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Research Progress on Conductive Fibers for Wearable Devices
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Research Progress on Conductive Fibers for Wearable Devices

With the continuous progress of electronic technology and the popularization of intelligence, wearable devices, by virtue of their close contact with the human body, have become a seamless bridge between the human body and the environment, and have great potential for application in the next generation of intelligent electronic devices. Considering the comfort of human body wearing and comparing with the traditional rigid materials, the research of flexible electronic materials in wearable devices is getting more and more attention.

Conductive fibers, as an important medium for transmitting electrical signals in fabrics, play a great role in the field of flexible smart textiles and smart wearable devices. There are various techniques for its preparation, such as direct processing of a single conductive material, fiber coating with conductive substances, blending conductive substances with fibers and so on.

Single material conductive fibers

1. Metal nanowires

The main metal nanowires that have been studied more are copper nanowires (CuNW) and silver nanowires (AgNW). Due to the high density of copper, high material cost and easy oxidation when exposed to air, the wide range of applications is limited. In contrast, silver nanowires have both excellent properties and the oxide is still electrically conductive, which is a widely used material. Polyol method is the most common and successful method for the preparation of silver nanowires.SIM et al. synthesized silver nanowires with high conductivity and low resistivity by using polyol method without adding capping agent. However, the cost of metallic silver is high, and how to control the cost of nanometallic wires needs to be continued research.

2. Electrically conductive polymers

Common intrinsic conductive polymer materials include polyaniline (PANI), polypyrrole (PPy), etc., which are low-cost and biodegradable, and provide new ideas for the green development of textiles. However, in practical applications, the long-term stability of the conductive effect of single-type conductive polymer materials is poor, which is not conducive to long-term use and large-scale production.
Research results show that metal fibers due to greater bending stiffness, poor spinnability; polymer class conductive fibers are equally unable to take into account the conductivity and tensile properties, so these materials usually can not be used alone, composite conductive fibers came into being.

Composite Conductive Fiber

Composite conductive fibers are mainly composed of insulating matrix and conductive substances, there will be conductive substances coated in the outer layer of fibers, or conductive substances filled with fibers or elastic polymers inside, the conductive substances and fibers blended spinning and other methods. Relative to a single material, composite conductive fiber conductivity, fastness and tensile deformation are better, and can be blended or interwoven with other fibers.

1. Coated Composite Conductive Fiber

Coated conductive fiber refers to the use of dip-coating, electroplating or chemical plating to make conductive materials coated on the surface of the fiber matrix. Zhan Lingzhong chemically plated aramid with metallic nickel and silver to improve the bonding strength and densification of its surface, and the dispersion uniformity of the coating agent needs to be taken into account, otherwise it will lead to uneven resistance values and make the conductivity worse.MONTAZERI-AN et al. first impregnated graphene on the surface of spandex to obtain flexible conductive fibers of SpX/GnP, and then wrapped silica gel around it by the wrapping method, forming wearable SpX/GnP/SR sensors. Wearable SpX/GnP/SR sensors were formed, and the resulting spandex filaments not only had high electrical conductivity, but also had excellent tensile strength due to the coating of silicone rubber. Overall, coating the conductive substance on the base fiber is easy to operate and the conductivity can be enhanced. However, there are some defects, such as conductive fibers after repeated rubbing and washing, friction, the surface of the conductive coating will be shed, resulting in conductivity and the use of performance is affected.

2. Spun Composite Conductive Materials

Spun composite conductive fibers are produced by solution spinning, melt spinning, and electrostatic spinning of conductive substances and insulating matrix. Wang Zhihui et al. used coaxial wet spinning technology to continuously prepare a composite skin-core conductive fiber with thermoplastic polyester elastomer (TPE) as the skin layer and PEDOT∶PSS as the core layer. The fiber not only has excellent mechanical and electrical properties, and the conductive layer is in the core layer, which can make the service life of the conductive fibers increase dramatically.SUN et al. proposed a coaxial wet-spinning method for the continuous preparation of superelastic LM (Liquid Metal) sheath-core microfibers with high and stable electrical conductivity, which are still highly conductive even when stretched up to 10 times the length of the ultrafine fibers, and the electrical resistance hardly changes under high strain conditions.
Compared with the coating method, the conductive fiber material produced by the spinning system has more stable conductivity. However, this technology is currently applied to synthetic fibers, and in the spinning process, the addition of conductive substances on the mechanical properties of fibers and other effects to be studied and investigated.

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