent properties such as good flexibility, conductivity, self-adhesion, and mechanical stability. Furthermore, when the H-hydrogel is used as an electrode to construct a TENG, due to the dynamic equilibrium state of the electric double layer between the nanosheets and water molecules inside the hydrogel, the TENG exhibits high electrical output performance, laying a solid foundation for its application in the field of energy harvesting and conversion.
3.3 Conductivity
The electrical conductivity of hydrogels plays a crucial role in the energy conversion efficiency and output stability of the constructed TENGs; improving conductivity can enhance charge storage and transfer capabilities
[25]. To endow hydrogels with good conductivity, NaCl
[26], KCl
[27], LiCl
[28], and ILs
[29]and other ionic conductive materials, or polyaniline (PANI)
[30], MXene
[31], multi-walled carbon nanotubes (MWCNTs)
[15], and silver nanoparticles (Ag NPs)
[32]and other electronic conductive materials are compounded with insulating polymer skeletons to prepare conductive hydrogels.
3.3.1 Add ionically conductive materials
Ionically conductive gels are prepared by integrating electrolyte salts (such as NaCl, LiCl) or ionic liquids into the three-dimensional network structure of the gel. The essence of their conduction mechanism is ion migration, with charge carriers being the dissociated cations and anions (such as Na+, Cl-, Li+). In the hydrated environment of the polymer network pores, these ions undergo directional migration under the drive of an external electric field (the induced electric field generated by TENG triboelectric charges), forming a conduction current. Their conductivity is significantly influenced by the ion hydration radius, polymer network pore size, and water content. When such hydrogels are used as electrode layers for TENGs, their working mechanism is primarily based on ion redistribution triggered by electrostatic induction. Electrostatic charges on the surface of the triboelectric layer induce opposite image charges in the nearby ionically conductive hydrogel electrodes. However, unlike the instantaneous response of free electrons in metal electrodes, ionic charges achieve electrostatic equilibrium through the physical displacement of ions, a process that forms a displacement current.
Patnam et al.
[33]Cross-linked with polyvinyl alcohol (PVA) and sodium carboxymethyl cellulose (CMC), and incorporating LiCl as a conductive substance, a CMC/PVA hydrogel was prepared. Subsequently, using this as the electrode layer and silicone rubber as the triboelectric layer, a single-electrode TENG (S-TENG) with flexibility and shape-adaptive characteristics was fabricated, as shown in
Figure 6. During the operation of the S-TENG, when the silicone rubber contacts the skin, electron transfer occurs due to the difference in surface electron affinity between the two materials, thereby generating triboelectric charges. Meanwhile, the presence of LiCl in the hydrogel allows ions to move rapidly between the electrode and the triboelectric layer interface during the triboelectrification process, effectively promoting charge separation and transport while reducing charge recombination losses. Testing revealed that the open-circuit voltage and short-circuit current of the S-TENG were 584 V and 25 μA, respectively, with a charging density of 120 μC/m².
2. Similarly, Xie et al.
[34]prepared a hydrogel (GNOH) using gelatin as the polymer and NaCl as the conductive substance. Furthermore, a single-electrode TENG was fabricated using GNOH as the electrode layer and Ecoflex as the triboelectric layer. During the triboelectrification process, Ecoflex acquires a negative charge; when Ecoflex contacts the GNOH electrode, the movement of NaCl ions within the GNOH helps maintain charge balance at the interface, promoting continuous charge separation and transport. Testing showed that the open-circuit voltage and short-circuit current of this TENG were 142 V and 3.8 μA, respectively, with a power output of 19.36 μW and a response time of 85 ms.
