Recent advances in functional fiber electronics

Rapid development of wearable electronics with various functionalities has stimulated the demand to construct functional fiber devices due to their merits of mechanical flexibility, weavability, miniaturization, and integrability. To this end, fiber components which can realize the functions of energy storage and conversion, actuating plus sensing have gained increasing concerns. Herein, we summarize the recent progress with respect to fiber material preparation, innovative structure design, and device performance in this review, also highlighting the possibility of integrated fiber electronics as an extension of application, the remaining challenges and future perspectives toward next‐generation smart systems and to facilitate their commercialization.

For flexible/wearable electronic applications, conducting fibers featuring high electrical conductivity and structural flexibility offer the great promise as essential building blocks for constructing flexible devices. Recently, conductive fibers have been made from metal fibers/nanowires, [8][9][10] carbon nanotube (CNT) fibers, [11][12][13][14][15][16] graphene fibers. 17 Specifically, CNT fibers comprising of aligned CNT bundles have been attracting great attention owing to their advantageous properties of good electrical conductivity, superior mechanical flexibility, light weight together with satisfactory chemical stability. 12,13,18 Besides, they can be easily tailored with desired properties via engineering their microstructures or incorporating functional material into the fibers, which present broad applications in the field of supercapacitors, [19][20][21] batteries, 22,23 actuators, 2,24-28 strain sensors, 29,30 etc. To date, flexible fibers with novel functions, such as high stretchability 22,31,32 and self-healing ability, 7 are also highly desirable to withstand the various deformations of bending, twisting, stretching plus the damages during daily usages, thus, satisfying the requirements of practical needs.
Among the plentiful applications, flexible fiber-shaped supercapacitors and batteries are regarded as promising power supplies for flexible/wearable electronics ascribed to their high power capability and high energy density, respectively. These fiber-shaped energy storage devices have been successfully realized via the rational incorporation of electroactive components into fibers, followed by device assembly in configurations of parallel, 19 twisted, 20,33 and coaxial patterns. 34 In addition, sensors 30,35 and actuators 36,37 are also indispensable parts for wearable devices as they can monitor environmental variations. In addition to single function, considerable efforts have been devoted to boost the realization of integration devices/systems which can be achieved by weaving/knitting different functional fibers into multifunctional textiles on the basis of textile technologies 4,5 or integrating all functional components on single fiber, 6,38-40 allowing for the accomplishment of self-powered smart system.
In comparison to well-established manufacturing approaches to planar 2D architecture electronics, it is more challenging to develop fiber-shaped ones. Critical aspects, including structural engineering and device assembly, were explored to obtain fiber components with targeted functions. Recent reviews on flexible electronics have mainly focused on the material preparation and their corresponding applications. [41][42][43][44] Here, this article presents a critical review of the current and significant progress in fiber-shaped electronic devices with a focus on fibrous materials, device configurations, and performance, also highlights the construction of multifunctionality and F I G U R E 1 Overview of functional fiber electronics integration system (as depicted in Figure 1). Particularly, conductive fibers are first introduced including metallic nanomaterials, CNT fibers, and graphene fibers with their corresponding fabrication methods and mechanical/electrical properties. Then, representative examples for fiber-shaped electronic devices covering supercapacitors, batteries (metal-ion batteries and metal-air batteries), actuators, and sensors are illustrated. Furthermore, integration systems for the purpose of multifunctionality have been descripted. The remaining challenges and future opportunities toward fiber electronics are also presented for next-generation wearable electronic devices.

FIBER MATERIALS
Extensive efforts have been exerted to explore fiber materials, including metallic nanostructures, [45][46][47][48] CNT fibers, 32,49,50 and graphene fibers, [51][52][53] with both high mechanical flexibility and electrical conductivity for the realization of wearable electronic devices. Conductive fiber materials comprising of metallic nanowires are mainly prepared via the methods of immersion, 9,47 dip coating, 54 wet spinning. 55, 56 Lee et al. 9 proposed an effective method to incorporate conductive Ag nanoparticles (AgNPs) into stretchable fibers with a multifilament structure, which involves two steps: (1) the polyurethane (PU)-based elastic fiber in multifilament structure was immersed in an ethanol solution containing 40 wt% AgCF 3 COO to effectively absorb Ag precursor induced by the ion-dipole interaction between CF 3 COO − F I G U R E 2 (A) Schematic to the fabrication process of BWY-Ag NWs fiber. Reproduced with permission from Ref. [57]. Copyright 2020, Elsevier. (B) Schematic to the wet-spinning process of conductive stretchable fiber. SEM: Ag particles, HRTEM: nAg-MWNTs. Reproduced with permission from Ref. [56]. Copyright 2014, American Chemical Society. (C) Schematic of the direct-spinning process. Reproduced with permission from Ref. [11]. Copyright 2004, published by American Association for the Advancement of Science. (D) SEM images of a CNT fiber dry spun from a nanotube array. Reproduced with permission from Ref. [15]. Copyright 2004, published by American Association for the Advancement of Science. (E) Schematic to the fabrication of CNT spring. Reproduced with permission from Ref. [32]. Copyright 2012, Wiley and -OH in the alcohol solvent. (2) Chemical reduction of the absorbed Ag + in fibers to AgNPs by hydrazine hydrate (N 2 H 4 ⋅4H 2 O) and finally Ag nanoparticle-based fiber strain sensors were obtained. Sun et al. 57 produced silver nanowires (AgNWs) composite fibers derived from braid-like weaved yarn (BWY) through a dip coating method ( Figure 2A). The BWY was cleaned by ethanol sonication and dried, followed by the dip coating in AgNWs ethanol solution. Various conductivities of yarns can be manipulated via tuning the dip-coating times. Afterwards, the BWY fibers covered with AgNWs were treated with H 2 plasma to remove residual insulating PVP and finally produce BWY-AgNWs conductive fibers. Another efficient approach is wet spinning to fabricate conductive fiber comprising of Ag particle, nAg-MWNTs, and polymer matrix in a fiber with approximately 100 μm diameter ( Figure 2B), which can be tuned by altering the nozzle size of wetspinning process setup. The coagulant of hexane has been demonstrated to offer the fiber with a maximum strain of 490% and a conductivity of 236 S/cm measured for 7.4 wt % Ag particles. 56 Baik et al. 55 also employed a wet-spinning strategy to synthesize stretchable and conductive Ag-PU fibers that used dispersion of Ag nanoflowers in the PU solution to form a spinnable dope and then extradited into deionized water coagulant. The as-spun fiber was finally collected on a winding drum and dried in air. The prepared Ag-PU fibers have obtained extremely high electrical conductivity (41 245 S/cm) and a maximum rupture strain of 776%. Though metallic fillers-based fibers possess extraordinary conductivity, the possible reactions between metals and some electrolytes restrict their usage in energy-related devices.
