Hierarchical Design of Textile-Based Sensor in Wearable Electronics: Comparison
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Smart textiles have recently aroused tremendous interests over the world because of their broad applications in wearable electronics, such as human healthcare, human motion detection, and intelligent robotics. Sensors are the primary components of wearable and flexible electronics, which convert various signals and external stimuli into electrical signals. While traditional electronic sensors based on rigid silicon wafers can hardly conformably attach on the human body, textile materials including fabrics, yarns, and fibers afford promising alternatives due to their characteristics including light weight, flexibility, and breathability. Of fundamental importance are the needs for fabrics simultaneously having high electrical and mechanical performance. 

  • textile
  • sensor
  • hierarchical design
  • wearable electronics

1. Introduction

Textiles are ubiquitous and indispensable to human lives. Owing to the light weight, high deformability, and breathability as well as the integrability with other materials, textiles have provided an optimal platform for the development of next-generation flexible electronics. In particular, textile-based sensors are a key component in these electronic devices designed for human healthcare, human motion detection, and intelligent robotics [1,2,3][1][2][3].
Flexibility or stretchability and electrical conductivity are two essential metrics for sensors in wearable electronics applications [4]. While textiles are easily stretched, compressed, bent, or twisted to allow for the high deformability, the electrical conductivity can be realized either extrinsically or intrinsically. The most straightforward way is to integrate rigid semiconductor and metal sensors to textile materials; however, the sensing performance is greatly limited by the mismatch of conformation at the interface [5]. Recently, the development of nanotechnology has enabled the direct coating, deposition, and printing of electrically conductive materials on textile supports. Although the conformable coating could maintain the conductive network within a low strain range, the coated fabrics always suffered from structural damage and interfacial delamination under large mechanical deformation, especially during cyclic loading conditions, leading to relatively poor sensing performance and service lifetime [3]. In this regard, constructing the fabric via hierarchical assembly of fibers or yarns made of intrinsically conductive materials, such as conductive polymers, metals, and carbon nanomaterials, represents a potent strategy to fabricate high-performance flexible sensors.
As known, textile materials can be classified as fiber, yarn, or fabric [6]. Specifically, fibers with a high aspect ratio act as element structure for the textiles. They can be further twisted into yarns, serving as the second level of textiles. Through some existing textile technologies such as weaving and knitting, yarns are leveraged to construct the third level of integration, namely, the fabrics (Figure 1). Considering the hierarchical architectures of textile materials, the overall performance of textile-based sensors is collectively determined by the interplay between the fiber properties and the geometrical or structural characteristics of their assembly at different length scales [7]. In detail, the fabric properties can be affected by several factors, including the selection of different fibers (metal, polymer, etc.), the use of various yarns (filament yarn, textured yarn, etc.), and the development of different patterns (woven, knitted, etc.). Therefore, delicate design of the microstructures and interfaces across multiple levels of hierarchy is anticipated to tailor the mechanical and functional properties of textile-based sensors.
 

2. Selection of Constituent Materials

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Figure 1. Schematic representation of hierarchical structures across multiple length scales in textiles [12,13,14]. Reprinted with permission from Ref. [12]. 2018, Royal Society of Chemistry.

2. Selection of Constituent Materials

The first step for the design of wearable electronics is the selection of building blocks, which are able to afford outstanding mechanical properties without sacrificing electrical performance.

2.1. Metallic Nanostructure

Thanks to the high free-electron density, metals are deemed as the most conductive materials on Earth, among which silver is the most widely studied given its highest conductivity [25,26][8][9]. However, as metal wires are relatively rigid and cannot be effectively woven into fabric, they are usually coated on polymer fibers or embedded in composite fibers. More intriguingly, when the size of metals decreases down to nanoscale, their physical properties become distinct from the bulk counterparts. This enables the metal nanostructures to be competent candidates for conductive-filled materials in textile-based sensors. Extensive studies have demonstrated the successful application of gold, silver, and copper nanowires in electronic textile devices [27][10].

2.2. Conductive Polymer

Conducting polymers with π-conjugated bond structures possesses superior electrical properties comparable to metals. Besides that, they have additional advantages including light weight, transparency, intrinsic flexibility, processability, cost-effectiveness, biocompatibility, and so forth. Commonly used conductive polymers include polypyrrole (PPy), polyaniline (PANI), polythiophenes (PTs), and poly(3,4-ethylenedioxythiophene) (PEDOT) [9][11], which satisfy different practical requirements. In particular, polypyrrole has been widely utilized in biomedical applications, attributed to its thermal stability, biocompatibility, and biodegradability [30,31,32][12][13][14]. Nevertheless, it is sensitive to moisture and prone to deterioration over time when exposed to a humid environment [33][15]. In contrast, PANI displays high environmental stability and is the optimal choice for corrosion protection, while its non-biodegradability, low processability, and low flexibility limit its application in biological fields [34,35,36][16][17][18].

