Optimal Design of Wireless Power Transmission Links for Millimeter-Sized Biomedical Implants

Abstract

This paper presents a design methodology for RF power transmission to millimeter-sized implantable biomedical devices. The optimal operating frequency and coil geometries are found such that power transfer efficiency (PTE) and tissue-loss-constrained allowed power are maximized. We define receiver power reception susceptibility (Rx-PRS) and transmitter figure of merit (Tx-FoM) such that their multiplication yields the PTE. Rx-PRS and Tx-FoM define the roles of the Rx and Tx in the PTE, respectively. First, the optimal Rx coil geometry and operating frequency range are identified such that the Rx-PRS is maximized for given implant constraints. Since the Rx is very small and has lesser design freedom than the Tx, the overall operating frequency is restricted mainly by the Rx. Rx-PRS identifies such operating frequency constraint imposed by the Rx. Secondly, the Tx coil geometry is selected such that the Tx-FoM is maximized under the frequency constraint at which the Rx-PRS was saturated. This aligns the target frequency range of Tx optimization with the frequency range at which Rx performance is high, resulting in the maximum PTE. Finally, we have found that even in the frequency range at which the PTE is relatively flat, the tissue loss per unit delivered power can be significantly different for each frequency. The Rx-PRS can predict the frequency range at which the tissue loss per unit delivered power is minimized while PTE is maintained high. In this way, frequency adjustment for the PTE and tissue-loss-constrained allowed power is realized by characterizing the Rx-PRS. The design procedure was verified through full-wave electromagnetic field simulations and measurements using de-embedding method. A prototype implant, 1 mm in diameter, achieved PTE of 0.56% ( -22.5 dB) and power delivered to load (PDL) was 224 μW at 200 MHz with 12 mm Tx-to-Rx separation in the tissue environment.

Silicon-Integrated High-Density Electrocortical Interfaces

“…In biological tissues, the tradeoff between coil design and tissue losses results in an optimal frequency for wireless power transmission where efficiency is maximized. For example, the inductance of coils located on miniaturized, millimeter-scale implants ranges from 10 to 100 nH [66], [88], [146], [147]. To compensate for reduced magnetic flux through the miniaturized receiving coil, the carrier frequency for wireless power transfer should be increased, often into the hundreds of megahertz to single-digit gigahertz range [66], [146]- [149].…”

Millimeter-scale integrated and wirewound coils for powering implantable neural microsystems

“…The coil is electrically connected to a T-shape PCB via the bondwires. Similar to the on-chip coil measurement, we used a deembedding technique [6] to remove parasitic effects from the bulky interconnects. The S-parameters of the short-circuited PCB with the bonding-wire and the open-circuited PCB were measured, converted to Z-parameters and Y-parameters, and subtracted from the coils Z-parameters and Y-parameters to eliminate the parasitic resistance/inductance and the parasitic capacitance, respectively.…”

“…2, 3-coil, etc.) [6]- [10]. For wireless power transmission to mm-scale IMDs, a 2 × 2.18 mm 2 integrated CMOS coil was reported in [7] providing good manufacturability, reliability, and cost competitiveness.…”

A polygonal double-layer coil design for high-efficiency wireless power transfer

“…[3][4][5][6][7] The Q-factors of other coil shapes have also been investigated, for example, the planar Polygonal Spiral Coil (PSC). [8][9][10][11][12][13][14] Currently, most of the PSCs consist of one or two coil layers. The single-layer type is most common and its design has been optimized.…”

“…The single-layer type is most common and its design has been optimized. [8][9][10][11][12][13] However, when the size of the single-layer coil is fixed, it is often difficult to increase its inductance without significantly affecting the Q-factor because of the limitation in the diameter of the coil wire. The double-layer polygonal type is less constrained as discussed in Ref.…”