A Dual-Band Wireless Power Transmission System for Evaluating mm-Sized Implants

Distributed neural interfaces made of many mm-sized implantable medical devices (IMDs) are poised to play a key role in future brain-computer interfaces because of less damage to the surrounding tissue. Evaluating them wirelessly at preclinical stage (e.g., in a rodent model), however, is a major challenge due to weak coupling and significant losses, resulting in limited power delivery to the IMD within a nominal experimental arena, like a homecage, without surpassing the specific absorption rate limit. To address this problem, we present a dual-band EnerCage system with two multi-coil inductive links, which first deliver power at 13.56 MHz from the EnerCage (46 × 24 × 20 cm³) to a headstage (18 × 18 × 15 mm³, 4.8 g) that is carried by the animal via a 4-coil inductive link. Then, a 60 MHz 3-coil inductive link from the headstage powers up the small IMD (2.5 × 2.5 × 1.5 mm³, 15 mg), which in this case is a free floating, wirelessly powered, implantable optical stimulator (FF-WIOS). The power transfer efficiency and power delivered to the load (PDL) from EnerCage to the headstage at 7 cm height were 14.9%-22.7% and 122 mW; and from headstage to FF-WIOS at 5 mm depth were 18% and 2.7 mW, respectively. Bidirectional data connectivity between EnerCage-headstage was established via bluetooth low energy. Between headstage and FF-WIOS, on-off keying and load-shift-keying were used for downlink and uplink data, respectively. Moreover, a closed-loop power controller stabilized PDL to both the headstage and the FF-WIOS against misalignments.