Fig. 1. Internet of things (IOT) based on sensor network^[1].

Fig. 2. (a) Schematic and (b) piezoelectric output response in different gas environments of ZnO NWs-PENG based self-driven gas sensor^[22]; (c) schematic and (d) output voltage at different concentrations of ethanol gas of olfactory electronic skin based on PANI/ PTFE/PANI sandwich nanostructure^[28].

Fig. 3. Self-driven H₂S gas sensor based on flexible PENG^[23]: (a) The sensing mechanism of vulcanization reaction; (b) response process; (c) recovery process.

Fig. 4. Self-driven pressure sensors based on flexible PENG: (a) The weight sensor based on the OPNG can distinguish the subjects with different weights^[37]; (b) the output response of vibration sensor based on the OPNG in different application conditions^[37]; (c) self-powered flexible pressure sensor based on P(VDF-TrFE) nanowire arrays can be used for acoustic detection^[43]; (d) sound pressure sensor based on P(VDF-TrFE)/BaTiO₃ piezoelectric reinforced nanocomposite microcolumn arrays can be used to monitor sound pressure changes^[44].

Fig. 5. [in Chinese]

Fig. 5. Self-driven biosensors based on flexible PENG are used to monitor (a) finger muscle movement, breathing, heartbeat pulses, and low-intensity sound waves^[43], (b) four different breathing modes: deep breathing, gasping, dyspnea, and normal breathing^[44], (c) human physiological activities such as blinking, vocalization, arm bending, radial artery pulse / heart beating, etc^[54].

Fig. 5. [in Chinese]

Fig. 6. E-skin (biological) sensors based on flexible PENG: (a)–(c) E-skin tactile sensor based on NiO@SiO₂/PVDF nanocomposite film^[57]; (d) self-powered E-skin based on the coupled process of piezoelectric-enzyme reaction can be used to detect glucose levels in body fluids^[11].

Fig. 7. Internal integrated SCPC with "sandwich" structure: (a) An integrated SCPC based on PVDF nano-film^[9]; (b) flexible SCPC based on PVDF-graphene nanosheets^[66]; (c) CuO / PVDF nanocomposite film based novel SCPC^[67]; (d) SCPC based on PVDF-PZT nanocomposite film^[68]; (e) mesoporous PVDF nano-film can be used as piezoelectric separator of SCPC^[69]; (f) all-solid-state flexible SCPC based on mesoporous PVDF-LiPF₆ film^[70].

Fig. 8. Microscopic electrochemical process of porous PVDF nano-film based SCPC^[69]: (a) In the initial stage, the lithium ion concentration in the electrolyte is in dynamic equilibrium; (b) when the external force F is applied to SCPC, the piezoelectric material deforms and a potential difference is generated inside the SCPC; (c) the piezoelectric electric field drives Li⁺ and electrons to conduct between the electrodes, promoting the electrode to undergo REDOX reaction; (d) the interior of SCPC gradually reaches dynamic equilibrium again; (e) when the external forceF is released, Li⁺ and electrons conduct in reverse; (f) the interior of SCPC tends to be stable and eventually reaches a dynamic equilibrium state.

Table 2. Summary of flexible substrate materials in SCPC.

Table 3. Summary of flexible electrode materials in SCPC.

Table 4. Summary of flexible piezoelectric materials in SCPC.

Table 5. Examples of component materials and output properties of flexible SCPC.