Inkjet-Printing Self-Powered Triboelectric Sensor on Fiber Substrate for Wearable Healthcare and Human-Interactive Applications

<br/>With the development of IoTs, big data, and artificial intelligence, various wearable electronics have been rapidly expanded, such as wearable smart watch/bracelet, biomedical applications, smart sportswear, and virtual reality. However, current wearable electronic devices inevitably require power sources and have to be charged or replaced frequently, which will limit the applications of wearable electronics. Recently, the textile TENG which integrates self-powered triboelectric sensors with daily clothes, brings a novel insight into wearable electronics and improves personal intelligence in the era of the IoTs. For example, Meng <i>et. al</i> developed a triboelectric textile sensing system for human pulse wave monitoring (<b><i>Matter</i></b>, 2020, 2, 794-804). Zhou <i>et. al</i> developed a machine-learning-assisted stretchable triboelectric strain sensor array for sign-to-speech translation (<b><i>Nature Electronics</i></b>, 2020, 3, 571–578). In this respect, 1D-shaped fiber TENGs have become a research hotspot due to the advantages of light weight, small size, good air permeability, compatibility with daily clothes, and adaptation to frequent mechanical deformations in daily life. Nevertheless, most current fiber-based TENGs are based on either a mixing or coating method to integrate an elastic fiber matrix with conductive electrodes, which always struggles to achieve the high-precision and customized fabrication of the triboelectric sensors on the small-diameter and large-curvature fiber surface.<br/><br/>In this work, we report an inkjet-printed fiber-based triboelectric strain sensor (IPFTS) for wearable applications. The helical-structure Ag ink was successfully inkjet-printed on the stretchable PU fiber surface with controllable thickness and turn number, and then encapsulated by Eco-Flex elastomer. The printable fiber diameter is as low as 500 μm with a large curvature of 4000 m^-1. Benefiting from the structural advantages, the SPFTS presents a large strain range (0-250%), excellent linear sensitivity (0.47), fast response (70 ms), and great mechanical stability (no decay was observed during the 40-minutes reciprocating 100% stretching-releasing process). Several system-level wearable health monitoring and human-interactive applications are demonstrated through our IPFTS, including heartbeat signal detection for arrhythmia screening, respiration signal detection for assessing sleep status, motion monitoring, and sign language gesture recognition. Benefiting from these features, the IPFTS shows huge application potential in wearable electronics, human-machine interfaces, and artificial intelligence.