A Comparative Study of Advanced Techniques for Wireless Power Transfer to Miniature Medical Implants

The elimination of batteries in implants enables their miniaturization to millimeter (mm) scale. This can be achieved by wireless power transfer (WPT), which is also beneficial for the long-term operation of implants. Current state-of-the-art WPT techniques include inductive coupling, ultrasound (US), and magnetoelectric (ME) modalities. US and ME links operate at acoustic wave resonance, working at low frequencies (100s of kHz to several MHz) while using mm-scale receivers. In contrast, mm-scale inductive links are efficient at high frequencies as they operate at electromagnetic wave resonance. While all three modalities are attractive, the literature lacks a comparative study in which these modalities are evaluated under similar conditions (size and depth of implant) when they are optimized and operate under regulatory safety limits. This paper compares the power receiving capability of ~ 6.2×2.2×1.4 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ME and US receivers, and an inductive coil with comparable 20 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> volume and 3 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> surface area. In measurements at a 30 mm depth within a tissue medium, the inductive, ME, and US WPT links could deliver 7.7 mW, 7 mW, and 15.55 mW of power considering the safety limits, respectively. Among these modalities, the ME link demonstrated the highest robustness against angular misalignment.