Prospects and Challenges of Flexible Stretchable Electrodes for Electronics

: The application of ﬂexible electronics in the ﬁeld of communication has made the transition from rigid physical form to ﬂexible physical form. Flexible electrode technology is the key to the wide application of ﬂexible electronics. However, ﬂexible electrodes will break when large deformation occurs, failing ﬂexible electronics. It restricts the further development of ﬂexible electronic technology. Flexible stretchable electrodes are a hot research topic to solve the problem that ﬂexible electrodes cannot withstand large deformation. Flexible stretchable electrode materials have excellent electrical conductivity, while retaining excellent mechanical properties in case of large deformation. This paper summarizes the research results of ﬂexible stretchable electrodes from three aspects: material, process, and structure, as well as the prospects for future development.


Introduction
In recent years, the application of flexible electronics in the field of communication has developed rapidly. Driven by the new generation of information and communication network technology, the manufacturing industry will rapidly complete the digitization, networking, and intelligence transformation, of which the key source technology is a perception [1]. Some flexible electronics are required to have a perception function to truly allow perception to be free from multidimensional space. Flexible perception can subversively change the rigid physical form of traditional information devices and systems, realize the efficient integration of information with people, objects, and the environment, realize the flexibility of information acquisition, processing, transmission, display, and even energy, and realize "all things are interconnected" [2]. Although there are many kinds of flexible electronic devices, and their applications are wide, the basic structures of most devices are similar. Generally speaking, flexible electronic devices are mainly composed of flexible substrates, electrodes, functional components, and packaging materials [3]. To achieve flexibility of the whole device, these functional materials need to be flexible enough under the premise of stable operation. Flexible electrodes are the core components of flexible electronics. They need not only good conductivity, but also excellent flexibility. Flexible electrodes usually match the mechanical flexibility of the substrate well. However, when large deformation occurs, the flexible electrodes break, which invalidates flexible electronics and restricts the further development of flexible electronics technology [4]. Therefore, there is a pressing scientific need to study flexible stretchable electrodes, which can guarantee high stability and reliability when the material of the electrodes undergoes large deformations [5,6].
The key to flexible stretchable electrodes is achieving flexibility and increasing their service life under repeated cyclic loading with certain stress (>>1%) [7]. Stretchability refers to the ability of flexible stretchable electrodes to maintain electrical conductivity under mechanical deformation. In the field of materials, it is a research hotspot to prepare materials with more flexibility and even stretchability; in mechanics, geometric graphic design is a shortcut to achieving overall structural stretchability; in micro-nano processing, the development of preparation process matching various new materials is a necessary step to achieve large-scale preparation.
This paper first introduces the concept, performance parameters, and functional adjustment of flexible stretchable electrodes, and then systematically introduces the research results of the flexible stretchable electrode from three aspects of material, technology, and structure. In the first part, the common flexible materials for flexible stretchable electrodes are summarized; In the second part, flexible stretchable electrodes with different stretchable structures are summarized. Electrodes with stretchable structures can reduce their limitations in the preparation of materials; the third part summarizes the current preparation techniques of flexible stretchable electrode materials and classifies them according to different principles in the preparation process. Finally, the existing problems of flexible stretchable electrodes are analyzed, and future development is prospected.

Concept
As shown in Figure 1, the ordinary electrode belongs to a rigid physical form and does not have flexibility or extensibility; the structure of the flexible electrode is integrated, which is flexible, but not stretchable; the flexible stretchable electrode is flexible and stretchable. Ordinary electrodes are generally prepared by mixing active materials, adhesives, and conductive agents, grinding them evenly, and then coating metal collectors with them. Due to the heterogeneous interface between the collector and the active material layer, such electrodes are prone to fall off or powder when folded or bent, leading to failure of the electrodes [8]. The development of flexible electronics requires high capacity, high stability, and high flexibility of the electrode materials. Flexible electrodes ( Figure 1) are mainly made up of conductive flexible self-supporting electrodes or by loading active materials onto flexible conductive materials [9]. Morphologically, the perception of multidimensional space is characterized by repeated stretching, compression, folding and torsion. Flexible electronics are required to maintain their original functions in any of the above-mentioned shapes, and it is not enough for electrodes to be flexible only.
The flexible electrodes that retain their function despite a strain of 10% are referred to as flexible stretchable electrodes, based on the elastic limit of the human skin [10]. In addition to being highly flexible, conductive, and loaded with active materials, flexible stretchable electrodes also need to meet the requirements of stretchability, strong adhesion, surface modifiability, and biocompatibility [11]. In the future, our perception will be greatly expanded by electronics built into our clothes and accessories, attached to our skin, and even implanted into our bodies, enabling us to blend highly efficiently with the perceived information. In addition, due to advances in flexible materials and flexible electronic products, great progress has been made in bioheuristic flexible robot technology [12][13][14][15][16][17].
From a biomedical perspective, this biocompatible electronic product, including wearable electronic devices [18,19], epidermal devices [20][21][22][23][24], and implantable electronic devices [25][26][27][28] (Figure 1), plays an increasingly important role in human health monitoring, which will significantly change the future of health care and our relationship with electronics; from the perspective of smart robots, a biomimetic soft robot with skin-like sensors and soft actuators (Figure 1) is expected to interact with human beings and their surroundings, significantly enhancing security, sensitivity, and adaptive responses, to achieve a complete human-machine cycle; from the perspective of the future industrial internet, it can achieve the multidimensional and high-fidelity representation of production feature data, and ultimately achieve the accurate construction and free regulation of the Internet through the establishment of the model database, the classification of production elements, and the correlation between each model and each other.  [30]. Image for "Wearable devices": Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://www.sohu.com/a/450746414_114822 (accessed on 2 April 2022)) [31]. Image for "E-skins": Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://www.sohu.com/a/226167819_460436 (accessed on 2 April 2022)) [32]. Image for "Soft actuators": Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (http://www.chinainc.org.cn/show-14-104546-1.html (accessed on 2 April 2022)) [33]. Image for "Sensors": Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://www.sohu.com/a/253004726_100101359 (accessed on 2 April 2022)) [34].

