Surging demand for minimally invasive electronics has given rise to the development of various bioelectronics systems for monitoring biological signals. These bioelectronics have now been extended to therapeutic use and are being extensively deployed clinically with suitable adaptation to 3D dynamically curved bio-interfaces. Conventional implantable power units, predominantly based on primary (single-use) batteries, are usually rigid and bulky, posing risks of immunological rejection and incision, while also failing to provide sufficient output power for long-term use. Therefore, to enable the conformable integration of advanced electronics with curved bio-interfaces and ensure the sustained operation of bioelectronic systems, there is an urgent need for a miniaturised power source system that offers enhanced performance and good long-term stability. Rechargeable, flexible energy storage devices integrated with wireless energy harvesting technology can provide a viable solution to enable sustainable stable bioelectronics systems.

Herein, research approaches for rational design and versatile fabrication of minimally invasive power sources with good flexibility and long-term stability for implantable electronics were proposed. Firstly, an injectable flexible needle-like micro supercapacitor was designed and fabricated, it achieved a high energy density of 32.56 µWh cm-2 at a power density of 45.05 µW cm-2, and long-term cycling stability (capacitance retention of 94 % after 10000 cycles) which can power micro-LED and sensors. The micro supercapacitor utilised activated carbon (AC) mixed with multiwall carbon nanotubes (MWCNTs) as electrode materials and increased the capacitance to reach 289.4 mF cm-2 at 0.1 mA cm-2, and employed water-in-salt electrolyte to extend the working voltage from 1V to 1.8V. Therefore, it can provide a high and stable power output and be integrated with functional electronics such as micro-LEDs and pressure sensors and could be injected into the targeted tissue areas to realise biomedical purposes with miniaturised incisions.

In addition, to further prolong the service time and avoid the pain caused by the secondary surgery for implantation and replacement procedure of bioelectronic system, a monolithic optoelectronic system integrated with sustainable wireless rechargeable power sources and programmable control was proposed, the implantable programmable micro-LED integrated with flexible wireless rechargeable micro supercapacitor on one piece can be applied for optogenetic therapy. Benefiting from photolithography microfabrication, the micro-coil and micro-supercapacitors were integrated on a flexible thin film substrate with the micro-LED and rectifying circuit. It can harvest wireless power to charge the micro-supercapacitors with high specific capacitance (382 mF cm-2 at 0.2 mA cm-2), high energy density (29.84 µWh cm-2 at a power density of 74.9 µW cm-2) and good cycling stability (capacitance retention of 91%) after 10000 cycles. As an application, the system with a voltage output of 3V can power the micro-LED to continuously work wirelessly under the subcutaneous area. Furthermore, after being connected with integrated circuit chips, the micro-LED could achieve programmable pulse light (with the frequency of 3Hz) for specific optogenetic purposes powered by the flexible wireless rechargeable system, which was proven to be biocompatible with in vitro cytotoxicity and in vivo animal model test and stably functional when immersed in PBS (phosphate buffer saline) solution for a long period.

These research works provide promising research approaches for conformal miniaturised power source systems with enhanced performance, good flexibility, long-term stability and monolithic all-in-one design for implantable biomedical electronics. This thesis aims to develop a new paradigm and prototype of minimally invasive implantation and sustained operation of power units for advanced implantable electronic systems. By integrating these power systems with other advanced functional electronics, a new generation of wirelessly powered, fully implantable and soft electronics will bring about a myriad of substantial advancements in sustained disease-diagnosis capabilities and point-of-care. Future work will focus on optimising material properties and exploring further integration with multifunctional bioelectronics for broader clinical applications.