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Preparation of Polymer Conductive Hydrogel and Its Application in Flexible Wearable Electronic Devices

Yue Gong , Yizhu Cheng , Yinchun Hu

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (3) : 616 -629.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (3) : 616-629 DOI: 10.7536/PC210329
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Preparation of Polymer Conductive Hydrogel and Its Application in Flexible Wearable Electronic Devices
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Abstract

Hydrogels are biological materials with various properties. Hydrogels are three-dimensional network polymer with high water content, high tensile strength and biocompatibility. In addition to having excellent properties of hydrogels, conductive hydrogels have good electrical conductivity, adjustable mechanical properties and self-adhesive characteristics. The appearance of conductive hydrogels enrich types of hydrogels, expand performance of hydrogels, and improve practical application value, so that hydrogels have entered into people's daily life. Conductive hydrogels have gradually become the best candidate materials for flexible wearable electronic devices. In recent years, conductive hydrogels with biocompatibility, mechanical flexibility and fatigue resistance have been extensively studied. Conductive hydrogels can monitor and covert a wide variety of physiological signals and physical signals. Flexible wearable electronic devices based on conductive hydrogel can monitor human health status in real time. Conductive hydrogels with exceedingly good performance promote the development of flexible wearable electronic devices. Flexible wearable electronic devices have gradually become main research direction in field of human-computer interaction technology and artificial intelligence. Conductive hydrogels are synthesized by using conductive polymers, conductive fillers, free ions and their mixtures. According to conductive mechanism, manufactured conductive hydrogels can be divided into electron conductive hydrogels, ion conductive hydrogels and mixed electron-ion conductive hydrogels. In this paper, preparation methods of conductive hydrogels are discussed. The research progress and application of conductive hydrogels in aspects of stretchability, conductivity, biocompatibility, self-repairing and other functions in flexible wearable electronic devices are summarized. It is expected that conductive hydrogels will get better development.

Contents

1 Introduction

2 Conductive hydrogels

2.1 Ion conductive hydrogels

2.2 Electronic conductive hydrogels

2.3 Electron-ion conductive hydrogels

3 Properties of conductive hydrogels

3.1 Mechanical property

3.2 Conductive property and strain sensitivity

3.3 Adhesive and self-healing properties

3.4 Biocompatibility

3.5 Anti-freezing and moisturizing properties

4 Application of conductive hydrogels in flexible wearable electronic devices

4.1 Application of conductive hydrogels in vivo

4.2 Energy storage components and converters

4.3 Human motion sensors

4.4 Biological electrode

4.5 Ionic skin

4.6 Electronic skin

5 Conclusion and outlook

Graphical abstract

Keywords

hydrogels / ionic conduction / electron conduction / flexible wearable electronic devices

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Yue Gong, Yizhu Cheng, Yinchun Hu. Preparation of Polymer Conductive Hydrogel and Its Application in Flexible Wearable Electronic Devices. Progress in Chemistry, 2022, 34(3): 616-629 DOI:10.7536/PC210329

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1 Introduction

Human body temperature, heartbeat, blood sugar, blood pressure and other physiological States are constantly changing. Flexible wearable electronic devices can be attached to the surface of biological tissues for health monitoring to obtain physiological signals and then judge the physiological state of the monitored tissues[1,2]. Flexible wearable electronic devices can detect and quantify physical signals such as humidity, temperature, vibration and touch according to different physical sensing mechanisms, such as piezoresistive, piezoelectric and capacitive. In recent years, flexible wearable electronic devices have been widely studied, and significant progress has been made in flexibility, signal detection and high sensitivity, which are widely used in human physiological signal, physical signal monitoring and bionic prosthesis and other medical and health fields[3]. In addition, flexible wearable electronic devices have important research significance in the fields of human-computer interaction, artificial intelligence and big data medical treatment. With the continuous development of medical detection and artificial intelligence, the requirements for flexible wearable electronic devices in terms of the diversity of physiological signals and the range of sensing accuracy are gradually improved, and new requirements are put forward in terms of flexibility, temperature sensitivity, biocompatibility, self-powering, self-healing and self-transmission. Traditional flexible wearable electronic devices are difficult to meet the current requirements in terms of structural design, sensor conduction and substrate flexibility, which also urges wearable electronic skin to explore new materials, new structures and new properties[4⇓⇓⇓⇓⇓~10].
Traditional electronic materials are made of metal, hard and rough, and can not form intimate conformal contact with soft and moist human tissues, which is the key factor for accurate physiological signal monitoring between human soft tissues and electronic devices. Table 1 compares the advantages and disadvantages of conductive hydrogels with those of conventional conductive materials. The rapid development of biosensors and conductive hydrogels has improved the sensitivity and portability of flexible wearable electronic devices, and greatly increased the practicality of wearable electronic skin[11,12]. In recent years, hydrogels have been developed unprecedentedly in the field of biomedicine due to their good biocompatibility, low immunogenicity and good mechanical adjustability[13]. The emergence of conductive hydrogels makes up for the lack of conformal contact between biological tissues and metal sensors, and accelerates the development of flexible wearable electronic devices and artificial intelligence[14⇓⇓⇓~18]. Conductive hydrogel not only combines the excellent characteristics of hydrogel, but also has good conductivity and excellent mechanical properties, which provides a new material for the design of flexible wearable electronic devices in close contact with biological tissues.It has broad application prospects in robotic sensing, human implantable devices, wound dressings and drug controlled release[19⇓⇓⇓⇓⇓⇓⇓⇓~28]. Conductive hydrogels can be synthesized by adding conductive polymers, conductive fillers, free ions and their mixtures into hydrogels. According to the conductive mechanism, the prepared conductive hydrogels can be classified into electronic, ionic and mixed electron-ion conductive hydrogels. The most common method to prepare conductive hydrogels is to directly dope the conductive components into the hydrogel matrix or coat them on the surface of the hydrogel. By changing the conductive fillers, dopants or hydration state, the electrochemical and mechanical properties of the conductive hydrogels can be adjusted[29,30]. In this paper, the preparation methods of conductive hydrogels are discussed firstly, and the research progress of conductive hydrogels in stretchability, conductivity, biocompatibility and self-repair are summarized.Then, the application of conductive hydrogel in flexible wearable electronic devices is introduced.Finally, the development of conductive hydrogel is prospected.