Carbon nanotubes (CNTs) are attractive for their high electrical conductivity, high mechanical strength, sufficient surface area, and self-supporting characteristics. 18,58 Unlike widely used carbon fibers, CNT in fiber format has good structural flexibility that can be bent and knotted. So far, the preparation of CNT fibers can be mainly divided into two methods: wet spinning 12,59 and dry spinning. 16,60 Wet spinning is a conventional method to manufacture fibers. In 2000, Vigolo 59 first assembled single-walled CNT (SWCNT) fibers via injecting dispersion of nanotubes in surfactant solution (SDS) into polyvinyl alchol (PVA) coagulation bath. The morphology of the as-spun fibers can be controlled with diameters from several micrometers to 100 μm by varying the parameters of injection rate, flow speed, and the needle, or capillary tube size. Due to the existence of the absorbed polymer (PVA), better alignment can be achieved after rewetting the fiber in an appropriate solvent (eg, acetone, poor solvent for PVA) and stretching treatment, thus, improving the fiber Young's modulus. 61 When replacing PVA with a polyethyleneimine (PEI) coagulant and cationic surfactant, the CNT fibers were obtained with improved mechanical properties and electrical conductivity ascribed to the strong interfacial binding effect of PEI with the nanotubes. 62 Even so, the residual polymers during the spinning process still limited the fiber conductivity, requiring complete removal upon thermal annealing but with the decreased mechanical properties of fibers. Neat SWCNT fiber without surfactant or polymer assistance can be further achieved from SWCNTs dispersion in superacids (102% sulfuric acid) to separate the nanotubes into individuals, and then coagulation in water by wet-spinning technique. 13 Compared with wet-spinning technology, dry-spinning method avoids the use of surfactants and strong acids. CNT fibers can be readily prepared by spinning from aligned CNT arrays 16,60 and directly spinning from a chemical vapor deposition (CVD) reaction. 11,63,64 The former preparation process involves the drawing of CNT sheets from a spinnable CNTs array grown on a silicon wafer coated with iron catalyst by CVD process to obtain continuous CNT fibers. The fiber can be engineered with controlled diameter by tunning the width of the array. By a similar spinning method, Jiang et al. 60 spun CNT fibers having length of centimeters from superaligned CNT array originated from van der Waals interactions between aligned nanotubes. Li et al. 11 proposed a direct spinning from CVD using liquid ethanol as the carbon source and an iron catalyst, and hydrogen as carrier gas to load into the reaction zone at a temperature range of 1050-1200 • C to form CNTs aerogel. The aerogel can be drawn directly from the hot zone to form CNT fibers ( Figure 2C). This direct-spinning process can be expanded with other carbon sources, for example, the mixture of aromatic-hydrocarbons and oxygen-containing molecules. Then, Zhang et al. obtained CNT fiber by introducing twist during a dry-spinning process ( Figure 2D) and made the resulting multiply fibers overtwisted into yarns, or knotted and knitted into complex shapes. 15 Due to the high strength and flexibility, this type of CNT yarn can be constructed into macroscopic structures for versatile applications. 65,66 Furthermore, the fiber can be converted to a spring shape, reaching the tensile strain up to 285%, which is much higher than the straight CNT fibers (the tensile strain at break is usually less than 10%) ( Figure 2E). 32 In addition, CNT fibers are also excellent electrical/thermal conductors. Compared with other flexible wires, they have a larger surface area, resistance to oxidation and corrosion. Therefore, by incorporating guest materials into CNT fibers, a flexible functional fiber can be manufactured. Benefitting from the unique structure and properties, the application of CNT fibers/yarns can be extended to a much broader area including sensors, actuators, artificial muscles, and supercapacitors.
Graphene, a 2D monolayer of sp 2 hybrid carbon atoms tightly bound to the honeycomb lattice, have excellent electrical properties, and widely used in energy storage materials. Integration of 2D graphene sheets into macroarchitectured fibers, has translated the excellent performance of individual graphene into advanced fibers. Up to now, the main methods to manufacture graphene fibers include wet spinning, 67,68 dry spinning, electrospinning. Injection of graphene oxide (GO) sheets dispersions into a coagulation bath containing hexadecyltrimethyl ammonium bromide (CTAB) surfactant solution to form GO fibers through wet-spinning technology and then chemical reduction can finally produce graphene fibers with a mechanical strength of approximately 182 MPa and electronic conductivity of approximately 35 S/cm. 17 The diameter of the as-spun fiber can be set by changing the nozzle size or the GO dispersion content. Xu et al. 67 obtained graphene fibers with aligned GO sheets from giant GO liquid crystals (LCs) using divalent ions (eg, Ca 2+ , Cu 2+ ) as coagulation bath through wet-drawing and chemical reduction ( Figure 3A proposed a method for continuous wet spinning of oriented GO fibers from high-concentration small flake GO (9 μm in diameter) LCs with stretching, yielding a high tensile modulus of 47 GPa. Various guest materials can be easily introduced into the graphene host, affording graphene-based composite fibers with new functionality or enhanced performance, which opens up the opportunities for exploring the potential applications. Through electrospinning technology, Matsumoto et al. 52 also successfully twisted from poly(acrylonitrile) (PAN) containing GO nanoribbons and subsequent pyrolysis to produce graphene nanoribbon/carbon composite nanofiber yarns. The as-spun yarn exhibited a tensile strength of 382.4 MPa and an electrical conductivity of 165 S/cm, respectively, which can be explained by well-dispersed nanoribbons highly oriented along the fiber axis and carbonization treatment, promoting the formation of ordered graphitic structure. Better physical properties can be expected by optimization of GO nanoribbon fraction and carbonization conditions.