Performance
Conductivity is one of the key indices of the electrical properties of flexible stretchable electrodes, which is related to the composition and structure of the material itself. The flexible stretchable electrode with good performance should show low resistance under the maximum operating strain, and the resistance value is almost unchanged during the cycle. Generally, the square resistance is the resistance value on the unit thickness and unit area of conductive materials, and the unit is Ω/sq or Ω/ . For a thin square film electrode with side length l and thickness d, the square resistance can be expressed as: Where the resistance R S (Ω/sq) is equal to the volume resistivity ρ (Ω·m) divided by thickness d, conductivity σ (S/m) is the reciprocal of volume resistivity. It can be seen from the above formula that the block resistance is independent of the sample size, directly proportional to the resistivity and inversely proportional to the thickness of the film sample. It is a parameter comprehensively reflecting the conductivity of the film. Among them, "square" has no practical significance, but is used to distinguish it from the resistance between two points.
As shown in Figure 2, the deformation types of flexible electronics in different use scenarios can be divided into bending, twisting, uniaxial tensile deformation, biaxial tensile deformation, circumferential tensile deformation, etc. As an indispensable part of flexible electronics, electrode materials can also withstand external forces and resist deformation, which we call the mechanical properties of materials. Under the action of external forces, the shape and size of the material change, which is called deformation. When a material is deformed, the relative positions and distances between molecules or ions within it change, and additional internal forces between atoms and molecules will be generated to resist external forces, and an attempt is made to restore to the state before the deformation. When the balance is reached, the additional internal forces are equal in size, and opposite in direction, to the external forces. The additional internal force on a material's unit area is called stress, and its value is equal to the external force on a unit area; that is, σ = F/A, where σ is stress, F is external force, and A is area. In the International System of Units, the unit of stress is N/m 2 , which is also written as Pa. If the area before the material is stressed is A 0 , then σ 0 = F/A 0 is the nominal stress. If the material is compressed by force and the area is A, σ T = F/A is the true stress. Nominal stress is commonly used in practice, and there is little difference in numerical values between materials with small deformation.  [38]. Image for "Uniaxial": Reprinted with permission from [39]. Copyright 2018 Nature materials.
Strain is used to characterize the relative displacement of particles within a material under stress. For isotropic materials, there are three basic types of strain: tensile strain ε, shear strain γ and compression strain ∆. For ideal elastic materials, elastic deformation occurs under stress. The relationship between stress and strain obeys Hooke's law, that is, the stress σ is proportional to the strain ε: σ = E·ε. The scale factor E in the formula is called the elastic modulus. Elastic modulus is the stress at which a material undergoes unit strain, which indicates the resistance of the material to deformation. The larger the E, the less likely the material will be to deform, indicating greater stiffness of the material. The unit of elastic modulus is the same as the unit of stress, both N/m 2 .

