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Advanced Fiber Materials

, Volume 1, Issue 1, pp 3–31 | Cite as

Advanced Functional Fiber and Smart Textile

  • Qiuwei Shi
  • Jianqi Sun
  • Chengyi HouEmail author
  • Yaogang Li
  • Qinghong Zhang
  • Hongzhi WangEmail author
Review
  • 1.1k Downloads

Abstract

The research and applications of fiber materials are directly related to the daily life of social populace and the development of relevant revolutionary manufacturing industry. However, the conventional fibers and fiber products can no longer meet the requirements of automation and intellectualization in modern society, as well as people’s consumption needs in pursuit of smart, avant-grade, fashion and distinctiveness. The advanced fiber-shaped electronics with most desired designability and integration features have been explored and developed intensively during the last few years. The advanced fiber-based products such as wearable electronics and smart clothing can be employed as the second skin to enhance information exchange between humans and the external environment. In this review, the significant progress on flexible fiber-shaped multifunctional devices, including fiber-based energy harvesting devices, energy storage devices, chromatic devices, and actuators are discussed. Particularly, the fabrication procedures and application characteristics of multifunctional fiber devices such as fiber-shaped solar cells, lithium-ion batteries, actuators and electrochromic fibers are introduced in detail. Finally, we provide our perspectives on the challenges and future development of functional fiber-shaped devices.

Graphic abstract

Keywords

Advanced functional fiber Energy harvesting Energy storage Chromatic fiber Actuator 

Introduction

Clothing is an interactive interface between human and environment [1]. People usually use different colors and styles of clothing to express and transmit information or adapt to the changes of the external environment [2]. In terms of function and application, clothing can be regarded as the second skin of human beings. With the development of flexible electronic technology and the closer interaction between people and the surrounding environment in the information age, smart clothing has gradually entered into people’s horizon [3, 4, 5]. For example, in science fiction movies, a dress can play music, videos, adjust the temperature, and even surf the internet at the same time. In addition, researchers and fashion design companies also predict the most important and basic performance of the future smart clothing, including the ability to collect energy, to store energy efficiently, to change color controllably, and to change shape at human will [6, 7, 8, 9]. If clothing can have both fashion design and the above special functionality, it will be very in line with the future needs of avant-garde consumers. Actually, the high value-added and high-tech integration characteristics of smart clothing have attracted a lot of capital and time investment from high-tech companies and researchers. As-mentioned smart clothing needs the support of flexible, elastic and stretchable electronic devices, especially for the fiber-shaped functional devices. More importantly, fiber is known as the most basic unit of clothing which can be used to design and obtain different patterns and styles of clothing by the weaving and knitting technology. In order to obtain the smart clothing described above with energy collection [10, 11, 12, 13, 14], energy storage [15, 16, 17, 18, 19], color and shape change, it is necessary to explore multi-functional fibers with such performance.

Figure 1 shows a brief timeline of the developments of manmade fibers. In 1764, the steam-driven Jenny spinning machine was invented and used to transform fibers from natural animals and plants (cotton, hemp, wool fibers, etc.) into longer fibers. Since then, fiber products are beginning to enter people’s lives, and constantly changing the style of people’s clothing. Following the development of chemistry and the improvement of fiber industry technology, natural and synthetic polymers have been synthesized into chemical fibers [20, 21, 22]. The length, thickness, and color of the chemical fibers can be adjusted in the production process. Different types of chemical fibers including cellulose nitrate fiber, viscose fiber, polyamide fiber, and polyester fiber, have the advantages of light resistance, wear resistance, easy drying and mildew resistance, respectively [23, 24, 25, 26, 27, 28]. In the information age, the functional fibers have been extensively studied and applied. The functional fibers refer to fibers with special functions including antistatic property, light-guide, ion exchange, thermal insulation, high elasticity, antibacterial, flame retardant, and radiation protection [29, 30, 31], in addition to their existing properties. The information age has changed these functional fibers from ordinary consumables to high-tech products and has also increased the fierce competition in the fiber industry. Nowadays, with the rise of artificial intelligence technology, researchers believe that fibers should also evolve to intellectualization. However, what should the next generation of fibers be? In response to the performance requirements of smart clothing, advanced fibers with energy collection, energy storage, chromatic-changeable, shape deformable, sensing and biometric characteristics have attracted much attention. In the past five years, various types of advanced fiber-shaped devices including fiber-shaped solar cells, lithium-ion batteries, electrochromic devices, and actuators, have been successfully developed [32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45]. To explore the practical possibilities of these smart fiber devices, these fibers have been attempted to weave or weave into textiles, as well as to integrate into clothing [46, 47].
Fig. 1

A brief timeline of the developments of fibers in modern era

Many researchers have made in-depth comments and reviews on energy harvesting and storage fibers [46, 47, 48, 49], and wearable electronics in recent years [50, 51, 52, 53]. These comprehensive reviews provide researchers with a more systematic understanding of the advances in energy fibers and wearable electronics. However, with the increasing demand for smart fiber products and the diversity of application research on functional fibers, the deformable, chromotropic, sensing and antimicrobial fibers have been developed and opened up the research of diversified smart fibers. This review focuses on the significant progress, related challenges, and future perspectives of flexible fiber-shaped multifunctional devices, including fiber-based energy harvesting devices, energy storage devices, chromatic devices, shape deformable devices, as well as the advanced fiber-based integrated textiles and clothes. Particularly, it summarizes fiber-shaped multi-functional devices and their potential applications for portable or wearable functional integrated electronics including newly developed manufacturing techniques and practical functional materials. Fabrication procedures and application characteristics based on different functional fibers are also discussed. Finally, the remaining problems/challenges and opportunities are then discussed to offer some helpful insights for the practical applications of future fiber-shaped functional devices.

Fiber-Based Energy Harvesting Devices

For the next coming smart textile, they should be more intelligent and independent to achieve their functional versatility. Stem from this purpose, how to realize the self-supply of energy is a vital step that must be taken. In daily life, there are various forms of underutilized energy such as solar energy, kinetic energy of human body and energy loss caused by the difference in temperature. Hence, the energy harvesting devices just like the solar cells, triboelectric nanogenerator and thermoelectric devices emerged as the times require [48].

As the most extensive natural clean energy, the solar energy reveals infinite potential in the fields of energy utilization and development. Solar cell is the most effective way to realize photoelectric conversion. Under the illumination condition, the electron–hole pairs generated in the semiconductor drift in different directions under the influence of the built-in electric field, forming potential difference and photocurrent between the two poles (Fig. 2a).
Fig. 2

Brief schematic diagrams of working mechanism for energy harvesting devices, a solar cells, b triboelectric nanogenerator, and c thermoelectric devices

In order to gather the kinetic energy, the triboelectric nanogenerators are happened to transfer mechanical energy to electricity as a stylish concept. During the process of periodic contact separation, friction charges are formed on the inner surface of two polymer sheets due to the contact electrification effect. Because of the electrostatic induction phenomenon, the corresponding inductive charge will be generated at the conductive material, thus forming an AC signal in the external circuit (Fig. 2b).

Thermoelectric conversion technology based on Seebeck effect has irreplaceable advantages in plenty of dispersed low-grade waste heat conversion power. When there is a temperature difference between the two ends of the material, the carriers inside the material will move from the hot end to the cold end driven by the temperature difference, thus forming a potential difference between the two ends of the material (Fig. 2c).

For the three sorts of emerging energy harvesting devices, their conventional configurations are almost heavy, rigid and limited with a planar structure, which impedes their potential in the fields of wearable technology. In order to be flexibly integrated with wearable electronics, the different sorts of efficient fiber-based energy harvesting devices must be explored and developed in the future.

Fiber-Shaped Solar Cells

As the second layer of human skin that most directly interacts with the external environment, the fabrics could protect the body simultaneously absorb the energy from sunlight. Solar cell, a kind of efficient solar energy harvesting device, is regarded as the most promising way to solve energy issues nowadays [49]. Conventional solar cells are almost fabricated on the 2D planar rigid substrate thus leads to the fairly limited applied circumstances. Accordingly, the 1D fiber-shaped solar cells are prone to the future application and facile configuration [50]. The combination of 1D fiber-shaped solar cells and natural fabrics could effectively harvest the clean solar energy in outdoor and then instantly power up wearable electronic or reserve the energy into energy storage devices.

