Energy Harvesting for Self-Powered Microsystems: A Critical Review of Materials, Power Management, and System Integration

The relentless proliferation of the Internet of Things (IoT), wearable bioelectronics, and cyber-physical infrastructure has rendered the conventional electrochemical battery the single most prohibitive bottleneck to long-term, maintenance-free autonomous microsystems. Energy harvesting, i.e. the conversion of ambient mechanical, thermal, and radiative energy into usable electrical power, has consequently emerged as a transformative paradigm to realize perpetual, battery-independent operation. This review critically synthesizes the most significant advances in energy harvesting technologies over the recent period, with a focus on triboelectric nanogenerators (TENGs), piezoelectric and pyroelectric transducers, indoor photovoltaics, radio-frequency (RF) rectennas, and their multi-source hybrid integrations. We highlight paradigm-shifting breakthroughs, including liquid-solid TENGs that eliminate mechanical wear, nonlinear piezoelectric oscillators that broaden operational bandwidth by over 300%, machine-learning-accelerated material discovery for high charge-density dielectrics, and multiband and broadband RF harvesting enabled by metamaterial architectures. Crucially, we move beyond conventional materials-centric narratives to critically interrogate the "unseen" system-level bottlenecks: ultra-low-voltage cold-start power management integrated circuits (PMICs), the impedance-matching challenges of hybrid energy sources, and the persistent degradation of micro-supercapacitors and thin-film batteries under realistic field conditions.