Functional Adjustment
Flexible electrodes can be divided into three levels according to their mechanical properties. The lowest level is bendable and coileable, the second level is plastic stretchable, and the highest level is flexible stretchable. As far as the degree of preparation difficulty, the higher a material is in the hierarchy, the more difficult the preparation. Concerning application requirements, the higher a material is in the hierarchy, the broader the scope of application. The development of flexible stretchable electrodes is a hotspot and difficult point in current research. A large stretching range (>1%) requires higher performance and structure of materials [40]. Flexible stretchable materials are usually prepared by two methods: one is to develop a new type of flexible stretchable material [41], and the other is to process the material into a geometric structure with scalability. As shown in Table 1, the process complexity of the two methods is different and the flexible stretchable materials prepared have different mechanical properties.  It is difficult for flexible stretchable materials to maintain absolute stability of resistance during stretching, and it is unavoidable that conductive materials undergo micro-structure fracture when they recover from stretching. Due to the low electrical stability of flexible stretchable materials, scientists have proposed geometric structures to achieve the stretchability of materials [42,43]. Conductors are fabricated into scalable geometries such as mesh [44] (Figure 3e), fabric [45] (Figure 3f), wavy [46] (Figure 3g), snake [47] (Figure 3h), and spiral [48] (Figure 3i) to form flexible stretchable electrodes. When an electrode with a stretchable structure is stretched, the conductor material can cleverly use its geometric shape to buffer the external stress, thus maintaining a stable conductivity at a larger stretch length and even lower than the resolution limit.
After the requirement of flexible stretchability was achieved in mechanical performance, with further research on flexible stretchable electrodes it was applied to epidermal electronic devices and implantable electronic devices [49,50], which puts forward new requirements for flexible stretchable electrodes. To achieve a close skin fit, flexible stretchable materials need to have a lower Young's modulus (<260 kPa [51]) than the human epidermis. In addition, long-time wear puts higher requirements on the biocompatibility and wearing comfort of the materials.

Varieties of Flexible Stretchable Electrode Substrate Materials
Flexible stretchable electrode materials usually need to be combined with active and base materials to improve their mechanical and electrical properties. As shown in Table 2, the conductivity of flexible stretchable materials is compared. Some active materials can act as collectors to make the polymer conductive and have electrochemical activity to contribute capacitance. Some active materials can only be plastic deformed, but not resilient. Loading this material on an elastic substrate yields a composite material which has the conductivity, electrochemical activity, and elasticity of the active material. From the base material, flexible stretchable electrode materials include carbon-based electrode materials, elastic polymerbased electrode materials, paper-based, fabric-based, and metal-based. The flexible stretchable electrodes made of different substrate materials are listed in detail below.

Carbon-Based Electrode Materials
From Table 3, carbon-based materials are widely used in the preparation of flexible stretchable electrodes due to their high specific surface area, high conductivity, and good chemical stability. In particular, carbon nanotubes (CNTs) and graphene with unique structures are well suited for flexible stretchable devices [58]. One-dimensional CNTs can be directly made into full-carbon thin films or fiber electrodes, and graphene can also be used to synthesize graphene thin films, graphene fibers, etc [59][60][61][62][63]. In addition, nanocarbon fibers, carbon paper and the composite of two carbon materials have been applied to stretchable electrode materials by researchers because of their high specific capacitance and good cyclic stability. CNTs/Silver nanoparticles Electrospinning 550% Maintain performance at 550% strain tensile range [72] Xiao et al. used the synergy between carbon nanotubes with good mechanical properties and vanadium nitride (VN) with the mesoporous structure to build a flexible membrane electrode with a self-supporting structure [73]. Niu et al. prepared a flexible composite film electrode with a "skeleton/skin" structure [74]. The electrode takes a single-wall carbon nanotube (SWCNT) with a network structure as the "skeleton", and polyaniline (PANI) is grown on the skeleton as the skin layer through in-situ chemical polymerization of aniline (ANI) monomer ( Figure 4a). Xu et al. prepared a composite material with high conductivity and high current capacity based on RGG (reduced giant graphene) doped with silver ions or silver fibers (Figure 4b), which can also be used to prepare flexible stretchable electrodes [75]. Zhou et al. designed a folded carbon electrode using pencil and sandpaper, which showed good tensile properties and was successfully applied to wearable electronic devices [76]. For the current electrode materials, the stretchability mostly comes from carbon materials, and most carbon materials have certain advantages and disadvantages. People will continue to develop new carbon or explore simple and economical new technologies and processes. Reprinted with permission from [75]. Copyright 2013 Advanced Materials. (c) Schematic illustration of the process to embed CNT ribbons in PDMS. Before encapsulating the CNTs, electrodes connected to the two ends of CNT ribbons are fabricated to enable conductivity measurement. Reprinted with permission from [77]. Copyright 2010 Advanced Materials. (d) Schematic illustration for the fabrication process of stretchable CNTs/PDMS substrates. Reprinted with permission from [78]. Copyright 2018, Nanoscale. (e) PDMS and LIG/PDMS substrate. Reprinted with permission from [79]. Copyright 2016 Advanced Energy Materials. (f) 3D scheme of the LIG transfer onto PDMS substrate and its elastomeric properties. Reprinted with permission from [80]. Copyright 2016 Advanced Energy Materials. Table 4 summarizes commonly used flexible stretchable electrode materials based on elastic polymers, including polydimethylsiloxane (PDMS), polyurethane (PU), thermoplastic copolymer (Ecoflex), and PEDOT:PSS. Polymer materials can be made into thin films by solution coating, showing good elasticity and tensile recovery, and can be stretched and used repeatedly. It can also be seen from Table 4 that there are many materials that can convert polymers into good fluid collectors and be used in various flexible electronics. The polymer is evenly coated on the carbon nano materials, which can be peeled off after curing into a film; metal materials, such as gold, silver, and copper, can be made into metal nanoparticles or metal nanowires, and then doped/deposited with polymers to prepare highly conductive current collectors; conductive polymers (PPy, PANI, polythiophene) can directly deposit a conductive layer on the polymer surface by electrodeposition. For example, the conductive filler carbon nanotubes (CNTs) are evenly spread on the glass substrate and then spin-coated with PDMS solution [92]. After cooling and curing, CNTs/PDMS film is peeled off (Figure 4d) to obtain a conductive collector [78]. As shown in Figure 4c, the CNT ribbons can also be directly placed on the surface of PDMS film, and then the electrodes at both ends of the CNT ribbons for conductivity measurement can be prepared. A layer of uncured PDMS can be cast on the CNT/PDMS to make a stretchable CNT/PDMS electrode film [77]. It is worth noting that the interaction between the carbon nanotubes and the PDMS is very strong. Once carbon nanotubes contact the surface of PDMS film, they cannot move by mechanical scratching. Therefore, this kind of electrode solves the problem of poor adsorption between the conductive components and the substrate materials. Laser-induced graphene (LIG) is also an effective conductive material [79]. The stretchable electrode can also be prepared by transferring LIG to PDMS ( Figure 4e). As shown in Figure 4f, the manufacturing process of LIG/PDMS electrode is described: a porous LIG pattern was obtained by directly writing a polyimide sheet with a laser; Then PDMS was completely injected into the 3D network, and LIG/PDMS thin film electrode was obtained after curing [80].

