Abstract:

Significance Wearable flexible strain sensors can facilitate all-round monitoring of human activities and thus have broad application prospects in fields such as healthcare, public health and human-computer interaction. Compared with traditional embedded rigid strain sensors, textile structures become an ideal structural platform for flexible strain sensors with the advantages of flexibility, comfort and hyperbolic effect. However, since the raw materials used in traditional textiles generally have electrical insulating properties, they should be modified into electrically sensitive materials before being used to construct flexible strain sensors. In addition, the textile structure design is targeted on the basis of the strain sensing mechanism. Although there are some basic researches on the application of textile technology in the field of smart wearables, it is still in its infancy in the actual market application. In order to further promote the development and application of smart wearable textiles and make full use of the textile structural advantages, this paper summarizes the design concepts and preparation methods of flexible textile-based strain sensors based on the current research progress. The paper is organized on progress made in fibers, yarns and fabrics.

Progress For fiber-based strain sensors, integrating fibers with electrically sensitive materials to achieve conductive fiber preparation is the primary issue which needs to be addressed in the preparation of strain sensors. Currently, there are three mainstream technologies to prepare fiber materials with good electrical conductivity which are fiber spinning, fiber surface coating and carbonization modification of fibers. Compared to fiber-based strain sensors, yarn-based sensors pay more attention to the macroscopic structural design to assemble multiple functional materials, achieving multi-dimensional upgrading of sensing performance. Yarn spinning technology, on the other hand, is an effective way to integrate functional fibers into yarns to achieve a good combination of structure and function. Spiral yarn and core-spun yarn are two commonly used yarn structures in strain sensors. Different fabric structures have their own advantages and disadvantages for creating strain sensors. Knitted fabrics have high stretchability, which can meet the size change ability required for strain sensors, but the structural stability is relatively poor. In comparison, woven fabrics have stable structure but the deformation is limited. The most common method for preparing strain sensors with braided structure is to use elastic yarns as core and conductive yarns as the braided sheath. Nonwoven structures provide an ideal template for the deposition of conductive materials, which can effectively construct three-dimensional interconnected conductive paths. The disadvantage however is that the strength is low and thus nonwoven fabrics are rarely used as a separate substrate for strain sensor. Finally, embedding flexible conductive yarn into textiles through the sewing process is also one way to prepare textile-based strain sensors. In principle, it can be embedded anywhere in clothing, providing preparation flexibility and potentially reducing costs.

Conclusion and Prospect Although significant progress has been made in the research of textile-based strain sensors, there are still some key issues that need to be further investigated in terms of structural design, mechanism analysis, and performance optimization before academic research can be used for practical applications. 1) In order to meet the requirements of high sensitivity and large strain range of sensors, the design concept is that any slight deformation will cause changes in the conductive network inside the material, and the conductive network is always connected under different strain levels. At the same time, the interfacial properties of the conductive filler and the substrate need to be improved to ensure the repeatability and stability of the sensor. 2)The inherent insulation, viscoelasticity, and complexity of the multi-scale structure make the mechanism study of textile-based strain sensors very complex. Establishing a theoretical relationship between the multi-scale structure and the sensing performance is a necessary foundation for optimization design and performance improvement of textile-based strain sensors. 3) The performance improvement of materials in terms of washability, comfort, and adaptability with the human body is an important research direction. It is also a key issue to fully leverage the structural and performance advantages of textile materials, and consequently promoting the practical application of textile sensors in the field of wearable electronics.

Key words: flexible strain sensor, textile structure, fiber, yarn, fabric, smart textile