Fiber materials are highly desirable for wearable electronics that are expected to be flexible and stretchable. Compared with rigid and planar electronic devices, fiber-based wearable electronics provide significant advantages in terms of flexibility, stretchability and breathability, and they are considered as the pioneers in the new generation of soft wearables. The convergence of textile science, electronic engineering and nanotechnology has made it feasible to build electronic functions on fibers and maintain them during wear. Over the last few years, fiber-shaped wearable electronics with desired designability and integration features have been intensively explored and developed. As an indispensable part and cornerstone of flexible wearable devices, fibers are of great significance. Herein, the research progress of advanced fiber materials is reviewed, which mainly includes various material preparations, fabrication technologies and representative studies on different wearable applications. Finally, key challenges and future directions of fiber materials and wearable electronics are examined along with an analysis of possible solutions. Graphical abstract

Polypyrrole (Ppy) hydrogels are a promising new avenue for developing cheap wearable electronics and biotechnology. In particular, the use of conducting polymer hydrogels can impart elasticity and a high specific surface area, leading to great potential for sensors, cell growth scaffolds, and energy storage. However, their significantly low conductivity (compared to Ppy films and carbon or metallic microstructures), hydrophobicity, and low adhesiveness mean that they are currently unsuitable for most biological and wearable applications. Here, we show that by electropolymerizing a covalently bonded polydopamine (PDA) phase within polypyrrole hydrogels, we increased the conductivity by 2720% and adhesion by 2140% compared to pure polypyrrole hydrogels. Pyrrole monomers provided π-bond stabilization and prevented a π-stacked, auto-oxidized layer of PDA from forming. Instead, through potentiodynamic polarization of polypyrrole gels after dopamine incorporation, we produced covalently bonded 5,6-dihydroxyindole, producing an additional phase of conjugated polymer that interacted with the polypyrrole through noncovalent bonding. The PDA’s unoxidized catechol groups also led to increased hydrophilicity and adhesiveness of the hydrogels. These results are a further step toward the realization of fully polymer wearable electronics made with a simple, scalable technique, thereby removing the need for expensive, biologically unfriendly metals or carbon structures.

Flexibility plays a vital role in wearable electronics. Repeated bending often leads to the dramatic decrease of conductivity because of the numerous microcracks formed in the metal coating layer, which is undesirable for flexible conductors. Herein, conductive textile‐based tactile sensors and metal‐coated polyurethane sponge‐based bending sensors with superior flexibility for monitoring human touch and arm motions are proposed, respectively. Tannic acid, a traditional mordant, is introduced to attach to various flexible substrates, providing a perfect platform for catalyst absorbing and subsequent electroless deposition (ELD). By understanding the nucleation, growth, and structure of electroless metal deposits, the surface morphology of metal nanoparticles can be controlled in nanoscale with simple variation of the plating time. When the electroless plating time is 20 min, the normalized resistance (R/R0) of as‐made conductive fibers is only 1.6, which is much lower than a 60 min ELD sample at the same conditions (R/R0 ≈ 5). This is because a large number of unfilled gaps between nanoparticles prevent metal films from cracking under bending. Importantly, the Kelvin problem is relevant to deposited conductive coatings because metallic cells have a honeycomb‐like structure, which is a rationale to explain the relationships of conductivity and flexibility.

Intelligent human-machine interfaces (HMIs) integrated wearable electronics are essential to promote the Internet of Things (IoT). Herein, a curcumin-assisted electroless deposition technology is developed for the first time to achieve stretchable strain sensing yarns (SSSYs) with high conductivity (0.2 Ω cm −1) and ultralight weight (1.5 mg cm −1). The isotropically deposited structural yarns can bear high uniaxial elongation (>>1100%) and still retain low resistivity after 5000 continuous stretching-releasing cycles under 50% strain. Apart from the high flexibility enabled by helical loaded structure, a precise strain sensing function can be facilitated under external forces with metal-coated conductive layers. Based on the mechanics analysis, the strain sensing responses are scaled with the dependences on structural variables and show good agreements with the experimental results. The application of interfacial enhanced yarns as wearable logic HMIs to remotely control the robotic hand and manipulate the color switching of light on the basis of gesture recognition is demonstrated. It is hoped that the SSSYs strategy can shed an extra light in future HMIs development and incoming IoT and artificial intelligence technologies.