Polymer-Based Electrode Material
Choi et al. embedded transparent retractable silver nanowires/reduced graphene oxide (AgNWs/rGO) electrode wires into polyurethane (PU) dielectric layer on PDMS substrate (Figure 5a), and directly manufactured thin-film electrodes on elastomer substrate to prevent capacitance change caused by stretching [93]. At the same time, PU material is also a polymer with excellent deformation ability, which can be used as an elastic matrix alone. NCNT/PU film was prepared by coating PU onto an N-doped CNT array (Figure 5b) [94]. In addition, researchers have uniformly grown ultra-thin manganese dioxide nanosheets on PU-NCNTs substrate by a simple microwave radiation method and prepared a new electrode with excellent performance [95]. Alternatively, the AgNWs conductive filler can be directly added to the PU elastic matrix to make a flexible AgNWs electrode [96]. PEDOT:PSS, as a conductive polymer, is also frequently prepared into thin films and gels, whose electrical properties are affected by different morphologies [100]. Choong et al. copied the micro pyramid PDMS substrate with silicon mold, and grafted PEDOT: PSS/PUD composite polymer (Figure 5d), and formed the structure of the piezoresistive electrode [98]. Zhu et al. compounded CNT and Ag-doped PEDOT: PSS materials to prepare flexible electrodes with a maximum tensile rate of 480% [101]. Zhao et al. [99] used PEDOT: PSS/SWCNT composite as an electrode and the elastic polymer PDMS was rotationally coated on the PEDOT: PSS/SWCNT electrode array to embed the electrode array in PDMS to obtain a flexible stretchable electrode (Figure 5e). PEDOT:PSS solution can also be spin-coated on SEBS elastomer substrate, and SEBS is selected as the substrate because. Unlike PDMS, its physical properties such as hardness, tensile strength, and elongation are not easy to change [102]. Some researchers fabricated stretchable electrodes such as carbon nanotube [103], multi-walled carbon nanotube (MWCNTs) [104], nanofiber composite (single-walled carbon nanotube/styrene (SWCNT/SBS)) ( Figure 5c) [97] on an Ecoflex elastic substrate with an ultimate tensile rate of 60%.