3.3.2 Add electronic conductive materials
Electronically conductive hydrogels are prepared by compositing intrinsic conductive materials (such as conductive polymers, carbon nanomaterials, or metal nanomaterials) as fillers or continuous phases with a gel matrix. Their conduction mechanism essentially involves the transport of electrons or holes, where charge carriers are free electrons or holes transmitted through the filler's own continuous conductive network or via quantum tunneling effects. Their electrical conductivity is primarily determined by the intrinsic conductivity of the fillers, their concentration (whether exceeding the percolation threshold), and interfacial contact resistance. They feature fast electron migration speeds, high response frequencies, and the ability to form current within extremely short timeframes. When used in TENGs, electronically conductive hydrogels serve mainly as high-performance electrodes or functionalized triboelectric layers. This is because their excellent conductivity meets the stringent requirements for electrode materials during charge generation, transport, and collection in TENGs, helping to improve the energy conversion efficiency and output performance of TENGs. Their working mechanism is based on rapid electronic cloud shielding and conduction. Image charges induced by triboelectric charges are balanced by the nearly instantaneous rearrangement (with response times reaching the microsecond level) of free electrons/holes within the filler network, forming a conduction current.
Long et al.
[35]Using PAM and PVA as polymers and MXene as the conductive material, a conductive hydrogel (MPP) was prepared, as
Figure 7shows. During the preparation of MPP, the introduced MXene, by virtue of its excellent intrinsic conductivity, constructed efficient electron transport channels for the hydrogel, significantly enhancing its conductivity; furthermore, due to the presence of surface functional groups on MXene, it promoted the formation of DELs between MXene and free water molecules within the hydrogel. When MPP is used as the electrode layer of a TENG, driven by the initial potential difference generated from contact electrification, ions in the DELs undergo directional movement under the electric field and rapidly reach a dynamic equilibrium state. This dynamic equilibrium enables the electrode surface to stably store and release charges, reducing charge recombination and loss, thereby significantly enhancing the electrical output performance of the TENG. The results indicate that the output voltage of the TENG fabricated with the MXene-added hydrogel increased by 4.5 times compared to the case without MXene. Feng et al.
[15]prepared a conductive hydrogel using PAM and SA as polymers and adding MWCNTs as the conductive material. The high aspect ratio of MWCNTs allows them to interconnect within the hydrogel, forming a continuous conductive network. Meanwhile, certain defects and functional groups exist on the surface of MWCNTs; these defects and functional groups can interact with PAM and SA molecular chains, enhancing the interfacial bonding force between MWCNTs and the hydrogel matrix, thereby ensuring the stability of the conductive network. When this conductive hydrogel is used as an electrode layer to assemble a single-electrode TENG, during the triboelectrification process, upon contact between the hydrogel electrode and the triboelectric layer, electron transfer occurs due to the difference in surface electron affinity between the two, generating triboelectric charges. The MWCNT conductive network within the conductive hydrogel can rapidly and efficiently transport the generated triboelectric charges to the external circuit, achieving charge collection and utilization. This efficient charge transport capability enables the TENG to continuously and stably generate an open-circuit voltage of 28 V. This self-powered characteristic demonstrates great potential for power supply applications in small electronic devices.
3.4 Environmental Stability
The output performance of hydrogel-based TENGs highly depends on the mechanical toughness and conductivity of the hydrogel, both of which are closely related to the state of water within the hydrogel. Since water is the main component of hydrogels, they are prone to evaporation (under high-temperature or dry conditions) or freezing (at sub-zero temperatures) during long-term use or in extreme environments, leading to loss of conductivity, reduced flexibility, and even structural embrittlement, ultimately causing performance degradation of the TENG. Therefore, enhancing the environmental stability of hydrogels, particularly their temperature resistance and moisture retention, is key to expanding their practical application scenarios. Existing research mainly focuses on constructing composite hydrogels with persistent water retention and anti-freezing capabilities by adding ionically conductive materials, zwitterionic osmolytes, or polyols to the hydrogel.
[36].
3.4.1 Add ionically conductive materials
Regarding the environmental stability of hydrogel-based TENGs, the mechanism of action of adding ionically conductive materials primarily relies on high-concentration ions (such as CaCl2, LiCl, [EMIM][BF4]]) and the strong hydration between them and water molecules. From a microscopic perspective, multiple water molecules are attracted around the ions, forming hydrated ion layers. This strong hydration effectively binds water molecules, restricting their free movement, thereby significantly reducing the water activity of the system. At this point, the moisture in the hydrogel is less likely to participate in chemical reactions or undergo changes in physical state, such as evaporation or freezing. Therefore, this characteristic of ionically conductive materials can significantly enhance the stability of hydrogels under complex environments, endowing them with excellent properties such as anti-freezing and anti-drying capabilities. In low-temperature environments, since water molecules are bound by strong hydration and difficult to form ordered ice crystal structures, the freezing of the hydrogel is prevented; in dry environments, water molecules are not easily lost through evaporation, ensuring the moisture content within the hydrogel and maintaining the stability of its structure and performance.
