# Wireless Power Transfer Approaches for Medical Implants: A Review
## 2 Different Approaches for a Wireless Power Transfer System
### Ultrasonic-Based Wireless Power Transfer  
The ultrasound imaging is a well-known tool for evaluating patients' physiological and pathological conditions. In the passive ultrasonic recorder, the backscattered echo is derived from the reaction of biological tissue's acoustic properties to sound waves. Additionally, the acoustic emission can be used for supplying energy wirelessly in the active biological environment [45]. The ultrasonic-based WPT system has a transmitter converting electrical energy to ultrasonic energy, and a receiver converting back the ultrasonic energy to electrical energy. \begin{table}
\begin{tabular}{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** \\ **Dimensions** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Receiver** \\ **Dimensions** \\ **(mm)** \\ \end{tabular} \\ \hline
[86] & 2020 & 1.47 GHz & 6.7 & 0.67 & 50 & \(6\times 6\) & - \\
[87] & 2020 & 0.403 GHz, & - & - & 30–350 & - & \(9.5\times 9.5\) \\
[88] & 2019 & 1.64 GHz, & - & 32, 1.1 & - & \(14\times 15\) & \(14\times 15\) \\
[13] & 2019 & 954 MHz & 10 & 65 & 110 & - & \(10\times 12\) \\
[84] & 2018 & 0.915, 1.9, & 0.398 & - & 4.5 & - & \(7\times 6\) \\
[38] & 2018 & 400 MHz & 19, 82 & - & 1, 3, 6, 12, 15 & \(\text{d}_{\text{outT}}=18\) & \(1\times 1\) \\
[89] & 2018 & 280 MHz & 44 & - & 3 & \(30\times 80\) & - \\
[90] & 2017 & 2.45 GHz & 2280, 600, & - & 1000–4000 & - & \(\text{d}_{\text{outR}}=63.6\) \\
[91] & 2014 & 2.4 GHz & - & 15–78 & 10–100 & \(63\times 39\times 50\) & \(63\times 39\times 50\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Existing microwave-based WPT approaches for implantable power applications. Figure 5: Photograph of the electrostimulator inserted in the lower epicardium of a rabbit via open-chest surgery [82]. The basic model of the implantable ultrasonic coupling WPT system is shown in Figure 6. The transmitting transducer powered by the transmitting module sends the ultrasonic waves, and the ultrasonic energy is transmitted to the receiving transducer through the human tissue. The receiving transducer converts the collected ultrasonic energy into electrical power. Accordingly, power is delivered to the implantable device through the receiving power module. The receiving power processing module mainly includes a voltage-stabilizing circuit and a rectifier circuit. A model of the ultrasonic principle of WPT in the transducer of the transmitter and receiver is shown in Figure 7[92]; the radiated sound power \(P\) is given by the below equation:  
\[P=\pi\rho_{o}c_{o}u_{a}^{2}a^{2}\int_{0}^{\text{a}}\frac{\left[J_{1}\left(ka \frac{1}{\sqrt{2}+d^{2}}\right)\right]^{2}}{l}dl \tag{11}\]  
where \(\rho_{o}\) is the density of the medium, \(c_{o}\) is the sound velocity of the medium, \(u_{a}\) is the amplitude, \(a\) is the sound source radius of the circular plane A, \(k=\frac{\omega}{c_{o}}\) is the wavenumber of a sound field, \(d\) is the distance along the z-axis, and \(J_{1}\left(ka\frac{1}{\sqrt{2}+d^{2}}\right)\) is the first-order Bessel function. The ultrasonic-based WPT system is an effective method for medical applications such as cardiac defibrillators and deep brain stimulators (DBSs) [93]. A mode of clinical therapy is a stimulation of excitable tissue for different disorders, such as Parkinson's disease, urinary incontinence, and heart arrhythmia. The traditional stimulus techniques use percutaneous cables to transport electricity to the electrodes. The classical techniques are dangerous because they can cause infection [12]. Figure 6: The basic model of the implantable ultrasonic coupling WPT system. Figure 7: The field model of the ultrasonic coupling wireless transmission system [92]. The ultrasound- or inductive-based WPT is an interesting solution for this application.