*Review* **Review of Fiber- or Yarn-Based Wearable Resistive Strain Sensors: Structural Design, Fabrication Technologies and Applications**

**Fei Huang <sup>1</sup> , Jiyong Hu 1,2,\* and Xiong Yan <sup>1</sup>**


**Abstract:** Flexible textile strain sensors that can be directly integrated into clothing have attracted much attention due to their great potential in wearable human health monitoring systems and human–computer interactions. Fiber- or yarn-based strain sensors are promising candidate materials for flexible and wearable electronics due to their light weights, good stretchability, high intrinsic and structural flexibility, and flexible integrability. This article investigates representative conductive materials, traditional and novel preparation methods and the structural design of fiber- or yarn-based resistive strain sensors as well as the interconnection and encapsulation of sensing fibers or yarns. In addition, this review summarizes the effects of the conductive materials, preparation strategy and structures on the crucial sensing performance. Discussions will be presented regarding the applications of fiber- or yarn-based resistive strain sensors. Finally, this article summarizes the bottleneck of current fiber- or yarn-based resistive strain sensors in terms of conductive materials, fabrication techniques, integration and performance, as well as scientific understanding, and proposes future research directions.

**Keywords:** resistive strain sensor; fiber-based sensor; yarn-based sensor; interconnection

### **1. Introduction**

As the demand for the real-time monitoring of human motion and physiological information has recently grown, miniature and intelligent wearable electronic devices have been rapidly developed. Nowadays, a variety of wearable electronic products, such as electronic skins, smart watches and sports wristbands, are becoming an indispensable part of our lives and changing our behavior patterns and lifestyles. Strain sensors are important components of wearable electronic devices, which register and transmit changes in human motion parameters and physical health indicators through electrical signal responses [1]. However, the rigidity of traditional semiconductor or metal sensors [2,3] limits effective interactions with the curved surface of the human body, resulting in the distortion or inaccuracy of the collected electrical signals. In addition, they are not deformable enough to meet the large strain requirements of the human body [4], as the tensile exerts stress and strain on the sensors in the various deformation modes (tensile, compression, bending, shear and torsion) [5]. Therefore, it is necessary to develop flexible and stretchable strain sensors.

According to the substrate structure, flexible sensors are divided into 1D fiber or yarn strain sensors [6–9], 2D film [10–12], fiber mat [13,14] or fabric [15–18] strain sensors, and 3D aerogel [19–21] or foam [22,23] strain sensors. Compared with 2D or 3D flexible strain sensors, fiber-based and yarn-based sensors are smaller in size and more flexible to better fit the soft and curved human body, and thus detect subtle movements more accurately.

**Citation:** Huang, F.; Hu, J.; Yan, X. Review of Fiber- or Yarn-Based Wearable Resistive Strain Sensors: Structural Design, Fabrication Technologies and Applications. *Textiles* **2022**, *2*, 81–111. https:// doi.org/10.3390/textiles2010005

Academic Editor: Laurent Dufossé

Received: 29 December 2021 Accepted: 4 February 2022 Published: 8 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Additionally, the multihierarchy nature of the fiber or yarn structure (fiber-yarn-fabric garment) shows outstanding softness and stretchability, enabling it to deform appropriately when subjected to additional stress or its own gravity. A large number of fibers can also disperse the stress to avoid excessive damage to the device structure. Moreover, they are easy to interconnect with the components of wearable electronics and hide in fabrics with different complex structures. Therefore, fiber or yarn sensors meet the requirements of excellent flexibility, air permeability and comfort for wearable electronic devices due to their advantages of softness, portability, ductility and easy implantation into complex structures. They are more suitable for the development of a new generation of flexible strain sensors. The characteristics of flexible strain sensors with different substrates are summarized in Table 1.


