Abstract
Soft and conformable electronics are emerging rapidly and is envisioned as the future of next-generation electronic devices where devices can be readily deployed in various environments, such as on-body, on-skin or as a biomedical implant. Modern day electronics require electrical conductors as the fundamental building block for stretchable electronic devices and systems. In this review, we will study the various strategies and methods of designing and fabricating materials which are conductive, stretchable and self-healable, and explore relevant applications such as flexible and stretchable sensors, electrodes and energy harvesters.
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1 Introduction
Stretchable electronic devices have received increasing attention by researchers globally as they have the potential to be applied in many innovative fields such as epidermal electronic devices [1, 2], biomedical engineering [3, 4], healthcare monitoring [5,6,7,8], soft robotics [9,10,25], Ecoflex [26] and polyurethane (PU) [27], are usually used as the substrate or matrix to integrate with other parts of the system. On the other hand, to achieve high conductivity, traditional metallic conductors, or conductive nanomaterials such as metallic nanowires [28] and carbon nanotubes (CNT) [29] which are dispersed in the matrix, work as conductive fillers to increase conductivity. Numerous works in the literature have studied the interactive behaviors between various conductive fillers and matrixes which have different mechanical properties [29,30,31,32,33]. Integration of the two different phases, which typically has very different mechanical properties, presents a significant challenge, and at the same time, excellent opportunities for researchers to tackle. To-date, various solutions have been developed, with each of them utilizing unique and creative techniques such as coating [34, 35] and patterning [36,37,39].
The strategies of fabricating stretchable conductors can be classified broadly into three types: (i) geometric engineering of non-stretchable components [5, 40,41,42,43], (ii) intrinsically stretchable material development [29, 33, 34, 44], and (iii) combination of first two types [45,46,47,48,49]. Table 1 shows a general comparison between the first two approaches of fabricating stretchable electronics based on the methods of mechanically geometric design and material synthesis respectively.
One of the first strategy to produce stretchable conductors is to utilize intrinsic conductive material synthesis. The most commonly used method to achieve this purpose is to disperse nanomaterial fillers into the elastomeric matrix, combining advantages of electrical conductivity of the nanomaterial fillers and mechanical stretchability of the matrix material [50, 51]. To achieve high performance in both mechanical and electrical behaviors, a balance needs to be maintained between stretchability and conductivity. This is because more fillers incorporated into the elastomeric substrate will increase electrical conductivity while causing the whole composite material to be stiffer [27, 52]. In addition, most elastomeric materials used to form the stretchable components are solution-based, and solution processing of multi-stacked layers in the functional devices must address problems associated with dissolution, mixing, or cracking of the underlying elastomeric layer. Therefore, methods of adding fillers and their dispersion process, as well as adhesive bonding chemicals like surfactants, play critical roles in improving the quality of the whole composite [50].
Another strategy uses geometric design of non-stretchable conductive material within the elastic matrix, which enables the composite to have the ability to stretch. Stretchable electronics can be created by a combination of rigid electronic islands and stretchable interconnects [53, 54]. Generally, composites produced by this method have high stretchability and remain stable under applied deformation. However, it has several disadvantages: the fabrication methods require multiple steps, have relatively high costs, are hard to control and it is difficult to achieve scalable manufacturing. Also, there are usually issues with integrating the stretchable interconnects and the matrix. Thus, solving these problems are critical challenges in this mechanical method. Since both strategies have their own advantages and limitations as shown in Table 1, researchers are exploring combinations of these two methods to achieve better performance in stretchable conductors.
Similarly, to achieve self-healing, different strategies can be adopted. In extrinsic self-healing systems, healing agents, usually monomers, can be encapsulated and dispersed within the matrix. When cracks rupture the material and hence the encapsulation, the agents flow out, polymerize, and heal the damage. Alternatively, intrinsically self-healing materials based on supramolecular interactions, like hydrogen bonds or Diels–Alder reactions, can be designed. Self-healing via encapsulated healing agents can occur without intervention at the damage site but face the challenge of repeatable healing as the healing agent is depleted. Attempts have been made to overcome this disadvantage, mainly by microvascular design, where a network allows healing agents to flow to that site. Intrinsic self-healing can occur repeatably but require a stimulus, for example, a mechanical force or elevated temperatures. To overcome this, materials utilizing other mechanisms like magnetic particles have been designed. Healing mechanisms have their own different limitations and characteristics, like hydrogen bonding can be impeded by aging or moisture but certain self-healing mechanisms are aided by water due to swelling of the polymer. There is a diverse range of self-healing materials based on different chemistries and interactions between the constituents in the composite.
