| Summary: | Wireless power transfer has experienced a rapid growth in recent years due to the need for miniature medical devices with prolonged operation lifetime. The current implants utilize onboard batteries as their main source of power. The use of batteries is not, however, ideal because they have constrained lifetime requiring periodic replacement. Energy can be supplied to the implantable devices through wireless power transfer approaches including inductive, ultrasonic, radio frequency, and heat. The implantable devices driven by energy harvesters can operate continuously, offering ease of use and maintenance. Inductive coupling is a conventional approach for the transmission of power to implantable devices. However, the inductive coupling approach is affected by tissue absorption losses inside the human body. To power implantable devices such as neural, cochlear, and artificial heart devices, the inductive coupling approach is being used. On the other hand, ultrasonic is an emerging approach for the transmission of power to implantable devices. The enhanced efficiency and low propagation loss make ultrasonic wireless power transfer an attractive approach for use with implantable devices. This paper presents a study on the inductive and ultrasonic wireless power transfer techniques used to power implantable devices. The inductive and ultrasonic techniques are analyzed from their sizes, operating distance, power transfer efficiency, output power, and overall system efficiency standpoints. The inductive coupling approach can deliver more power with higher efficiency compared to the ultrasonic technique. On the other hand, the ultrasonic technique can transmit power to longer distances. The advantages and disadvantages of both techniques as well as the challenges to implement them are discussed.
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