2 Conductive hydrogel

Hydrogel is a network polymer with abundant water content, the hydrophilic groups on the polymer molecular chain ensure the swelling property and high water content of the hydrogel, and the intermolecular interaction force between the crosslinked network of the hydrogel and the polymer molecular chain generates cohesive force to prevent water molecules from further penetrating[31⇓⇓⇓~35]. The porous structure of the hydrogel allows water molecules to freely pass through the polymer network, and the high-frequency flow of molecules in water provides a feasible method for the preparation of conductive hydrogels.
In nature, conductivity is derived from the transport of electrons or ions. For example, inorganic materials such as metals often conduct current through electrons, while in organisms, electronic information is usually transmitted through ions[18]. Conductive hydrogels mainly achieve ionic conduction by incorporating charged salts or conductive materials into the elastic three-dimensional hydrogel network, or by adding intrinsic conductive polymers, metal nanoparticles and carbon-based nanomaterials into the hydrogel to achieve electronic conduction[36]. Unlike conventional conductive materials, the conductivity of ionically conductive hydrogels is generated by the directional movement of free charged ions in the hydrogels produced by charged salts or conductive materials[37⇓⇓⇓⇓⇓~43]. Conductive materials that generate charged ions in water can generally be divided into three categories: acids (e.g., HCl, H2SO4, H3PO4); Metal salts (e.g., LiCl, FeCl3, AlCl3); Ionic liquids (e.g., 1-ethyl-3-methylimidazolium chloride), free charged ions generated by these conductive materials move directionally in the hydrogel, providing conductivity to the hydrogel[36⇓⇓⇓⇓⇓⇓~43]. The most widely used preparation method of electronic conductive hydrogels is to directly incorporate electronic conductive materials into the hydrogel matrix. Electronic conductive materials are generally divided into three categories: metal (silver, copper, etc.) nanoparticles/nanowires; Carbon-based materials (e.g., carbon nanoparticles/carbon nanowires, graphene, graphene oxide) and conductive polymers (e.g., polystyrene)[18,36]. In the ionic conductive hydrogel network, the free moving charged ions electrolyzed from the ionic conductive material are used as the main solvent in the hydrogel network, and the high concentration of the free moving charged ions provides continuous conductivity for the hydrogel during the deformation process. However, due to the high concentration of ions, there are risks of ion leakage, evaporation at high temperature and freezing at low temperature during the deformation of hydrogels, which prompt researchers to conduct more in-depth research on ion-conducting hydrogels[18,36,49]. The inclusion of a high concentration of non-transparent conductive filler material in the electronically conductive hydrogel results in poor transparency of the electronically conductive hydrogel[44⇓~46].
At present, a variety of conductive hydrogels have been developed by combining different conductive materials and hydrogels. The basic method for preparing the conductive hydrogel comprises the following steps of: (1) forming a single network hydrogel through self-polymerization of a conductive material; (2) the conductive material is self-assembled into an ion conductive channel to form the conductive hydrogel; (3) by incorporate a conductive filler or char salt into a non-conductive hydrogel matrix; And (4) construct that interpenetrating network conductive hydrogel by dope the conductive polymer or the filler with the non-conductive polymer. Lithium chloride (LiCl) and ferric chloride (FeCl3) are highly soluble salts, which can produce high hydrated ionic Li+ and Fe3+ in water to provide good conductivity for hydrogels, and become the best metal salts for preparing ionic conductive hydrogels[47]. Graphene oxide (GO) and carbon nanotubes (CNTs) have been widely used as conductive fillers to improve the electrical conductivity of electron-conductive hydrogels, while being able to improve the mechanical properties of hydrogels to some extent. Researchers have conducted extensive research on conductive hydrogels, increased the functional diversity of conductive hydrogels, improved their electrochemical and mechanical properties, and increased the structural optimization and functional diversification of flexible wearable electronic devices. Table 2 shows the conductive components, network structure, conductive properties and applications of the conductive hydrogels introduced in this part.