In this review, we present an update and summary on the mechanics, strategies and potential applications of stretchable and self-healable conductors over the last couple of decades. We hope that this review will aid interested readers to better understand this area, and gain an appreciation of the importance of conductors and self-healing in stretchable electronics.
2 Intrinsically conductive materials
A widely used strategy to realize stretchable conductors is to combine conductive fillers, which are generally rigid and brittle, with a stretchable elastomer [55], forming a blended composite block or thin film [56, 57]. The different properties of each component, mainly the electrical conductivity of a filler and stretchability of an elastomer, complement each other and generates synergy. There is a diverse range of conductive fillers, including carbon-based (e.g., CNT, carbon black (CB), graphene, graphite, carbon fiber), metal-based (e.g., silver (Ag), gold (Au), copper (Cu)) materials, and conductive polymers (e.g. poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), polyaniline (PANi), polypyrrole (PPy)) [58]. There is also a wide range of geometrical shapes and sizes of the fillers (e.g. wire, flakes, particles, etc.). For the elastomer that fillers are dispersed into, PDMS, PU, Ecoflex, poly(styrene–butadiene–styrene) (SBS) are the most widely used materials [26]. The dispersed fillers in the polymeric matrix form conductive pathways where electrons can transport, rendering the composite electrically conductive. This percolation dependent conductivity is heavily influenced by the type and shape of the fillers. This determines the amount of junction contact resistances induced [59] and is also closely associated with the stretchability of the composite. Therefore, the selection of fillers and elastomer is of great importance.
Another important consideration would be to choose an appropriate fabrication method. Commonly used methods are represented in Fig. 2a. Dry process such as direct chemical vapor deposition (CVD) and array drawing can create a high-quality film, but they are accompanied by potential high costs. In contrast, although wet process is relatively inexpensive and simple, it is generally difficult to obtain a high-quality film. However, in terms of industrial application, solution-based processing has a great advantage. Solution-based processes such as mixing, drop-casting and spin-coating generally allow for low-temperature processing, large scale production, and deposition onto various substrates. These methods can reduce the cost and overcome limitations in substrate materials and size compared to conventional processes in semiconductor manufacturing. However, polymer surfactants or ligands that aid homogeneous dispersion of fillers are likely to interfere with charge transport which limits the conductivity. Meanwhile, inappropriate fabrication process can nullify advantages of the filler morphology. For example, usage of ultrasonication for homogeneous dispersion of fillers may shorten the length of the fillers, which negates the efficacy of the high aspect ratio fillers for stretchable conductors.
a Commonly used fabrication process of a stretchable conductor. Reproduced with permission [60]. Copyright 2004. American Association for the Advancement of Science. Reproduced with permission [61,62,63]. Copyright 2006, 2014, 2012. Wiley–VCH. Reproduced with permission [64]. Copyright 2018. IEEE. Reproduced with permission [65]. Copyright 2014. Royal Society of Chemistry. Reproduced with permission from [66]; published by MDPI, 2018. b The fabricated stretchable conductors made of 1D SWNTs. Reproduced with permission [67]. Copyright 2008. American Association for the Advancement of Science. c Superaligned CNT forest (left), CNT ribbons pulled out from CNT forest (middle), and optical image of folded CNT ribbon film embedded in PDMS. Reproduced with permission [68]. Copyright 2010. Wiley–VCH. d SEM image of the fabricated 3D CNT ropes and stress–strain curves for cycle 1st, 10th, 100th, and 1000th with a set strain of 20%. Reproduced with permission [69]. Copyright 2012. Wiley–VCH. e A transparent SWNTs thin film on a PET substrate with a meter-scale and their SEM image. Reproduced with permission [70]. Copyright 2018. Wiley–VCH. f Effect of acid treatment of CNT on [71]. Copyright 2007. American Chemical Society
For these reasons, to realize the best performing stretchable conductor, diverse factors ought to be considered. The selection of materials for fillers and elastomer, and the fabrication process both determines the electrical and mechanical performances of the conductors. The list of stretchable conductors based on different conductive fillers is provided in Table 2. In the following section, we will discuss the strategies for high-performance stretchable conductors based on the materials.
