fig2

Figure 2. Main concern of self-powered IoT sensors. Materials of sensors contain flexible materials, Reproduced with permission^[38], Copyright 2021, Springer Nature, nanomaterials, Reproduced with permission^[106], Copyright 2019, IOP Publishing Ltd, and degradable materials, Reproduced with permission^[221]. Copyright 2022, Elsevier Ltd; Sensor operation modes contain physical sensing, Reproduced with permission^[222]. Copyright 2018, American Chemical Society, chemical sensing Reproduced with permission^[223], Copyright 2022, John Wiley & Sons, Inc., and combined sensing, Reproduced with permission^[149], Copyright 2020, American Chemical Society; Techniques in sensors contain TENG Reproduced with permission^[224], Copyright 2022, John Wiley & Sons, Inc., PENG, Reproduced with permission^[192], Copyright 2023, Elsevier Ltd, and machine learning Reproduced with permission^[225], Copyright 2022, John Wiley & Sons, Inc. PENG: Piezoelectric nanogenerator; TENG: triboelectric nanogenerator.