2.1 Ion-conducting hydrogel

In living organisms, the conduction of electrical signals through the transport of freely charged ions is ubiquitous and extremely important[48]. The conductivity of ionically conductive hydrogels results from the directional movement of free charged ions in the hydrogel network. Ion-conducting hydrogels have similar flexibility to biological tissues, making them ideal candidates for soft substrates of flexible wearable electronic devices[40,49⇓⇓ ~52].
Flexible wearable electronic devices usually consist of energy storage components and conversion devices, such as supercapacitors and triboelectric nanogenerators. Acid is dissolved into the hydrogel, so that the hydrogel has ionic conductivity, and the ionic conductive hydrogel obtained by the method is generally used for preparing a portable supercapacitor. Zhi et al. Crosslinked polyacrylamide (PAM) with vinyl hybrid silica nanoparticles (VSNPs) to form hydrogels, added phosphoric acid (H3PO4),H3PO4 to the hydrogels to provide ionic conductivity, and used the hydrogels to prepare stretchable and compressible supercapacitors[53].
The most common method of preparing ion-conducting hydrogels is to dissolve a metal salt (e.g., LiCl, FeCl3) into the hydrogel[37,54]. Wang et al. added cellulose nanofibers (CNF) and lithium chloride (LiCl) into acrylamide (AM) solution, and synthesized PAM/CNF/LiCl hydrogel by one-step polymerization[55]. The hydrogel has anti-freezing water retention property, and the low-temperature working range is -80 to 25 deg C, and the supercapacitor based on the PAM/CNF/LiCl hydrogel can work under the low-temperature condition. Hu et al. prepared a hydrogel with sensing, UV resistance, water retention and anti-freezing properties by uniformly dispersing tannic acid-coated hydroxyapatite nanowires (TA @ HAP NWs) and polyvinyl alcohol (PVA) in a binary solvent of ethylene glycol (EG) and water (W)[56]. In this combination, TA @ HAP NWs play a role in filtering ultraviolet rays and improving the mechanical properties of the material. AlCl3 can be fully ionized in water, but not in EG. The binary system of EG and water helps Al3+ to move freely in the molecular-level ionic water channel separated by EG molecules, which makes the hydrogel more sensitive to strain changes. At the same time, the binary system of EG and water alleviates the problem that water is easy to freeze below zero and evaporate at high temperature.
Constructing ion channels in hydrogels to transport ions can improve the conductivity of ion-conducting hydrogels[57]. Wu et al. Prepared a highly transparent and ultra-stretchable ionic conductive hydrogel, which has stable conductivity under large deformation. Its principle is to couple ions with dynamic crosslinking network through molecular synergy (Fig. 1), which solves the problem of incompatibility between conductive region and elastic region.Ammonium 3-dimethyl (methacryloyloxyethyl) propane sulfonate (PDMAPS) and 1-ethyl-3-methylimidazolium ethyl sulfate (IL) were assembled into ion-rich nanochannels, and the polyzwitterion with ester carbonyl group was a potential hydrogen bond acceptor.Polyacrylic acid (PAA) containing carboxylic acid groups was selected as the hydrogen bond donor to fabricate the dynamic hydrogen bond network, and the ammonium persulfate (APS) initiated nanochannels to couple with the hydrogen bond network, avoiding the risk of electrolyte aggregation or leakage[40].
Using ionic liquids to provide conductivity for hydrogels is also an effective way to prepare ionic conductive hydrogels. He et al. Designed and synthesized two kinds of soft and hard monomers of Polymerizable eutectic solvents (Polymer izable deep eutectic solvent, PDES), acrylamide/choline chloride (Am/ChCl) and maleic acid/choline chloride (MA/ChCl), and constructed supramolecular network structures by intramolecular and intermolecular multiple bonds.The transparent conductive elastomer Poly (Am/ChCl-MA/ChCl) prepared by photoinitiated in-situ copolymerization has high transparency (95.1%), good elongation (strain up to 450% and stress up to 0.3 MPa), good ionic conductivity (4.0×10-4S·cm-1) and excellent self-healing ability (self-healing efficiency of 94% at room temperature and 73% at -23 ℃)[58]. This ionic conductive hydrogel can still maintain high transparency, stable mechanical properties and good conductivity in a wide temperature range of -23 ~ 60 ℃, which provides a guarantee for the stable and continuous work of electronic skin in extreme environments. He et al. Introduced phytic acid (PA) into acrylic acid/choline chloride (AA/ChCl) PDES on the basis of PDES conductive materials.In a supramolecular dynamic network formed by high-density hydrogen bonds in a system, the ionic conductive hydrogel with high transparency, super stretchability and super self-repairing capability is prepared by utilizing an in-situ ultraviolet light polymerization mode, the ionic conductivity of the ionic conductive hydrogel can reach 7.8×10-4S·cm-1, and the scheme greatly improves the conductivity of the ion conductive hydrogel[59]. Jiang et al. prepared a hydrophobic ionic conductive gel by using poly (ethyl acrylate) (PEA) elastomer and ditrifluoromethylsulfonyl imide ( [NTf2]) ionic liquid, and there is a strong hydrogen bond between the [NTf2] anion and the elastomer.The hydrophobic ionic conductive hydrogel exhibits ultrahigh tensile properties (strain up to 5000%, stress up to 1.75 MPa), high optical transparency (average visible light transmittance up to 95%), and ultrahigh toughness (4.7 kJ·m-2)[60]; In addition, the ion-conductive hydrogel can maintain its own excellent performance under extreme environments and can work stably without causing corrosion to metal electrodes. Moon et al. Synthesized a series of poly (methyl methacrylate-butyl acrylate) (PMMA-r-PBA) copolymers with different molar ratios and the corresponding homopolymers by reversible addition-fragmentation chain transfer polymerization using ethyl 2-phenylcarbonylthio-2-phenylacetate as a chain transfer agent to prepare ionic conductive hydrogels.The hydrogel obtained by the method has high elongation (when the strain is 850% and the stress is 2×105Pa), the elastic modulus can reach 3.1×105Pa, and the deformation recovery rate is as high as 96.1% after 500 times of tension-recovery cycles[61].
The preparation method of the ionic conductive hydrogel provides a reliable experimental scheme for preparing the conductive hydrogel which has the advantages of super stretch, high conductivity, high transparency and capability of stably working in an extreme environment. The preparation method of the hydrogel solves the problems that the ionic conductive hydrogel taking water as a solvent is easy to freeze at low temperature and easy to lose water at high temperature, and the like; But also provides an effective implementation scheme for solving the problems that the electrolyte solution causes chemical corrosion to the metal electrode, the gel prepared by the organic solvent is flammable, the conductivity of the ionic conductive hydrogel is incompatible with the mechanical property, and the like; At that same time, the invention provide a new solution for the problem that the ion conductive hydrogel is easy to cause ion leakage in the large deformation process.

