Wireless Recharging of Implanted Batteries via Ultrasound

In order to expand the applications for implanted rechargeable batteries, and to reduce the frequency of battery replacement procedures, we are investigating a recharging technique complimentary to and improving on the current RF recharging technique. Although the first applications deal with batteries that could be implanted in human bodies to power neurostimulators, sensors, and drug pumps, non-medical applications may exist. Using a transmitter-receiver arrangement, we have recharged batteries wirelessly using ultrasound at several frequencies between 0.75 and 3.0 MHz. Rechargeable implantable batteries of 35, 200 and 600 mA-hr were charged at rates of up to 0.75 C, where C is the charging rate (charging current/maximum battery charging current). Typically the intervening medium was one centimeter of a tissue mimicking liquid (TML), however some in vitro experiments have also been performed. Charging was accomplished at distances of up to 20 centimeters in water, and even through millimeters of plastic and centimeters of aluminum. Temperature measurements were made on both transmitting and receiving transducers, and in the TML. As expected there were significant increases in temperature at the higher charging currents. Experimentally we determined that the “overall efficiency” of the charging process, viz. $E=(Ibatt*Vbatt)/(net electrical power)input$⁠, was closely correlated with the observed heating. That is, the lower the efficiency, the higher the input electrical power required, the more transducer heat was produced and conducted into and through the medium. The critical issues were the coupling of the transmitter and receiver to the medium, and the efficiency of conversion of the receiver output to charging power by the charging circuitry. These depend on the mechanical and electrical impedances, and we improved the efficiency considerable by appropriate impedance matching. Active and passive methods of cooling the transducers and intervening medium have been constructed and successfully tested. With our system, recharging times will be limited not by heating considerations, but only by the optimum rate at which a given battery can accept charge.