2.1 Carbon-based fillers
Usage of carbon materials as a filler has led to substantial progress for the development of intrinsically stretchable electrodes. CB, CNT, graphene, carbon nanofibers etc. are commonly used carbon-based fillers for stretchable conductors [76, 89,90,91]. Their main advantage lies in their outstanding electrical and mechanical properties as well as cost-effectiveness.
Among carbon-based conductive fillers, CNT may hold the most promise. This is attributed to the efficacy of high aspect ratio, a wide range of structures (e.g., ribbon, yarns), availability of mass-production, and cost-effectiveness [92]. 1D CNT may hold more promise than nanoparticles or flakes (e.g., CB) due to junction-less conductive pathways, which have high conductivity without compromising the stretchability. This is because high loading of fillers to have an efficient pathway [93, 94] may adversely affect the stretchability [95].
When commercially available CNTs are embedded into stretchable elastomer, homogeneous dispersion is necessary as homogeneous dispersion holds a key role in realizing the ultimate properties of the resulting composites and enables patterning by e.g. 3D printing. In practice, due to the high van der Waals forces, multi-walled nanotubes (MWNTs) normally exist in clusters and it is difficult to disperse them into the solvent. Dispersion homogeneity can be resolved with the help of a surfactant or by choosing an appropriate solvent [92, 93, 96].
The representative example of CNT/elastomer blended composite for highly stretchable CNT-based conductor was introduced by Sekitani et al. [67] (Fig. 2b). They fabricated a single-walled nanotube (SWNT) based elastic conductor by dispersing SWNTs/ionic liquid mixture into a fluorinated copolymer. The developed SWNT film exhibits a conductivity of 57 S cm−1 when unstrained. To increase the stretchability, they perforated the SWNTs film so it becomes a net-shaped film, which allowed them to stretch up to ~ 130%. Due to the high aspect ratio of the SWNTs (3 nm in diameter and > 1 mm in length), the conductivity is barely changed under uniaxial stretching by 38% or less, and the conductivity is still as high as 6 S cm−1 when stretched up to 134%.
Interestingly, CNTs in a specially assembled form such as CNT-ribbon or CNT-yarn have been developed due to their unique benefits: e.g. better mechanical strength or higher transmittance. CNT ribbons refer to the sheet of continuous CNTs that can continue up to meter scale with unidirectional alignment along the drawing direction (Fig. 2c). They are obtained simply by being pulled out from vertically grown super-aligned CNT forests (Fig. 2c) [2f) [71, 103]. Acid treatment is done by simply immersing CNTs into acids, resulting in reduction of sheet resistance. In an example by Geng et al., HNO3 treatment for 60 min reduces sheet resistance by a factor of 2.5 times with negligible change in transparency [71]. Meanwhile, Kim et al. carried out chemical do** with gold trichloride solution (AuCl3), demonstrating an increase in conductivity of the SWCNTs/PU web composite [103].
2.2 Metal-based fillers
Despite the tremendous popularity of the CNTs for stretchable conductors, the relatively higher conductivity of metal (CNT based composite: < 100 S cm−1, metal based composite: > 100 S cm−1) led to substantial progress of metal-based stretchable conductors [104]. The representative metal fillers used for stretchable conductors are silver, gold, and Cu. Their shapes are diverse, ranging from nanoparticle (NP), nanowire (NW), nanosheets, to nanoflakes. The fabrication approach for stretchable conductors is similar to that of carbon-based, most of which are solution-based approaches including vacuum filtration, Meyer rod coating, and solution coating followed by transfer. They are generally achieved at relatively low temperature. Of the numerous metal-based nanofillers, the most popular metal used is silver due to its lower price than gold, and higher stability than copper. In addition, the relatively low fusing temperature of AgNWs which could lower the electrical resistance has made it more attractive. These advantages have pushed significant advances in Ag-based stretchable conductors.