2.2 Lectron-conducting hydrogel

One common strategy to improve the conductivity of hydrogels is to add conductive fillers, such as carbon nanotubes, graphene, or metal nanoparticles/nanowires, to the hydrogels[62]. The introduction of electronic conductive materials not only provides conductivity for the hydrogel, but also improves the mechanical strength of the hydrogel.
Conducting polymers have received increasing attention in the preparation of electronically conductive hydrogels. Jin et al. Made a conductive hydrogel with dual synergistic network for wearable resistive strain sensor, which is very easy to stretch. The design of this scheme improves the poor transparency of traditional electronic conductive hydrogel.The double synergistic network is composed of a polyacrylic acid (PAA) network and a rigid conductive polyaniline (PANI) network, wherein the PAA network is crosslinked by an amphiphilic block copolymer, and the PANI network is chemically doped and ionically crosslinked by phytic acid (PA).And the two networks are further interlocked through physical entanglement, hydrogen bonding and ionic interaction, and the obtained hydrogel has super tensile property (the strain can reach 1160%,The stress can be up to 0.3 MPa) and the conductivity (5.12 S·m-1) can be controlled, the strain sensor fabricated for this electronically conductive hydrogel has good sensitivity and sensing width[63]. Zeng et al. Used pyrimidinone (UPy) groups as crosslinks to incorporate into a fragile polyaniline/poly (4-styrenesulfonate) (PANI/PSS) network. The formation of the PANI/PSS network provided an electron conduction path for the hydrogel (fig. 2). The electron conducting hydrogel exhibited 13 S·m-1 conductivity[64]. In addition, the hydrogel showed complete self-healing within 30 s after damage and could accurately monitor human movement.
Carbon nanotubes (CNT) and graphene oxide (GO) are commonly used conductive materials for the preparation of electronic conductive hydrogels. A typical experimental protocol for the preparation of CNT/GO-based conductive hydrogels is to disperse the conductive component (CNT/GO) in the hydrogel matrix. Lu et al., inspired by the anti-freezing/anti-thermal behavior of nature and based on the chemical principle of mussel, developed a self-adhesive electronic conductive hydrogel with lasting water locking ability and extremely high temperature tolerance.Polydopamine (PDA) -decorated carbon nanotubes (CNTs) were incorporated into hydrogels, which are heat-and frost-resistant and can retain all properties over an extremely wide temperature range of -20 or 60 ° C, even for long-term preservation[65]. Guo et al. Designed GT-DA/CS/CNT composite hydrogels with antibacterial, antioxidant and conductive properties by using gelatin-grafted dopamine (GT-DA), polydopamine-coated carbon nanotubes (CNT-PDA) and chitosan (CS) through oxidative coupling of catechol groups.The GT-DA/CS/CNT adhesive hydrogel not only can be used for treating full-thickness infected defect wounds, but also has potential application value in the field of multifunctional bioactive dressings[66].
Direct addition of metal materials to hydrogels can affect the elasticity of hydrogels, and metal fillers are usually introduced in the form of nanoparticles or nanowires. Silver nanomaterials have excellent electron transport ability. Devaki et al. prepared nano-silver/polyacrylic acid conductive hydrogel by in-situ polymerization of acrylic acid (AA) at room temperature using hydroxymethyl urea (MU) as a reducing agent to reduce silver ions (Ag+), and the conductivity of the hydrogel was 5.7×101S·m-1[67].
MXene has shown great potential in the preparation of conductive organic hydrogels due to its excellent conductivity, large specific surface area, excellent hydrophilicity and excellent mechanical properties[68]. Chen et al. introduced highly conductive MXene nanosheets into a tannic acid-modified cellulose nanofiber/polyacrylamide (TA @ CNF/PAM) hybrid gel network, and prepared a nanocomposite organic hydrogel by infiltrating glycerol (Gly)/water binary solvent[69]. Due to the incorporation of Gly, the prepared organic hydrogels showed excellent flexibility and conductivity in a wide temperature range (from -36 to 60 ℃) and long-term stability in an open environment (> 7 d). In addition, the organic hydrogel has excellent mechanical properties (tensile strength of 160 kPa, tensile strain of 1500%), tissue adhesion and self-healing, and the conductivity is 1.1 mS·cm-1 at 36 ° C. The organic hydrogel can be used as an ideal candidate material for the preparation of sensors for monitoring large range and fine movement. The combination of the hydrogel network and the conductive polymer enables the hydrogel to be conductive, and the performance of the electronic conductive hydrogel is further optimized by modifying, grafting and modifying the conductive filler. The design scheme of the electronic conductive hydrogel provides the feasibility for preparing multifunctional electronic conductive hydrogels with high conductivity, ultra-stretchability, good biocompatibility and the like.

2.3 Electron-ion conducting hydrogel

One possible way to improve the conductivity of hydrogels is to add electronic conductivity to their ionic conductivity[69]. Zheng et al. designed and synthesized a novel all-polymer conductive hydrogel with an interpenetrating polymer network (IPN) structure, which is composed of conductive PEDOT: PSS polymer and zwitterionic poly (HEAA-co-SBAA) polymer, and realizes the combination of high mechanical property, biocompatibility and sensibility[70]. The poly (HEAA-co-SBAA)/PEDOT: PSS hydrogels exhibit ultra-high elongation of 4 000% ~ 5 000%, tensile strength of 0. 5 MPa, rapid self-healing of 3 min and adhesion on different soft and hard substrates due to the existence of hydrogen bonding, electrostatic interaction and IPN structure. The hydrogel is further designed as a strain sensor with high sensitivity and stability, which can quickly and accurately monitor the fine deformation and human movement caused by strain and pressure.

3 Properties of Conductive Hydrogel

In addition to the excellent properties of hydrogel (such as high water content and good biocompatibility), conductive hydrogel also has good mechanical properties, electrical conductivity, frost resistance, water retention, self-adhesion and self-healing. By continuously improving and developing the properties of conductive hydrogels, we can meet the different requirements of flexible wearable electronic devices in different environments.

3.1 Mechanical Properties

Construction of double network structure is an important method to improve the mechanical properties of conductive hydrogels. In most cases, the "first network" of double-network (DN) hydrogels is usually rigid, brittle, and tightly cross-linked, which acts as a "sacrificial bond" that dissipates a large amount of energy during deformation, while the "second network" is usually soft, ductile, and loosely cross-linked, which acts as a "hidden elasticity" that protects the integrity of DN hydrogels after the first network is broken[71].
Niu et al. Constructed chemically and physically cross-linked polyacrylamide-polyvinylpyrrolidone (PAM-PVP) DN hydrogels, which obtained conductivity by introducing LiCl electrolyte[72]. The DN conductive hydrogel has extremely high extensibility (> 18 000%, tensile strength of (179 ± 28.1) kPa), high toughness, good compression recovery, and electrical conductivity (2 S·m-1). Zhou et al. developed a DN hydrogel by integrating an ionically cross-linked agar network, a covalently cross-linked acrylic acid (AA) network, and a dynamic and reversible ionically cross-linked coordination between AA chains and Fe3+[73]. During large deformation, the interaction between the hydrogen-bonded cross-links of agar and the AA-Fe3+ can be temporarily sacrificed to facilitate the dissipation of energy, while the reversible AC-Fe3+ interaction can be regenerated after stress relief, thus greatly improving the toughness of the material. The developed DN hydrogel exhibited remarkable extensibility with a strain at break of 3174.3% and a tensile strength of 69.6 kPa.
Xu et al. used aramid nanofibers (ANF) crosslinked with polyvinyl alcohol (PVA) to construct DN hydrogels[74]. Highly branched aramid nanofibers (ANF) provide a robust three-dimensional (3D) framework similar to that found in load-bearing soft tissues. Silver nanoparticles (Ag NPs) were embedded in the DN structure to increase the conductivity of the DN hydrogel. The prepared conductive hydrogel has good mechanical properties.The water content of the conductive hydrogel is 76. 4 wt%, the tensile strength is 7. 5 MPa, the elongation is 407%, and the conductivity is 2 S·m-1.