**Table 1.** Features of flexible strain sensors with different structures.

According to the sensing mechanism, textile strain sensors are mainly categorized as resistive, capacitive [24–27], piezoelectric [28,29], inductive [30,31], triboelectric [32], or optical [33]. In terms of fiber- and yarn-based strain sensors, resistive and capacitive sensors are the most widely studied, as shown in Table 2. Resistive sensors realize strain detection by detecting changes in resistance. They have the advantages of a simple assembly process and easy signal identification, but their linearity is low. Capacitive sensors are composed of a dielectric layer and two electrode layers. The dielectric layer is sandwiched between the two electrode layers and deformed under the applied strain. This kind of strain sensor has a good linear response to strain, but it is easily affected by the environment, considering aspects such as temperature and humidity. This review mainly focuses on the research progress of resistive strain sensors.

There are two common methods for preparing 1D flexible resistive strain sensors. One is to prepare stretchable conductive composite fibers by a spinning method. The other is to coat conductive materials on the surface of a substrate to form stretchable conductive strain sensors by methods such as dip coating, in situ polymerization, layerby-layer assembly and so on. The performances of strain sensors (in terms of mechanical properties, workable strain range, sensitivity, repeat stability, response time, linearity, etc.) are affected by the conductive materials and elastic matrix, conductive network and yarn structure. Although plenty of studies have been conducted and great progress has been made in the field of flexible strain sensors, most of the reported sensors are far from being implemented in practical applications due to technical obstacles and challenges. At present, there is still a lack of comprehensive reviews covering the selection of conductive materials, the preparation and structural design of fiber and yarn strain sensors, and

interconnection packaging and applications. In this review, the latest research progress and various modification strategies of fiber and yarn resistive strain sensors are first introduced, and the emphasis is placed on the influence of conductive materials on the electrical performance of the sensor, as well as the influence of different fabrication technologies and structural design on sensing performance. Moreover, the integration strategy and application prospects of existing fiber and yarn strain sensors are discussed. Finally, the limitations and prospects of fiber- and yarn-based strain sensors in terms of performances and scientific understanding are summarized and analyzed.


**Table 2.** Features of resistive and capacitive strain sensors.

#### **2. Conductive Materials**

In terms of the resistive flexible strain sensors, the conductivity of conductive materials and the structure of the conductive network not only determine their initial resistances, but also affect the range of resistance variation and the working strain. On the one hand, if the initial resistance of the strain sensor is too large, its resistance will easily increase beyond the range of the test instrument under large strain, which limits the application range of the strain sensor [34]. Additionally, it will cause an excessive static load and large power consumption. Therefore, the initial resistance range of the sensor should preferably not exceed megohms. On the other hand, if the initial resistance of the strain sensor is too small, it can easily be affected by other external resistances, such as interconnected contact resistance, which leads to a lower sensitivity and inaccurate measurement. Apart from the electrical resistance, the stability and the compatibility with the elastic matrix should also be considered when conductive materials are selected. At present, the common conductive materials include intrinsic conducting polymers, such as polypyrrole (PPy) [35], polyaniline (PANI), polythiophene (PTh) and PEDOT:PSS [36]; advanced carbon-based materials, such as carbon black (CB) [37], carbon nanotubes (CNTs) [38–40] and graphene (Gr) [4,41]; metal materials, such as gold (Au) [42], silver (Ag) [43–45], copper (Cu) [46,47] and liquid alloys [48]; and a new transition metal carbon/nitride 2D nano-layered material, MXene [16,49]. The characteristics of each conductive material are summarized in Table 3.