Hybrid fibers of 1D CNTs and 2D graphene sheets can display improved properties of electrical conductivity and mechanical flexibility that exceeded of single constituent component, efficiently reducing the π-π stacking inter-action among graphene sheets. Chen et al. 70 developed a hybrid carbon-based fiber in hierarchical structure comprising of nitrogen-doped RGO and acid-oxidized SWC-NTs ( Figure 3D and E). The resultant fiber possesses not only high packing density, but also abundant electron and ion transport pathways to realize a microsupercapacitor with satisfactory volumetric performance. By embedding graphene fibers with Fe 3 O 4 nanoparticles as catalysts to grow CNT under CVD method, Cheng et al. 71 reported CNT/graphene hybrid fiber with high surface area and flexibility, demonstrating its promising application for the construction of flexible textile supercapacitors. Zhong et al. 51 also used the CVD method combined with a poststretching approach to obtain the CNTs and graphene (GNS) hybrid multiple-thread yarns. The tensile strength and electrical conductivity of the achieved composite fiber are 300 MPa and 1000 S/cm, respectively, presenting the prospects for application in various fields. Researches of metal, graphene, or CNT-based fibers have received considerable attention as summarized in Table 1. Although metallic fibers possess quite high electrical conductivities, their rigid structure and high mass density are unsatisfactory for construction of flexible electronics. In contrast, CNT or graphene fibers are more suitable ascribed to their  features of being flexible, stretchable, and lightweight. Despite the great achievement mentioned above, it is still challenging to exploit novel fiber materials and fabrication technique for scale-up production and applications.

FIBER ELECTRONICS
Recent development in fiber materials has inspired the research of fiber-shaped configuration successfully utilized in energy storage devices, wearable displays, smart actuators, sensors, and so forth. In this section, the construction of fiber electronic devices for energy storage and actuating/sensing applications and integrated systems will be discussed.

Fiber energy storage devices
Nowadays, enormous efforts have been directed toward fiber-shaped energy storage devices, such as supercapacitors and rechargeable lithium-ion batteries (LIBs), to power portable and wearable electronics. These fiber devices are actively pursued with light weight, flexibility and miniaturization, to meet the requirements of wearable electronics. Commonly, flexible fiber-shaped supercapacitors/batteries can be categorized into parallel, twisted, and coaxial types ( Figure 4). 80 Among them, devices in a parallel structure are just simply assembled by coupling two fiber electrodes uniformly coated with gel electrolyte on a planar support to keep a space between them, 81 without extra operation. Besides, multiple devices fabricated in parallel structures can be readily grouped in series or parallel to further increase the output potential or current for wider applications. 82 Another effective design is to construct a flexible device by twisting two fiber-shaped electrodes via a rotation-translation setup. 83 Compared with the parallel pattern, the twisted device is capable of self-standing without the assistance of polymer support (eg, PET, PDMS) and can adapt to various mechanical deformations when being woven into textiles for practical usage. 84 Special care needs be paid to achieve a small distance between twisted fiber electrodes, which is crucial for charge transport and device performance. Differently, the coaxial configuration can be realized by sequential assembly of an inner fiber electrode, gel electrolyte, and the outer electrode in a core-shell structure. Compared to parallel or twisted structure, the coaxial structure can ensure sufficient contact with each component with a low internal resistance, and can withstand mechanical deformations to keep structural integrity, favoring stable device behavior. However, the capacity match between the electrodes still remains challenging to realize a maximal performance and the electrodes are too bulky to weave, inconsistent with the development of miniaturized devices with energy storage ability.

Fiber supercapacitors
Supercapacitors can be classified into electrochemical double-layer capacitors (EDLCs) and pseudocapacitors according to their energy storage mechanism. 85,86 The capacitance of EDLCs comes from the accumulation of ions formed at the electrode-electrolyte interface. [87][88][89] Differentially, the energy storage of pseudocapacitors relies on fast and reversible redox reactions on the surface of electroactive materials (eg, metal oxides and conducting polymers), 90 therefore, delivering a considerably higher capacitance than the values of carbon-based EDLCs. Carbon-based fiber materials, specifically CNT or graphene fiber, possess excellent electrical conductivity and good physical properties, endowing them great potential as electrodes for fiber supercapacitors. 34 In addition to carbon materials, pseudocapacitive materials, such as transition metal oxides/hydroxides (popular examples including MnO 2 , RuO 2 , Co(OH) 2 ), and conducting polymers (typical representatives, such as polyaniline [PANI] and polypyrrole [PPy]), were also combined with carbonbased fibers to realize high performance. Among the metal oxides, MnO 2 possessing a high theoretical specific capacitance (1380 F/g), wide voltage window, and low cost, is regarded as the most promising electrode material. However, its poor electrical conductivity and inferior cycling performance, have largely limited the practical applications. In combination with carbon nanomaterials, such as CNTs, the composite electrodes enable high electrical conductivity and capacitive performance. A fiber supercapacitor was assembled by pairing two buckled MnO 2 /oxidized CNT hybrid electrodes in parallel (Figure 5A). 19 The fiber-shaped electrodes were made of MnO 2 nanoparticles uniformly anchored to the oxidized CNT fibers ( Figure 5B) which were parallel laid on PDMS film to form a buckled structure after a prestretching-releasing process. The resulting devices exhibited excellent specific capacitance (409.4 F/cm 3 tested at a current density of 0.75 A/cm 3 in LiCl-PVA gel electrolyte) and nearly no curve change compared with the original cyclic voltammetry (CV) plots measured under different conditions of deformations (stretching and folding), plus stable capacitance retention especially at a prestrain of up to 40% ( Figure 5C).