3.2 Conductivity and strain sensitivity

Conductivity and strain sensitivity are the key properties of conductive hydrogels. Strain sensitivity is reflected by the strain factor (GF). GF=((R-R0)/R0)/ε, where ε is the strain, and R0 and R are the electrical resistance of the conductive hydrogel at 0% strain and tensile strain, respectively[75,76].
Guo et al. added a mixture of catechol lignin DAL and reduced graphene oxide (rGO) (DAL/rGO) to sodium alginate/polyacrylamide (SA/PAM) DN hydrogel, and the obtained DAL/rGO composite hydrogel could stably adhere to the skin and rapidly respond to physiological signals[77]. When the conductivity of the hydrogel increases with the increase of DAL/rGO content, the conductivity can increase to 0.15 S·m-1, and the GF of the conductive hydrogel increases from 1.67 to 7.97 in the cyclic strain variation from 100% to 500%, and the hydrogel can maintain stable conductivity in long-term storage below 4 ℃. The DAL/rGO composite hydrogel has the characteristics of preparing skin sensors and outdoor sports equipment. Based on the sol-gel transition of polyvinyl alcohol (PVA) and cellulose nanofibers (CNFs) in a mixed solvent system of dimethyl sulfoxide and water, Jiang et al. synthesized a stretchable ion-conducting hydrogel with good toughness, strength and transparency[78]. The ionic conductivity of the ionically conductive organic hydrogel was 3.2 S·m-1, and it remained flexible and conductive even at − 70 ° C (conductivity was 1.1 S·m-1. When the strain is more than 300%, the GF finally remains at 1.5, and the relatively high GF indicates that the PVA-CNFs ionic conductive hydrogel is sensitive in the measured strain range. The multifunctional sensor prepared by the ion conductive hydrogel has high sensitivity, stability and durability to monitor human motion. Ouyang et al. used a hydrogel formed by reduced graphene oxide (rGO) and carbon nanotubes (CNTs) as the sensing layer of a strain sensor[79]. In the range of strain from 0 to 60% or from 60% to 120%, the curve of ΔR/R0 versus ε is almost linear, and the values of GF are 7.2 and 89, respectively. The strain sensor prepared by the conductive hydrogel is used for accurately monitoring small-strain and large-strain body movements, including various joint movements and muscle movements.

3.3 Self-adhesion and self-healing

The self-adhesion of hydrogels on various devices can provide more reliable performance and functionality for flexible wearable electronic devices, especially for the adhesion of dynamic and highly deformable tissues, such as skin[80,81]. The self-healing ability can extend the service life of conductive hydrogels, which is essential for the durability of flexible wearable electronic devices.
The mussel-inspired conductive hydrogel showed good self-adhesion. Wang et al. developed a dopamine-triggered gel (DTG) strategy for the preparation of mussel-inspired, transparent, conductive hydrogels[82]. One of the greatest advantages of the conductive hydrogel prepared by this strategy is its strong self-adhesiveness.The micro-adhesion properties of DTG hydrogel were characterized by shear adhesion test, and it was found that the adhesion strength of DTG hydrogel increased with the increase of dopamine content, and the highest adhesion strength was up to 20. 4 kPa. In addition, the DTG hydrogel showed high adhesion to tissue surfaces such as pig skin (adhesion strength of 10.3 ± 0.5 kPa). DTG hydrogels have unique dopamine-mediated dynamic interactions and are self-healing. When two broken DTG hydrogels come into contact, they will automatically heal into a complete block within a few seconds. Zussma et al. developed a new type of DN hydrogel consisting of poly (4-styrenesulfonate-co-methyl-uracil-imidazolium) chloride (PSS-MUI), gelatin, and Fe3+[83]. The hydrogel has good extensibility and good ionic conductivity.The reversibility of the dynamic hydrogen bond and the coordination interaction between ions and metals make the hydrogel have self-healing ability and strong adhesiveness.The maximum adhesive stress provided by the hydrogel is (40. 9 ± 1.5) kPa. The rheological method was used to study the self-healing property of the hydrogel.When the structure of the conductive hydrogel was seriously damaged, the recovery process was very fast, and the dynamic modulus value tended to be stable within 20 s. Huang et al. prepared a conductive hydrogel composed of polyacrylic acid, polyvinyl alcohol, borax, ethylene glycol and water. The hydrogel has high transparency and conductivity in the temperature range of -60 ~ 60 ℃, and can self-heal within 1 min. It can be repeatedly adhered to various substrates including glass, metal and rubber, and the adhesion strength exceeds 18 kPa[84].

3.4 Biocompatibility

Biocompatibility of conductive hydrogels, including non-immunogenic reactions and non-toxic side effects[85]. Zhang et al. used single-walled carbon nanotubes (FSWCNTs), polyvinyl alcohol (PVA), and polydopamine to prepare a conductive, healable, self-adhesive hybrid network hydrogel[86]. The hydrogel has good biocompatibility. The results of in vitro cytotoxicity test showed that the hydrogel had no cytotoxicity and could promote cell adhesion and proliferation. Ma et al. developed a self-healing conductive injectable hydrogel based on chitosan-grafted aniline tetramer (CS-AT) and benzaldehyde polyethylene glycol (PEG-DA) as a cell delivery vehicle for myocardial infarction[87]. The conductive hydrogel with good biocompatibility can be obtained by mixing the CS-AT and PEG-DA solutions. The proliferation of C2C12 myoblasts in the hydrogel showed that the hydrogel had good biocompatibility. The survival rate of C2C12 myoblasts in the hydrogel after injection was not significantly different from that before injection.