By substituting the CNT fiber for conventional metal wire as an anode, the mechanical performance of fibrous solar cell presents a marked improvement [52, 53]. In view of this, a sort of double-twisted perovskite solar cell was fabricated by a pristine CNT fiber and another CNT fiber that was coated with compact n-TiO2, meso-TiO2, CH3NH3PbI3−xClx, poly(3-hexylthiophene)/single-walled carbon nanotube (P3HT/SWNT), and silver nanowire network from the inside out (Fig. 3a). As shown in Fig. 3b, the double-twisted fibrous perovskite solar cell showed a maximum power conversion efficiency (PCE) of 3.03%, accompanied with Jsc, Voc and FF of 8.75 mA cm−2, 0.615 V and 56.4% respectively. Similarly, established on the foundation of intrinsic outstanding electroconductivity and mechanical strength of CNT fiber, a kind of novel CNT fiber electrode consisted of hydrophobic aligned CNT core and hydrophilic aligned CNT sheath was applied on the fibrous dye-sensitized solar cell (DSSC) and concurrently obtain a supreme PCE of 10%. Based on the composite CNT fiber electrode with the high electroconductivity, mechanical strength and incorporation with other active phases, the assembled fibers only losted 18% of the initial PCE after bending at 90o for 2000 cycles, indicating a promising application regarding flexible wearable electronics (Fig. 3c, d). As displayed in Fig. 3e, several fiber-shaped DSSCs could be flexibly woven into the normal textile, meanwhile, there was no fracture and separation under distinct deformations (e.g., bending and twisting). In outdoors, the integrated flexible self-powering device kept a pedometer working steadily under sunlight.
Fig. 3

a Schematic illustration for the structure of double-twisted fiber-shaped perovskite solar cell. b The J-V curves of the double-twisted perovskite solar cell. a, b Reproduced with permission [51]. Copyright 2015, Wiley–VCH. c Schematics for the fibrous solar cell with the enlarged structure of a CS-CNT electrode fiber by the asymmetrical twisting. d The J-V curve of fibrous solar cell, together with the power conversion efficiencies of the fiber-shaped solar cell as a function of cycle. e The fiber-shaped solar cells are integrated with fabrics and the combined items could power up pedometer in outdoor. ce Reproduced with permission [11]. Copyright 2018, The Royal Society of Chemistry

Fiber-Shaped Triboelectric Nanogenerator

In daily life, the human body is performing various actions or motions all the time. Nevertheless, a majority of them are just for coordination of physical moves and superfluous. Triboelectric nanogenerators (TENGs) that convert mechanical energy into electricity have received extensive attention owing to its potential as a continuous, self-sufficient and sustained power-supplying source [14, 54]. Benefits from the sensitive sensing of electrical signals for TENGs, it also has broad applications in the fields of sensors or monitors. Recently, due to the rapid development of smart wearable textile, the fibrous TENGs have drawn widespread attention and research.

In order to obtain a fibrous TENG with excellent flexibility and stretchability, a highly stretchable sensing fiber with triboelectric sheath-core structure was constructed by a built-in wavy core fiber and intrinsically stretchable sheath tube. The fabrication process and detail structures were shown in Fig. 4a. The core fiber was made of conductive metal wire and the wrapped nylon fiber on the outside surface. For the sheath tube, the silicon rubber grafted by fluoroalkylsilanes (FAS) fiber was wrapped by elastic bamboo fiber which could protected the outer electrode. After the dip-coating of conductive layer silver nanowires (AgNWs) and further spraying of PDMS, the sheath fiber tube presented tensile strain and elastic strain of ~ 600% and ~ 120% respectively. The potential application concerning sensor was investigated by a combination between the SSCTEF and a soft knee-pad. As displayed in Fig. 4b, the combined textile responsed to the various motion states which were created by persons with diverse exercising routines. In Fig. 4c, a SSCTEF was fixed with an elbow to simulate a more realistic state of motion. During the process of bending and straightening, the periodic changes with different output voltage were revealed in that a stream of electrons was induced back and forth between the inner electrode and the electrode of the SSCTEF [55]. The output signal was reflected by joint movements involved stretching, compressing, and bending, which indicating the SSCTEF was closer to reality in the motion monitoring compared with traditional sensors.
Fig. 4

a Schematic diagram for the fabricating process of a stretchable sheath-core structural triboelectric fiber (SSCTEF). b The self-powered fabrics weaved by SSCTEF could collect motion signals of different people. c The different voltage signals collected by SSCTEF indicate different states of elbow bending. ac Reproduced with permission [55]. Copyright 2017, Elsevier. d The single electrode triboelectric yarn (SETEY), the nether digital photos show the scalable SETEYs fabricated by the industrialization process. e Mechanism illustration of dynamic polarization process when a single SETEY work in water, digital photo shows that the LCD is lit up by the tensile of SETEY underneath water, together with the output voltages as a function of different depth underneath water. f Fabrication sketch of the energy textile (e-textile) based on SETEY. g The e-textile displays decent stretchability consequently harvests biomechanical energy to power e-devices handily. dg Reproduced with permission [14]. Copyright 2019, Nature Publishing Group

At present, vast of manufacturing processes of fibrous TENGs are immature and complicated, which hinders the further practical applications and real achievement of e-textile. Hence, it is profound to develop a kind of continuous and large-scale preparation process. Using a modified melt-spinning method, a scalable highly stretchable triboelectric yarn was successfully prepared that based on intrinsically elastic silicone rubber tubes and extrinsically elastic built-in stainless-steel yarns lately [14]. The sketch for the structure and mechanism of SETEY and optical photos of rolled SETEYs were given in Fig. 4d. During the stretching process, in-plane charge separation and alteration of the charge distribution were triggered by the changes of the contact area between the sheath tube and the core yarn, the electrons flow from the ground electrode into the SETEY under the motivation of the increased holes which anchor onto the highly conductive core yarn. The reverse released movement of SETEY drived electrons flowing back. Thus, the repeated stretching and release of SETEY could obtain an alternating current. Moreover, the water durability of SETEY was also evaluated in this work. Figure 4e depicted the working state and principle of SETEY underneath water. A stable output performance under different depth in water was also observed. The coupling effect was created between the surface potential from triboelectrification and induced surrounding bulk water, which greatly promoted the amount of charges generated from SETEY. Even the SETEY was immersed in the water, it still could power up a LCD and the output voltage was not affected by water depth. Figure 4f illustrated the fabrication of e-textiles based on the SETEY. Double-plied yarn was twisted by commercial stainless-steel yarn and water-resistant modified polyacrylonitrile yarn. Then the composite yarn was weaved with SETEY to form an integral e-textile. The e-textile exhibited exceptional flexibility and stretchability, simultaneously, the self-powering system could harvest biomechanical energy and instantly supply power for portable electronic devices (Fig. 4g).

Fiber-Shaped Thermo-Electric Devices

The thermoelectric devices have increasingly attracted extensive attention in which large-area waste heat recovery, sensor, heat management of the human body and as a kind of burgeoning energy-harvesting and energy-transfer system [56, 57, 58, 59]. Further dimension reduction from bulk or planar thermoelectric functional materials or devices to 1D fiber-shaped configuration could facilitate the scalable manufacture, lightweight and structural diversification design, meanwhile, the outstanding endurance for intuitive and evident mechanical deformation has been demonstrated by many flexible devices from assembling functional fiber to some extent [60, 61]. Therefore, the fiber-based flexible thermoelectric energy generators are expected to be applied to stable portable energy supply.

Based on the silk yarn with excellent mechanical properties, the outstanding electroconductivity was endowed by strongly adhering PEDOT:PSS onto the surface of the yarn. The optical photo of comparison with respect to neat and PEDOT:PSS dyed silk yarns were given in Fig. 5a. A uniform blue could be observed on the surfaces of treated silk yarns. In the meantime, the bulk electrical conductivity of silk yarns was significantly increased to a peak at 14 S cm−1 according to log-normal distribution. In Fig. 5b, an in-plane thermoelectric textile integrated with 26 p-type legs was placed upon a reservoir with hot (hot plate) and cold (steel heat sink) temperature ending to test thermoelectric performance. The obtained output voltage of Vout/ΔT ≈ 313 μV K−1 (i.e., 12 μV K−1 for each element) was relatively close to the speculative value of Vout/ΔT ≈ 351 μV K−1. The ΔT of 66 °C was operated with Rload ranging from 1 to 27 kΩ, hence a current of 1.25 μA and a maximum power output of about 12 nW was measured [62].
Fig. 5

a The optical photo of neat and dyed PEDOT:PSS silk yarns, together with the distribution of electrical conductivity of PEDOT:PSS prepared with different solvent. b The in-plane thermoelectric textile integrated with 26 p-type legs is placed between the hot and cold temperature end, the right performance plot presents Vout as a function of ΔT and P, where P = VoutI as a function of measured current I for ΔT = 66 °C. a, b Reproduced with permission [62]. Copyright 2017, American Chemical Society. c Schematic diagram for the thermal drawing process of thermoelectric fiber, the as-prepared fibers in a core-sheath structure show the good flexibility and well integration with textile. d Schematic and demonstration for electricity generation mechanism of the TE-based devices with the corresponding voltage and power density evaluated as a function of the ΔT. c, d Reproduced with permission [61]. Copyright 2017, Elsevier. e Sketch of the fabrication process of thermoelectric yarn. f Optical photos for the procedure of tiger TE yarn. g The output power of thermoelectric textile woven by tiger yarns. eg Reproduced with permission [63]. Copyright 2016, Wiley–VCH

The decent mechanical properties could be varied from thermoelectric fibers integrated with textile as shown in Fig. 5c, which a type of intrinsically flexible thermoelectric was fabricated by novel thermal drawing technology. The glass cladding was skillfully loaded onto the thermoelectric core. After the final coating of polymer, the scalable thermoelectric fiber with good flexibility in core-sheath structure was finished. The thermoelectric fibers could be well combined with wearable fabrics which are profited by intrinsic flexibility of fibers. Two different thermoelectric devices based on as-prepared thermoelectric fiber were established to examine fiber’s capability of interaction between heat and electricity (Fig. 5d). Under the temperature difference of 19 K, an output voltage of 29 mV was obtained by the cup which was equipped with 7-pair p-n thermoelectric legs. Moreover, the output voltage and output power density were 97 mV and 2.34 mW cm−2 respectively under a temperature change of 60 K. For another architecture, an output voltage of 15 mV could be gained by thermoelectric pipe with a temperature difference of 11.4 K. Furthermore, the pipe structure thermoelectric device delivered output voltage of 70 mV and output power density of 1.46 mW cm−2 based on 60 K difference in temperature [61].