The above-mentioned parallel configuration suffered from relatively low specific power/energy density due to the use of flexible substrate support to introduce additional volume and weight. To overcome these shortcomings, a twisted supercapacitor was developed directly  Figure 5D). 91 First, the Kevlar fiber was twisted around the straight plastic wire fixed on a stage. Then, the gel electrolyte was coated onto two electrodes to separate them to avoid the short circuit. As observed in Figure 5E, the flexible plastic wire had a diameter of approximately 200 μm. A magnified SEM showed ZnO nanowires in hexagonal shape covered on the substrate uniformly. To further enhance the capacitance, the fiber supercapacitor operated in a gel PVA/H 3 PO 4 electrolyte delivered a specific capacitance approaching approximately 2.4 mF/cm 2 under a scan rate of 100 mV/s after MnO 2 coating ( Figure 5F). Notably, the twisted fiber electrodes had limited electrical contact to produce a high internal resistance and would be easily detached from each other when bent or knitted into textile structure. To solve the critical issues, a coaxial fiber supercapacitor was developed, where aligned CNT fiber served as an inner layer wrapped with CNT sheet outer layer coated with a gel electrolyte ( Figure 5G). 34 The cross-sectional view of as-fabricated device ( Figure 5H) showed the distinct morphologies of CNTs in the inner and outer part which were distinguished by two concentric cycles, evidencing a coaxial structure. The CNT fiber can be modified with a diameter ranging from 6 to 40 μm by controlling the width of spinnable CNT arrays. CV profiles of the device were overlapped well and kept a rectangular shape at 50 mV/s increased to a high scan rate of 1000 mV/s, indicating a double-layer capacitor behavior and stable electrochemical performance under high charge-discharge rates. Additionally, the coaxial supercapacitor yielded a discharge specific capacitance of 59 F/g with no obvious capacitance loss over 11 000 long-term cycles, and operated stably when bent under a curvature range of 0-31.25 cm −1 ( Figure 5I) or stretched.
Two different electrodes can further form a hybrid supercapacitor to broaden operational voltage range for performance enhancement. For instance, a fiber-shaped asymmetric supercapacitor assembled from the hybrid fibers of MnO 2 nanosheets onto the PEDOT:PSS-coated CNT (MnO 2 /PEDOT:PSS/CNT) fiber and ordered microporous carbon (OMC)/CNT as positive and negative electrodes can achieve the voltage window expanded to 1.8 V along with an energy density as high as approximately 11.3 mWh/cm 3 . 20 Supercapacitors with extra functions, such as sealhealing capability and stretchability, are actively pursued for their importance in smart and wearable electronics. Huang et al. 3 designed self-healable yarn-based supercapacitor by wrapping magnetic electrodes with a self-healing polymer (carboxylated PU) shell. The stainless steel yarn grown with magnetic Fe 3 O 4 particles and subsequently electrodeposited with PPy layer can heal the damage upon cutting and restore the mechanical and electrical properties, thus, restoring the capacitance, which was contributed to the synergetic effect of vast hydrogen bonds in carboxylated PU network and the magnetic interaction between the yarn electrodes. Another research applied reduced GO-based fiber springs coated with self-healing carboxylated PU shell, can also reconnect the broken electrodes with a capacitance retention of 54.2% after three cutting-healing cycles. 7 Apart from seal-healing property, Peng and coworkers 31 developed stretchable fiber supercapacitor by sequentially wrapping CNT sheets on an elastic rubber fiber as inner and outer electrodes. Due to the combined advantages of elastic fiber and aligned CNT, the fabricated device exhibited high stretchability and electrical conductivity, which can be easily stretched after 100 cycles with a strain level of 75% without obvious structure degradation and capacitance fade. Although the use of elastic fibers endows the fiber devices with excellent stretchability, it adds extra volume and weight of the device, resulting in the limited capacitance and energy density for the entire device. To realize the fiber supercapacitor without the support of rubber fiber, Zhang et al. 22 proposed a stretchable fiber supercapacitor with springlike CNT fiber electrodes comprising of twisted CNT fibers with coiled loops. Upon stretching, the coil structured loops started to elongate while maintaining the CNTs in high alignment. After releasing, the spring fiber recovered to its initial coiled shape. To construct stretchable fiber supercapacitor, two fiber springs were placed in parallel with a coating layer of gel electrolyte to avoid short circuits. When stretched to 100% for 100 cycles, the specific capacitance of stretchable fiber supercapacitor is still retained at more than 90%, indicating the excellent stretchability and electrochemical stability of the resulting supercapacitor.

Fiber batteries
Typically, flexible batteries contain a positive electrode (anode, a lithium-containing compound), a negative electrode (cathode, carbon material), and an electrolyte. Flexible fiber batteries especially flexible LIBs have been extensively studied to power wearable electronics. Sharing similar configuration with fiber supercapacitors, fiber batteries actually operate in a different charge storage mechanism, which relies on the Li + insertion/extraction in bulk electrodes instead of charge accumulation at the electrode-electrolyte interface in supercapacitors. During charging, Li + deintercalated from the cathode moves through the electrolyte to the anode. During discharging, Li + embedded in the anode moves back to cathode. The electrons in the outer circuit flow in an opposite direction of ion motions. The reversible movement of Li + between the cathode and anode or called Li + insertion/extraction process allows flexible LIBs for the realization of energy storage and conversion. In 2012, Kim et al. designed the first fiber LIB, a mechanically flexible coaxial LIB cable 92 by respectively utilizing a hollow-spiral anode (Cu wire coated with Ni-Sn) and a conventional LiCoO 2 (LCO) cathode ( Figure 6A and B). The hollow-spiral anode was obtained by winding the twisted bundles of Ni-Sn coated Cu wires around a circular rod with a diameter of 1.5 mm and stretching axially to reach an outer diameter of approximately 1.2 mm after removal of rod. Then, Al wire as cathode current collector was wound on the hollow-spiral anode, followed by coating LCO cathode slurry. Finally, the liquid LiPF 6 -containing organic electrolyte was injected into the empty space to produce a cable-type battery. As shown in cross-sectional optical image ( Figure 6C), the cable battery was constructed with a hollow-spiral anode, modified PET layer as separator, Al wire, tubular cathode, and packaging tube, with an outer diameter of several millimetres. Compared with nonhollow anode system, the cable-type battery with the hollow structure anode exhibited superior battery performance, including a higher capacity, more stable cycling, and lower internal resistance ( Figure 6D and E), attributed to the hollow structure with enlarged surface area and the better electrolyte permeability to active materials. In addition, the cable battery still exhibited stable electrochemical performance under large strain, such as bending and twisting deformations ( Figure 6F), showing its excellent mechanical flexibility.
