Abstracts

Rationale: The ability to miniaturize implantable neural sensing and stimulation devices for treatment of epilepsy has potential benefits to the patient population such as reduced surgical invasiveness, increased reliability, and improved MRI compatibility. Wireless powering reduces the implant s dependency on a battery allowing for a reduction in overall size of the device. Far-field radio-frequency powering (FFRFP) improves upon near-field coupling by: significantly increasing implant-to-charger range (ease of patient use), decreasing sensitivity to external charger placement (improving patient compliance), and requires only a miniature high-frequency antenna rather than a large coil. The FFRFP system developed in this work is capable of delivering sufficient power for potential neural stimulators targeted at treatment of epilepsy.Methods: An application specific integrated circuit (ASIC) is designed to rectify a radio-frequency (RF) signal (received from a wireless source) to a DC supply (to satisfy neural stimulation power requirements). The circuitry is fabricated on a 0.057 mm2 ASIC and designed to maximize the efficiency of the RF-to-DC conversion.Results: Devices targeting the vagus nerve for epilepsy treatment have stimulation settings that vary widely; however, when averaged across the patient population and over time, the power requirements are around 6 W. The ASIC achieves a measured maximum power conversion efficiency (PCE) of 11.3% when the power input is at 20 mW. To operate at the maximum PCE point, a pulsed FFRFP technique is used where 20 mW instantaneous power bursts are delivered. The duty cycle of these bursts are optimized such that the average power delivered to the stimulation circuitry is the minimal necessary to achieve 6 W. With an 11.3 % PCE, the average power required at the input to the ASIC is 53 W, which is achieved using a 20 mW power (measured at the input of the ASIC) pulsed at a duty-cycle of 0.266%. A link-budget is analyzed for a case which considers a device implanted close to the vagus nerve, a few mm beneath the neck surface. A 1 W 2.4 GHz external RF power source is considered with a 6 dBi gain antenna (satisfying FCC and FDA regulations) and operating at distances of up to 1 m (facilitating patient-use). With a 1 W source and the 53 W requirement, the link can support 42.8 dB of loss. The derived power loss, considering attenuation through the thin tissue layer and impedance mismatches at the tissue-air interface, is about 6 dB. The overall power transfer including the gain of a miniature PCB antenna (+0.5 dBi), an external antenna gain (+6 dBi), tissue loss (-6 dB), and 1 m free-space path loss (-40 dB), is -39.5 dB. This gives 3.3 dB of margin when delivering the necessary power of 53 W at the input of the ASIC to supply the 6 W power for stimulation.Conclusions: The FFRFP technology developed in this work shows to ability to wirelessly transfer sufficient power to satisfy neural stimulation electrical requirements, without the need for a battery, which allows for a miniaturized and patient friendly solution for the treatment of epilepsy.