Inspired by the concept of through-thickness thermoelectric power generation, the thermoelectric textiles were successfully fabricated by tiger yarns that were combined by separate and alternative n- and p-type segments. The schematic illustration for the fabrication of thermoelectric tiger yarns was shown in Fig. 5e. To start with, the highly aligned sheets of polyacrylonitrile (PAN) nanofibers were gathered onto two parallel wire collectors by electrospinning. Then the thermoelectric active materials were alternately deposited on both sides of the as-prepared sheet using a stencil mask. Afterward, the interconnected area between thermoelectric strips was sputtered by gold. Eventually, the thermoelectric tiger yarn was obtained by the further twist of coated yarns. As depicted in Fig. 5f, the initial tiger structure was kept well during the process of twist insertion or subsequent complete yarn untwist. The successive n-p junctions were founded on dense plain-weave thermoelectric textile and the integral textile presents a stable high output power above 0.62 W m−2 and 1.01 µW under the temperature difference of 55 °C (Fig. 5g). In the light of the mentioned three fiber-shaped energy harvesting devices, Table 1 summarized the materials, designing structure, processing techniques, and performances of relevant researches.
Table 1

Summary for materials, designing structure, processing techniques, and performances of fiber-shaped energy harvesting devices

Fiber device

Materials

Designing structure

Processing techniques

Performances

Refs.

Fiber-shaped solar cells

CNT fiber, CNT/TiO2 composite fiber (N719 incorporated)

Double-twisted

Twisting accompany with dip-coating and heating process

Normal: PCE = 2.94%, flexible test: ~ 100% PCE retention with the increasing incident light angle from 0 to 180°

[52]

Hydrophobic aligned CNT, hydrophilic aligned CNT, dye-absorbed Ti/TiO2 wire

Core–sheath fibrous structure

Twisting accompany with immersion and heating process

Normal: PCE = 10.00%, flexible test: ~ 86% PCE retention at 90o for 2000 cycles

[11]

CNT fiber, TiO2, P3HT/SWNT, Perovskite

Double-twisted

Spun-twisted, h eat-assisted coating

Normal: PCE = 3.03%, flexible test: ~ 100% PCE retention after 1000 bending cycles

[51]

Aligned MWCNT fiber, TiO2 nanoparticle P3HT:PCBM, PEDOT:PSS, Ti wire

Intertwined structure

Post-processing (heat treatment, coating) and twisting

Normal: PCE = 1.78%, flexible test: ~ 85% PCE retention after 1000 bending cycles, ~ 80% PCE retention after 1000 bending cycles (woven into flexible substrate)

[53]

Ti wire, TiO2 compact layer, TiO2 porous layer, perovskite layer, Spiro-OMeTAD layer, thin gold film

Hierarchical entangled structure

Spin coating together with vapor-assisted deposition and sintering in the substrate

Normal: PCE = 10.79%, flexible test: ~ 100% PCE retention after 500 bending cycles

[64]

Fiber-shaped triboelectric nanogenerators

Metal conductive wire with nylon fiber, silicone rubber tube with FAS, bamboo fiber, AgNWs, PDMS

Core-sheath structure

Coiling the functionalized layer by layer

Ultrahigh working strain (100%), a maximum sensitivity of 17.4 per unit strain

[55]

Silicone rubber, stainless-steel yarn

Core-sheath structure (the core is extrinsically elastic built-in stainless-steel yarn)

Continuously rolling by a specialized spinning equipment, together with blow-molding and compaction

Scalable manufacture, large working strain (200%), superior performance in liquid, all-weather durability

[65]

Silicone rubber, conductive yarn

Coaxial core-sheath structure

Winding the internal core and external sheath respectively and inserting them

Extensive multi-scenario application (such as gesture-recognizing, large-area energy-harvesting and sustainably charging)

[66]

PDMS, CNT, PMMA

Hierarchical Coaxial structure

Wrapping and dip-coating along with pre-stretch operation

Flexible, stretchable, weavable, converting multidirectional mechanical energies to electricity, sensing diverse mechanical stimuli

[67]

AgNWs, PTFE, PU, PDMS-AgNW film

Coaxial core-sheath structure

Comprising wrapping and coating

A maximum peak power density of 2.25 nW cm−2 with reliable durability (4000 testing cycles), versatile physiological signals monitoring

[68]

Stainless-steel fibers, dielectric fibers

Core–shell structure

Covering fibers are twined around core fibers to fabricate core–shell yarns

Scaled-up for fabrication, machine washing up to 120 times

Can be further processed by cutting and sewing for garment design, higher or comparable output voltage/current.

[9]

Stainless steel, polyester fiber, PDMS

Hierarchical core-sheath structure

Twisted inner structure with PDMS protection

The maximum peak power density of 3D textile (woven by energy-harvesting yarn) can reach 263.36 mW m−2 under the tapping frequency of 3 Hz, large-area energy-harvesting and sustainably charging, monitoring the movement signals

[69]

Fiber-shaped thermo-electric devices

Dyeing silk (from Bombyx mori), PEDOT:PSS

Core–shell structure

Dip-coating with dyeing process

Bulk electrical conductivity of 14 S cm−1, high Young’s modulus of approximately 2 GPa, scaled up to 40 m long

[62]

Bi0.5Sb1.5Te3, Bi2Se3, borosilicate glass

Core-sheath structure

Thermal drawing technology (melting spinning)

Output voltage and power density are 97 mV and 2.34 mW cm−2 (ΔT = 60 K, 7-pair p-n legs), output voltage and power density are 70 mV and 1.46 mW cm−2 (ΔT = 60 K, 5-pair p-n legs)

[61]

Bi2Te3, Sb2Te3, PAN, Au

Composited hierarchical twisted yarn structure

Electrospinning, together with depositing by stencil-mask, and post-twisting process

Nanofiber: ZTSb2Te3/PAN = 0.48

ZTBi2Te3/PAN = 0.14

Yarn: ZTSb2Te3/PAN = 0.24

ZTBi2Te3/PAN = 0.07

Output power is 8.56 W cm−2 (ΔT = 200 K)

[63]

CNT carbon nanotube, N719 cis-diisothiocyanato-bis (2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) bis (tetrabutylammonium), PCE power conversion efficiency, P3HT poly(3-hexylthiophene), P3HT:PCBM poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), FAS fluoroalkylsilanes, AgNWs silver nanowires, PDMS polydimethylsiloxane, PMMA polymethyl methacrylate, PTFE polytetrafluoroethylene, PU polyurethane, PAN polyacrylonitrile

Fiber-Based Energy Storage Devices

With the emergence of a myriad of flexible electronic devices, it is of great concern to search the flexible energy storage devices which are safe, environmental and high- efficient [70, 71]. Nevertheless, the traditional and commercially available energy storage devices like lithium-ion battery and supercapacitors, are almost rigid, bulky and in poor adaptability of complicated conditions, which hinder the further development of wearable fields. For being better combined with fabrics or other smart wearable electronic products, bendable, stretchable and deformable fiber-based energy storage devices are expected to be well designed and developed found on these two sorts of energy storage devices.

