From flexible electronics technology in the era of IoT and artificial intelligence toward future implanted body sensor networks

Flexible electronics technology dramatically changes the capability of sensors, which allows us to detect human biological signals not only on the skin but also inside the human body. Wearable sensors that stick to the skin surface can detect various biomechanical movements, biological signals, and byproducts such as useful elements from sweat and body temperature. On the other hand, implantable sensors directly or indirectly contact with biological components inside the body, such as tissue, organs, or muscles, to support or treat bodily functions or problems. With the development of these advanced sensors, we can live together with a huge number of sensors in the future. Toward body sensor networks that can be fully implanted in the future, sustainable energy sources that support the operation of sensors as well as the development of materials that enable long-term use inside the body remain challenges. In this review, we first summarize various state-of-the-art sensors in terms of flexible and wearable platforms. Additionally, we review the recent research trends of energy harvesters in mechanical and thermal energy conversion into useful electricity for the operation of the sensors. Furthermore, we cover recent studies in the aspect of materials for implantable sensors. Finally, we discuss future direction of the sensors that may enable implanted body sensor networks in the body.