3.5 Freezing resistance and water retention

The freezing resistance and water retention of conductive hydrogels are important characteristics for their stable operation in extreme environments. By introducing highly hydrated lithium chloride (LiCl) into the cross-linked network structure of polysulfobetaine and polyacrylic acid, Zhang et al. Prepared a new ionic conductive hydrogel with anti-freezing, water retention and self-regeneration capabilities[88]. The ionic conductive hydrogel can be stored at ultra-low temperature (-80 ℃) for more than 30 days without freezing and can keep about 100% of its initial water content. In addition, when the ionic conductive hydrogel is dehydrated by vacuum drying and placed in the environment of -40 deg C, the dehydrated ionic conductive hydrogel can also realize self-regeneration by spontaneously collecting water molecules from the surrounding environment, which can not be realized by the previously reported conductive hydrogel. These properties allow the hydrogel to operate over a wider temperature range and have a longer service life. Song et al. prepared an antifreezing, safe, and non-toxic conductive hydrogel based on a polyvinyl alcohol (PVA)/zinc/ethylene glycol system[89]. The optimized conductive hydrogel has high ionic conductivity and good anti-freezing performance, with conductivity of 9.05 mS·cm-1 at -20 ℃, 15.03 mS·cm-1 at room temperature, and 3.53 mS·cm-1 at -40 ℃. Dai et al. Used a simple solvent replacement method to replace part of the water in the synthesized polyacrylamide/montmorillonite/carbon nanotube hydrogel with glycerol, and obtained the frost resistance and water retention, which maintained its original performance in the range of -60 ~ 60 ℃, and the conductive hydrogel could be stored for 30 days at room temperature[90].

4 Application of Conductive Hydrogel in Flexible Wearable Electronic Device

In recent years, flexible wearable electronic devices have attracted more and more attention, and gradually become a research hotspot, and have been widely used in various artificial intelligence and biomedical fields[11]. Flexible wearable electronic devices have been developed rapidly, which not only have foldability and scalability, but also can maintain a high level of electrochemical performance and working stability[91]. So far, flexible wearable electronic devices can accurately complete the monitoring of human heart rate, blood pressure, muscle tension and other physiological parameters, and can be used as flexible tactile sensors on robot fingers, arms or artificial prostheses to transmit touch feelings to the outside world. In the past, the stretchability of wearable electronic devices was mainly achieved by strain engineering technology to prepare stretchable conductors, that is, non-stretchable inorganic materials, such as metals, were designed to be wavy attached to elastic substrates. When the substrates were stretched, metals could expand, but this greatly limited the application of wearable electronic devices[92⇓~94]. Conductive hydrogels, as a new type of conductive materials, not only have conductivity, but also have many characteristics of hydrogels, such as flexibility and biocompatibility, which are conducive to their applications in skin dressings, drug release, biosensors and flexible wearable electronic devices[95⇓~97]. Conductive hydrogels have shown great value in the fields of biomedicine, tissue engineering and intelligent medicine[98⇓⇓~101]. Ionically conductive hydrogels generally have ionic gradients and transparency, which are advantageous for the preparation of energy storage elements and conversion devices such as actuators, nanoengines, displays, and electronic touch screens. Electron-conducting hydrogels have been widely used in biomedical engineering, such as biosensors, tissue engineering and drug delivery, due to their excellent biocompatibility[36].

4.1 Application of Conductive Hydrogel in Vivo

For applications in vivo or as implantable devices, conductive hydrogels need to have good biocompatibility and matching elastic modulus, and be able to firmly attach to and remain stable in highly dynamic organs such as the heart, stomach, and lungs. Therefore, it is of great significance to develop conductive hydrogels with ideal elastic modulus, toughness, biocompatibility and tissue adhesion[102].
Zhao et al. developed a graphene nanocomposite conductive hydrogel assembled on bioelectronic devices[103]. First, graphene oxide (GO) was introduced into polyvinyl alcohol (PVA) hydrogel to prepare GO nanocomposite hydrogel. The GO-PVA hydrogel was further interpenetrated with a cross-linked network of polyacrylic acid and N-hydroxysuccinimide ester (PAA-NHS ester) grafted to provide bioadhesion capability. Then, the resultant GO-PVA-PAA-NHS ester hydrogel was dried on a bioelectronic device. A bioelectronic device based on the conductive hydrogel can be adhered to a living tissue. The nanocomposite conductive hydrogel based on GO can be used as an electric bioadhesive, which can form adhesion within 5 seconds, and the interface toughness is greater than that of 400 J·m-2, so that bioelectronic devices can be adhered to different wet dynamic tissues. The electrical conductivity (>2.6 S·m-1) of the electrobioadhesive further allows for bi-directional bioelectronic communication.

4.2 Nergy storage element and conversion device

Conductive hydrogels are commonly used in the preparation of energy storage elements and conversion devices in flexible wearable electronic devices, such as supercapacitors and nanogenerators, because of their high transparency and good conductivity. Ding et al. developed a transparent ionically conductive hydrogel with hydrophobicity and extremely high temperature resistance, and developed a capacitive pressure sensor by using the prepared ionically conductive hydrogel. Two ionically conductive hydrogels were overlapped to form a variable capacitor, and the capacitance of the variable capacitor changed with the change of applied pressure[104]. The ionic conductive hydrogel has super stretchability, can be used for simply preparing a variable capacitor, and can also be used as an electroluminescent device of an electrode and a flexible transparent conductive substrate. Zhai et al. Developed a soft skin-friendly three-wave electric nanogenerator (STENG), which realizes biomechanical energy collection and tactile sensing by hybridizing elastomers and ionic conductive hydrogels into electrochemical layers and electrodes[105]. The STENG can be connected to a liquid crystal display (LCD), convert the energy pressed by a finger into electric energy, and endow the screen with electric energy, which can be used as a capacitor to supply power for an electronic watch, and STENG can be used as a charging energy storage to supply power for different portable electronic devices.STENG is sensitive to pressure, so it can be used as an artificial electronic skin for touch/pressure sensing, which provides a new development opportunity for the application of multi-functional power supply and flexible wearable electronic products. The ionic conductive hydrogel prepared by Vlassak et al. Does not freeze in the environment of -57 ℃, and the hydrogel can still be stretched to more than four times of its initial length in this temperature range, and the stretchable ionic touch sensor based on the hydrogel can still work stably in the sub-zero temperature[106].

