Battery-free antenna sticker for wireless sensing joint rotation

Bio-integrated electronic devices are skin-mounted accurate tools for acquiring biological signals. The knee joint is one of the most important joints for human movement, playing a crucial role in functions such as walking, standing, sitting and containing important physiological health information about orthopedic disease diagnosis, prevention, and rehabilitation. However, it is still challenging to precisely and directly monitor joint rotation via skin-mounted bio-integrated electronic devices because joints are hidden inside the skin. In this study, we design a reusable, sustainable, battery-free, wireless, and flexible antenna sticker to adhere to the skin that can directly measure knee joint rotation degree. The antenna sticker is adhered to the lower leg and powered by electromagnetic radiation from a radio frequency identification (RFID) reader. When rotating the knee, the distance between the antenna sticker and the reader changes, leading to an alteration in the received signal strength indicator (RSSI). Meanwhile, the 3D optical motion capture system collects joint rotation data as a “gold standard”. After matching two series of real-time data using the Gaussian method, one RSSI can represent one specific joint rotation degree. The antenna sticker is stable at a variety of humidity and temperatures. The antenna sticker shows great potential to sense joint rotation and human motions. The scientific insights and engineering principles revealed in this antenna sticker will be applicable to a broad range of applications including skin-mounted epidermal electronics and wearable devices.