A parallel-trace high-Q planar spiral coil for biomedical implants

High-Q inductive coils are essential components in biomedical implants for efficient wireless charging and effective wireless sensing. The planar spiral coil (PSC) that can be easily optimized and reliably fabricated by lithographic tools is a preferred candidate. To support the inductive coupling at MHz range, the size of PSCs used in implants is much larger than those used in wireless communication circuits. Therefore, to achieve high Q, it is imperative to reduce the metal trace's unit-length-resistance. In this paper, multiple parallel-connected metal traces, instead of a conventional single trace, have been employed to reduce the unit-length-resistance by mitigating the skin effect. Although the approach was used to make stranded wires for mega-watts transmission systems, it has been used to design PSCs for the first time. The parallel-trace PSC exhibits 38%~53% improvements in Q when it resonates with a capacitor at ~10 MHz. Measurement results also indicate that there is ~10% inductance reduction in the parallel-trace PSC compared to the single-trace PSC of the same design. Measurement results also indicate that, in a parallel-trace PSC, the length difference between the parallel-connected, side-by-side traces when they are winded into a coil, and the dielectric environment difference when they are placed in different layers, can be neglected when the operating frequency is less than the PSCs self-resonating frequency. Utilizing widely-available planar fabrication technologies, the parallel-trace PSC can be widely adopted in biomedical implants.