CNTs have been extensively explored as current collector and active materials in flexible energy-related devices contributed to the unique features of outstanding electrical and mechanical properties. CNTs in a fiber format can be easily dry spun from spinnable CNT arrays having tunable diameters and lengths. Ren et al. 93 94 The Si/CNTs fiber was obtained by deposition of Si onto aligned CNT sheets, followed by twisting composite sheets to form a flexible composite fiber ( Figure 7D-F). The as-designed Si/CNTs fiber showed combined merits of high energy capability of Si and super mechanical/electrical properties of CNTs, as well as the inhibited volume expansion of Si during the processes of charging and discharging. The assembled fiber battery showed good flexibility with no obvious capacitance decay over continuous 100 cycles of bending deformation. To replace rigid lithium metal wire, a full fiber-shaped battery by coupling LiMn 2 O 4 /CNT (LMO/CNT fiber, cathode) and Li 4 Ti 5 O 12 /CNT (LTO/CNT fiber, anode) paralleling, without the need of currentcollecting metal substrate and polymer binder 95 (Figure 7G-I). The resulting fiber-shaped battery (length: 1 cm) delivered a specific capacity of 70 mAh/g at 0.05 mA with a capacity retention of about 85% after 100 charge-discharge cycles. Again, this fiber battery was highly flexible enough to be deformed into diverse shapes or woven into electronic textile and even performed stably after 1000 cycles of bending.
Furthermore, an elastic battery with stretchability was assembled in a spring structure by simply winding two composite fibers, LMO/CNT and LTO/CNT as the cath- (I) charge-discharge curves for the full battery at 0.05 mA (length: 1 cm). Reproduced with permission from Ref. [95]. Copyright 2014, Wiley ode and anode, around an elastic substrate and subsequently coated with gel electrolyte, displaying a capacity retention of over 80% after 200 cycles of stretching by up to 100%. 95 When applied PDMS fiber as elastic substrate, a full battery exhibited a discharge specific capacity of 91.3 mAh/g under 0.1 mA/cm with a capacity retention of 88% after stretching at a strain of 600% (Figure 8A-C). 96 The maximal stretching strain can be engineered by tuning the radius of the fiber substrate, that is, an increased radius of the fiber substrate gives a large strain. To avoid the use of elastic substrates, spring CNT fibers with coil-structure loops were further applied to construct elastic electrodes bearing LMO and LTO as cathode and anode to efficiently reduce both the volume and weight of devices. 22 In addition to the mechanical flexibility, fiber batteries are also desired to heal itself to recover the electrical/electrochemical properties operated under various deformations (eg, folding, bending, and twisting) during practical uses. Recently, inspired by the self-healing phenomenon in nature, polymers with self-healing functionality have been explored to heal mechanical fracture by reconnecting the damaged interface with reversible chemical or physical interactions. Gao et al. 97 designed a flexible, all-fiber LIBs with self-healing ability in diameters of hundred micrometers, thick enough to observe the reconnection of the broken parts ( Figure 8D). The assembled battery was composed of porous RGO fibers containing SnO 2 quantum dots as anode, spring-like RGO fibers containing LCO nanoparticles as cathode, LiClO 4 -containing gel electrolyte, and a self-healing carboxylated PU packaging layer. The length of the spring-like cathode can be readily tuned to match the capacity of the anode. Similarly, the self-healing mechanism can be explained by abundant hydrogen bonds existing in the supramolecular network F I G U R E 8 Super-stretchy fiber-shaped LIB: (A) schematic of the fabrication process; (B) photographs of fiber-shaped batteries with 40 and 3 cm in length before and after stretching to 200%, respectively. (C) Charge-discharge profiles before and after 600% strain at 0.1 mA/cm. Reproduced with permission from Ref. [96] Copyright 2014, Royal Society of Chemistry. Self-healing fiber-shaped LIB: (D) schematic of the selfhealable mechanism; (E) the detailed self-healing process upon cutting and reconnection; (F) long-term performance with different healing times at 0.1 A/g. Reproduced with permission from Ref. [97]. Copyright 2018, Elsevier. (G) Schematic of the fiber-shaped Na-ion battery using aqueous electrolyte. Reproduced with permission from Ref. [99]. Copyright 2017, Elsevier. (H) Schematic fabrication of the fiber-shaped Al-air battery. Reproduced with permission from Ref. [100]. Copyright 2016, Wiley which can reconstruct at the broken surface after cutting. The as-fabricated LIB showed stable and good recovery of electrochemical performance, delivering a specific capacity of 82.6 mAh/g with an 82.2% capacity retention over 50 cycles under bending and twisting, and retaining a specific capacity of 50.1 mAh/g with a 50.3% capacity retention over five healing cycles at a current density of 0.1 A/g ( Figure 8E and F). These above results reveal remarkable flexibility and self-healing property, providing great possibility to design flexible and self-healing power sources well performing under complex deformations toward nextgeneration wearable electronics.
Impressively, rational incorporation of other active materials into aligned CNT fibers can also realize other novel types of batteries, such as the hybrid fiber of aligned CNT, mesoporous carbon, and GO layer as fibrous sulfur cathode to fabricate a cable-shaped Li-S battery. 98 Notably, many reported flexible batteries use flammable or toxic organic electrolytes. When it comes to practical applications in wearable electronics, the unwanted leakage of electrolytes can create great safety hazards.
To explore safe batteries, Peng and coworkers 99 designed flexible, fiber-shape aqueous Na-ion batteries by dropping the suspensions of Na 0.44 MnO 2 (NMO) and carbon-coated NaTi 2 (PO 4 ) 3 (NaTi 2 (PO 4 ) 3 @C, NTPO@C) onto stacked CNT sheets to produce composite fibers (CNT/NMO cathode and CNT/NTPO@C anode) and employing Na 2 SO 4 solution as the electrolyte ( Figure 8G). The designed fibershaped Na-ion batteries display remarkable electrochemical performance, as well as high flexibility and safety. In addition, the batteries with Na + -containing biocompatible solution, such as normal saline solution, cell-culture medium, can still operate well. These above results indicate the potential in wearable and implantable electronic devices. Also, the as-fabricated fiber electrode can consume dissolved oxygen in aqueous Na + solutions, through an electrochemical deoxygenation process and change the local pH, implying promising applications in biological or medical field.