Fiber-Shaped Lithium Ion Batteries

Recently, various forms of energy storage devices are extensively studied due to their ample storage and lower price. However, they still have a long way off the real world due to their lower output voltage, energy density, higher reactivity, unstable cycling performance, also the premature manufacturing technique and matching system of electrode/electrolyte. By comparison, based upon the distinctive properties of lithium metal with high theoretical capacity (~ 3860 mAh g−1) and the lowest electrochemical potential (~ 3.04 V), various lithium-based energy storage systems are boosted to be applied to ubiquitous energy grids [72, 73, 74]. Meanwhile, its lower density (~ 0.53 g cm−3) and excellent metal ductility make lithium-based anode be a candidate in more flexible and lighter structures [75, 76, 77]. Rechargeable lithium-ion batteries (LIBs) have drawn a wide range of research interests during the past few decades because of their predominant features like long cyclic life-span, high electrochemical window, high energy density, and coulombic efficiency when it comes to those conventional lead-acid and Ni–Cd batteries (Fig. 6a) [78].
Fig. 6

a The comparison of the different kinds of batteries in the light of volumetric and gravimetric energy density. Reproduced with permission [78]. Copyright 2001, Nature Publishing Group. b Typical structure of the lithium-ion battery and the transport direction of ions during the charging/discharging process. c The possible evolution process about the configuration of batteries

In general, the typical structure of LIB consists of electrode materials, current collector, separator and electrolyte. Just like the intrinsic mechanism of majority chemical energy storage systems, the lithium ions are deintercalated from cathode to anode via electrolyte in the charging process. Meanwhile, in order to maintain the charge balance in the system, the electrons flow to the anode by an external circuit. As to the discharging process, the ions and electrons exhibit opposite movement against charging process respectively. The LIBs delivery a reversible cycle according to the intercalation/deintercalation of lithium ions (Fig. 6b).

Gradually, in that the demand of human being for energy storage devices is not limited within a fixed location. The structure of the battery is evolving from a three-dimensional bulk to two-dimensional planar structure. Meanwhile, the flexibility and lighter weight of 2D planar configuration greatly excite the potential of the battery in sundry aspects, especially the flexible smart wearable electronics. However, the 2D structure still lacks in the more complicated service environment such as winding, stretching and weaving. Except that, the comfort of it just like gas permeability is still to be considered. In view of the mentioned above, it makes sense for researchers to develop lithium-ion batteries with 1D fibrous shape, which facilitates the application of energy storage devices in the field of flexible wearable electronics (Fig. 6c).

In order to achieve fibrous structure, one of the most versatile approaches is that the mixed active materials, binder and conductive agent slurry are coated or deposited onto the linear current collector. Then the separator with liquid electrolyte/polymer electrolyte are wrapped/dipped onto the electrode. Ultimately, the LIB with typically cable coaxial configuration is fabricated by further winding and sealing. Benefited from the intrinsic advantages of lithium metal, it is also an ideal choice in the 1D lithium-based batteries. Carbon nanomaterials have great potential as electrode materials and current collectors because of their good electrical conductivity, electrochemical stability, and excellent mechanical strength [79]. A sort of composite fiber anodes, combined with aligned multiwalled carbon nanotube (MWCNT) and silicon, and lithium wire were integrated to obtain flexible, wire-shaped lithium-ion battery (Fig. 7a). According to the SEM images (Fig. 7b), the highly aligned MWCNTs fibers showed no obvious aggregates with a uniform Si deposition on the surface, which ensured high tensile strength and electrical conductivity. The prepared LIB based on MWCNT/Si composite fiber showed decent specific capacity retention (Fig. 7c).
Fig. 7

a Schematic illustration of fiber-shaped LIB made up with MWCNT/Si composite fiber and lithium wire. b SEM images regarding the aligned MWCNTs fibers and composite fiber electrode. c Variation of specific capacity with the cycle number. ac Reproduced with permission [80]. Copyright 2014, Wiley–VCH. d Schematic illustration and digital photos of 1D cable lithium–sulfur battery. e The open circuit voltages of cable lithium–sulfur battery at different bending angles and the LED can be lighted up at different situations. f Comparison between cable lithium–sulfur battery with other kinds of energy storage devices on energy density. df Reproduced with permission [81]. Copyright 2015, Wiley–VCH. g Schematically showing the fabricated process of stretchable fiber-like lithium metal battery. h The inner structure of stretchable fiber-like lithium metal battery and flexibility representation. i Charge and discharge profiles with the strain being increased from 0, 25, 50, and 75% to 100% at the current density of 50 mA g−1. gi Reproduced with permission [82]. Copyright 2019, Elsevier

The lithium–sulfur battery system exhibits a promising future in energy storage due to a high theoretical energy density of 2600 Wh kg−1 and the lower the cost of sulfur cathode. In view of this, a 1D cable-shaped lithium–sulfur battery was fabricated by wire-shaped composite cathode and lithium wire anode (Fig. 7d). A hybrid fiber consisting of aligned CNT fibers and sulfur worked as cathode. The lithium wire acted as anode. The assembled fiber-shaped Li–S battery was able to light up a red light-emitting diode (LED) (ignition voltage of ≈ 1.8 V). Meanwhile, it could be integrated into the fabric cloth and power the LEDs instantly (Fig. 7e). In a rate range from 0.1 to 1 C, the energy density of fiber-shaped lithium–sulfur battery could peer with planar lithium–sulfur batteries yet overwhelms flexible lithium-ion batteries and supercapacitors, which was advantageous to practical application in flexible energy storage devices (Fig. 7f). In order to meet more versatile applications, a kind of stretchable lithium metal battery was fabricated on the elastic fiber by the structural design of electrode (Fig. 7g). The 1D stretchable lithium metal battery showed a hierarchical ring structure and decent flexibility (Fig. 7h). Although the stretchable fiber-shaped battery with a series strain state of 0, 25, 50, 75, and 100%, it showed a negligible capacity loss in the in situ discharge/charge profiles, emphasizing the desired promising application of stretchable battery (Fig. 7i).

For now, the mentioned fiber shaped LIB involved with lithium wire is still in its infancy because of the inevitable safety issues, just like the metallic activity and sensitivity of lithium to oxygen and humidity. Moreover, it is better to introduce safe electrolyte or solid electrolyte for the sake of wearable application [83, 84]. Inspired by the emerging and efficient manufacturing process, as shown in Fig. 8a, an all-fiber LIB were fabricated by 3D printing technology. The electrode materials, lithium iron phosphate (LFP) and lithium titanium oxide (LTO), were mixed with carbon nanotubes and polymer to obtain highly viscous printable inks. Both as-printed fiber electrodes demonstrated good flexibility and electrochemical performance. All-fiber LIB was assembled by twisting the as-printed LFP and LTO fibers electrodes together with gel polymer electrolyte. As shown in Fig. 8b, the all-fiber device could light up a LED at straight or bending states without brightness failure, because of the good flexibility and proper mechanical strength of as-printed electrode fibers. They could be easily woven into fabrics as well. The initial charge and discharge capacities were about 141.3 and 110 mAh g−1 respectively, with Coulombic efficiency of 77.1%. The charge and discharge capacities gradually stabilized at 91.7 and 89 mAh g−1 with a high capacity retention of 81% after 30 cycles (Fig. 8c).
Fig. 8

a Schematic fabrication process of all-fiber LIB by 3D printing. b Display of the all-fiber battery lighting up a red LED in the straight and bending state. c Cycling performance of 3D printed all-fiber device. ac Reproduced with permission [86]. Copyright 2017, Wiley–VCH. d Schematic illustration of preparation for fibrous electrodes and the assembly process of the LIB. e Digital photos of an all-fiber quasi-solid-state with good integration for fabrics. f Cycling performance at different working states. df Reproduced with permission [85]. Copyright 2018, Elsevier. g Schematic of all-solid-state flexible LIB. h A yellow LED can be lighted up by cable type all-solid-state LIB at normal and knitting states. i The cyclic capacity retention of all-solid-state flexible LIB for 100 cycles. gi Reproduced with permission [87]. Copyright 2019, American Chemical Society

Nowadays, the flexible power devices require not only the improvement for high energy density and power density, but also the safety and flexibility for practical application. Thus, it is particularly important to develop the energy storage devices that can work under some sudden conditions. Figure 8d presented the diagrams of the manufacturing steps for fibrous electrodes and the assembly process of a kind of self-healable fiber-shaped LIB. The self-healing property was mainly attributed to the abundant hydrogen bonds in the supramolecular network at the broken surface. The stress induced by the recovery of the PU will pull the disconnected rGO fiber to the interconnected state [85]. In Fig. 8e, the fiber-shaped LIB could power up LED display board under various states. The batteries could also be successfully weaved into textile. The fair cycling performance of self-healable fiber-shaped LIB indicated the potential application in wearable electronics (Fig. 8f). Based on multilayered coaxial structure, Fig. 8g showed a kind of all-solid-state flexible 1D LIB. The micro coaxial batteries over the CF surface were fabricated via electrophoretic deposition and dip-coating methods. After charging at appropriate current density, a red LED could be driven by the micro-coaxial battery at ordinary and knotting states (Fig. 8h). On the premise of excellent flexibility, the microfiber battery also exhibited a stable potential window of 2.5 V and retains up to 85% discharge capacity even after 100 charge/discharge cycles (Fig. 8i).

The progress on 1D fiber-shaped LIBs study is exciting. Nevertheless, the considerable efforts with aspect to energy density, the endurance of water or other more complicated application conditions and industrialized manufacture still need to be paid in the future work.