Wang et al.
[37]By combining a conductive rigid cellulose/Zn
2+/Ca
2+network with a soft PVA/borax network, a dual-network PVA/cellulose hydrogel was constructed, as
Figure 8shows. Among them, the cellulose in the rigid cellulose/Zn
2+/Ca
2+network possesses high crystallinity and excellent mechanical strength. Zn
2+and Ca
2+act as ionic crosslinkers, capable of undergoing complexation reactions with functional groups on the cellulose molecular chains to form stable ionic crosslinked structures, providing rigidity and strength support for the hydrogel. In the soft PVA/borax network, the PVA molecular chains exhibit good flexibility and hydrophilicity. Borax undergoes an esterification reaction with the hydroxyl groups on the PVA molecular chains, forming dynamic reversible borate ester bonds, which endow the hydrogel with certain flexibility and self-healing capabilities. The synergistic interaction of the two networks enables the hydrogel to possess excellent mechanical properties, self-adhesive characteristics, transparency, and recyclability. Additionally, the high concentrations of ions such as Zn
2+and Ca
2+introduced into the hydrogel undergo strong hydration with water molecules. This interaction effectively reduces the water activity of the hydrogel system, endowing the hydrogel with excellent antifreeze properties. Under low-temperature conditions, water molecules are tightly bound by ions, making it difficult to form ice crystals, thereby avoiding volume expansion and structural damage caused by freezing, and ensuring the stability and integrity of the hydrogel in low-temperature environments. Furthermore, using this dual-network PVA/cellulose hydrogel as the electrode layer and PDMS as the triboelectric layer, a single-electrode TENG was fabricated. The results indicate that within a wide temperature range from -40 °C to 50 °C, the TENG maintains stable electrical output performance, with an open-circuit voltage reaching up to 211.3 V. In low-temperature environments, due to the antifreeze property of the hydrogel, its internal structure and ionic conductivity are not significantly affected, allowing it to continue generating and transporting charges normally, thus enabling the TENG to operate stably. In high-temperature environments, the stable structure of the hydrogel and the presence of high-concentration ions also prevent excessive evaporation of water and volatilization of ions, ensuring smooth charge transport. This result fully demonstrates the reliability and practicality of the dual-network PVA/cellulose hydrogel under extreme temperature conditions.
3.4.2 Add zwitterionic permeate
Zwitterionic osmolytes can achieve efficient "water locking" at the molecular scale through strong hydration, thereby directly inhibiting the migration of water molecules. Specifically, zwitterionic molecules possess unique structural characteristics, carrying both positive and negative charges simultaneously. This coexistence of positive and negative charges makes zwitterionic molecules powerful "water molecule adsorption centers," capable of binding tightly to a large number of water molecules through strong electrostatic interactions. When zwitterionic molecules encounter water molecules, the positive charges attract the oxygen atoms (negatively charged) in the water molecules, while the negative charges attract the hydrogen atoms (positively charged), subsequently forming a dense "hydration shell" around the zwitterionic molecules. This "hydration shell" acts like a sturdy "cage," firmly trapping water molecules within it, effectively restricting their free movement, and greatly enhancing the water retention capacity of the hydrogel. Even in relatively dry environments or when facing certain degrees of temperature changes, water molecules find it difficult to break free from the constraints of the "hydration shell" and escape, thus ensuring the stability of the water content within the hydrogel. Commonly used zwitterionic osmolytes include betaine, sulfobetaine, etc.
Lei et al.