2.3. Carbon Nanomaterial

Carbon nanomaterials (e.g., carbon nanotubes (CNTs), graphene and its derivatives) hold enormous potential in the application of flexible sensors. This is due to their low-dimensionality, huge specific surface area, and a collection of fascinating physical properties, including excellent flexibility, ultrahigh strength, and electrical conductivity.

2.4. Two-Dimensional (2D) Materials

The intensive study of graphene has drawn worldwide attention to other 2D materials as well. Some emerging 2D materials such as transition metal dichalcogenides (TMDs) [49[19][20][21],50,51], transition metal carbides and nitrides (MXene) [52][22], black phosphorus [53][23], and metal-organic frameworks (MOFs) [54][24] have been synthesized and used for the preparation of flexible sensors.

3. Microstructural Control of Fiber

Fibers act as the building blocks of textiles to create various styles of clothes through knitting or weaving technologies. They are usually prepared by a spinning method (e.g., wet-spinning and electrospinning) and present a straight configuration with limited stretchability [58,59,60][25][26][27]. To this end, substantial efforts have been dedicated to improving the deformability of fiber sensors while maintaining high electrical conductivity.

3.1. Helical or Twisted Structure

The creation of the helical structure of fibers affords an effective way to increase the stretchability, which can be achieved by mechanically twisting or coiling straight fibers. When the helix fiber is under tension, it is subject to a straightening process first, so that the tensile strain directly imposed on the fiber is minimized. It has been documented that the spiral-arranged textures along twisted graphene fibers improve their tear resistance and fracture toughness [61][28]. From a mass production perspective, twisting and coiling is also a relatively common process in industry and is simple to scale up.

3.2. Core–Sheath Structure

The design of coaxial fibers provides an alternative pathway to gain higher stretchability. Coaxial fibers commonly include a conductive core wrapped by an elastic shell. Distinct from the twisting structure, the core–sheath structure offers a shorter ion transport path and a higher stability upon external loading due to the strong interface bonding within the layer-by-layer configuration [65][29].

3.3. Buckling Structure

Introducing buckling structure via the pre-stretch-and-release method has been manifested as a facile way to realize the high stretchability and flexibility in electronic devices [70,71][30][31]. Similar to helical configuration, the buckling structure is flattened first under tensile loading, thus giving rise to a stable conductance during deformation. According to the literature [3[3][6],6], a buckling structure can be created in either conductive core fibers in TPE or conductive coating materials wrapped on pre-stretched elastomeric fibers.

4. Configurational Engineering of Yarn

Yarn is defined as an assembly of a bunch of fibers held together through the insertion of twist to form continuous strands. From the structure point of view, typically, there are three main technologies involved in the production of yarn [77][32]: (1) Yarns are produced by placing the fibers in parallel that are bonded together via mutual frictional forces. Given the weak friction resistance dominated by van der Waals forces, the tensile loads imposed to the yarn easily induces mutual slippage when the yarn suffers from severe bending or twisting deformation. This is an irreversible process that can cause easier failure at low strains and faster degradation of conductivity. (2) Densely aligned CNT bundles can be prepared by twist spinning, typically showing a high conductivity of 300 S/cm) and high strength of more than 100 MPa, whereas the interfibrillar slip phenomena are still inevitable. As a result, the sensitivity of the helical fiber strain sensors is relatively low and strain range is also lower less than the maximum sustained strain. (3) In light of the limited stretchability of straight or twisted yarns, substantial efforts have been directed towards configurational engineering by introducing multiply or coiled structures via overtwisting the yarns under tension (Figure 6) [78][33].  
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4.1. Coiled Configuration

Figure 6. SEM images showing CNT yarns with different configurations, including: (a) single CNT yarn, (b) two-ply CNT yarn, (c) coiled CNT yarn, and (d) two-ply coiled CNT yarn. Reprinted with permission from Ref. [78]. 2020, Wiley-WCH.

4.1. Coiled Configuration

Leveraging high flexibility of CNTs, Cao’s group [79][34] demonstrated the controlled fabrication of a yarn-derived spring-like CNT rope, where the loops were uniformly arranged. Inside each loop, the CNT bundles were observed to twist with slight alignment. These CNT ropes exhibited outstanding axial stretchability up to strain of 285% by loop unfolding and straightening during the stretching. Accordingly, the fracture toughness was also as high as 28.7 J/g. More importantly, the electrical conductivity of such CNT yarns was nearly 440 S/cm at the initial state and no degradation was noted after repeated loading cycles. Later on, Cao et al. [80][35] modified the spinning strategy and accurately controlled the loop position and number, and successfully fabricated a partial-helical structure. In contrast to the fully helical configuration, the partial one exhibited a better elasticity up to the strain of 25% as well as a linear resistance–strain relationship that is important for strain sensor applications.

4.2. Two-Ply Coiled and Supercoiled Configuration

In order to further improve the strain range and failure limit, Cao’s group [82][36] proposed a two-level hierarchical composite structure composed of double-helix CNT yarn segments. During the stretching process, a two-stage fracture behavior could be observed, which gave rise to higher tensile strain and effectively delayed the failure process. When the first yarn fractured at around 75% strain, the resistance of double-helix CNT yarns exhibited a sudden change; after that, it kept increasing steadily until the eventual failure at around 170% strain. Apparently, such a hierarchical design would prolong the service lifetime of the double-helix CNT yarn-based sensors. Moreover, the toughness could be significantly enhanced to guarantee the device stability.

