Wireless ultrasonic power transfer using a pre-charged CMUT structure with a built-in charge storage capacitor

Capacitive micromachined ultrasonic transducer (CMUT) technology is a potential candidate to implement an ultrasonic power receiver for implantable medical devices (IMDs) because CMUT technology employs photolithography-based microfabrication techniques amenable to miniaturization, integration with electronics, and biocompatibility. Pre-charged CMUTs operating in constant-charge mode eliminate the DC bias and this mode of operation is more suitable for ultrasound power transfer to IMDs. We designed and fabricated a novel pre-charged CMUT structure with a built-in charge storage capacitor. This new configuration features a floating electrode between the upper and lower electrodes. Charges are stored on this floating electrode prior to implantation by directly bringing the floating electrode into contact with the bottom electrode while applying a DC bias between the top and bottom electrodes of the CMUT. After pre-charging the CMUT, the charges are retained without any leakage, as confirmed by occasional measurements over the course of about two years. We have also demonstrated that this device allows operation without a DC bias and can be used as a power receiver in an IMD. In the presented design, the CMUT can be pre-charged at a desired precise charge level. The amount of trapped charge can be controlled by holding the floating electrode in contact with the bottom electrode by applying external ultrasound pressure and simultaneously maintaining a DC bias. The maximum received power was 10.1 mW, corresponding to a received power density of 3.1 mW/mm², with a 14.5% efficiency. We have achieved an acoustic-to-electrical power conversion efficiency as high as 29.7% at lower input power levels.