A high-efficiency ultrasonic power link for implantable medical devices

We propose to develop a high-efficiency transcutaneous ultrasound link to provide power to implanted hearing devices. This requires a cohesive combination of engineering expertise to develop new technology, and clinical/physiological expertise to realise its potential in terms of new surgical options and to test its ability to deliver functionally. At present, implanted hearing devices are powered with a set of radiofrequency induction coils that transfer power through the magnetic field. Such systems have some serious and fundamental limitations: Induction coils trade off size and efficiency, with smaller devices being less efficient, and are quite sensitive to alignment and positioning errors. The high sensitivity to alignment error forces the use of alignment magnets which make it impossible to perform MRI scans on the implanted patient without either removing the magnet, or accepting large shadow areas in the images. MRI scanning is the definitive test for many disorders, and manyimplantees are children, 40% of whom will require an MRI at some point in their lifetime, so addressing this issue is of great concern. The other major limitation of RF coils is the size/efficiency tradeoff. In order to achieve acceptable coupling efficiencies (typically 25% to 30% in commercially-available implants) coils must be 50 mm or more in diameter. This limits the range of possible implantation sites and requires a large and uncosmetic external unit. The stigma attached to the large and visible external unit is a serious concern, particularly in young patients. The present proposal aims to address these shortcomings in present technology by moving to a different power delivery modality: acoustic ultrasound energy. In our system an external ultrasound transducer is driven from a high-efficiency RF oscillator. This creates an ultrasound wave thatpropagates through the tissue. A second, implanted transducer converts the ultrasound energy into electrical energy and a high-efficiency rectifier converts the RF to DC to recharge an implanted battery or capacitor. Recent advances in piezoelectric materials and processing techniques make it possible to achieve very high electromechanical coupling efficiencies in ultrasonic transducers. Unlike induction coils, however, there is no inherent size/efficiency tradeoff with an ultrasound link and so it is possible to deliver power to an implant with comparable efficiency in a device one tenth the diameter of an inductive link. Three specific technologies that we are working on will be key to creating a competitive ultrasonic link. First, we will be using the ultra-high-efficiency piezoelectric PMN-PT composite, a material which we have developed an extensive expertise in manufacturing and working with. Second, we will be manufacturing focused transducers to reduce the alignment sensitivity of the device. Third, we will be constructing high-efficiency impedance matching layers to reduce the amount of power reflected at the transducer-tissue interface. As a proof-of-concept of the idea of ultrasonic power delivery we have constructed and tested two 5 mm-diameter air-backed PMN-PT uncurved and unmatched transducers. We were able to demonstrate transfer of up to 200 mW of power with an efficiency of 32% through 6 mm of water and 15% through 5 mm of tissue. We have developed mathematical models to understand the loss sources and find that with improved matching layers, a composite transducer and focusing, efficiencies of up to 50% through 5 mm of tissue are theoretically achievable. The device development will be complemented by clinical research into how best to take advantage of the newsurgical possibilities available with a much smaller device and how to secure the device without the use of an implanted magnet. In particular, the feasibility of implanting the device in a mastoid recess and in the pinna will be examined.