Although 1D nanowires are generally regarded as the most effective fillers, gold nanoparticles exhibited outstanding performance as a highly stretchable and conductive material due to their unique behavior. By depositing AuNPs onto polyurethane through layer by layer (LBL) or vacuum-assisted flocculation (VAF) methods [81], films presented excellent electrical conductivity of 11,000 S cm−1 and 1800 S cm−1 under unstrained condition respectively, with corresponding maximum strains of 115% and 486%. When the respective films are stretched to a strain of 60%, the conductivity decreased to 3500 S cm−1 and 210 S cm−1, and when stretched to 110%, the conductivity decreased to 2400 S cm−1 and 94 S cm−1. The conductivity values still remain high even after strained, unlike other carbon-based conductors in spite of unfavorable low aspect ratio morphology. This is attributed to the self-assembly behavior of the AuNPs when stress is induced by stretching. As seen in Fig. 3a, as the tensile strain is applied, NPs are re-organized, aligned along the tensile direction. This re-organization and formation of conduction band facilitate the conductive pathway along the strain direction.
a Self-organizing behaviour of Ag nanoparticles under strain. Reproduced with permission [81]. Copyright 2013, Nature Publishing Group. b Synthesis process for very long Ag nanowires, and images (middle) and the NW length distribution graph (right) of lengthened Ag nanowires formed by the SMG method. Reproduced with permission [62]. Copyright 2012, Wiley–VCH. c Sheet resistance versus time of CuNWs based films to show the oxidative stability. Reproduced with permission [82]. Copyright 2014, Royal Society of Chemistry. d Laser-nanowelded stretchable CuNW conductor. Reproduced with permission [84]. Copyright 2014, Wiley–VCH e Schematic illustration of the PEDOT/STEC (left). Conductivity change under strain for PEDOT incorporated with three different enhancers for stretchability and electrical conductivity (STEC) (right). Reproduced with permission [86]. Copyright 2017. American Association for the Advancement of Science
Apart from the special case of self-assembly behavior of NPs with effective mobility, 1D metal NWs normally show better performance as stretchable conductors. Going one step further, superior stretchability with relatively high electrical conductivity is realized with help of very long AgNWs synthesis called successive multistep growth (SMG) which enables an increase of the length to > 500 µm (Fig. 3b). The films with lengthened AgNWs accommodate strains above 460% while significant resistance change is not observed. The ductility that Ag intrinsically possesses, together with the more effective percolation network induced by long AgNWs under strain, produces a highly stretchable and conductive electrode.
Although AgNWs are the most extensively used fillers among conducting metal fillers, better cost-effectiveness (by 100 fold) and the higher abundance (by 1000 fold) of copper have attracted great interest in it as a promising conductive filler [105]. Nevertheless, a crucial drawback of copper that limits a wider use of copper fillers is susceptibility to oxidation which reduces the conductivity. In order to fully realize the advantages of copper (e.g., comparable electrical conductivity with silver), a diversity of strategies to avoid the oxidation problem without compromising electrical conductivity have been devised. They include coating with graphene oxide, nickel or chitosan [105, 106], or ligand [107], reduction of the oxide layer to metallic copper by acid solution [108], or embedment into elastomer [71, 82].
In addition, the welding process to reduce junction resistances of nanofillers would be hindered by the oxidized layer of copper fillers or give rise to oxidization of the fillers. Annealing processes under hydrogen or hydrazine [109, 110], or with the help of a protective layer (e.g., chitosan) have been carried out to overcome this oxidization problem. An example of embedment of CuNWs into the underlying substrate (e.g., elastomer) whereby oxidation is suppressed is represented in [82] (Fig. 3c). The polymer can effectively protect them from being oxidized by the surrounding air [82]. Compared to the CuNW film on glass, the films where CuNWs is partly embedded demonstrate excellent oxidative stability (Fig. 3c). This simple tactic allows for a transparent and stretchable conductor with chemical stability.
An example of nanowelding process with minimization of oxidation was introduced by Han and co-workers [84]. They conducted a plasmonic laser welding process to weld the nanowire junctions together. Such a process offers a reduction of resistance by minimizing the problem of oxidation incurred by conventional thermal annealing process. Furthermore, the prepared CuNW films show good stretchability (Fig. 3d).
Nanofillers play an important role to form conductive pathways that determine electrical conductance. Therefore, various post-treatment techniques to adapt to nanofillers can help facilitate the formation of conductive pathways. The as-synthesized nanofillers are normally capped with stabilizing ligands that hinder the conductive pathway. Processes of ligand washing to remove the thick ligand layer or ligand exchange to those with lower chain length can be carried out to lower contact resistance [104]. Also, soldering with additives [111] and welding [112] have also been performed to enable electron transfer at the junction.