Fiber-Shaped Supercapacitor

Different from the conventional capacitors and commercial LIBs, supercapacitors (SCs), also known as ultracapacitors, are promising energy storage devices that can be safely charged/discharged within seconds, simultaneously accompanied with extremely long cycle life (more than 100,000 cycles). Combined with the properties of high-power density (often more than 10,000 W kg−1) and simple structures, SCs are developing into one of most potential candidates for future wearable electronic devices [88, 89, 90].

To be closer to the practical operation of wearable fabrics device, the common planar sandwich structures of SCs is necessary to evolve into the fibrous configuration [91]. 1D fiber-shaped SCs are usually miniaturized with diameters ranging from micrometers to millimeters. Their smaller size, lighter weight, and unique wire-shaped structure properties allow them to be various desired shapes and woven or knitted into fabrics. The low dimensional carbon-based materials are also suitable for the electrodes of SCs because of excellent electronic conductivity, high theoretical specific capacity, and outstanding electrochemical stability [92]. Utilizing the excellent intrinsic properties of graphene, the porous graphene ribbons were prepared by spinning composite graphene oxide supramolecular hydrogel, directly reducing and removing the additive (Fig. 9a). The high-performance yarn SC was fabricated by the combination of 1D porous graphene ribbons and gel electrolyte. The wrinkles and pores could be easily observed in this porous graphene ribbons with novel structure, which was appropriate to be integrated or woven into textile (Fig. 9b). As depicted in Fig. 9c, the yarn SC was weaved into a glove and there was only 5% capacitance loss after 100 bending cycles which demonstrate an ideal electrochemical and mechanical stability. Recently, a kind of hybrid fiber consist of graphene oxide and cellulose nanocrystal was prepared through non-liquid–crystal spinning and following reducing treatment (Fig. 9d). The obtained hybrid GO/CNC fibers exhibited unobstructed channels for the transfer of carriers which ensured the further electrochemical performance for the whole device. The capacitance retention was around 100% although under different bending angles (Fig. 9e). Meanwhile, the capacitance could remained about 97.2% after 500 cycles at a 180° bending angle, which indicated a foreseeable operationality for flexible electronics.
Fig. 9

a Schematics for the formation of porous graphene ribbons and all-solid-state yarn supercapacitor. b SEM images of porous graphene ribbons and yarn SC. c Flexible and cyclic valuation and of yarn supercapacitor woven into a glove. ac Reproduced with permission [93]. Copyright 2015, Elsevier. d Schematic illustration concerning the preparation of rGO/CNC composite fiber. (Inserts is the ross-section SEM image of rGO/CNC-20 hybrid fiber). e Stability study of the device at various bending states. d, e Reproduced with permission [94]. Copyright 2018, Elsevier. f Schematics of the fabrication process of fluorescent hybrid fibers and the digital photos of multicolor fiber electrodes. g Photographs of the integration between fluorescent fibrous supercapacitors and fabric. h Cycling performance of fluorescent supercapacitor fiber and inset is spectrum before and after cycles. fh Reproduced with permission [95]. Copyright 2017, Wiley–VCH

In some special cases, the multifunctionality of single parts will significantly promote the environmental adaptability of the whole device. For instance, the colorful fluorescent fiber-shaped supercapacitors with several different colors from red to purple were fabricated by introducing the fluorescent dye particles on top of the surface of aligned multiwalled carbon nanotubes (Fig. 9f). The colorful fibrous supercapacitors made up of fluorescent fiber electrode are well integrated with fabrics. Besides, the multicolor improve the visibility so that play a warning role in the darkness (Fig. 9g). Moreover, according to Fig. 9h, the working stability of fluorescent supercapacitors on energy storage predict satisfactory practicability in real life.

Stretchability is also an important index to evaluate flexible energy storage devices, the textile with better stretchability has greater deformation potential. Lately, the stretchable all-gel-state fibrous supercapacitor was established by 3D hybrid hydrogel found on concept of all-hydrogel design [96]. Figure 10a presented the fabrication process of hybrid hydrogel fibers and all-hydrogel-state fiber devices. The hybrid hydrogel was formed by immediately mixing GO and PANI dispersion solution. The hydrogel was molded into fiber shape after further reduction and then assembled to form a device. Due to the outstanding flexibility of hybrid fiber electrode, a spring-like SC was prepared by structural design. The good capacitance retention of the spring-like SC was achieved under various elastic changes and long tensile cycles (Fig. 10b). The 86% capacitance retention of hybrid fiber could be observed in Fig. 10c after 17000 cycles, indicating the eminent stability. Back to practical reality, continuous and scalable preparation of the high-performance materials are profound. In Fig. 10d, the ultralong MoS2 modified rGO fibers (rGMF) were fabricated by a facile wet-spinning method. The addition of MoS2 nanosheets increases active sites and simultaneously keeps the wrinkle structure of rGO thus lead to a high capacitance. In this case, an all-solid-state fiber-shaped supercapacitor was designed by the combination of rGMFs and PVA-H3PO4 gel electrolyte. The device could be woven into fabrics and textile with decent electrochemical stability under bending states (Fig. 10e). Next to the above topic, using MnO2 core–shell nanorod and MoO2@C nanofilm as positive and negative electrode respectively, PVA-LiCl gel acted as the electrolyte, a kind of wire-type asymmetric pseudocapacitor has been fabricated. Because of the simple configuration, it thus could be easily scaled up to 100 cm in length. At the same time, the device exhibits fair flexibility and stable power output (Fig. 10f). Likewise, the pseudocapacitor gives expression to an outstanding cyclic stability with 97.36% capacitance retention of initial state after 100,000 times [97]. As shown in Fig. 10g, there was no obvious capacitance loss after bending and recovery.
Fig. 10

a Schematic for the formation of PANI/GO hybrid hydrogels. b The variation of capacitance retention under stretching/compressing states and cyclic stretch, this kind of stretchable SC can be well woven into yarn to power up LEDs. c Cyclic stability of SC at a current density of 1.26 A g−1. ac Reproduced with permission [96]. Copyright 2018, Wiley–VCH. d Schematic diagram for the formation of hybrid fiber electrode, together with the SEM images of it. e The flexibility and weavability displays of fibrous all-solid-state SCs. d, e Reproduced with permission [17]. Copyright 2019, The Royal Society of Chemistry. f Schematic for the structure of asymmetric pseudocapacitor, a scalable spring-like device could light up an LED display board. g Electrochemical stability was proved under various deformed states. f, g Reproduced with permission [97]. Copyright 2019, Wiley–VCH

Table 2 lays down a set of guidelines considering the important parameters with regards to fiber-shaped energy storage devices research. The state-of-the-art works have performed well on the aspect of safety, energy density, output voltage, cycling performance under normal or complicated states, flexibility, weather durability, and scaled-up manufacturing etc. [98, 99, 100, 101, 102]. Nevertheless, the restriction about integrity of those single excellent performance still hinder the practical utilization and commercialization. The comprehensive concerns should be well considered in the next future development [103].
Table 2

Summary for configuration, device capacity, cycling retention, and flexibility of fiber-shaped energy storage devices

Fiber device

Configuration (cathode//anode electrolyte)

Device capacity

Cycling retention

Flexibility

Refs.

Fiber-shaped lithium ion battery

MWCNT/MnO2 fiber//Li wire

109.62 mAh cm−3 or 218.32 mAh g−1 @5 × 10−4 mA

  

[104]

CNT@α-Si composite yarns//Li metal

2200 mAh g−1 @0.2C

86% after 30th

 

[105]

Aligned MWCNT/Si composite fiber//Li wire

1670 mAh g−1 @1A g−1

1042 mAh g−1 @3A g−1

58% after 50th (1A g−1)

After 100 cycles bending, 80% capacity after 20th @2A g−1

[80]

CNT-LMO composite yarn//CNT-Si/CNT composite yarn

106.5 mAh g−1 @1C

87% after 100th

Woven into fabrics and working normally

[106]

MWCNT/LMO composite fiber//MWCNT/LTO composite fiber

91.3 mAh g−1 @0.1 mA cm−1

95% after 50th 78% after 100th

92% capacity retention @180o bending, > 88% capacity retention @strain of 600%

[107]

Aligned CNT/GO/CMK-3@S//Li wire

1051 mAh g−1 @0.1C (coin typed)

~1000 Wh kg−1 @0.1C (cable lithium–sulfur battery)

600 mAh g−1 retained after 100th (coin typed)

Normally work @180o bending

[81]

3D printed LFP//3D printed LTO

~110 mAh g−1 @50 mA g−1

81% after 30th

No failure of the LED brightness at bending

[86]

LCO nanoparticles@ rGO//SnO2 quantum dots@rGO

101.1 mAh g−1 @0.1A g−1

~80% after 50th

82.2% capacity retention after 50 cycles @ bending and twisting

[85]

