Figure 21.

The working principle of WPT. a) Electromagnetic waves can be classified into near-field and far-field regions depending on the distance from the electromagnetic source. The area within λ/2π is called the reactive near-field region, λ/2π ~ λ is the radiative near-field region, λ ~ 2 λ is the transition region, and over 2 λ is the far-field region. b) The schematics of different WPT techniques: near-, mid-, and far-field WPT. Inductive coupling near-field WPT employs coils as antennas for a power transmitter and a receiver, and the power transfer happens through magnetic field coupling. Capacitive coupling near-field WPT employs a pair of electrodes as antennas, and the electric field coupled between the electrodes transfer the energy from one to the other. Mid- and far-field WPT uses antennas (e.g., monopole, dipole, loop antennas) that can emit and receive radiative electromagnetic field. c) The penetration depths of electromagnetic or RF waves with different frequencies are shown.^[453] The level of attenuation of RF waves varies slightly depending on tissue types. Adapted with permission.^[453] Copyright 2016, Society for reproduction and fertility.