[38]prepared a multifunctional polyvinylpyrrolidone/poly(AAm-
co-1-vinyl-3-butylimidazolium bromide)/cellulose nanofiber-betaine ionic conductive hydrogel (PVP/P(AAm-
co-VBIMBr)/CNF-BA, abbreviated as PPAVC-BA), as
shown in Figure 9. During the preparation of this hydrogel, P(AAm-
co-VBIMBr) formed a covalently crosslinked network via free radical polymerization, providing stable structural support for the hydrogel. Meanwhile, the introduction of the zwitterionic betaine BA significantly enhanced the conductivity and antifreeze performance of the hydrogel. In terms of conductivity, the ions within betaine BA can move freely inside the hydrogel; when an electric field is applied, these ions move directionally to form an electric current, thereby significantly enhancing the hydrogel's conductivity. This excellent conductivity enables the hydrogel, when used as an electrode material, to efficiently transmit charges, reduce charge loss during transmission, and improve the efficiency of electrical signal transmission. Regarding antifreeze performance, the "hydration shell" formed between betaine BA and water molecules plays a key role. In low-temperature environments, due to the restraining effect of the "hydration shell" on water molecules, it becomes difficult for water molecules to arrange into ordered ice crystal structures, effectively inhibiting the freezing phenomenon of the hydrogel. This significantly enhances the antifreeze performance of the hydrogel, ensuring its normal function. Benefiting from the superior properties of the PPAVC-BA hydrogel, the single-electrode TENG fabricated from it (PPAVC-BA-TENG) still exhibits good electrical output performance under extreme temperature conditions (as low as -50 °C and as high as 80 °C), demonstrating its potential for applications across a wide temperature range.
3.4.3 Add polyols
Polyol molecules are rich in multiple hydroxyl groups, which play a critical role in enhancing the environmental stability of hydrogel-based TENGs. Firstly, the hydroxyl groups in polyol molecules can form hydrogen bonds with water molecules; this additional hydrogen bonding interferes with the original hydrogen bond network of water. When the temperature drops and water molecules attempt to arrange themselves regularly to form ice crystals, the hydrogen bonds formed between polyol hydroxyl groups and water molecules disrupt this arrangement order, making it difficult for water molecules to form complete, regular ice crystal structures, thereby effectively inhibiting ice crystal formation. Additionally, as strong hydrophilic groups, hydroxyl groups exhibit a powerful attraction to water molecules. Water molecules tightly bound by polyol hydroxyl groups have their thermal motion significantly restricted; escaping this binding through evaporation requires overcoming a greater energy barrier. Therefore, in dry environments, the presence of polyols significantly reduces the rate of water loss and drying in hydrogels, enabling them to maintain their water content and shape stability for extended periods. Commonly used polyols include glycerol, ethylene glycol, and polyethylene glycol.
Han et al.
[39]In a glycerol (Gly)/H
2O binary solvent containing NaCl and gelatin, a double-network conductive gel (PASG/NaCl/Gly) was successfully synthesized by polymerizing acrylamide (AM) with the zwitterion [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA), as
Figure 10shows. The synergistic effect among polyols, NaCl, and zwitterions enables PASG/NaCl/Gly to maintain good flexibility and internal structural stability even at low temperatures. At low temperatures, although the movement of water molecules is somewhat restricted, the hydrogen bonds formed between polyol hydroxyl groups and water molecules, along with the "hydration shell" structure of the zwitterions, effectively inhibit ice crystal formation, ensuring that the hydrogel does not become hard and brittle due to freezing. Using this conductive gel as the electrode layer and Ecoflex as the triboelectric layer, a single-electrode TENG (gel-TENG) was fabricated. Tests showed that in a low-temperature environment of -18 °C, the gel-TENG exhibited a fast response capability (47 ms) and a short recovery time (47 ms), indicating its ability to achieve efficient sensing in low-temperature environments, making it suitable for application scenarios requiring precise and rapid responses.
3.5 Collaborative Optimization Strategy
In the performance optimization of hydrogel-based TENGs, a single strategy often fails to balance multiple requirements such as mechanical properties, electrical properties, and environmental stability. Therefore, constructing an integrated structure-function hydrogel system through the synergy and integration of multiple optimization strategies has become an important approach to enhancing their comprehensive performance. For instance, combining multi-network structures with nanofillers not only enhances the mechanical strength and toughness of the hydrogel but also simultaneously improves its conductivity and environmental adaptability, achieving a comprehensive improvement in performance.