MoS2@CNT fiber//Li-ZnO@CNT fiber

1169 mAh g−1 @50 mA g−1

96% after 100th

90.3% capacity retention at a strain of 100% for 100 times

[82]

LFP @carbon fiber//LTO

4.2 μAh cm−2 @13μA cm−2

85% after 100th

Normally work under multiple bending states

[87]

Fiber-shaped super capacitor

Biscrolled PEDOT/MWNT yarn/Pt wire (PVA/H2SO4 solid electrolyte)

~179 F cm−3 @0.01 V s−1

99% after 10,000th (sewed into gloves)

92% after 10,000th @bending

99% after 10,000th @sewing

[108]

NiCo2O4 nanosheets/stainless-steel wire (PVA/KOH gel electrolyte)

10.3 F cm−3 @ 0.08 mA

78% after 5000th

~ 90% capacitance retention after 500 cycles bending

[109]

Porous graphene ribbons (H3PO4/PVA gel electrolytes)

208.7 F g−1 @0.1 A g−1

99% after 5000th

> 95% capacitance retention after 100 cycles bending

[93]

Biscrolled MnO2/CNT yarns (PVA/LiCl gel electrolyte)

166 F g−1 @2.3 mA cm−2

CA = 889 mF cm−2

CV = 155 F cm−3

EA = 35.8 μW cm−2

EV = 5.41 mWh cm−3

92% after 1000th

~ 100% capacitance retention after 1000 bending cycles from 0° to 165°

[110]

rGO nanosheets/stainless-steel wire (PVA/H3PO4/Na2MoO4 polymer gel electrolyte)

18.75 mF cm−1 (CA = 38.2 mF cm−2) and 2.6 mWh cm−1 (EA = 5.3 mWh cm−2) @0.5 mA

~ 100% after 2500th

~ 100% length capacitance retention under different bending conditions

[111]

Pt/CNT@PANI composite yarn (PVA/H3PO4 gel electrolyte)

CA = 91.67 mF cm−2

EA = 12.68 mWh cm−2

@0.8 mA cm−2

80% after 5000th

Negligible loss of capacitance at different bending angles from 0° to 180°

[112]

Hybrid GO/CNC fibers (PVA/H2SO4 gel electrolyte)

123.2 F g−1 (CV = 155.8 F cm−3 EV = 5.1 mWh cm−3) @0.1 A g−1

92.1% after 1000th

97.2% capacitance retention after bending at 180o for 500 times

[94]

PANI/rGO fibers (PVA/H2SO4 gel electrolyte)

EV = 8.80 mWh cm−3

PV = 30.77 mW cm−3

86% after 17,000th

Subtle capacitance loss during the stretching process

[96]

Fluorescent hybrid MWCNT fiber

CV = 11.98 F cm−3 @ 10 mA cm−3

98.4% after 10,000th

Stable color intensity maintained, accompanied by stable specific capacitance with little variation

[95]

MnO2 core–shell nanorod array//MoO2@C nanofilm (PVA/LiCl gel electrolyte)

CA = 31.7 mF cm−2

CV = 13.45 F cm−3

EV = 9.53 mWh cm−3

PV = 22720 mW cm−3

~97.36% after 100,000th

Scaled up to 1 m with good flexibility (can be readily bent with different degrees from0° to 360°)

[97]

NaDC/rGO/MoS2 hybrid fiber (PVA/H3PO4 gel electrolyte)

134.38 F g−1 (CA = 332.85 mF cm−2 CV = 21.9 F cm−3) @ 50 mA

~73% after 800th (tested with the bending process)

~ 73% capacitance retention for 800 bending cycles at bending angle of 60o

[17]

LMO LiMn2O4, LTO Li4Ti5O12, LFP LiFeO4, LCO LiCoO2, CA areal capacitance, Cv volumetric capacitance, EA areal energy density, Ev volumetric energy density, PV volumetric power density, rGO reduced graphene oxide, NaDC sodium deoxycholate

Fiber-Shaped Chromatic Devices

Color is very important to nature and human society. People can design brilliant and colorful patterns and styles to express their emotions and stories through simple and imaginative combinations of colors. In recent years, with the rapid development of flexible electronic materials and wearable products, high flexible, portable, intelligent and multi-functional equipment has become a hot spot of consumption growth. Decorations and clothes with constant color can no longer meet people’s demand for fashion and novelty. Therefore, flexible intelligent chromatic materials and devices have become a hot topic and have been widely studied. According to the different physical or chemical mechanisms, chromatic devices can be divided into electrochromic, thermochromic and structurally colored devices [33, 34, 113]. This section will introduce the latest research progress of fiber-shaped chromatic devices.

Electrochromic Fibers

The sandwich-like structure is considered to be a typical structure of electrochromic devices, which consists of two transparent conductive electrodes with an internal electrochromic active layer [114, 115]. There are two kinds of electrochromic active layers for different electrochromic materials. One is the mixed structure of electrochromic materials and electrolytes. The other is the layered structure of electrochromic materials, electrolytes, and electrodes. However, a series of problems are encountered in the preparation of electrochromic fibers, including the difficulty of miniaturization of electrodes, the uneven distribution of the electric field, and the poor stability during a small curvature radius. Nevertheless, efforts have been made to fabricate electrochromic fibers by spirally rolling the narrow electrochromic films, paralleling the coil electrode on a fiber substrate, and wrapping fiber electrode on a fiber device [32, 35, 116].

Current electrochromic fibers include three typical structures: coiled, helically coaxial and wrapped structures. A stretchable coiled electrochromic fiber with display components was made with three main parts [116]. First, the electrochromic bands were prepared on the thin plastic membrane by patterning the electrochromic active materials of PEDOT:PSS and insulation layer of cytop, and then casting ionic liquid and PVDF-HFP based solid electrolyte. Second, the thread shape of display elements was formed through the slit and release of fabricated electrochromic bands. Finally, the stretchable electrochromic fiber was obtained by helically rolling the slit films around polyurethane rubber. The extensibility of the fabricated coiled electrochromic fiber was demonstrated. 4 pixels were contained in one fiber device. The pitch angle of the yarn was 45 degree. The selected one pixel changed its color form pale to dark blue through the voltage application. However, this kind of fiber-shaped electrochromic devices was relatively large and exhibited monotonous color, and could not effectively meet the requirements regarding weavability.

In order to improve the practicability of electrochromic fibers, Lin et al. prepared a multicolor electrochromic fiber which shows red, green, and gold, turning by the applied voltages [35]. As shown in Fig. 11a, the multicolor electrochromic fiber was prepared by a template method. WO3 and poly(3-methylthiophene) were used as electrochromic active substances, and they were selectively deposited onto the electrodes. Figure 11b exhibited digital pictures of the coloration and discoloration of the fiber. The color of cathode changed to dark green when a +1.5 V bias was applied. When a − 1.5 V reverse bias was applied, the colors of two electrodes exchanged. Figure 11c displayed the reflectance spectra of the electrochromic fiber. With − 1.5 V bias, the reflectance of the WO3 layer was quite high, and the removal of bias voltage significantly decreased the reflectance. Although this modified electrochromic fiber showed much improved multicolor performance, the golden color of the electrode reduces the applicability.
Fig. 11

Electrochromic fiber devices. a Schematic of the preparation process of parallel coil electrodes [116]. b Digital pictures to display the color change of the fiber device. c Reflectance spectrum of the active material tungsten oxide (WO3) during coloring process. ac Reproduced with permission [35]. Copyright 2018, Wiley. d Structure diagram of the electrochromic fibers and the electrochromic mechanism of the PEDOT based electrochromic layer. e The photographs and f reflectance spectra of PEDOT based electrochromic fiber under reduced (left) and oxidized (right) states. df Reproduced with permission [32]. Copyright 2014, American Chemical Society

The superposition of red, green and blue can produce various colors. The π-conjugated organic polymers including Poly(3,4-ethylenedioxythiophene), poly(3-methylthiophene), and poly(2,5-dimethoxyaniline) have been discussed and used as active materials to obtain red, green, and blue electrochromic devices [32, 117]. As shown in Fig. 11d, the electrochromic polymers were deposited on the surface of the core stainless-steel wires [32]. Then the gel electrolyte was coated on the electrochromic layer, followed with another stainless-steel wire wrapping on. It can be seen that the PEDOT based electrochromic fiber are exhibited between red and blue (Fig. 11e, f) when different voltages were applied. These electrochromic fibers could respond quickly to voltage changes. Furthermore, these fibers were flexible and could be implanted into textiles.

Thermochromic Fibers

Thermochromic materials are compounds or mixtures that change their visible absorption spectra when heated or cooled. In particular, reversible thermochromic materials have the function of color memory [118]. It has the characteristics of discoloration at a specific temperature, showing a new color, and restoring to the original color when the temperature is restored to the initial temperature [119, 120]. Therefore, reversible thermochromic materials can be used to prepare fibers with chromic properties.

