Fig. 3.

Working principle of wireless power transfer techniques. a Schematic of inductive coupling in which a current in a coil, represented by the red arrow, creates a magnetic field going through a secondary coil. b Schematic of utilization of an ultrasound probe to send energy to implanted device. Reproduced with permission [72].

Copyright 2019, AAAS. c Schematic of capacitive coupling, in which internal and external electrodes forms capacitors using biological tissue as the dielectric