Electrode Materials with Other Substrates
In addition to the flexible electrodes in the above two substrates, some researchers have used paper-based, fabric-based, metal-based [105][106][107], etc., to prepare flexible electrodes and electronic devices. For example, a paper-based flexible stretchable electrode was prepared by interfacial polymerization of pyrrole monomers to make its active material, polypyrrole (PPy), grow on the flexible dustless paper [108], or to make CNTs/MnO 2 composite grow on the dustless paper [109]. Hu et al. prepared a highly conductive carbon nanotube/cotton fabric flexible electrode by repeatedly soaking cotton fabric in carbon nanotube solution by a simple dipping and drying method [82]. Supercapacitors (Figure 6a) made from these conductive textiles show high areal capacitance, up to 0.48 F/cm 2 , and high specific energy. Metal-based electrode materials can be divided into two types, one of which is ordinary metal materials. Xia et al. deposited cobalt hydroxide (Co (OH) 2 ) nanosheets on porous nickel membranes (Ni) by electrochemical deposition [111], using the porosity and flexibility of the nickel membranes to achieve the flexibility of the electrodes. Li et al. directly used gold wire as a flexible matrix to deposit carbon-based materials on the surface of their wire by electrochemical deposition and apply them to flexible supercapacitors [112]. The other is liquid metal. Liquid metals possess high stretchability, good biocompatibility, and excellent electrical conductivity, making them promising candidates for flexible and stretchable electrodes. Liquid metal-based flexible and stretchable electrodes have been used in electronic devices such as nanogenerators [113], diodes [114], and actuators [115]. Ma et al. report a stretchable conductor that is fabricated by simply coating or printing liquid metal onto an electrospun elastomeric fiber mat [116]. The liquid-metal fiber mat shows good biocompatibility and smart adaptiveness to omnidirectional stretching over 1800% strain. Dong et al. proposed that liquid metal electrode has high biocompatibility and good performance in neurite growth and long-term implantation [110]. Figure 6c demonstrates the fabrication process of stretchable electrode arrays (SEA). As shown in Figure 6b, Due to the high stretchability and softness of the liquid metal electrodes, it could expand and form an intimate interface with the targeted cortical surface.