2.3 Conductive polymer based filler
Conducting polymer fillers in conjunction with carbon and metal fillers have been at the center of development of soft electronics. The main benefit of conducting polymer fillers is that they possess tunability of electrical and mechanical properties, through engineering the molecular structures or transforming into block copolymers that consist of the rigid electronic blocks and soft blocks [113]. Of all the conducting polymers, PPy, polyaniline (PANi), polyindole, polythiophene (PTh) are the most typical examples of conducting polymers studied [114, 115]. Unfortunately, their poor solubility may limit their wider use in applications.
On the other hand, a water-soluble conducting polymer, PEDOT:PSS has been developed, which comprises of positively charged PEDOT, and negatively charged and water-soluble PSS. Their solution processability, together with high conductivity, flexibility in tuning electrical and mechanical properties, transparency and commercial availability have made it an ideal material that can lead to significant innovation in stretchable conductors [113]. The additional advantage of solution processability is to enable large-scale production in soft electronics.
Although the high electrical conductivity of > 1000 S cm−1 can be achieved, coupled with good transparency of PEDOT:PSS (comparable to indium tin oxide, ITO), the fracture strain of PEDOT:PSS is as low as 5% and Young’s modulus is ~ 2 GPa [85]. The typical methods to improve mechanical compliance are the addition of plasticizers to lower the elastic modulus, examples of which include glycerol [116], Triton X-100 [117], and ionic liquids [118], or to form a polymeric blend with water-soluble polymers (e.g., poly(ethylene glycol) (PEG), poly(ethylene oxide) (PEO)).
The usage of architectures by buckling and kirigami methods discussed in the following section can be also considered. In an example (Fig. 3e) [86], an ionic liquid plasticizer allows PEDOT:PSS to dissolve in water and also plays the role of a dopant for PEDOT, improving its conductivity and elastic modulus. Excellent electrical and mechanical properties were obtained where the maximum conductivity is 3390 S cm−1 at the strain of 100%, and the fracture strain is 800% when supported on styrene ethylene butylene styrene (SEBS).
3 Structural designs
Structural design is another important strategy in the fabrication of stretchable conductors. Certain structural designs can enable originally non-stretchable conductive materials to possess outstanding stretchability. Generally, different structural configurations can achieve different stretchabilities and remain relatively stable under applied deformation due to their specific geometric design. However, several challenges of applying this strategy have yet to be resolved by researchers in this field: (1) the fabrication methods of making stretchable electronic devices are complicated, (2) it often entails relatively high cost, (3) some of processing techniques are hard to control, making it difficult to achieve scalable manufacturing, (4) there usually are complications in the integration between stretchable interconnects and matrix.
In this section, an intelligent pre-strain method to realize buckling design by Huang and Roger’s group is highlighted first as this method can be precisely controlled with an understanding of the buckling mechanism on a thin-film substrate, and thus it is considered as the most popular way in the area of making stretchable electronics. Then, different innovative structural designs developed by various research groups to realize stretchable functions are reported, including 2D in-plane, 3D out-of-plane, textile, kirigami and origami designs. Overall, these designs are summarized in Fig. 4a with the technique’s resolution and performance to give a brief comparison of these structural configuration designs.
Structural design for stretchable conductors. a Comparison between 2D in-plane [119], 3D out-of-plane [120], textile [121], kirigami and origami designs [122] with aspects of maximum stretchability and microstructure dimension. b Buckling strategy and wrinkle conductor. Reproduced with permission [123]. Copyright 2006, Nature Publishing Group. c 2D in planed self-similar serpentine design. Reproduced with permission [124]. Copyright 2013, Elsevier B.V. d 3D out of planed helical design [125]. e Textile structural design. Reproduced with permission [121]. Copyright 2016, Nature Publishing Group. f Kirigami structural design. Reproduced with permission [126]. Copyright 2015, Nature Publishing Group
3.1 Buckling design
The buckling phenomenon was firstly applied by Huang and Roger’s groups in the stretchable electronics field [123]. It was found that a stiff ribbon which has poor stretchability can be attached to a pre-stretched elastomer surface at certain positions, and they become wrinkled after release, shown in Fig. 4b [44]. Copyright 2014, Royal Society of Chemistry
One of the important parameters for good performance is linearity as it makes calibration relatively simple. Unlike many resistive type strain sensors which have poor linearity and hysteresis, capacitance type sensors generally exhibit good linearity. Capacitance type strain sensors are generally made of a deformable dielectric layer sandwiched by a pair of stretchable electrodes. Change in capacitance is encoded to strain. An example of capacitance type strain sensor with high sensitivity and good linearity is represented in [44] (Fig. 6b). The device detected diverse human motions well as its sensitivity is enough to detect target motion. An example that detects even subtle blood pulse signal and motions like blinking and clenching was introduced by Wang et al. [6], realized by graphene woven fabrics on PDMS. Monitoring of blood flow pulse is useful to check medical conditions like high blood pressure. Another performance parameter, increased sensitivity, which is usually quantified as a high gauge factor, is also important.
