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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.

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This paper proposes that transmitter-toreceiver resonator efficiency can be enhanced by the novel combination of resonator and switching controller at the receiver side. More specifically, the switching controller modulates the load resistance such that the receiver resonance is amplified. This increases the loaded-Q, reflected resistance, and, subsequently, overall efficiency and distance range. The efficiency and distance range are superior than resonator-only receivers, despite of losses from the switching controller itself. This breaks the common routine that typical switching converters only lower the power flow and efficiency when they are inserted in wireless power chain. Moreover, the scheme solves the common problem of loadvariation-induced efficiency degradation. More specifically, if the present load value is deviated from optimal point, the proposed controller adjusts the effective load resistance to amplify the reflected resistance. The loaded-Q amplification is easily controlled simply by changing the duty ratio of switching controller. This is more feasible than traditional impedance transformation network whose control requires large array of capacitor-switch matrix or movement of coil position. The efficiencies with and without the switchingcontrolled resonance amplification are 60.2% and 51.7%, respectively, for a 20-W loading at 15-cm distance for a 20 cm × 16 cm receiver.