4.3 Human motion sensor

Ion-conducting hydrogels usually have excellent mechanical and electrochemical properties. Ion-conducting hydrogels have certain advantages in the fabrication of flexible wearable electronic devices, which can maintain stable operation under large deformation and stretching, showing minimal resistance changes. Most of the electronic conductive hydrogels have good biocompatibility, self-adhesion and conductivity. The electronic conductive hydrogel can be self-adhered to the surface of the biological tissue without an external medium to form conformal contact with the surface of the biological tissue, so that the noise of a monitoring signal is reduced, the signal monitoring accuracy of the flexible wearable electronic device is improved, and the wearing comfort of the flexible wearable electronic device is improved.
Wan et al. Developed a functional network hydrogel, which is made by "soft" homogeneous polymer network and "hard" dynamic trivalent iron (Fe3+) cross-linked and interconnected, with cellulose nanocrystal (CNCs-Fe3+) network and excellent mechanical properties.Uch as excellent mechanical strength (tensile strength of 2.1 MPa), strong toughness (9.0 MJ·m-3), reliable elongation (maximum strain of 830%) and good self-recovery performance[107]. The hydrogel showed the ability of self-healing in only 5 min without any stimulus or healing agent. Tunable electromechanical behavior with sensitive, stable, and repeatable resistance changes upon mechanical deformation. Based on the adjustable electromechanical behavior, the hydrogel can be used as a wearable strain sensor to monitor the movement of finger joints, respiration, and even the slight pulse of blood.
Lu et al. Used the hydrophilicity of dopamine to improve the hydrophobicity of polypyrrole, and combined polydopamine-polypyrrole nanoparticles (PDA-PPy NPs) with polyacrylic acid (PAA) through in-situ polymerization to form conductive PDA-PPy-PAA hydrogel.The conductivity of the hydrogel is 12 S·m-1, and the visible light transmittance of the electron-conductive hydrogel is as high as 70% even with a thickness of more than 1 mm due to the light transmittance of the nanogrid interwoven with the nanofibers[108]. In addition to being transparent to visible light, the hydrogel also has UV shielding capabilities and can be used as a skin dressing to protect the body from UV radiation. The hydrogel is also flexible and extensible, able to withstand large strains of more than 2000% with a tensile strength of 130 kPa. The results of cell culture showed that PDA-PPy-PAA hydrogel had good biocompatibility and cell affinity. After cultured on PDA-PPy-PAA hydrogel for 3 days, the number of cells increased significantly. Compared with the general opaque electronic conductive hydrogel reported in the past, PDA-PPy-PAA hydrogel can be used as a self-adhesive and transparent bioelectrode, which can be directly adhered to the skin without additional assistance to detect the bending and stretching of the human body, such as wrist and knee joint movements (Fig. 3). The PDA-PPy-PAA hydrogels were stably attached to the body, and they repeatedly detected body movements even after undergoing large deformations. Lee et al. prepared a conductive hydrogel based on single-walled carbon nanotubes (SWCNTs), which has a fast self-healing ability and can recover (98 ± 0.8)% of its initial conductivity within a healing time of 3.2 s without any external stimuli, such as heat, pH, light or catalyst[109]. A self-healing strain sensor was fabricated based on SWCNT conductive hydrogel.The rapid self-healing of SWCNT conductive hydrogel ensures the rapid recovery of the electrical properties of the sensor after it is released to the relaxed state, and avoids the degradation of the device performance under large deformation. Self-healing strain sensors are capable of monitoring strain, bending, and torsional forces. In addition, it can measure and withstand strains up to 1000%, with a high GF (0.24 at 100% strain, increasing to 1.51 at 1000% strain) and excellent cyclic stability. Based on these key features, the self-healing strain sensor can be embedded in gloves, clothing, or directly attached to the skin to accurately detect large-scale human movement.

4.4 Bioelectrode

Bioelectrode is an important embodiment of the application of conductive hydrogel in flexible wearable electronic devices. Majidi et al. Used micron-sized silver flakes suspended in a polyacrylamide-alginate hydrogel matrix to form a conductive hydrogel after a partial dehydration process[110]. The hydrogel has high electrical conductivity (>350 S·cm-1) and is capable of providing direct current while maintaining flexibility (Young's modulus < 10 kPa) and deformability. The hydrogel can be used as an electrode to provide high-frequency electrical signals for neuromuscular electrical stimulation.
Monitoring of vital signs underwater, such as respiratory rate, blood pressure, and cardiac status, is essential for healthcare and health management. One commonly used stretchable substrate is polydimethylsiloxane (PDMS) film, which is coated on gold nanolayers (Au/PDMS). Their mechanical stability and conductivity are not affected by water. However, due to its hydrophobicity, Au/PDMS does not adhere to the skin in water. In order to reapply the waterproof Au/PDMS under water, an additional adhesive layer was introduced to connect the membrane and the skin. Chen et al. Designed and manufactured an ionically conductive hydrogel containing dopamine, which was used as a waterproof electrode for underwater ECG monitoring[111]. Dopamine methacrylamide (DMA), acrylic acid (AA) and methoxyethyl acrylate (MEA) were copolymerized to form a random copolymer P (DMA-co-AA-co-MEA) (PDAM), which was then drop-cast onto the surface of Au/PDMS to form a waterproof PDAM/Au/PDMS electrode. DMA containing dopamine provides underwater adhesion, while AA provides ionic conductivity when deionized in water, enabling the adhesive polymer to monitor and deliver electrophysiological signals. MEA prevented the destruction of PDAM by forming a water-resistant scaffold. In the presence of water, the electrode is tightly bound to the skin and has stable conductivity. After drying, the electrode can be easily peeled off without leaving any residue on the skin and can be reused. Due to the good biocompatibility of these monomeric polymers, there was no skin irritation within 2 H. Stable underwater conductivity and firm skin adhesion enable the electrode to collect accurate ECG signals under various conditions in the water. The results show that the wearable device with waterproof electrodes can collect ECG signals in real time during swimming, which can be used to reveal the heart condition. These electrodes realize the underwater detection of ECG signals, which can be used for health monitoring and motion management of human body.

