Ultrasonic power transfer using piezoelectric devices is a promising wireless power transfer technology for biomedical implants. However, for sub-dermal implants where the separation between the transmitter and receiver is on the order of several acoustic wavelengths, the ultrasonic power transfer efficiency (PTE) is highly sensitive to the distance between the transmitter and receiver. This sensitivity can cause large swings in efficiency and presents a serious limitation on battery life and overall performance. A practical ultrasonic transcutaneous energy transfer (UTET) system design must accommodate different implant depths and unpredictable acoustic changes caused by tissue growth, hydration, ambient temperature, and movement. This paper describes a method used to compensate for acoustic separation distance by varying the transmit (Tx) frequency in a UTET system. In a benchtop UTET system we experimentally show that without compensation, power transfer efficiency can range from 9% to 25% as a 5 mm porcine tissue sample is manipulated to simulate in situ implant conditions. Using an active frequency compensation method, we show that the power transfer efficiency can be kept uniformly high, ranging from 20% to 27%. The frequency compensation strategy we propose is low-power, non-invasive, and uses only transmit-side measurements, making it suitable for active implanted medical device applications.

Ultrasonic power transfer is a promising alternative to electromagnetic induction for providing wireless power to active implanted medical devices. In a portable ultrasonic power link for devices like cochlear implants, there is a key requirement for an inverter circuit that is capable of driving the transmitting piezoelectric transducer with high efficiency in order to maximize battery life and overall device reliability. In the following paper, a high efficiency Class E amplifier is presented for use on the transmit side of an ultrasonic transcutaneous energy transfer (UTET) link. The amplifier is characterized by a peak efficiency of 90.8% at an output power level of 293 mW and drive frequency of 1.275 MHz.

No abstract

We will report on development of a system for efficiently powering implanted hearing aids by transmitting ultrasonic acoustic energy across the skin. As compared to traditional magnetic induction coil power delivery systems, ultrasound-based systems offer a more compact form factor for the same power handling capability and lower electrical loss. Part of the challenge of building such a system for implanted hearing aids is developing efficient modulation and demodulation electronics that can deliver both electrical power and an acoustic frequency signal to the implanted device. We present the design and implementation of an amplitude modulated system in which the power is delivered on the carrier and signal in the modulation sidebands. The transmitter consists of an efficient PWM encoder driving an LC resonator tuned to the ultrasound transducer resonance frequency. The receiver consists of an efficient rectifying demodulator that provides supply voltages to internal electronics as well as the acoustic signal. Power loss mechanisms, form factor considerations, linearity, and overall system performance will be discussed.

We present on the development of a 12.8 mm diameter ultrasonic transcutaneous energy transfer system for powering implanted hearing devices. The system was based on two custom 8mm diameter PMN-PT 1-3 composite transducers operating at 1.25MHz and featured three significant innovations. First, to ensure long-term biocompatibility and reliability of the implanted portion of the link and to ensure robust attachment to bone, the implanted transducer was designed into a 12.8 mm diameter, 3.5 mm thick hermetically sealed titanium package in an easy-to-implant form factor. The transducer was designed to deliver sound efficiently through the casing walls using a mass-spring acoustic matching technique. Second, for the external unit a “dry” acoustic coupling system based on a silicone pressure sensitive adhesive was designed that combined efficient acoustic coupling, robust adhesion to skin and that was comfortable to wear. The external unit was aligned with the internal unit using a magnetic alignment system. Third, we developed and demonstrated an efficient Class E transmit amplifier and receive electronics that incorporated compensation for changes in acoustic separation between the transducer to maintain robust, high efficiency power transmission. The system achieved 33% DC-to-DC electrical conversion efficiency through 5mm of water and 19% DC-to-DC efficiency through 5mm of porcine tissue.