Aligned and cross-stacked CNT sheets offer rich void space for efficient gas diffusion and oriented microchannels for electrolyte infiltration, continuous pathways of electron transport due to its outstanding electrical properties (10 2 -10 3 S/cm) and high mechanical strength for selfstanding. Therefore, CNT sheets-based air cathode endows the metal-air batteries with stable cycling performance at high current and structural stability against mechanical deformations. Typical examples of flexible fiber metalair batteries include aligned CNT sheet air electrode for fiber Li-air battery, 101,102 aligned cross-stacked CNT sheet air cathode for fiber Zn-air battery, 103 Ag-coated crossstacked CNT sheet air cathode for fiber Al-air battery ( Figure 8H). 100 CNT-based fiber actuators can be designed to response to different stimuli, for example, electric, 25,104 solvents/vapors, 1 moisture. 26,105 Popular mechanisms to explain the actuation are molecular order (eg, LC elastomers, shape memory polymers, and dielectric elastomers) and volume change due to the mass transport, thermal volume expansion, or phase transitions. Foroughi and coworkers have pioneered the exploration of CNT fibers as electromechanical actuators that realize the electrical-to-mechanical energy conversion. 2 The CNT fiber functioned as a torsional actuator, exhibiting a rotation of 15 000 o and a rotation speed of 590 rpm in a three-electrode system filled with an organic electrolyte ( Figure 9A and B). The simultaneous contraction and rotation were driven hydraulically by internal pressure associated with ion insertion when immersed in the electrolyte, similar to electrochemical double-layer charge injection occurred in CNT-based supercapacitors (Figure 9C and D). In addition to electrolytes, CNT fibers can further produce electromechanical torsion (lengthwise contraction and rotary torsion) in air or liquid media, such as water and organic solvents, resulted from electromagnetic attraction between individual CNTs upon applying the current. 106 Compared to 2D membrane actuators with simple bending and unbending behaviors, the abovementioned fiber actuators can deliver distinctive torsional rotation, which is more desirable for the implementation of complex tasks. However, they suffered from the limi-tation of the use of the electrolytes, thus, leading to the restriction of operation voltage or temperature. Therefore, electrolyte-free actuating system is demanded for practical applications.

Fiber actuators
CNT yarns obtained from spinnable nanotube arrays have attracted wide investigation for fiber-shape devices, benefitting from their super mechanical strength, flexibility along with nanotube arrangement. Inserting twist to a CNT yarn can enlarge their mechanical strength, which is contributive to the development of high-performance artificial muscles. Chen et al. 1 designed hierarchically arranged helical fibers (HHFs) constructed from the twisted multiply fibers of helical assembly of MWCNTs ( Figure 9E). The massive multiscale gaps, nanoscale gaps among the MWCNTs, and micrometer-scale gaps among the adjacent twisted fibers, are favorable for the rapid diffusion of solvents/vapors, thus, enabling a facilitated actuating performance. Upon the absorption of ethanol droplet, a twisted 20 primary fibers reversibly rotated a 570 times heavier copper paddle (mass: 75 mg) generating the largest rotary speed and contraction of 6361 rpm and approximately 10% during the process of rotation. These mechanically flexible and strong HHFs can be further woven into a smart textile capable of lifting a 100 times heavier copper ball (mass: 240 mg) 4.5 mm rapidly (within milliseconds) upon the spraying of ethanol (Figure 9F), generating the output power of 49 W/kg during the initial 50 ms, which is comparable to mammalian skeletal muscles (50 W/kg). In another work, Di et al. 24 showed an incandescent tension annealing process (ITAP) can stabilize the coil-structure CNT yarns via strengthening the mechanical property of the yarn structure to prevent unwanted irreversible untwist, thus, eliminating the need to tether torsional artificial muscles ( Figure 9G). This ITAP involves incandescently heating twisted CNT yarns under electric current to a temperature of approximately 2000 • C while a weight is attached at the yarn end to offer tensile load. Upon exposure to acetone vapor, a coiled ITAP yarn with the dimension of 24 mm in length and 100 mm in thickness reversibly rotated a 6100 times heavier rotor by 630 o (corresponding to a rotation of 26 o per mm of muscle length) ( Figure 9H). The maximum rotational speed of the rotor was 44 rpm, and the muscle lifted a weight corresponding to a 2.9 MPa load by about 0.7% of the yarn length. Such CNT twisted yarns show promising application of torsional actuator with high torsional speed and large torque, and reversible torsional actuation.
In addition to the response to organic solvents/vapors, CNTs-based fiber actuators can also be made into water/ moisture driven actuation. He et al. fabricated hierarchically helical CNT fiber with hydrophilic ability when treated pristine fibers with oxygen plasma ( Figure 9I). 105 The hydrophilic secondary fiber (HSF) from 10-ply hydrophilic primary fibers (HPFs) with an oxygen level of 10.9% generated a contractive stress of approximately 13.6 MPa and the corresponding peak stress rate of 21.7 MPa/s upon absorption of a water droplet. The contractive stress outputs could be further optimized by tuning the oxygen contents in the HPF. Besides, the HSF was also sensitive to moisture, producing a contractive stress of approximately 22.8 MPa when exposed to a relative humidity of ≥80% ( Figure 9J). Besides of the contractive actuation, the HSF also generated a rotation upon exposure to water, with a maximal torsional torque of 0.4 N m/kg and high reversibility of rotary actuation, demonstrating the possibility in smart window ( Figure 9K). Such fast mechanical response can be attributed to the hierarchy of channels at nano-and micronscales, which provide adequate capacity for water infiltration driven by capillary force, and the resulting volume expansion of the helices finally triggers the contraction and rotation.
Particularly, hybrid CNT yarns with volume-change guest infiltration into a twisted yarn has been demonstrated to develop electrolyte-free muscles which do not need the device packaging and reduces the actuator weight. Changing guest dimension causes torsional rotation and contraction of the twisted yarn host. For example, actuator based on a wax-filled CNT yarn due to the thermal volume expansion of paraffin wax by electrical, chemical, or photonical excitation, 23 [115]. Copyright 2020, American Chemical Society. (E) Schematic to a bisheath buckled structure designed for strain sensor; (F) the bisheath buckled fiber strain sensor can monitor different walking patterns. Reproduced with permission from Ref. [116]. Copyright 2017, Wiley (PDDA) guest-CNT yarn upon water absorption/relative humidity change or even magnetic particle-filled CNT yarns by external magnetic field 107 can be driven. Intriguingly, hybrid CNT yarn can be developed into bioactuators to sense glucose concentration driven by the volume expansion of glucose-sensitive guest hydrogel, 108 suggesting the promising application in implantable, self-actuating drug delivery systems.

Fiber sensors
Strain sensor is an indispensable part for wearable electronics, which has been developed for human motion detection and healthcare monitoring. So far, various types of strain sensors have been constructed based on the transduction methods of piezoelectricity, capacitance, resistivity, etc. Among them, resistive sensors are of great research interest attributed to their simplified fabrication, low energy consumption, and easy measurement, etc.
