**5. WPT system overview**

The basic topology of magnetic field resonant coupling is shown in **Figure 11** [16, 20, 21]. The transmitter and receiver sides are coupled through the magnetic field (**Figure 11a**) and electric field (**Figure 11b**). The DC power supply generates the source power. The inverter is responsible for DC/AC conversion to produce the highfrequency AC power. The AC power is transferred to the receiver side and converted from AC to DC power by a rectifier. Then, the power is transferred to the load. In the WPT operating frequency for implantable devices, the common frequency is a at low frequency because of the reduction of tissue heating effect [22]. The typical frequency is 13.56 MHz within the license-free industrial, scientific, and medical (ISM) band [23].

**Figure 11.** *System overview for (a) magnetic field coupling and (b) electric field coupling methods.*

The class E power amplifier is commonly applied in MHz-range WPT applications because of its excellent power efficiency from the properties of zero voltage switching and zero voltage derivative switching [24]. The class-E power amplifier can be depicted in **Figure 12**. In theory, it has a maximum efficiency of 100% [25], according to Eq. (48). The overall circuit consists of the power supply Vdc, RF-chuck, and passive load network. The RF-chuck with high reactance ensures DC voltage pass through the system. The passive load network is responsible for minimizing the overlap between drain voltage and drain current and further reducing the output power loss [26].

$$\mathbf{P\_{dissipation}} = \frac{1}{\mathbf{T\_0}} \int\_{\mathbf{T\_0}} \mathbf{I\_{DS}} \ast \mathbf{V\_{DS}} \,\mathrm{d}\mathbf{t} = \mathbf{0} \tag{48}$$

Some group has proposed a class E power amplifier-based WPT system for implantable biomedical in practical applications. Mutashar *et al.* [27] designed a wireless magnetic resonant coupling system using a class-E power amplifier and inductive power links via two spiral transmitters and receiver coils (**Figure 13a**). The 13.56 MHz carrier frequency within the ISM band range was selected to avoid tissue damage. The proposed class-E power amplifier can reach up to 87.2% power efficiency. The overall inductive coupling system achieves a power transmission efficiency of 73%. The compact structure could be implemented in bio-implants in the future. In practical applications for the electrical field coupling method, Narayanamoorthi *et al.* [24] developed a class-E-based capacitive coupled wireless power transfer system for biomedical implants (**Figure 13b**). They optimize the class E-based capacitive coupling WPT systems at an operating frequency of 13.56 MHz. Moreover, the matching circuit consisted of inductive-capacitive-inductive (LCL) impedance was used to improve the power efficiency of the power amplifier up to 96.34%.