There are two main ways to prepare thermochromic fibers: composite fibers and surface coatings. The mechanical properties of composite thermochromic fibers are usually poor, which makes it difficult to meet the requirements of weaving. Lu et al. made an electrothermal chromatic fiber by coating the thermochromic polymers onto a conductive fiber. As shown in Fig. 12a, the elastic electrothermal chromatic fiber was fabricated by in situ polymerization of diacetylene monomer on the SWCNTs based elastic conductive fiber and following with coating a protected layer of silicone [33]. The electrothermal chromatic fibers exhibited highly flexible and stretchable properties. The chromatic transition of the fiber was reversible even under stretching up to 80% and bending angel of 180°. As exhibited in Fig. 12b–d, even if the electrothermal chromatic fiber was woven into a Chinese knot, wrapped around the glass rod and stretched during deformation, it was still well worked. However, this kind of thermochromic polymers-based fiber devices were only changing between blue and red.
Fig. 12

Electrothermal chromatic fibers. a Preparation process of the stretchable electrothermal chromatic fiber. b An electrothermal chromatic Chinese knot prepared from the electrothermal chromatic fiber. c An electrothermal chromatic fiber coiled on a glass rod. d In situ photographs during stretching. ad Reproduced with permission [33]. Copyright 2016, The Royal Society of Chemistry. e Schematic illustration of the structure of the stretchable electrothermal chromatic fiber. f Schematic diagram of color change mechanism during electric heating. g Electrothermal chromatic fibers with patterns of letters are woven into fabrics. Reproduced with permission [121]. Copyright 2017, The Royal Society of Chemistry

In order to obtain abundant color change of thermochromic fibers, Li et al. prepared a series of thermochromic fibers which show orange, red, green, yellow, blue and white, turning by the electric heating. As shown in Fig. 12e, the as-prepared electrothermal chromatic fiber has a multiple layered sheath-core structure [121]. The inner and outer layers of sheath-core fibers were the core layer of elastic polyurethane, the conductive layer of PE-RGO-TiO2, the protective layer of PDMS, and the thermochromic ink in turn. Schematic diagram of electrothermal chromatic mechanism was given in Fig. 12f. When the circuit was switched on, the temperature increase and lead to the color change of the thermochromic ink layer. Besides, a series of thermochromic fibers that were prepared using varies inks with diverse colors and response temperatures are woven into a fabric (Fig. 12g). The three different colors fibers could change their colors twice respectively during the electric heating.

Structurally Colored Fibers

Structurally colored fibers refer to a kind of fibers with color on the surface or in the interior due to periodic structure [113, 122]. Different from the traditional fiber color, the color of structurally colored fibers is mainly produced by the interaction of micro-nanostructure and light on its surface or inside [123, 124]. The color of the structurally colored fibers can be changed by adjusting its inner or surface periodic structure. In addition, there is almost no energy consumption in the process of color changing of structurally colored fibers. Therefore, the preparation and application of structurally colored fibers have attracted extensive interest of researchers.

For the preparation of structurally colored fibers, the construction of periodic alignment of micro-and nano-materials within the fibers is a difficult and key problem. Shang et al. proposed a magnetically assisted self-assemble process to obtain the structurally colored fibers [125]. Induced by an external magnetic field, the superparamagnetic colloidal spheres were arranged into the one-dimensional chainlike structure and embedded in the stretchable PDMS matrix. Figure 13a schematically showed the color-changing process of the Fe3O4@C colloidal spheres embedding structurally colored fibers during the stretching and squeezing. As shown in Fig. 13b, the digital photos exhibit the real reflective colors of the stretched and squeezed structurally colored fibers. The mechanical strain sensitivity of the structurally colored fiber can be explained in Fig. 13c. The tuning of the color of the fiber depends on the Bragg’s law,  = 2nd sinθ. The increase and decrease of lattice space in the chain-like structures caused the red- and blue-shift of the diffraction.
Fig. 13

Structurally colored fibers. a Schematic illustration of the color change of the structurally colored fiber during squeezing and stretching. b The digital photos of the stretched, initial and squeezed fibers. c Schematic illustration of color change of the structurally colored fiber during mechanical strains. ac Reproduced with permission [125]. Copyright 2016, The Royal Society of Chemistry. d Schematic illustration of the constructing of the multilayer structurally colored fibers. e Optical micrographs of the stretchable structurally colored fiber displaying the color changing upon mechanical strain. f The reflection spectrum of the structurally colored fiber corresponding to the optical micrographs in e. df Reproduced with permission [126]. Copyright 2013, Wiley

As we all know, there are abundant and colorful structural colors in nature. Therefore, the study of structurally colored fibers is also imitating natural organisms. Kolle et al. presented a fiber rolling technique to fabricate the multilayer structurally colored fibers with an adjustable band-gap center frequency [126]. As schematically shown in Fig. 13d, the bilayer film consisting two elastomeric dielectrics of PDMS and PSPI was firstly assembled. Following with rolling the above bilayer film on a thin glass fiber, the multilayer structurally colored fiber was obtained. The multilayer structurally colored fibers exhibited the tunable reflective colors during stretching it along its axis (Fig. 13e). The reason for the reflection band blueshifts of the fiber was that the Poisson’s ratio of two consisted elastic materials were comparable whereas the axial elongation renders the decrease of its diameter and thickness of each layer. Over 200 nm peak wavelength shift has been measured of the multilayer structurally colored fiber during stretching its length over 200% (Fig. 13f).

Shape Deformable Fibers

Shape deformable materials can reversible change its position or shape in response to external stimuli, such as magnetic field, electricity, irradiation, heat, and atmosphere [37, 38, 127, 128]. Since the 21st century, new advanced deformable materials have been widely studied and gradually applied in biomimetic devices, biomimetic technology, and fashion decoration, such as shape memory polymer and alloy, phase change materials [129, 130, 131, 132]. Fiber, as the unit of weaving fabric and further designing clothing, is the basis of clothing. Design and construction of controllable shape deformable fiber device is the key to realize the development of deformable clothing. Recently, researchers have devoted a lot to the study of shape deformable fibers, and have made great progress.

Electrically Controlled Deformable Fibers

In the era of electronic information, electric energy is the most convenient and easily controlled energy. People try to fabricate and construct electric-driven fiber materials when studying shape deformable fibers [37, 38, 131]. There are usually two ways to obtain electro-deformed fibers: one is dielectric elastomer-based electrically controlled deformable fibers, which drive the elastic deformation of the dielectric polymer by high voltage; the other is to drive the fiber deformation by Joule heat generated by electric energy. We will introduce the studies on deformable fibers based on above two principles.

For the shape deformable fibers, there are three main manifestations of their shape change: shrinkage, elongation and rotation. Liu et al. constructed a hierarchically buckled sheath-core deformable fibers consisting of an elastic styrene(ethylene-butylene)-styrene (SEBS) rubber core and a parallel CNTs sheath [40]. As shown in Fig. 14a, the SEBS rubber core was firstly stretched above 1000% strain for preparing the elastic conductive fiber. For the preparation of electrically controlled deformable fibers, the SEBS rubber layer acted as a dielectric was wrapped on a layer of paralleled CNT without stretch. The shape deformable fiber actuator was fabricated by twisting the sheath-core fiber. This electrically controlled deformable fiber could operate isobarically and exhibit both torsional and tensile actuation (Fig. 14b). The dependence of the retraction stroke and rotation behavior with the different twisted density were studied in Fig. 14c. The rotation angle and speed showed a linear behavior with twisted density. The tensile stroke was hardly changed with increasing twisted density.
Fig. 14

Electrically controlled deformable fibers. a The fabrication process of a hierarchically buckled sheath-core fibers with a SEBS rubber core and a parallel CNTs sheath. b The rotation angle and tensile stroke change of a twisted sheath-core fiber with different electric field. c The shape changes performance and tensile stroke characteristic of the twisted sheath-core fiber with different twisted density. ac Reproduced with permission [40]. Copyright 2016, AAAS. d The photographs of 5 strands of the double helix twisted and coiled fiber lifting 1 kg load by Joule heating. e Schematic and SEM images of the double helix twisted and coiled fiber. f Schematic showing the function of a double helix twisted and coiled fiber in an artificial limb. g Flexion motion of the artificial limb. dg Reproduced with permission [133]. Copyright 2013, Wiley

Joule heating is one of the common ways to drive the deformation of a fiber. Kim et al. proposed a double helix twisted and coiled fiber actuator with both spandex and nylon which can be driven by Joule heating [133]. Figure 14d exhibited the schematic of the double helix twisted and coiled fiber. The spandex sheath wraps around the nylon core and twisted to form a coiled shape. As displayed in Fig. 14e, 5 strands of the double helix twisted and coiled fiber can lifted a displacement of 75 mm of 1 kg load up during the Joule heating. Figure 14f and g schematically revealed the potential application of the double helix twisted and coiled fiber. Using the double helix twisted and coiled fiber bundles, the artificial limb for grasping/flexion motion was obtained.