Flexible Stretchable Structure
As shown in Table 5, stretchable structures can be roughly divided into five types: spiral structure, wave structure, fabric structure, mesh structure, and snake structure. Among them, the devices made of spiral structure are generally fibrous and have certain rigidity, such as CNT yarn and stainless steel wire, with a tensile rate ranging from 20% to 400%; the tensile ratio of electrodes or devices with wave structure is generally low, ranging from 30% to 100%; fabric structure can be stretched through special textile structure, such as knitted fabrics and sweaters, with a stretching rate of 20-120%; the ultimate elongation of the electrodes/devices with mesh structure varies greatly from 10% to 2000%, 10% is a normal sponge structure, and 2000% is a paper-cut work of art. The snake-like structure has poor mechanical performance and cannot be restored. The device can only reach 20-30% stretch ratio. Spiral structure and fabric structure are moderately stretchable, and one-dimensional fiber-like devices can be well integrated into fabric clothing, and the fabric structure itself is a part of fabric clothing, which is an ideal structure form in the development of smart flexible devices [117]. Yu et al. coated carbon nanotube (CNT)@MnO 2 core fibers with gel electrolyte and CNT@PPy one-dimensional fibrous supercapacitor (FSS) with asymmetric nuclear sheaths was prepared by coating the composite film [118], and then an extensible spiral structure was formed by over-twisting (Figure 7a). Chu et al. synthesized layered carbon tubular nanostructures (hCTNs) on the surface of stainless-steel spring (SSS) with helical structure [119], and then polymerized the nano spherical polyaniline (PANI) onto the hCTNs network in turn to form a hCTNs/PANI hybrid stretchable electrode (Figure 7b). There are two preparation methods for stretchable devices of fabric structure: one is simple, and the stretchable electronic device is prepared by depositing active material on the fabric structure substrate by dipping coating or screen printing. The fabrication process of composite fabric electrodes is shown in Figure 7c and the structural diagram [132] is applied to supercapacitors. Another is to fabricate fiber-like electrodes and then weave them into fabric structures to obtain stretchable devices, as shown in Figure 7d is a full-solid symmetric yarn supercapacitor with two PEDOT:PSS/CNF (carbon nanofiber)/CF (carbon fiber) electrodes in parallel, and a schematic diagram [133] of its mechanical deformation.
The electrode with wave structure has a simple preparation method and low cost. However, the surface of the prepared material is wrinkled and undulating, so the wearing comfort is poor. There are usually two ways to prepare wavy structures: First, the elastic base is pre-stretched, on which a 2D planar electrode material is deposited, and then the prestretched elastic base is released [134] (Figure 8a). The electrodes deposited on it show an undulating wavy structure. The electrodes deposited on it will show an undulating wavy structure. Based on the above method, the Xie group [135] found that porous graphene can be wavy by pre-stretching the substrate, thereby improving the defect of porous graphene with poor stretchability. The wavy PANI/Graphene electrodes were then prepared and packaged to obtain a stretchable supercapacitor.  Secondly, there are fiber wavy electrode materials. Pre-stretch the elastic yarn and wrap the electrode material around it. After the elastic yarn is released, the wavy electrode material is obtained. For example, Ren et al. proposed a method to fabricate composite electrodes by tightly combining CNT/PANI composite membranes with elastic filaments [136]. The wavy structure can be obtained by stretching the elastic filaments, attaching them to the CNT membranes, and then releasing them (Figure 8b). Another stretchable supercapacitor with a pre-stretching strategy introduced PEDOT into SWCNT thin film to obtain (SWCNT/PEDOT) hybrid fiber [137]. A biaxially stretchable supercapacitor with a unique structure is obtained by combining hybrid fibers with PDMS and obtaining wavy fiber electrodes under the pre-elongation strategy.
The mesh structure and the S-shaped structure have the best stretchability and can reach thousands of elongation rates, but they require the high maneuverability of materials [138,139]. If the stretchable structure has no resilience after stretching, it is difficult to restore to its original state, which limits its practical application. Kim et al. produced a supercapacitor consisting of two-dimensional planar SWCNT electrodes and ionic gel electrolytes [140], which rely on narrow, long snake-shaped metal interconnects to ensure mechanical stability during deformation. The micro-supercapacitor array shows good performance when stretched up to 30% (Figure 9a). Yun et al. [141] designed a dual-axis micro-capacitor (MSC) array with integrated solar cells (SCs) and strain sensors (SS), as well as a snake-like interconnect structure and a stretchable substrate (Figure 9b). Twodimensional scalable electronic devices have limited specific surface area capacitance and are incompatible with three-dimensional wearable devices. To overcome this limitation, it is urgent to develop three-dimensional stretchable electronic devices with higher quality loads and customizable shapes. The Pu team [142] found that a scalable micro supercapacitor (MSC) on a 3D honeycomblike PDMS substrate can maintain its capacitance stability even when subjected to large strains such as stretching, bending, and warping. The stretchability of the MSC array system is also controlled by the distortion of the honeycomb-like PDMS structure (Figure 9c). Inspired by the honeycomb lamp structure, the Lv team [143] proposed a three-dimensional scalable supercapacitor with an expandable honeycomb structure, whose electrodes were prepared by electrodeposition of porous PPy/black phosphorus oxide (BPO) composite onto carbon nanotube (CNT) thin films. Figure 9d shows the manufacturing process of the capacitor. Based on the manganese dioxide nanowire composite electrodes, the team also developed an editable supercapacitor with customizable shape and stretchability [144] ( Figure 9e). Its shape is more complex and finer, and therefore it is more controllable. Figure 9. (a) Photograph of a stretchable 2D planar micro-supercapacitors array on a PDMS substrate without electrolyte (Inset image shows the tape-transferred micro-supercapacitor array) and photographs of the entire device of the 3 × 3 micro-supercapacitor array before (Lower left) and after application of strain of 30% (Lower right). Reprinted with permission from [140]. Copyright 2013 ACS Nano. (b) Optical image of the biaxially stretchable MSC array with integrated SS and SCs. The inset shows the circuit diagram. Reprinted with permission from [141]. Copyright 2018 Nano Energy. (c) Schematic illustration of a stretched and bent device; exploded view of various structural layers of the device; digital photograph of a compressed (Lower left) and twisted and stretched (Lower right) device. Reprinted with permission from [142]. Copyright 2016 ACS Nano. (d) Schematic drawing of the fabrication process for 3D stretchable supercapacitors: (I) The PPy-based electrodes was first to shaped into identical electrode strips with predefined shapes; (II) the electrode strips were adhered by adhesives (PVA/H 3 PO 4 gel electrolyte) and connected into two electrode arrays (array A and array B); (III) the edge of the array A and B were coated with a silver paste to ensure conductive connections, and then the array A were intersected with array B; (IV) two arrays were separated by a thin nanocellulose separator (about 20 µm). (V) Array A and B were coated with PVA/H 3 PO 4 gel electrolyte to form a 3D stretchable supercapacitor with expandable honeycomb structures. (VI) The 3D stretchable supercapacitor was stretched to form a honeycomb structure and packaged with a thin-layer PDMS. Reprinted with permission from d. Copyright 2018 Advanced Materials. (e) Supercapacitor basket ((1), supercapacitor pyramid conformal to 3D objects), four pyramid-like supercapacitors (2) connected in series, forming a stretchable pop-up spring structure. (Right) Four symmetric supercapacitors connected in series and tailored into more complicated and delicate paper-cutting. Reprinted with permission from [144]. Copyright 2017 Advanced Materials.

