Efficient Four-Coil Wireless Power Transfer for Deep Brain Stimulation

This paper demonstrates a closed-loop wireless power transfer (WPT) system for deep brain stimulation-implanted biomedical applications. A novel two-layer PCB FR4 coil design is proposed as a second coil for enlarging the WPT efficiency. High-efficiency implantable WPT technology was achieved by applying an inner dual-layer printed spiral coil (PSC) and outer helical coil and this coordinated with a T-type impedance-matching network. To reduce the loading effect, which lowers Q factor in implanted devices, four-coil WPT systems are recommended compared with two- or three-coil WPT systems. A practical four-coil system was implemented for verification, for which a dual-layer printed circuit board composed of a PSC, with a load loop on the other side of FR4, was designed and implemented around the system on a chip. PSCs for biomedical implants can be optimized by designing the geometric parameters. The implant coil occupies an area measuring 5 × 5 mm, and is implemented on both an FR4 board and an implantable flexible substrate with the optimized geometric parameters. Finally, the transmission efficiencies of using FR4 and flexible printed circuit boards, at a distance of 10 mm, were 19.1% and 14.8% through the air and 11.7% and 7.7% through the tissue, respectively, thus enabling the supply of milliwatts of power to the stimulation circuits, and successfully demonstrating the stimulation tissue model with a V/I output current of up to 180 μA.