**4. Near-field WPT methods**

#### **4.1 Capacitive coupling transfer**

The capacitive coupling link approach is used to transfer data and power in short wireless communications to the implanted devices. The basis for this approach is two parallel plates which behave like condensers. The first plate is attached to the skin outside of the body; the second plate is implanted inside the body and attached as shown in **Figure 2** to the implanted device. The electric field is used as a carrier by the capacitive coupling to transfer data and power to the skin which acts as a dielectric divider between these two plates [11].

In **Figure 2**, the voltage transmission rate was analyzed as follows: The voltage of the *Vin* and the *C*<sup>1</sup> and *C*<sup>2</sup> between the implanted and the outside plates is the input capacitance equivalent, *Cin* is the implanted circuits' input capacitance and RL is the equivalent "ac" of the loading system's resistance. The corresponding *Ceq* condensers are given

$$\mathbf{C}\_{eq} = \mathbf{C}\_1 + \mathbf{C}\_2 \tag{1}$$

Assuming *Cin* < < *Ceq*, then

$$V\_{out} = V\_{in} \left[ \frac{R\_L^2}{R\_L^2 + X\_{ceq}^2} + j \frac{R\_L X\_{eq}}{R\_L^2 + X\_{ceq}^2} \right] \tag{2}$$

and the voltage transfer rate is given by

$$\left| \frac{\mathbf{V\_{out}}}{\mathbf{V\_{in}}} \right| = \left( \frac{\mathbf{R\_{L}^2}}{\mathbf{R\_{L}^2} + \mathbf{X\_{ceq}^2}} \right)^{14} \tag{3}$$

Therefore, when *XCeq* <*RL*, *V*out is maximized. The main drawback of the method is that the tissue temperature of the plates can be increased, causing patient discomfort. The human body is also a non-magnetic material. Negligible losses in the magnetic field indicate that the electrical field is absorbed by human tissue [12].

#### **4.2 Inductive coupling transfer**

Inductive coupling transfer is now an attractive technology for the development of short communication biomedical applications. The magnetic coupling is used as the communication environment, common to techniques for radiofrequency

**Figure 2.** *Simplified capacitive coupling transfer [12].*
