Ultrasonic wireless links for next-generation miniaturized implantable sensors
Keywords: Electrical Engineering.

Today's implantable medical devices rely on bulky batteries for energy supply, requiring invasive implant procedures, which limits their use to only last-resort therapies. New applications of implantable devices are being proposed, such as: peripheral nerve stimulation for treating chronic pain and inflammatory disease, optogenetic stimulation for mapping of neural pathways, and even sensing applications of neural potential, pressure, and temperature monitoring. To realize these new applications, and to achieve widespread use of implantable device therapy, a new energy paradigm must be designed to replace the batteries and allow for extreme implant miniaturization for minimally invasive implant procedures. Extreme implant miniaturization is especially critical for developing sensors for continuous monitoring, due to a higher threshold for implantation. Towards this goal, we propose ultrasound for efficient power transfer to deeply implanted and miniaturized implantable devices, and demonstrate ultrasonic link utility with design of an implantable pressure sensor and optogenetic stimulator. A comprehensive analysis and methodology is presented for designing ultrasonic receivers for efficient powering of miniaturized implantable medical devices. Key ultrasonic receiver efficiencies, resonance characteristics, and the inductive band are defined, and a theoretical model is presented for first-order design insights. Methods are described to accurately characterize the ultrasonic receiver impedance and acoustic-to-electrical aperture efficiency. Ultrasonic receivers are shown to achieve favorable source resistances from kOhm to 100's of kOhm, for matching to typical implant loads, and high aperture efficiencies of 40-90%. An iterative design methodology, using the theoretical model and impedance measurements is demonstrated for an example ultrasonic receiver design. An ultrasonically powered, millimeter-sized implantable device is proposed for future optogenetic peripheral nerve stimulation in large animal models. Through system level analysis, the effective impedance match between the ultrasonic receiver and implant electronics is shown as the dominant component of the power recovery efficiency. We present a numerically solved time-domain impedance match analysis of the non-linear power electronics to derive the optimal ultrasonic receiver impedance, and the design of the ultrasonic receiver is shown to meet this specification. The implantable stimulator is characterized through both electrical and optical measurements, demonstrating high peak optical intensities of 1-15 mW/mm2 with safe levels of ultrasonic power. Finally, a high-precision implantable pressure sensor with ultrasonic power-up and data uplink is presented for applications in continuous pressure monitoring. The fully integrated implant measures 1.7 mm x 2.3 mm x 7.8 mm and includes a custom IC, a pressure transducer, an energy storage capacitor, and an ultrasonic transducer. In order to reduce overall dimensions, unique circuit and system design techniques are presented to enable a single time-multiplexed ultrasonic transducer for both power recovery and data uplink transmission. Implant performance is characterized at significant depths, 12 cm in a tissue phantom, offering a > 13x improvement over the state of the art in the depth/volume figure of merit, while demonstrating a robust ultrasonic data uplink with better than 1e-5 bit error rate. A transient charging analysis is presented to derive the optimal ultrasonic receiver impedance and charging specifications to maximize overall harvesting efficiency. The IC features a front-end with a 10-bit SAR ADC, achieving a pressure full-scale range of 800 mmHg with a pressure resolution of 0.78 mmHg, exceeding the requirements for a wide range of pressure sensing applications, while accounting for various nonidealities. The pressure sampling rate is fully externally controlled, up to 1 ksps, in order to significantly decrease implant energy consumption and to allow for adaptable programming for specific applications. The implantable sensor is packaged in biocompatible materials and wirelessly characterized in a custom-built pressure chamber, and in situ, using sheep tissue.