Preparation Process
Selecting an appropriate method to prepare flexible and conductive electrodes is crucial for the construction of flexible electronics. The preparation methods reported in the literature are generally divided into the following categories, and the advantages and disadvantages are summarized in Table 6.

Phase Inversion Methods
Phase inversion methods can be divided into precipitation from vapor phase, precipitation by controlled evaporation, thermally induced phase separation, and immersion precipitation. One or more conductive fillers are mixed with dispersants and flexible base materials to form conductive nano-composite materials, which are then prepared into a conductive film by scratching or rotating coating. This method is one of the most common methods to prepare flexible electrodes, which is easy to operate and has the advantage of high yield. Zhao et al. [145] facilitated the preparation of polyethersulfone/activated carbon (PES/AC) flexible membrane electrodes by the phase inversion method. The transparent and uniform solution was dispersed into the membrane by rotating coating technology and immersed in deionized water immediately in a condensation bath, thus enabling good contact between the component materials ( Figure 10a). Sachse et al. [146] prepared a transparent conductive nano-silver wire electrode with 85% visible light transmission and 15 Ω· −1 average square resistance on glass by immersion coating, as shown in Figure 10b. Reprinted with permission from [150]. Copyright 2017 Advanced Materials interfaces.

Chemical Oxidation Polymerization Method
This method is generally used to grow conductive materials on a suitable flexible matrix by a chemical in situ polymerization of monomers [151]. The key point of this method is to control the polymerization conditions of monomers and the synergistic forces between monomers and flexible matrices. Wang et al. [147] attached polyaniline nanowires formed by oxidative polymerization of aniline monomers to flexible carbon nanotubes as yarns (Figure 10c). Carbon nanotube yarns are used as both electrodes and high-toughness materials. In-situ polymerization of polyaniline nanowires on the surface of carbon nanotube yarns can further improve the energy storage capacity of supercapacitors. Similarly, Yu et al. [152] prepared flexible polyaniline/graphene composite electrodes using in situ oxidative polymerization of aniline.

Electrochemical Deposition
Electrochemical deposition is the most common among them, which mainly causes the monomers in the electrolyte to polymerize on the selected matrix under the action of an applied current to obtain a flexible electrode. This method is more suitable for conductive polymer and carbon-based materials. For example, Chen et al. [148] used rubber fibers as flexible substrates then attached carbon nanotubes to the flexible substrates, and finally used the electrochemical deposition to deposit a layer of polyaniline on the surface of carbon nanotubes to obtain a flexible composite electrode (Figure 10d). Amade et al. [153] obtained a composite electrode with a thin MnO 2 layer wrapped in a thick carbon nanotube structure by electrochemical deposition of MnO 2 on carbon nanotubes, and obtained an ultra-high specific capacitance of 642 f·g −1 at a scanning speed of 10 mV·s −1 . Hu and Lei [154] prepared nitrogen doped porous carbon fibers (NCF) using commercial carbon fibers by electrochemical deposition and calcination. The NCF was subjected to azide hydrothermal treatment to obtain a self-supporting NNCF flexible electrode.

Chemical/Physical Vapor Deposition
The "physical vapor deposition (PVD)" and "chemical vapor deposition (CVD)" method combines conductive material with flexible substrate, and the flexible stretchable electrodes obtained are mostly in membrane shape [155]. Compared with electrochemical deposition, vapor deposition is generally suitable for the preparation of some carbon-based materials such as graphene and carbon nanotubes. Niu et al. [156] deposited single-walled carbon nanotube thin films onto polydimethylsiloxanes by chemical vapor deposition, and then prepared stretchable flexible transducer devices using PVA/H 2 SO 4 as gel electrolyte. Wu et al. [149] deposited silver on biomimetic elastomer microporous membranes by crosslinked enhanced ELD (electroless deposition) technology (Figure 10e). The bond strength between the PDMS substrate and the silver layer was significantly increased to 3.1 MPa by the low-cost upgraded ELD process and cross-linking enhancement. Although the materials prepared by physical vapor deposition have the advantages of small electrochemical impedance and uniform surface properties, their disadvantages are their expensive costs.

Electrospinning Technology
The material obtained by this technology has a high porosity, and its application in the electrochemical field will greatly increase the wettability between the material and the electrolyte, expand the accessible surface area of ions and help to improve the electrochemical performance of the device. Iqbal et al. [150] used the core-shell structure of carbon nanofibers/polyaniline composite as raw materials for spinning and obtained nanofibers composite membrane (Figure 10f) by "spinning-carbonization-spinning" three-step method. The supercapacitor prepared by this method also has good electrical properties. Zhang et al. [157] prepared a highly flexible transparent conductive electrode based on copper sulfide by electrostatic spinning and metal deposition. Nano-channel network (CuS NN) thin films have good chemical stability and significant mechanical stability.