Conventional strain sensors constructed from metals and semiconductors have the limitations of poor stretchability (<5%) and low sensing ability (low gauge factor), which are far from the requirements of wearable sensors. Recently, high-performance fiber strain sensors have been studied by embedding conductive fillers, such as AgNWs, CNTs, and grapheme, into elastic polymer matrix for their integrated merits of flexibility, stretchability, and wearability. Zhu et al. 72 integrated AgNWs into PU fibers in a structure of conductive AgNW filled into the surface layer of the PU matrix to form densely conductive networks. The manufactured PU/AgNW fibers manifest good electrical conductivity (3.1 S/cm), high breaking elongation (265%), wide response range (43%), and fast response (49 ms) and durability, contributed to the combined effects of highly stretchable PU matrix and the conductive AgNW network plus the enhanced interfacial stability ( Figure 10A and B). In another design to further improve the electrical conductivity and breaking elongation, Lee et al. 109 proposed highly stretchable conductive fibers consisting of AgNWs and AgNPs embedded in elastomeric polymer poly(styrene-block-butadiene-block-styrene, SBS) by wet spinning of an AgNW-dispersed SBS solution, followed by Ag precursor absorption and hydrazine reduction for strain sensors. The composite fibers with 0.56 wt% AgNW-AgNP deliver a maximum breaking elongation of 900% and the highest conductivity of 2450 S/cm, attributed to the addition of rigid and conductive fillers of AgNWs and AgNPs to the SBS polymer matrix. To prove the practicality of the composite fiber in stretchable electronic devices, the composite fiber was attached to an artificial glove as a strain sensor to monitor the various motions of fingers with good response speed and recoverability ( Figure 10C).
In addition to metal nanowires, CNT has been recognized as ideal conducting filler for fabricating conductive fiber with its excellent electrical or mechanical properties. CNT-based strain sensors are usually made from random CNT mixed with polymers, for example, cellulose nanofibrils (CNFs) and SWCNTs by the combined three-roll-mill and wet-spinning strategy. 110 Meng et al. 111 developed a coaxial fiber containing cellulose wrapping oriented CNTs by coaxial wet-spinning technology using a rotating coagulating bath. The resultant coaxial fiber exhibited excellent mechanical strength ascribed to the existence of hydrogen bonding and van der Waals interactions. The coaxial fiber functioned as strain sensor which can be explained by the changes of contact conductive paths under stretching. However, these strain sensors still exhibit limited stretchability and low strain range. To further achieve high stretchability and wide detection range, strain sensors have been fabricated from elastic polymers, for example, CNT/thermoplastic PU fiber with a multilayer-hollowmonolith structure, 112 PU/CNT@Fe 2+ fibers. 113 By a coating approach, Zhi et al. 114 developed a CNT/cotton/PU core-spun yarn strain sensor that is capable of identifying the motions of finger and elbow, and even the eye winking. By virtue of physical interaction between CNT and the cotton/PU yarn, the as-obtained composite yarn can undergo the stretching up to 300% and can be stably operated for a long-term test (ca. 300 000 cycles) under 40% strain. Zhao et al. 115 reported a stretchable helical CNTs/PU composite yarn comprising of the electrospun aligned PU nanofibers film, spray coating of CNT dispersion onto PU nanofibers film, and continuous twisting process to form a helical yarn ( Figure 10D). Benefitting from the winding-locked CNT network and helical structure design, the helical CNTs/PU yarn has a maximum elongation of 1700%, stable conductivity and resistance recoverability within 900% stretching. Moreover, the helical yarn can work as a strain sensor to capture human motions (eg, walking) with stable signal response, presenting the possibility in applications of wearable electronics and large-strain sensors.
Various microstructures have been proposed for fiber strain sensors such as the bisheath buckled structure, 116 a twistable sandwich fiber composed of buckled CNT electrodes, 117 the layer-by-layer assembly of graphene on spiral elastic fiber, 118 and buckled graphene ribbon 119 to enhance the strain detecting performance. Buckling structures have been demonstrated as a straight structural design to achieve fiber strain sensor with high stretchability and linearity. Through a stretch-release process, the buckling structures can be formed to keep the conductivity stable along the prestretching direction. Baughman et al. 120 reported a fiber strain sensor in hierarchically buckled sheath-core structure, by wrapping CNT sheets on a prestretched rubber fiber core with CNT orientation parallel to the fiber direction, achieving a quite high stretchability up to 1320%. Furthermore, Liu et al. designed a bisheath buckled structure of buckled CNT sheets and buckled rubber on the elastic fiber ( Figure 10E). 116 The as-fabricated resistive strain sensor can be reversibly stretched high to approximately 600%, displaying a linear and large resistance increase of 102 and 160% for strains of 0-200% and 200-600%, respectively. Such large resistance change arises from the decreasing contact area between neighboring CNT buckles during stretch. Moreover, the bisheath buckled fiber strain sensor can identify different walking patterns of extending, flexing, squatting, marching, and jumping ( Figure 10F). The sensitivity of the fiber strain sensor can be altered by modifying the formed buckling structure via simply changing the fabrication strain.

