Efficient and Easy to Fabricate Silicon-Based Planar Micro-Coils for Wireless Power Transfer Applications

Wireless power transfer (WPT) has been recognized as a feasible solution to expand the lifespan of wearable and implants. This study proposes two topologies for planar micro-coils, namely, non-spiral and quasi-spiral, for WPT applications. The proposed micro-coils require a simple microfabrication process, due to the positioning of both terminals outside the coil loops at the same layer. Advantageously, by using a single mask microfabrication process the micro-coils can be fabricated on the silicon substrate. By simulation, we demonstrate that in the non-spiral topology the electrical potential and magnetic flux density distribute uniformly. For comparison purposes, a commercial circular wound coil which occupies 4.35 times higher volume in a circuit than the fabricated micro-coils is employed. Experimental results indicate that the non-spiral and quasi-spiral micro-coils wirelessly receive 3.6 and 1.3 times higher power, respectively, than the commercial coil. This efficiency is not confined to the amount of the scavenged power, while the micro-coils can effectively and efficiently charge a super-capacitor in a shorter period. Furthermore, the capability of the micro-coils to re- charge battery of a vital sign monitoring wearable is demonstrated. The ability to integrate the micro-coils in wearables and implantable medical devices will enable body-worn or implanted systems to be powered or recharged wirelessly while keeping their size miniaturized.