Wang et al.
[40]By compounding PVA and PAM with Zn
2+-crosslinked and polyaniline (PANI)-anchored CNFs (CNFs-Zn
2+), a multi-network conductive hydrogel (PBACA-Zn
2+) was prepared, as
shown in Figure 11. In this system, CNFs-Zn
2+ are crosslinked with the polymer network via hydrogen bonds and metal coordination bonds, forming an energy dissipation phase that fractures prior to the covalent network, thereby significantly enhancing the strength and toughness of PBACA-Zn
2+. Meanwhile, the aligned CNF crystalline domains impart PBACA-Zn
2+ with the ability to inhibit crack propagation, thus improving fatigue resistance. Furthermore, PANI anchored on the CNF surfaces forms continuous conductive pathways during mechanical training, increasing the conductivity of PBACA-Zn
2+ to 12 S/m. Additionally, the BF
4)
2 salt introduced contains BF
4- anions that form hydrogen bonds with water molecules, disrupting ice crystal growth and effectively lowering the freezing point, enabling PBACA-Zn
2+ to maintain stable structure and performance at low temperatures. A single-electrode TENG (S-TENG) assembled using this hydrogel as the electrode layer and Ecoflex as the negative triboelectric layer demonstrated stable electrical output over 3000 cycling tests, validating the effectiveness of this synergistic strategy in enhancing device durability and environmental stability.
In summary, aiming at the surface characteristics, mechanical properties, electrical conductivity, and environmental stability of hydrogels, the advantages and disadvantages of various optimization strategies were comparatively analyzed, such as
Table 1shown. In the research process, the performance of different optimization strategies in improving the output power of hydrogel-based TENGs, reducing energy loss, extending service life, and expanding their application scenarios should be fully considered.
4 Application of Hydrogel-Based TENGs in Human Health Monitoring
Hydrogel-based TENGs demonstrate unique advantages in monitoring human physiological signals and motion states due to their excellent flexibility, biocompatibility, high sensitivity, and environmental adaptability. By converting minute human mechanical deformations (such as muscle contraction, joint movement, and breathing fluctuations) into electrical signals, they enable non-invasive, real-time, and continuous health monitoring, holding great promise in the fields of wearable medical devices, remote health management, and sports rehabilitation.
4.1 Human physiological signal monitoring
Human physiological signals (such as ECG and respiratory signals) are core indicators reflecting health status. Hydrogel-based TENGs, by adhering to the human skin surface, can efficiently collect mechanical deformation and convert it into electrical signals, achieving comfort and long-term stability in monitoring.
4.1.1 ECG Monitoring
Electrocardiogram (ECG) signals are important physiological signals reflecting the heart's electrical activity and hold significant value for the diagnosis and monitoring of heart diseases. Hydrogel-based TENGs can be attached to the human chest or pulse points, converting the tiny mechanical vibrations generated by pulse beats and heartbeats into electrical signals, thereby enabling real-time, continuous monitoring of ECG signals and providing strong support for the early diagnosis and prevention of heart diseases.
Wang et al.
[41]prepared a conductive hydrogel (h-CAPAM-Li
+) based on SA, AM, and LiCl, and fabricated a single-electrode TENG using it as the electrode layer and PDMS as the triboelectric layer. When attached to the human chest, this TENG captures periodic heartbeat vibration signals with sinusoidal waveforms and high signal-to-noise ratios under resting conditions. During movement, despite baseline fluctuations caused by body motion, it successfully captures accelerated heartbeat signals while maintaining stable peak resolution.
4.1.2 Breathing Monitoring
Respiration is a vital process of human life activities, and respiratory monitoring holds significant importance for diagnosing respiratory system diseases and assessing sleep quality. Hydrogel-based TENGs can be attached to the human chest or abdomen to collect mechanical deformations generated by respiratory movements, converting them into electrical signals to achieve monitoring of parameters such as respiratory rate and depth.
Li et al.