INTEGRATED FIBER ELECTRONICS
Apart from single function has been successfully demonstrated, integrated systems are required to follow. Flexible fiber energy storage devices particularly supercapacitors and batteries are essential part in wearable electronics. It is well known that supercapacitors have high power density whereas batteries have high energy density. To realize high energy and power densities in one device, a twisted fiber hybrid energy device ( Figure 11A) 121 fabricated by twisting three hybrid fibers of CNT/ordered mesoporous carbon (OMC), CNT/LTO, and CNT/LMO, demonstrating the combined advantages of a LIB and a supercapacitor with both high energy (50 mWh/cm 3 or 90 Wh/kg) and power densities (1 W/cm 3 or 5970 W/kg). To meet the requirement of self-powered system, it is desirable for integrated function of energy conversion and storage. Therefore, a coaxial "energy fiber 38 " contains two parts of photovoltaic conversion and energy storage, comprising of TiO 2 nanotube-modified Ti wire and aligned CNT sheet as two electrodes, delivering an entire photoelectric conversion and storage efficiency of 0.82% with CNT layer thickness of 20 μm ( Figure 11B F I G U R E 1 1 (A) Schematic structure of the devices comprising of LIB and supercapacitor segments. Reproduced with permission from Ref. [121]. Copyright 2015, Wiley. (B) Schematic diagram of the circuit connection in charging and discharging process. (C) Charging-discharging curve at 0.1 μA during the discharging process. Reproduced with permission from Ref. [38]. Copyright 2013, Wiley. (D) Schematic structure of the solar-powered coaxial-fiber stretchable sensing system. Reproduced with permission from Ref. [40]. Copyright 2019, Elsevier. (E) Schematic structure of coaxial fiber CF@TiO 2 @MoS 2 electrode for energy harvesting and storage. Reproduced with permission from Ref. [6]. Copyright 2016, Wiley and C). Besides, the "energy fiber" can be further woven into flexible textiles. In addition to the solar energy, another renewable energy of wind can also be stored to charge the battery (Zn-Ag 2 O battery). 122 A coaxialfiber integrated system ( Figure 11D) 40 was assembled by integrating solar cells (the outer layer), an aqueous Zn-MnO 2 battery (PEDOT@MnO 2 /CNT fiber as cathode and Zn/CNT fiber as anode, the middle layer), and a stretchable MWCNT/TPE strain sensor (the inner layer) to realize multifunctionality of energy harvest, storage, and utilization. Specifically, the solar cell harvested solar energy that further stored as chemical energy in the Zn-MnO 2 battery, affording a continuous power source for the fiber strain sensor. This self-powered strain sensing system can operate under a strain up to approximately 180% and stably respond to both static and dynamic strain, presenting the effectiveness in real-time monitoring of human behaviors. These aforementioned integration systems were built up by connecting several monofunctional device together, which still suffered from the entire device of light weight and small size challenges. Therefore, a MoS 2 -based fiber electrode (CF@TiO 2 @MoS 2 ) 6 was devel-oped to achieve versatile applications to fulfill all purpose in a single electrode, including dye-sensitized solar cell (DSSC), supercapacitor, LIB, and electrocatalytic HER ( Figure 11E), suggesting a great potential for designing onedevice-multiple-functions for smart wearable electronics.

CONCLUSION AND OUTLOOK
To date, carbon-based fiber materials especially CNT fibers and graphene fiber have been the focus of research in fiber devices considering their superior electrical and mechanical properties, allowing them as flexible building blocks in constructing wearable devices. Compared to conventional metallic fibers, CNT fibers/yarns have become attractive for their fascinating structural flexibility, light weight and chemical stability. Ascribed to the van der Waals interaction between CNT bundles, continuous CNT fibers can be readily drawn from their spinnable arrays under twisting or aqueous suspensions, called dry-spinning and wetspinning approaches. However, the high cost of aligned CNT arrays and limited fiber length have restricted the scalable manufacturing of CNT fibers to a certain extent. Besides, the fiber alignment and impurities removal are of importance to the fiber density, enabling the production of a high-performance CNT-based fiber. The doping of intrinsic CNT has also been proved to be an efficient strategy for improving the electrical conductivity. To realize more functions, CNT fiber has been incorporated with guest materials, but the maintaining alignment of CNTs in composite fiber during the fabrication process should also be taken into account. Functional fibers are in great demand due to their advantageous properties of small size, flexibility, knittability, and integrability, which pervade a diverse range of wearable electronic devices including batteries, supercapacitors, actuators, sensors, displays, solar cells, etc. Although encouraging progress has been made, it is still challenging for industrial applications. Flexible fiber supercapacitors/batteries have served as essential power supply for wearable devices, whereas several critical issues need to be addressed. Although organic electrolyte can expand electrochemical window which is correlated with device performance, it suffers from the safety problem. Therefore, employment of solid-state electrolyte is demanded to avoid the electrolyte leakage problem. However, the unwanted heavy encapsulating materials add extra weight/volume, and greatly reduce the energy density of the electrochemical device. Besides, the existing energy harvesting/storage devices are still too bulky for wearable and flexible application. Research on the material manufacturing and innovative structure design should be directed to being miniaturized in the future. Crucial parameters to evaluate actuator include response speed, displacement, and durability. More attention should be focused to design electrolyte-free fiber actuator to eliminate the need of the liquid electrolyte and complicated three-electrode electrochemical setup. Extra efforts need to be further exerted to develop torsional actuators as artificial muscles with high performance in terms of controllable actuating direction (which can translate mechanical motions to accomplish specific tasks), novel actuator structure (eg, high degree of inserted twist) and actuating mechanism (eg, piezoelectric and electrostrictive effects), high-speed response (rotational speed), and large torsional strokes as well as prolonged cycling stability without fatigue. Massive production techniques are also required for the commercialization of actuating systems. Incorporation of the CNT hybrid yarn with guest materials showing thermally, electrically, or chemically induced volumetric change (eg, moisture-sensitive PDDA, thermosensitive paraffin wax) can produce yarn rotation and contraction. Moreover, biocompatible materials should be carefully selected for their great potential in surgical robotics and medical devices. The operational stability of the strain sensor is strongly dependent on the interfa-cial adhesion between active materials and polymer substrate. More enthusiasm needs to be devoted to fabricating sensors in special structure design, ensuring a good balance of sensing performance with simultaneous high sensitivity (high gauge factor) and broad sensing range as well as good stability for long-term operation. Besides, extra attention also goes to excellent endurance to various mechanical deformations of bending, twisting, and even stretching to allow for detecting various human activities. Other critical properties need to consider are the accurate detection of both subtle and large strains and the resistance performance of the sensor in atmosphere to assess its practical lifespan. Furthermore, enough breathability to ensure comfort during long-time wearing and the tolerance to washing treatment are also essential for practical utilization.
Fiber devices are easily being integrated with multiple functions or woven into textile electronics. For instance, energy storage devices (eg, supercapacitors and batteries) have successfully been incorporated with extra stretchability and self-healing features for advanced wearable electronic or integrated with energy conversion devices, such as solar cells and nanogenerator, to produce self-powering systems. 5 In addition to the connection of single function device in series, novel material and architecture design are desired to integrate multiple functions into a single device to meet the requirement of light weight and minimization. Although the material science and technology of CNT fibers are still in their infancy, future research effort has been directed toward the full potential of CNT fibers for flexible and wearable electronics.