4.5 Ionic skin

As the largest organ of human body, skin can sense a variety of external stimuli and convert them into physiological signals for neural recognition[112]. With the continuous development of artificial intelligence, soft robotics and flexible wearable devices, artificial skin is expected to become an important medium for human-computer interaction[113⇓⇓⇓~117].
Liu et al. designed and synthesized an antifreeze hydrogel for multimodal artificial skin based on poly (ionic liquid) (PIL)[118]. The zwitterionic hydrogel can maintain high elongation (900%), high conductivity (1.1 S·m-1) and self-healing property even in the environment of -20 deg C. Based on the PIL-based conductive hydrogel, three sensing modes of capacitance, resistance and triboelectricity can be realized, and stable operation can be maintained in a wide temperature range (-20 ~ 60 ℃). In capacitive mode, this PIL skin can detect pressures ranging from 1.1 to 45 kPa. In resistive mode, temperature (-20 ~ 60 ℃) and strain (0 ~ 160%) can be detected. In addition to this, the combination of triboelectric nanogenerator (TENG) with this resistive mode enables PIL skin to sense not only temperature and strain, but also pressure (0.5 ~ 40 kPa) at -20 ° C simultaneously. Feng et al. Prepared a series of highly transparent organogels with adhesion and self-healing properties by adjusting the content of monomers and crosslinkers using long-branched acrylate as functional monomers and paraffin oil as medium[119]. The full gel capacitive ionic skin with ionic conductive hydrogel as conductive layer and organic gel as dielectric layer was obtained by in-situ polymerization. The ionic skin can not only sense physical stimuli such as pressure and tension, but also has temperature sensitivity. The prepared ionic skin can be used to monitor various movements of the human body, such as finger extension, wrist flexion, and throat movement. In addition, the ionic skin can act as a prosthetic skin on the pneumatic soft prosthesis to achieve intelligent tactile perception, as shown in Figure 4.

4.6 Electronic Skin

It is of great significance for the development of human-computer interaction and artificial intelligence to imitate the comprehensive function of human perception through electronic skin. Some electronic skins with high sensitivity and high stability have been developed, but few people pay attention to their comfort, environmental friendliness and antibacterial activity. Peng et al. developed a breathable, biodegradable, and antibacterial e-skin based on an all-nanofiber triboelectric nanogenerator, which is made by sandwiching silver nanowires (Ag NWs) between polylactic-co-glycolic acid (PLGA) and polyvinyl alcohol (PVA)[120]. By adjusting the concentration of Ag NWs and the ratio of PVA and PLGA, the antibacterial performance and biodegradability of e-skin can be adjusted, respectively. An all-nanofiber electronic skin with a multilayer interwoven nanofiber network and numerous microscale hierarchical micropores provides a high specific surface area for contact charging and pressure sensing, as well as multiple interfiber capillary channels for heat and moisture transfer. The electronic skin has remarkable antibacterial activity on Escherichia coli and Staphylococcus aureus, and can effectively inhibit bacterial growth and prevent bacterial infection. In addition, by selecting and configuring degradable PLGA and PVA, the electronic skin can adjust its service life from hours to weeks. The all-nanofiber-based e-skin enables real-time, non-invasive, and self-powered monitoring of physiological signals (such as microexpression, pulse, respiration, and vocalization) and joint movements (such as elbow, knee, and ankle). The prepared electronic skin has air permeability, biodegradability and antibacterial activity, and is comfortable, safe and pollution-free to wear, and has energy collection and high sensitivity. Han et al. Prepared a conductive hydrogel using MXene and polyvinyl alcohol (PVA), and designed a highly stable electronic skin based on this conductive hydrogel.MXene and PVA are connected to form a layered network structure through hydrogen bonding, and PVA intercalation is used to adjust the electrical properties and mechanical deformation ability of the material. The electronic skin can record heart signals in vivo in real time and continuously monitor gastric peristalsis[121].

5 Conclusion and prospect

With the development of biomaterials, conductive hydrogels have made great progress in mechanical properties, biocompatibility, self-repair and temperature sensitivity. However, the development of conductive hydrogels still faces the following challenges: (1) how to maintain stable conductivity under working conditions; (2) Whether they can work stably for a long time in the face of sudden changes in the environment; (3) Whether the conductive hydrogel can avoid causing damage or inflammation to biological tissues when it forms conformal contact with biological tissues. Therefore, the preparation of conductive hydrogels with excellent mechanical properties, good tissue adhesion, injectability and biocompatibility is very important for the development of flexible wearable electronic devices. Thanks to the rapid development of flexible materials, manufacturing technology and sensing technology, flexible wearable devices have been widely used in the fields of human physiological parameter monitoring and robot tactile perception. Flexible wearable electronic devices have made breakthroughs in transparency, flexibility, pressure sensitivity, spatial resolution and versatility.
High conductivity, adjustable mechanical properties and high stretchability are the basic performance requirements for the preparation of flexible wearable electronic devices, and it is found that the conductive hydrogel with multiple additional functions is urgently needed to be developed in the practical application of flexible wearable electronic devices.For example, conductive hydrogels that combine antibacterial properties, good biocompatibility, self-adhesion and self-healing properties also need to consider the high water retention rate of conductive hydrogels and the packaging technology related to hydrogel sensors and other device units. With the advent of the era of big data and 5G, the development of precision medicine and health care has become the focus, and it is very important to prepare conductive hydrogels with specific functions to meet different needs according to the practical application of different needs.

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Funding

National Natural Science Foundation of China(11802197)

Key R&D Program of Shanxi Province (International Cooperation)(201903D421064)

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