Solvent Responsive Deformable Fibers

For the solvent-responsive deformable fiber, solvents are usually adsorbed and desorbed on the surface of the fiber to drive its deformation which is the direct way to change to shape or length of a fiber [39, 43, 45]. Chen et al. fabricated a hierarchically arranged helical fiber that could respond to solvent and vapor and display an elongation and rotation [39]. The hierarchically arranged helical fiber was constructed by helically assembling carbon nanotubes into fibers and then twisting the fibers together (Fig. 15a). As shown in Fig. 15b and c, the organic solvent or vapor successively infiltrate through the microscale and nanoscale gaps, which resulted in a rapid rotation and shrinkage of the hierarchically arranged helical fiber. The contractive stress of the fiber by absorbing ethanol was measured (Fig. 15d). The contractive stress could reach up to about 1.0 MPa within 0.5 s, exhibiting a rapid response. An actuating textile was also obtained by weaving 18 pieces of hierarchically arranged helical fibers and displayed driving behavior.
Fig. 15

Solvent responsive deformable fibers. a Schematic illustration of the twisting process for creating the hierarchically arranged helical fiber actuator. b Schematic of the infiltration of the organic solvent into the fiber’s helical gaps. c Scheme of the contraction and rotation of the hierarchically arranged helical fiber. d Contractive stress curves for the hierarchically arranged helical fiber and single-ply helical fiber. ad Reproduced with permission [39]. Copyright 2015, Springer Nature. e Scheme of a drawing–twisting procedure to fabricate continuous twisted graphene oxide fibers. f The rotation of solvent-driven twisted graphene oxide fibers. g The rotation speeds of twisted graphene oxide fibers driven by different polar solvating species. h Scheme of the responses of two united twisted graphene oxide fibers under the wetting of acetone. eh Reproduced with permission [43]. Copyright 2019, The Royal Society of Chemistry

In addition to the CNTs-based shape deformable fibers, 2D structural graphene has also been used to fabricate solvent responsive deformable fiber. Fang et al. proposed a handedness-controlled fiber-shaped actuator responding to polar solvating species, such as acetone, methanol, and ethanol [43]. As schematically shown in Fig. 15e, the continuous twisted graphene oxide fibers were processed by twisting and drawing the flexible graphene oxide belts. The actuation behavior of a suspended twisted graphene oxide fibers was observed by wetting the fiber with 0.05 mL acetone (Fig. 15f). The fiber was rotated with an angular speed of 633 rad s−1 in 0.7 s. The speeds of the forward and reverse rotation of twisted graphene oxide fibers were measured by using polar solvating species (Fig. 15g), indicating that the torsional rotation of the twisted fiber was repeatable. The twisted graphene oxide fibers exhibited a controllable actuation by configuring two fibers in a homochiral unit.

Light-Induced Deformable Fibers

Light inducing is a non-contact method to achieve material shape change, which has long-distance controllability [135, 136, 137]. Chen et al. introduced a contractile muscle-like actuation of a supramolecular material formed by the self-assembly of a photo-responsive amphiphilic molecular [41]. As shown in Fig. 16a, the UV light (λ = 365 nm) irradiation applied on the stable motor isomer induces photochemical isomerization on the central alkene bond. As schematically shown in Fig. 16b, the unidirectional alignment nanofiber bundles displayed a bending behavior during UV irradiation. The nanofiber bundle could be bending towards the photoirradiation of light source with a speed of about 1.8° s−1 (Fig. 16c). The in situ SAXS measurements was conducted to detect the structural changes of the nanofiber bundle during photoirradiation (Fig. 16d and e). After 60 s irradiation, the diffraction of (001) plane increased from 0° to 65°, corresponding to the bend angle of the nanofiber bundle.
Fig. 16

Light Induced deformable fibers. a The photochemical and thermal helix inversion steps of hierarchical supramolecular organization. b Scheme of bending behavior of the unidirectional alignment supramolecular bundles. c Photographs of the photo-deformation under irradiation of UV light. d, e 2D small-angle X-ray scattering (SAXS) images of fiber-shaped supramolecular bundle after irradiation for 0 s and 60 s. Reproduced with permission [41]. Copyright 2018, Springer Nature. f Schematic drawing of the rotation and reverse rotation of twisted PASS/GO fiber with simplified loading forces during the water evaporation and adsorption. g Scheme of the simplified loading forces of a piece of the twisted PASS/GO fiber in a cotton fabric. The bending behavior of the cotton fabric during near infrared irradiation. Reproduced with permission [134]. Copyright 2017, The Royal Society of Chemistry

Light-induced deformable fibers not only show simple rotation and bending, but also can be knitted into ordinary fabrics to drive fabric deformation. Shi et al. fabricated a pre-deformed twisted PASS/GO fiber which could display various actuation phenomena with the irradiation of a near-infrared light [134]. The metastable structured PASS/GO fiber was prepared by directly axial rotation of the hierarchically wrinkled PASS/GO fiber. The component of the PASS/GO fiber was mainly including the hydrophilic PAAS and GO. As shown in Fig. 16f, the twisted PASS/GO fiber exhibited the contraction and rotation phenomena during the irradiation on the fiber, because of the photothermal effect of GO evaporated the adsorbed water molecule on the surface and slit of the fiber. Based on the light-responsive fiber, the light-induced deformable fabric was obtained by directly weaving 3 pieces of twisted PAAS/GO fibers in a cotton fabric. The fabric could be folded upward to 90 degrees by the near infrared light irradiation (Fig. 16g). The remote controllable light-induced deformation fabric has potential use in shape changeable clothing.

Summary and Perspective

In this review, we first discuss the demands and the potential applications including portable devices, miniature devices, and wearable electronics of functional fiber devices, as well as the historical developments of manmade fibers. The recent progress in advanced functional fiber-shaped devices that consist of fiber-based energy harvesting devices, energy storage devices, fiber-shaped chromatic devices, and shape deformable fibers are systematically summarized. Particularly, the fabrication procedures and application characteristics of representative fiber-shaped multifunctional fiber devices are also discussed in details. These previous studies have opened up initial ideas for design, preparation and characterization of functional fiber devices and made tremendous progress, showing the advantages of fiber-shaped devices, including desired flexibility, miniaturization, weavability, and wearability. It promoted the research upsurge on multifunctional fiber-shaped devices. These efforts also make people aware of the application prospects of functional fiber devices, and more look forward to the opportunity to use these functional fibers.

Despite the present research achievements on the fabrication of multifunctional fiber-shaped devices, there are still many difficult problems to be solved to promote the commercialization of functional fibers. For example, application stability, safety, and scale-up fabrication can be regarded as the critical challenge for the real application of functional fiber devices, especially for the integrated process of the functional fibers with the traditional textile industries and internal stress problems in complex application environments. To further improve the practical applicability of the four representative functional fiber devices (energy harvesting, energy storage, color tuning, and shape deformation), the following research issues should be seriously studied. (1) For the energy harvesting fiber devices, the energy conversion efficiency needs to be further improved, and how to effectively store and use these collected energy needs to be further studied in detail. (2) Regarding the energy storage fibers, the stability and safety of electrodes and electrolytes of fiber-shaped batteries need to be guaranteed. Besides, as a wearable energy storage device, the fiber-shaped batteries should be washable and stable under complex stress. For high power density energy storage fibers, the self-discharge of fiber-based supercapacitors need to be solved. (3) Concerning the chromotropic fibers, the multi-color characteristics of chromotropic fibers need to be studied and obtained. The stability of chromotropic fibers during light irradiation and exposing to high humidity should be seriously considered. (4) For the deformable fibers, how to improve the sensitivity of deformable fibers and obtain the large deformation, as well as the output force during deformation. Finally, large-scale fabrication can be considered as the most critical issues for multifunctional fiber devices. In order to realize their practical application, scale-up fabrication has to be carried out. In the longer-term view, the materials used in these multifunctional fiber devices are environmentally friendly and recyclable.

Notes

Acknowledgements

This work was supported by the Science and Technology Commission of Shanghai Municipality [16JC1400700], the Program of Introducing Talents of Discipline to Universities [No.111-2-04], and the Innovative Research Team in University [IRT_16R13]. C. H. thanks the Natural Science Foundation of China [No. 51603037], DHU Distinguished Young Professor Program [LZB2019002], and Young Elite Scientists Sponsorship Program by CAST [2017QNRC001].

Compliance with ethical standards

Conflicts of interest

The authors declare no competing financial interests.

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Copyright information

© Donghua University, Shanghai, China 2019

Authors and Affiliations

  1. 1.State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and EngineeringDonghua UniversityShanghaiPeople’s Republic of China
  2. 2.Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of EducationDonghua UniversityShanghaiPeople’s Republic of China
  3. 3.School of Materials Science and EngineeringNanyang Technological UniversitySingaporeSingapore

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