Fig. 1. Design concept of self-powered flexible biomedical sensor: (a) Active biomedical sensors directly collect various physiological signals and convert them into electrical signals; (b) energy-type biomedical sensors collect energy and provide energy for commercial sensors.

Fig. 2. Working mechanism of piezoelectric nanogenerator^[34]: (a) Crystal model of ZnO; (b) piezoelectric potential of ZnO nanowire; (c) finite element analysis of piezoelectric potential of ZnO nanowires; (d) mechanism of piezoelectric nanogenerator.

Fig. 3. Four working modes of triboelectric nanogenerator^[36]: (a) Vertical contact separation mode; (b) lateral sliding mode; (c) single-electrode mode; (d) freestanding triboelectric-layer mode.

Fig. 4. Working mechanism of thermoelectric generator based on spin Seebeck effect^[41].

Fig. 5. Self-powered flexible respiratory sensor: (a) Wearable self-powered active sensor for respiration monitoring based on a flexible piezoelectric nanogenerator^[27]; (b) wearable respiration sensor based on a pyroelectric nanogenerator integrated with an N95 respira-tor^[46]; (c) blow-driven triboelectric nanogenerator as an active alcohol breath analyzer^[47].

Fig. 6. Self-powered flexible pulse sensor: (a) Self-powered, one-stop, and multifunctional implantable triboelectric active sensor for real-time biomedical monitoring^[49]; (b) flexible self-powered ultrasensitive pulse sensor based on triboelectric effect^[50]; (c) self-powered ultra-flexible biosensor based on nanograting-patterned organic photovoltaics^[51].

Fig. 7. Self-powered flexible temperature sensor. (a) Self-powered temperature sensor based on a PyNG^[52]; (b) self-powered temperature-pressure dual-parameter sensor fabricated by organic thermoelectric materials^[53]; (c) wireless temperature sensor system based on hybridized nanogenerator^[8].

Fig. 8. Self-powered flexible artificial sense organ: (a) Self-powered triboelectric auditory sensor for social robotics and hearing aids^[59]; (b) self-powered triboelectric tactile sensor with metallized nanofibers for wearable electronics^[60]; (c) an artificial triboelectricity-brain-behavior closed loop for intelligent olfactory substitution^[61].

Fig. 9. Core research directions of self-powered flexible biomedical sensor: (a) Multifunctional sensing system^[63]; (b) wireless signal transmission^[65]; (c) flexible man-machine interface^[66].