**1. Introduction**

Wireless power transmission (WPT) is an emerging technology to transmit power without wires [1, 2]. It is widely studied in various applications such as consumer electronics charging [3], radio-frequency identification (RFID) system [4], electric vehicle charging [5], biomedical applications [6], etc. Especially in implantable devices, the market benefit of global implantable medical devices achieved a value of US\$ 120.5 billion in 2021 by IMARC Group. The market can be expected to reach US\$ 168.3 billion by 2027, exhibiting at a compound annual growth rate (CAGR) of 5.54% during 2022–2027. In practice, implantable devices solve today's problems using advanced medical technologies such as a well-known pacemaker and capsule endoscopy [7, 8]. The pacemaker and capsule endoscopy invention brings new perspectives for cardiovascular disease therapy and direct health diagnosis of the whole small bowel. Although several miniatured battery systems have been developed to provide several years of power supply for implantable devices, a battery replacement would induce patients' inconvenient and unexpected surgery risks [9, 10]. Recently, the WPT technique has been widely applied in implantable medical devices that are expected to power supply issues in the future [11, 12].

WPT technique can be mainly divided into the coupling and radiative approaches [13, 14], as shown in **Figure 1**. The WPT coupling can be categorized into magnetic and electric field methods. The radiative approach can be categorized into

**Figure 1.**

*WPT classification, including nonradiative and radiative methods.*

electromagnetic waves and optical power transmission. In the radiative method (far-field), power is transferred by electromagnetic radiation in terms of microwave, laser, and light. Although radiative techniques can transfer power over longer distances, the large environmental loss-induced transfer efficiency is considerable due to poor air medium. The nonradiative method (near field) utilizes magnetic field and electric field coupling to approach wireless power transfer. The main advantage of nonradiative techniques is high transfer efficiency; however, its technical limitation is that the transfer distance is limited.

The nonradiative method is also called coupling-based WPT, which can be described as the power-transmitting side (primary side) and power-receiving side (secondary side). The coupling type can be classified as a magnetic field (**Figure 2a**) and electrical field coupling (**Figure 2b**). Both magnetic and electric field coupling methods can be further categorized as resonance mode, based on LC resonance (**Figure 2c** and **d**).

**Figure 2.**

*Magnetic and electric field coupling with and without resonant circuit.*