### *2.1. Conducting Polymer*

Conducting polymers have a conjugated long-chain structure and the delocalized π electrons on the double bond migrate to the molecular chain to form a current, and thus the material exhibits conductivity. Due to the general solubility of their corresponding monomers, conducting polymers can be formed in situ in a soft polymer matrix and are flexible in processing and compatible with elastomeric polymers. However, their charge/discharge stability and ramp voltage are low, since the electron transfer of the conducting polymers are controlled by the doping concentration (10–50%). The conductivity of conducting polymers is much lower than that of metal. Furthermore, conducting polymers are more brittle and rigid than linear aliphatic polymers, because their π-conjugated main chain structure is composed of olefin bonds or aromaticity [55]. Seyedin et al. reported PU/PEDOT: PSS elastomeric composite fibers by a wet-spinning method and their resistance shifted towards higher resistances with the increase in the stretching–releasing cycle period [56]. In actual applications, the stability of conducting polymers is not good enough, especially doping materials when considering air oxidation stability. Therefore, the combination of conducting polymers and carbon-based nanomaterials as conductive sensing materials is another used method [57]. For example, Li et al. proposed a wearable strain sensor by using thermoplastic polyurethane fibers as the core support, aligned and interconnected carbon nanotubes in the sub-outer layer as conductive filaments and the outer layer of PPy coating as the cladding layer [58]. It has a wide detectable range (from 0.1% to 50% tensile strain) and performs a multichannel detection of deformation capabilities (tension, bending and torsion). Wu et al. prepared a PEDOT: PSS/CNT/TPU composite fiber strain sensor by dip coating. In this layered microstructure, PEDOT: PSS is used as a sensing material to reduce the initial resistance and improve the sensitivity of the sensor, while the CNT aggregate acts as a conductive bridge to ensure conductivity at large strains, providing a larger sensing range for the sensor [59].


**Table 3.** Common conductive materials and their characteristics [4,50–54].

#### *2.2. Carbon-Based Materials*

Carbon-based materials with excellent conductivity and multidimensional structures are suitable for manufacturing large-strain, high-sensitivity flexible strain sensors. Seyedin et al. used a variety of conductive fillers (such as spherical CB, rod-shaped SWC-NTs and chemically converted Gr sheets) to prepare different conductive fibers by wetspinning technology [60]. It was found that the electrical and mechanical properties of composite fibers depend on the length and the length-diameter ratio of fillers as well as the interaction between the fillers and the elastomer. Overall, CB has a lower cost and better dispersion than CNTs and graphene. Doping CB in CNTs and Gr can improve sensor performance while reducing manufacturing costs. For example, Zhang et al. prepared a simple and low-cost strain sensor by sequentially coating CNTs and CB on the PU yarn by a layer-by-layer assembly method [61]. Under small strain, the conductive network of the CB layer breaks, while the conductive network of the CNT layer does not break until a large enough strain is reached. The CB layer and CNT layer rupture successively with strain, so that the sensor exhibits super stretchability and a large linear range (15–150%). Compared with CNTs with a high length-diameter ratio, 0D CB has a higher degree of freedom of deformation, so the point-to-point conductive network will be destroyed more obviously during the stretching process, giving the strain sensor better sensitivity [62–64].

CNTs tend to aggregate and entangle with each other when mixed with polymers due to the high length-diameter ratio and large specific surface area. Consequently, it is difficult for them to uniformly disperse in the polymer matrix, not allowing for the excellent conductivity they show in composite fibers. In addition to using dispersants, CNTs can also be modified with polar functional groups, such as carboxyl (-COOH) and hydroxyl (-OH), to improve the dispersibility and adhesion of CNTs in the matrix. However, the graphitized structure of CNTs will be destroyed, resulting in a decrease in electrical conductivity [65], and the same is true for reduced graphene oxide. Compared with CNT sensors, graphenebased strain sensors generally have a higher sensitivity and lower sensing strain due to their small size, sheet-like structure, which is easy to slide, and poor stretching ability. Therefore, appropriate materials should be selected according to the actual requirements in terms of prepared strain sensors. Additionally, the viscoelasticity of the stretchable substrate and the fracture of the carbon material will cause the hysteresis of the sensor under large strain [66], leading to a low sensitivity and poor repeatability and stability. In summary, it is still a challenge to manufacture flexible carbon-based strain sensors with good sensitivity and a broad strain range at low cost.
