Various electronic devices with high performance and advanced structures have been effectively integrated into a functional electronic system owing to their miniaturization. In particular, extensive research on miniaturized devices that gradually lose their functionality by dissolution in water/biofluid has been recently conducted, which would accelerate the development of eco-friendly as well as bio-implantable devices for transient electronics. To satisfy the technological demand for continuous powering of implanted devices in the human body regardless of the time and place, the development of high performance wireless power generation and energy storage devices is required. Supercapacitors, so-called next-generation energy storage devices, can be used in the event of unexpected power-off owing to their superior power density and long cycling lifetimes compared to batteries. Therefore, the application of transient supercapacitors as devices essential to integrated electronic circuits in the human body is expected to play an important role in the advancement of independent and sustainable transient electronics.

Here, we introduce the fabrication and applications of fully biodegradable microsupercapacitors (MSCs) consisting of interdigitated water-soluble metal (W, Fe, and Mo) electrodes, a hydrogel electrolyte (agarose gel), and a biodegradable poly(lactic-co-glycolic acid) (PLGA) substrate, encapsulated with thin films of PLGA and polyanhydride. In particular, this unusual energy storage device based on 300 nm thick Mo electrode exhibits electrochemical performance with a areal capacitance of 1.6 mF/cm², energy density of 0.14 µWh/cm², and power density of 1.0 mW/cm². Such high performance is attributed to the pseudocapacitance originating from the metal-oxide film generated by electrochemical corrosion at the interface between the metal electrode and the water-containing agarose gel electrolyte, during repeated charge/discharge cycles. The oxidation states of those oxide layers are confirmed to be WO₃, Fe₂O₃, and MoO₃, respectively, via taking scanning electron microscope images and x-ray photoelectron spectra. The dissolution kinetics of all components of the MSCs are measured in phosphate-buffered saline at body temperature and an encapsulation strategy is suggested to control the functional lifetimes of the MSCs. Finally, we demonstrate the potential applications of the fabricated biodegradable MSCs as transient energy storage devices by illuminating a light-emitting diode and as a transient capacitor in integrated circuits for wireless power transmission.