# Approach-Based Analysis on Wireless Power Transmission for Bio-Implantable Devices
## 3 Analysis of Various WPT Approaches
### WPT Based on Role of Impedance Matching Approach and Hybrid Technique  
This section discusses another group of WPT systems listed in Table 2, which have been proposed with the focus of an impedance-matching role or hybrid technique. [PERSON] et al. [29] developed a WPT system with its equivalent circuitry and showed the realization of impedance matching. This method is capable of reaching the input power utmost 10 mW and efficiency of up to 46% without the use of impedance-matching circuits, which is shown in Figure 4.  
In order to raise the energy conversion in the implants using ultrasound receivers, a suitable complex conjugate among the piezoelectric receiver circuit and the energy transformation circuit is perceived as a worthy power transfer. Placing a boost converter before the rectifier circuit allows near perfect resistance. This boost converter converts the alternating current potential to a square wave, which is modulated using the PWM method. This helps to avoid the additional work of transforming impedance between the receiving circuit and the module of rectifier in the WPT system [30]. Hence, the overall efficiency of the WPT gets improved. In [31], the WPT link was examined with the impact of various titanium alloys at the operating frequency of 13.56 MHz. Usually, poor resistivity in the IMDs decreases the efficiency of the connection. Here, a simple analytical representation has been conferred to understand that effect by comparing the alloys of two different grades. Here, the link \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Approach** & **Operating Frequency** & **Performance** & **Remarkable Observation** \\ \hline \multirow{3}{*}{[29]} & Impedance-matching & & & 10 mW input power achieved \\  & technique in magnetic & 144 MHz & 46\% PTE & without affecting power \\  & coupling resonance & & & efficiency provided without \\  & & & & using matching circuits \\ \hline \multirow{3}{*}{[30]} & Resistive matching & 987 kHz (Piezo- & AC boost method: load & Power transfer attained \\  & (complex conjugate match) & electric receiver) & power = 1 mW; 74\% PTE & maximum level in providing \\  & with an AC boost converter & & & ultrasonic power to IMDs \\ \hline \multirow{3}{*}{[31]} & Titanium alloy on the & & Connection link & Grade 2 gives more efficiency \\  & WPT link & 13.56 MHz & Efficiency improved & than Grade 5 titanium alloy. \\ \hline \multirow{3}{*}{[32]} & Powering through & & For air kapton gap of & (i) & DTR up to 170 kbps \\  & 2-contact resonant & 1 MHz & capacitive link, up to 90 & achieved; 12 mW of PDL; \\  & capacitive link (Hybrid & mW of power and 70\% & (ii) & Suitable for high-density \\  & ASK-FSK technique) & & PTE attained & micro-stimulation implants \\ \hline \hline \end{tabular}
\end{table} 
Table 2: WPT systems focusing on an impedance-matching role. Figure 4: WPT system without using any impedance-matching circuits. efficiency attained was greater for the Grade 2 alloy than for the Grade 5 alloy at 13.56 MHz. So, the electrical properties of the titanium alloy should be carefully considered when designing the WPT link in the system to meet the desired target for link efficiency. [PERSON] et al. proposed and demonstrated in [32], for the first time, an experimental workbench model of an approach of supplying power wirelessly for biomedical IMDs over a capacitive connection, which is two-contact resonant. Here, the data were simultaneously sent in hybrid form (ASK-FSK) of encoding, i.e., amplitude shift plus frequency shift, keying on the single chosen medium. It is highly suitable for the implants of micro-stimulation, where the telemetric rate for data transmission does not depend on the frequency of the service provider. For realization purposes, a concept tabletop setup with flexible and adaptable patches of capacitance of 20 mm \(\times\) 20 mm with a 3 mm thick portion of beef meat was used. Proper impedance matching is also attained by the capacitive coupling at resonant.