[42]prepared a PEGDA/Lap nanocomposite hydrogel based on polyethylene glycol diacrylate (PEGDA) and laponite (Lap) composites, and constructed a single-electrode TENG (BS-TENG) using it as the electrode layer and polylactic acid (PLA) as the triboelectric layer. When attached to the human abdomen, this TENG can record repetitive signals during breathing, detecting and distinguishing between shallow and deep breaths through the amplitude of voltage signals, thereby assessing sleep status.
4.2 Human Motion Status Monitoring
In addition to monitoring human physiological signals, hydrogel-based TENGs can also monitor human motion states by collecting dynamic deformations such as joint movements, gait, and neck movements, providing support for sports injury prevention, rehabilitation training assessment, and human-computer interaction.
4.2.1 Joint Movement
Joint movement is a crucial component of human motion. Monitoring joint movement holds significant importance for preventing sports injuries and guiding rehabilitation training. Hydrogel-based TENGs can be attached to human joints to collect mechanical deformation and friction generated during joint movement, converting them into electrical signals to monitor parameters such as joint angle, velocity, and acceleration. By analyzing these parameters, the motion state and function of the joint can be understood, providing a basis for diagnosing sports injuries and conducting rehabilitation training.
Chen et al.
[43]Using PDA/MXene/
N-isopropylacrylamide (NIPAM) hydrogel as the electrode layer and PDMS as the triboelectric layer, a PMN-TENG was fabricated. By attaching it to human joints such as the elbow and knee, the PMN-TENG can effectively monitor various movements of athletes. For instance, during knee movement, the output voltage of the PMN-TENG increases from 6.24 V during walking to 17.16 V during jumping. Furthermore, there is a direct relationship between the peak output voltage of this TENG and the intensity of human movement; the intensity can be evaluated based on the fluctuation of the output voltage signal corresponding to each action, thereby helping athletes better understand their training effects.
4.2.2 Gait Analysis
Gait analysis is an important method for studying the patterns and characteristics of human walking, holding significant value for diagnosing neurological diseases and evaluating rehabilitation outcomes. Hydrogel-based TENGs can be integrated into insoles or shoes to collect contact forces and motion parameters between the foot and the ground during walking, converting them into electrical signals to achieve gait analysis. By analyzing gait parameters such as step length, cadence, and gait cycle, one can understand human walking ability and health status, providing references for disease diagnosis and rehabilitative treatment.
Wang et al.
[44]prepared a graphene oxide-polyacrylamide (GO-PAM) hydrogel as the friction layer, with silver nanowires as the top electrode and copper as the bottom electrode attached to the upper and lower surfaces of the GO-PAM hydrogel, respectively, to form a "sandwich-structured" TENG for monitoring human gait movements, such as
Figure 12shown. Four TENGs were installed on the soles and heels of the left and right feet, respectively, to collect analog voltage signals generated by the four TENGs. After converting the analog voltages into digital signals, they were transmitted to a computer by a microcontroller (MCU). Subsequently, the collected raw signals were filtered to remove noise while preserving the useful features of the original signals. Finally, artificial neural network (ANN), decision tree, and random forest models were constructed to recognize and analyze gait data. These three models were applied to identify Parkinson's disease, left hemiplegic gait, and right hemiplegic gait to monitor patient recovery in real time.
4.2.3 Neck exercises
Neck motion monitoring is of great significance for preventing cervical spine problems caused by long-term use of digital devices and a sedentary lifestyle. Hydrogel-based TENGs can be attached to the human cervical spine to collect mechanical deformation and friction generated during neck muscle movement, converting them into electrical signals to achieve monitoring of cervical muscle changes. Through precise recognition of neck movements, they provide a basis for preventing cervical spine problems.
Qu et al.
[45]A single-electrode TENG (GH-TENG) was fabricated using a glycerol (GL)/PVA hydrogel as the electrode layer and silicone rubber as the triboelectric layer. Subsequently, a smart neck collar based on GH-TENGs was worn on the human neck to monitor neck muscle movement states, such as
Figure 13shows. Electrical signals generated by neck movements are captured by the smart neck collar and transmitted to a microcontroller. The microcontroller converts the collected analog electrical signals into digital signals, which are then wirelessly transmitted to a computer via a Wi-Fi module for visualization. With this smart neck collar, six distinct neck movements can be clearly observed: looking down, looking up, turning left, turning right, tilting left, and tilting right. Each movement exhibits unique waveform characteristics in its corresponding channel, facilitating intuitive differentiation and identification of various changes in neck movements. Based on this information, timely adjustment of neck movements can effectively prevent cervical spine problems.