One strategy for this is to use conductive fillers with lower density networks whereby they facilitate the disconnection between filler junctions under strain and thus achieving higher gauge factors. Another approach to enhance sensitivity is to use treatment of nanofillers, which was conducted by Kim et al. [182]. The treatment of intense pulsed light irradiation onto composite where Ag flakes/Ag nanocrystals are embedded into PDMS leads to an increase in the gauge factor. In order to achieve high performance skin-mounted operation, apart from the stretchability, other properties such as lightness, biocompatibility, low power consumption are also desirable.
5.2 Electrodes
Electrodes refer to electrical conductors that interface with a non-conducting part of the circuit. Examples of use include electrochemical cells, electrolysis and electrophysiology.
In lithium (Li)-ion batteries, although lithium metal anodes have a high theoretical specific capacity, there are safety issues because dendrites tend to form and cause a short circuit. Another material with high specific capacity is silicon, but the large volume change during lithiation and delithiation requires mechanisms to alleviate the stresses produced. A stretchable carbon/silicon anode for lithium-ion batteries was designed by coating with a self-healing polymer [183]. The fabrication process is illustrated in Fig. 7a. Graphitic carbon was grown by CVD on a nickel foam using vaporized hexane at ambient pressure. The nickel was subsequently removed, and a uniform layer of amorphous Si was deposited onto the 3D carbon skeletal structure using silane as a precursor at low pressure. The self-healing polymer was then drop-casted on the foam. It is a modified version of the hydrogen bonding based polymer in Sect. 4.2, but instead of reacting with urea, adipic acid was chosen.
Illustrations of healable conductive electrodes. a Synthesis and microscope images of carbon/Si/polymer electrode. Reproduced in part with permission [183], Copyright 2016, Wiley–VCH. b Schematic illustrating volume change during lithiation/delithiation. Reproduced in part with permission [184], Copyright 2018, Wiley–VCH. c Schematic of self-layering mechanism of Ag nanosheets Reproduced in part with permission [185], Copyright 2018, American Chemical Society. d Schematic of capacitor fabrication. Reproduced in part with permission [186], Copyright 2018, The Royal Society of Chemistry
Although the polymer had only partial healing when cut due to the breaking of non-reversible covalent bonds, stretching within 50% is unlikely to fully fracture the material and it can self-heal. The average strain beyond which the foam coated with 4× weight polymer is 46.2%, and 1000 loading/unloading cycles at 25% strain caused an increase in resistance of less than 400 Ω. A higher percentage of polymer increases stretchability but decreases specific capacity and efficiency due to its poor Li ion conductivity and increased diffusion length. The optimal was 4× weight polymer which showed only a slight decrease in specific capacity from 719 mAh g−1 to 584 mAh g−1 over 100 charge/discharge cycles.
In another work, poly(acrylic acid)- ureido-pyrimidinone (PAA-UPy) was chosen as the binder polymer due to the ability of UPy dimers to form quadruple hydrogen bonds [184]. Si NPs and conductive CB were dispersed in the polymer with weight ratio 6:2:2. The composite shows a high initial capacity of 4194 mAh g−1 and a capacity of 2638 mAh g−1 after 110 charge/discharge cycles. The adhesion force at cut surfaces was measured to be 36.2 mN m−1 after 1 min of contact, demonstrating self-healing. Combined with good adhesion with Si NPs and carbon black, crack formation was limited to widths of 0.3 µm after 110 cycles.
Recently, a printable conductive electrode was demonstrated by mixing gelatin with a Ag salt (silver trifluoroacetate) dissolved in ethanol and a reducing agent, dopamine, obtaining a gel ink [185]. A self-layering effect is shown in Fig. 7c, and Ag NP sheets were obtained after curing at 200-300 °C. The electrical conductivity could be tuned by varying the concentration of ethanol to control the surface morphology of the Ag NP layer. After 100,000 bending cycles of 2 mm radius (3.2% strain), the resistance of the electrode increased by less than 4%. Cracks produced by extreme bending could be healed by heating, and 100% electrical recovery was obtained at 150 °C for 5 min. This material was demonstrated as a printed flexible electrode for heating as well as for a micro-supercapacitor.
