The heterogeneous and conductive nature of biological tissue render near-field inductive coupling ineffective in powering devices if implanted deeply. Similar to Lone Star Neuro's pulse-generator platform, SCMR (Strongly Coupled Magnetic Resonance) could overcome this limitation. If further device miniaturization is required, such as a device with dual electrodes and no batteries, mid-field power transfer can now be an option:
Power transfer in the mid-field region can (around a wavelength away from the source) transfer enough power to a millimeter scale implant deep inside a tissue. Further, mid-field region allows power flow lines to be manipulated with an interference pattern for focusing them in a specific spot, and well within the SAR (Specific Absorption Rate) safety limits.
Ho et al explain the theory behind mid-field power transfer, which is straightforward. Design and implementation of a mid-field power transmitter and a miniaturized receiver is also well within the capabilities of today's semiconductor components and manufacturing technologies, promising new millimeter-scale medical implants for a variety of therapy modalities which have not been feasible until recently.