4.3. Entangled Configuration

In addition to the coiled or double-helix yarns with relatively regular configurations, extreme overtwisting can produce entangled CNT yarns with a higher complexity [85][37]. With increasing overtwisting, the straight yarn gradually self-assembled into single-helix and then double-helix structures and finally developed random at multiple sites along the yarn axis while intertwining to generate amorphous configurations. An aggregation containing multiple self-interlocked twists were eventually generated. Such an entangled yarn structure was highly stable and could be stretched to 500% strain, with no change of resistance when disentangled. According to the stress–strain curves, multiple stress peaks were visible, corresponding to the resolving events of the double-helix segments within the entanglement. In addition to the large sensing range, the entangled coiled CNT yarn presented a cyclic stretching–releasing stability at 500% strain for 600 cycles.

5. Pattern Design of Fabric

A fabric structure is made by the interlacement of yarns or intermeshing of the loops to act as 2D flexible materials [86][38]. The engineering design of patterns applied to smart textiles are based on mature textile manufacturing and garment assembly routines, such as knitting, weaving, braiding, embroidery, and sewing. From a mechanical and structural perspective, different fabric structures are entirely dissimilar to each other. For instance, the yarns in woven pattern are nearly immobile and form a dense and stable structure, so that the fabric is almost inextensible with limited deformations in the yarn structure. By comparison, the interlocked loops in a knit pattern usually deform and slide readily, giving rise to a high stretchability with significant changes in small-scale structure [87,88][39][40]. From the application perspective, each fabric manufacturing technique has its own pros and cons in light of specific substrate properties, of which the selection depends on the end-use of the electronic textile.

5.1. Weaving

The most commonly used are the woven fabrics, consisting of yarn interlacements mutually in orthogonal directions [90][41]. The length, frequency, and distribution of interlacements in a woven fabric structure collectively decides its mechanical and functional characteristics. The above parameters are critical to the electrical performance as well since the consecutive points in interlacements influence the electrical contact [91][42]. According to the weave patterns, woven fabrics can be classified into plain weave, satin weave, twill weave, and so on, which can satisfy the needs of different applications.

5.2. Knitting

Different from weaving, knitted fabric is created by interlocking loops of neighboring threads [97][43]. In contrast to woven fabrics that require elastic yarn with accessible extension < 10%, knitted fabrics offer high elasticity and can develop large extensions as high as 100% even based on non-elastic yarns. As mentioned above, although the stretching deformation of knits that involves a flattening of yarn loop curvature usually quickly recovers, the further deformation beyond such elastic stage would lead to yarn sliding against each other, which cannot be recovered immediately. The relevant energy dissipation accounts for higher fracture toughness and impact resistance of knitted fabric compared to the woven fabric.

5.3. Non-Woven

A non-woven textile is produced by the physical or chemical bonding of fibers without any restriction. Both staple and filament fibers from different generic groups can be used to produce the non-woven fabrics (NWF). Apparently, the properties of NWF depend on the selection of fiber materials, the arrangement of fibers, and the bonding types and intensity. So far, some studies have demonstrated the application of NWF in wearable sensors, leveraging its specific functions including softness, resilience, flame retardancy, washability, and so forth.

6. Coating Technology

The simplest way to manufacture textile-based sensors is spinning conductive fibers and yarns, followed by weaving and knitting. Alternatively, coating of conductive materials on fiber, yarn, or fabric affords another facile and efficient approach for the large-scale production of electronic textiles. Commonly used coating methods include in situ polymerization, vapor-phase polymerization, dip coating, spray coating, vacuum filtration, and rod coating. Chemical polymerization is suitable for making fabrics coated with conductive polymers such as PPy and PEDOT:PSS. A common practice is soaking the fabric in a solution containing the monomer, the oxidant, and the dopant to initiate the polymerization process. For example, Lycra fabric-based stretchable and conductive sensor was prepared by coating a PPy layer using a chemical polymerization method [104][44]. In contrast, vapor-phase polymerization provides a higher homogeneity of coating than chemical polymerization. Therein, the textile substrate is immersed in a solution of oxidant and dopant and then exposed to the vapor of the monomer to form the polymer layer. As a result, the sensitivity is higher but the sensing range is lower compared to the fabric coated with the in situ polymerization method [105][45]. Dip coating represents a simple way to prepare the conductive textiles. Soaking a Spandex fabric in aqueous PEDOT:PSS dispersion was demonstrated to endow the fabric with an electric conductivity of 0.06 S/cm [106][46]. Multiple dip coating can further increase the conductivity up to 1.7 S/cm; however, over-coating would lead to the interfacial delamination failure which has a negative effect on the electric performance. Spray coating has an advantage of better thickness controllability compared to dip coating.  

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