Coating, Printing, and Spraying
This method has the advantages of low cost, easy scale production, and good substrate selectivity [158]. It can prepare a homogeneous suspension of active substances and coat them on flexible substrates by some means. For example, Xu et al. [159] used screen printing technology to print polyaniline/graphene composite slurry and apply it to the membrane electrodes of flexible supercapacitors (Figure 11a). Li et al. [160] prepared a foldable flexible supercapacitor by writing graphene slurry on a flexible substrate with a brush (Figure 11b). Lim et al. [161] were the first to obtain a flexible transparent conductive nano silver wire film, with an average square resistance of 38.7 Ω· −1 and transmittance of 87.62%, on a PET substrate by brushing (Figure 11c).

3D Printing
A new method of manufacturing parts directly from digital models by stacking layers of materials is called 3D printing [163]; It can use almost all types of materials, such as metals, ceramics, polymers, composites, and biomaterials. This method can enhance the specific capacity/capacitance, energy density and power density of energy storage equipment, and improve mechanical properties, especially high flexibility. Sun et al. [162] used 3D printing technology to manufacture a micron level micro battery with lithium iron phosphate as the positive electrode and lithium titanate as the negative electrode (Figure 11e). This micro battery has a high area specific capacity, which is the perfect combination of 3D printing technology and flexible energy storage devices. Li et al. produced a graphene composite aerogel microgrid electrode for flexible supercapacitors by 3D printing technology [164]. The 3D printing graphene composite aerogel has the advantages of light weight, high conductivity, and excellent electrochemical performance. In particular, the flexible supercapacitors using 3D graphene composite aerogel electrodes show exceptional capacitance retention and power density. This work has extensive guiding significance for the manufacture of high-performance and integrated flexible energy storage devices.

Vacuum Suction
Compared with the previous methods, vacuum filtration is easier to operate, and the suspension of active substances deposits on the flexible substrates by gravity and vacuum pressure [165]. During the filtration process, the thin areas where the active substances accumulate are concentrated, and the velocity of the dispersed liquid is larger than that of the relatively thick areas, which results in a faster and thicker filtration rate in the thin areas, thus improving the uniformity of the membrane formation of the composite flexible electrodes. De et al. [166] fabricated a conductive network of nano-silver wires using the filter transfer method and attached it to the PET substrate. The resulting flexible transparent conductive film of nano-silver wires has 85% transparency, 13 Ω· −1 average square resistance, and less than 2% resistance change after 1000 bending tests. Zeng et al. [167] prepared transparent conductive nano-silver wires thin films on glass substrates by filtration transfer method, then transferred the conductive network of nano-silver wires to polyvinyl Alcohol (PVA) substrates to obtain a flexible transparent conductive film with a transmittance of 87.5% at 550 nm and a mean square resistance of 63 Ω· −1 . The conductivity of the film can also be improved by alternating vacuum filtration, such as Wu et al. [168] using manganese dioxide (MnO 2 )/carbon nanotube composite as active material and using alternating vacuum filtration method to prepare the self-supporting flexible membrane electrodes (Figure 11d).

Summary and Future Prospect
In this paper, the research progress of flexible stretchable electrodes and recent achievements in the application of flexible electronic devices have been introduced. The preparation of flexible stretchable electrodes with high linearity and high strain, and the study of sensing technology, provides the basic materials for the wide application of flexible electronic products. Flexible stretchable electrodes with high linearity and strain are characterized by flexibility, folding, twisting, compression, stretching, and arbitrary deformation while maintaining high conductivity and reliability. Its applications have penetrated many fields, such as industrial production, ocean exploration, environmental protection, medical diagnosis, bioengineering, space development, and smart home technology. For example, flexible stretchable solar cells, highly flexible actuators, stretchable supercapacitors, electronic skin, artificial muscles, and so on have been developed. High conductivity, stretchable and editable flexible conductive materials are the key materials to produce wearable electronic devices in the future, especially biomedical electronic devices that are perfectly compatible with human organs. The analysis and summary of this article have far-reaching significance for the development of these areas.
Although the research of flexible electrode technology has made great progress in the past few years, most of the preparation techniques are still in the preliminary stage. There are still some practical applications. In summary, there are three bottlenecks in the preparation of flexible stretchable electrode materials: (1) Low stability, including air stability and tensile stability; (2) Low reliability, which is mainly caused by the mismatch between the conductive substrate and the device interface; (3) Low integration, which is manifested by the low uniformity of large-size film-forming and device processing. To make a breakthrough in the future, the research on the combination of flexible electrodes with their application devices needs the new technology of integrating materials, electronics, machinery, and other disciplines and the cooperation of researchers from all disciplines. Data Availability Statement: Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.