To systematically compare the performance of different hydrogel-based TENGs in human health monitoring, the key parameters of the aforementioned typical research cases are summarized in
Table 2.
Although hydrogel-based TENGs demonstrate significant potential in the field of health monitoring, they still face certain limitations in practical wearable applications. Firstly, long-term signal stability poses a challenge: evaporation of water within the hydrogel or changes in environmental humidity can easily cause variations in its conductivity and mechanical properties, thereby affecting the long-term consistency and accuracy of signal acquisition. Secondly, wearing comfort needs improvement: the elastic modulus of some high-strength hydrogels still differs from that of human skin, and prolonged contact may lead to local pressure sensations or a feeling of foreign bodies. Furthermore, signal interference in dynamic environments cannot be ignored: sweat secreted during human movement alters the contact state at the hydrogel-skin interface, interfering with the signal-to-noise ratio of weak physiological signals such as ECG and pulse, thus increasing the difficulty of signal extraction and analysis.
5 Conclusion and Outlook
Hydrogel-based TENGs have become a key research direction in the field of flexible electronics due to their excellent flexibility, biocompatibility, and environmental adaptability. This paper systematically reviews the research progress of hydrogel-based TENGs for human health monitoring. First, the basic working principle of hydrogel-based TENGs is introduced. Second, enhancement strategies for hydrogel materials in terms of dielectric properties, surface characteristics, mechanical performance, electrical conductivity, and environmental stability are elaborated. Subsequently, applications of hydrogel-based TENGs in human health monitoring are summarized. Although significant progress has been made in the development of hydrogel-based TENGs, their future research still faces several key challenges. Specifically, further deepening and expansion can be pursued in the following three directions.
(1) Enhance the long-term durability and environmental stability of hydrogels. Achieve material self-healing by introducing dynamic covalent bonds (such as borate ester bonds) to delay micro-crack propagation; slow down water evaporation and extend device lifespan under extreme environments (such as high temperature and high humidity) through measures like regulating hydrophobic-hydrophilic microdomains and introducing low-volatility solvents (such as ionic liquids and glycerol); and improve the anti-fatigue performance of hydrogels under dynamic loads such as repeated stretching and bending via strategies including multi-level crosslinking, nanocomposites, or surface encapsulation.
(2) Optimize the interfacial adhesion and integration between hydrogels and heterogeneous materials. Systematically investigate the adhesion mechanisms of hydrogels with various substrates such as metals, polymers, and textiles, and develop universal interfacial technologies based on biomimetic adhesive layers like dopamine and polydopamine; design interfacial transition layers with gradient moduli or micro-structured patterning to alleviate stress concentration and enhance interfacial stability of devices under dynamic deformation; meanwhile, explore electrode-skin interface materials and structures featuring low impedance and high biocompatibility to improve the signal-to-noise ratio of physiological signal acquisition and wearing comfort.
(3) Promote the integration of smart responsive hydrogel-based TENGs with systems. By combining functional hydrogels with pH-responsive and temperature-responsive properties, develop intelligent sensing TENGs capable of autonomously adjusting to environmental or physiological signals; further integrate micro-energy management circuits, low-power wireless transmission modules (such as Bluetooth and NFC), and embedded signal processing units to construct flexible, self-sufficient "sensing-powering-communication" integrated systems; additionally, fuse machine learning algorithms to achieve real-time analysis of multimodal physiological signals (such as ECG, EMG, and respiration) and early anomaly warning, thereby enhancing the intelligence and practicality of health monitoring systems.
National Natural Science Foundation of China(22378253)
China Postdoctoral Science Foundation(2024M761895)
Shaanxi Provincial Department of Education Science Research Program(24JK0357)
Shaanxi Provincial science and technology plan project(2025JC-YBQN-146)
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