Typically, capacitors have interfaces between the electrodes and dielectric, which tend to delaminate during deformation, leading to interfacial contact issues. A self-healing capacitor was designed to overcome this problem by in situ integration of the electrode into a hydrogel electrolyte [186]. The hydrogel was synthesized from a co-polymer of vinylimidazole (VI) and hydroxypropyl acrylate (HPA), with ammonium persulfate (APS) and methylenebisacrylamide (MBAA) in an aqueous solution of H2SO4, forming PVH-H2SO4. It was then immersed in a solution of aniline (ANi) and APS added, where the ANi diffused into the hydrogel and was polymerized into PANi. The hydrogel was lastly cut to reveal the capacitor structure, shown in Fig. 7d. Self-healing was achieved due to the hydrogen bonding between the imidazole and hydroxy groups within the hydrogel, with additional cation–π and π–π interactions in the PANi layer electrode. After 10 cut/heal cycles at 25 °C for 20 min, the capacitance recovery was 96% while recovery of mechanical stress was 88.6%.
5.3 Energy Harvester
Another important application of stretchable conductors is in energy harvesting devices. Development of energy harvesting devices is a step forward in the quest for an environmentally sustainable society. Emerging with stretchable electronics, energy harvesting devices have been the focus of researchers in the past decade. Prior to using stretchable conductors, most of the flexible energy harvesters can only operate under a low strain ratio (~ 0.1%) [187]. Thus, highly flexible and stretchable energy harvesting devices (over 15% strain ratio) are studied by applying the strategies of material development, structural configuration design and integration methodology. Based on their specific potential applications, mainly in wearable and stretchable electronics, energy harvesting devices have been developed, converting mechanical energy to electrical energy mainly in two ways: triboelectricity or piezoelectricity, which are also called nanogenerators [188, 189].
Recently, a hyper-stretchable elastic-composite generator (SEG) realized by very long Ag nanowires (VAgNWs) stretchable electrodes is reported to have high stretchability (around 200%) and about seven times higher power output than piezo-generators [190]. Figure 8a shows the generated voltage and current from the SEG on the stocking serving as a wearable device by bending and straightening the knee. A scalable approach for highly deformable and stretchable energy harvesters and self-powered sensors is also reported by fabricating a shape-adaptive triboelectric nanogenerator (saTENG) with conductive liquid contained in a polymer cover. It is claimed that this energy harvesting device can function as deformable and stretchable power sources and potentially applied in fields such as robotics and biomechanics. Figure 8b illustrates a saTENG looped around the arm of a subject to harvest energy from tap** motion and serve as a self-powered arm motion sensor.
Stretchable and self-healable conductors for applications of energy harvesting. a A type of hyper-stretchable elastic-composite energy harvester. Reproduced with permission [190]. Copyright 2015, WILEY–VCH. b A highly shape-adaptive, stretchable design based on conductive liquid for energy harvesting and self-powered biomechanical monitoring. Reproduced with permission [191]. c Stretchable porous CNT-elastomer hybrid nanocomposite for harvesting mechanical energy. Reproduced with permission [192]. Copyright 2015, WILEY–VCH. d Highly transparent, stretchable, and self-healing Ionic-skin triboelectric nanogenerators for energy harvesting. Reproduced with permission [193]. Copyright 2017, WILEY–VCH
A stretchable energy harvester can also be developed through combination with nanomaterials [194]. In this work, stretchable porous nanocomposite (PNC) based on a hybrid of a multiwalled CNTs network and PDMS matrix for harvesting energy from mechanical interactions is reported. A cross-section view of a bent PNC is illustrated in Fig. 8c. In addition, an innovative method of using an ionic conductor as the current collector in the energy harvester is demonstrated to enhance stretchability, transparency and self-healing properties [195]. The resulting ionic-skin TENG (IS-TENG) has a high transparency (92% transmittance), ultra-long uniaxial strain (700%) and good self-healing performance that can recover its performance after 300 times of complete bifurcation. Figure 8d shows the digital photo of this IS-TENG attached to the human palm as an energy harvester to power 40 LEDs.
