# Wireless Power Transfer Approaches for Medical Implants: A Review
## 2 Different Approaches for a Wireless Power Transfer System
### Hybrid Wireless Power Transfer
In [108], the authors proposed a coupled WPT-Power Line Communication (PLC) system that consists of a two-coil resonator system. A four-port system has been designed with two ports dedicated to power transfer and the other two to data transfer. Capacitors have been used to tune the power channel to the desired frequency and improve the matching of the filtering stage to the coupled inductors. A hybrid inductive-based and a microwave-based WPT system are also presented in [109]. One of the current challenges for wireless transfer for small sensors is to minimize the system size. [PERSON] et al. [109] decreased the size of the compact system, at the same time implementing multi-functionality. This goal was obtained by designing an antenna with 14 mm \(\times\) 15 mm dimensions and having 20 mm \(\times\) 20 mm dimensions for the hybrid system including the antenna and coil. The coils operating frequency was 510 MHz and the antennas worked at 2.48 GHz and 4.66 GHz. [PERSON] et al. [110] developed a hybrid inductive-ultrasonic WPT link to power biomedical implants over bone, air, and tissue. They optimized cascaded inductive and ultrasonic links for WPT applications. The hybrid link was designed for an air-tissue medium to operate at 1.1 MHz with a power transmission efficiency of 0.16%. Recent works related to the HWPT system are shown in Table 5. The receiver dimension should be as small as possible to make it more convenient for implant applications. In case the receiver dimension is larger than 40 mm, such design can be printed on a flexible material substrate such as Kapton to facilitate the surgery procedures [111]. It is worth mentioning that rotational/lateral misalignment and bending are two conditions that may happen because of changes in the implanted antenna location or the person's movement. Therefore, the bending of a flexible substrate and the misalignment effect must be investigated precisely [112]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Reference** & **Year** & **Frequency** & \begin{tabular}{c} **Output** \\ **Power** \\ **(mW)** \\ \end{tabular} & \begin{tabular}{c} **Efficiency** \\ **(\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Active** \\ **Range** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Transmitter** \\ **Dimension** \\ **(mm)** \\ \end{tabular} & **Receiver** \\ \hline
[109] & 2019 & \begin{tabular}{c} 510 MHz, \\ 2.48 GHz, \\ 4.66 GHz \\ \end{tabular} & 0.0004 & 3.7, 2.2, 1 & 20–60 & \begin{tabular}{c} d\({}_{\text{outf}}\) = 39.75 \\ \end{tabular} & \
\begin{tabular}{c} d\({}_{\text{outf}}\) = 20 \\ \end{tabular} &
\begin{tabular}{c} Inductive and \\ Microwave \\ \end{tabular} \\
\begin{tabular}{c} \({}_{\text{-}}\) \\ \end{tabular} \\
[114] & 2020 & \begin{tabular}{c} 415 MHz, \\ 905 MHz, \\ 135 MHz \\ \end{tabular} & - & \begin{tabular}{c} 10, 0.5, \\ 4.6, 6.5 \\ \end{tabular} & 15–110 & \begin{tabular}{c} d\({}_{\text{outf}}\) = 79.6 \\ \end{tabular} & \
\begin{tabular}{c} d\({}_{\text{outf}}\) = 31.5 \\ \end{tabular} &
\begin{tabular}{c} Inductive and \\ Microwave \\ \end{tabular} \\
[115] & 2017 & \begin{tabular}{c} 13.56 MHz/910 \\ \end{tabular} & - & 17 & 16 & \begin{tabular}{c} d\({}_{\text{outf}}\) = 83.2 \\ \end{tabular} & \
\begin{tabular}{c} d\({}_{\text{outf}}\) = 24.2 \\ \end{tabular} &
\begin{tabular}{c} Inductive and \\ Microwave \\ \end{tabular} \\
[110] & 2017 & \begin{tabular}{c} 1.1 MHz \\ \end{tabular} & - & 0.16 & 60 & \begin{tabular}{c} d\({}_{\text{outf}}\) = 100 \\ \end{tabular} & \
\begin{tabular}{c} d\({}_{\text{outf}}\) = 15 \\ \end{tabular} &
\begin{tabular}{c} Ultrasonic \\ \end{tabular} \\
[116] & 2012 & \begin{tabular}{c} 200 kHz \\ \end{tabular} & 8 & 1 & 70 & \begin{tabular}{c} d\({}_{\text{outf}}\) = 39 \\ \end{tabular} & \begin{tabular}{c} d\({}_{\text{outf}}\) = 33 \\ \end{tabular} & \
\begin{tabular}{c} d\({}_{\text{outf}}\) = 33 \\ \end{tabular} &
\begin{tabular}{c} Inductive and \\ Ultrasonic \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}