# Approach-Based Analysis on Wireless Power Transmission for Bio-Implantable Devices
## 3 Analysis of Various WPT Approaches
### WPT Systems Based on Different Transmitting and Receiving Coils Technique  
Normally, to attain a highly efficient power gain in WPT systems, the transmitting and receiving coils should be strongly coupled. However, in many cases, the presence of weakly coupled WPT cause an inefficient power transfer. Some of the WPT systems proposed for better efficiency on the basis of modified transmitting coils are grouped here and listed in Table 1.  
In article [19], the authors introduced an array technique to boost the system's gain for weakly coupled systems, which uses a multi-coil transmission array (Tr) to supply the receiving devices (Rr) in near-field. This is achieved by increasing the gain of the power at the self-resonant frequency and creating the effective phase factors of the elements of the transmission array. Here, a (1 \(\times\) 1) cm\({}^{2}\) Rr coil at a space of 20 cm of power gain has been implemented along with a (3 \(\times\) 3) Tr coil array consisting of nine (3 cm \(\times\) 3 cm) elements, plus a single big Tr coil with the equivalent dimensions as the matrix for a neutral comparison. With this set-up, the power gain of the converter increased to up to 16.4 dB. Figure 1: Basic WPT model. Another approach of double-sandwiched coils for both Tr and Rr is discussed in [20] in which a low-resonant frequency of 160 kHz is used so that the safety level of patients is extended. This system is especially useful for wireless charging of the battery of medical micro-robots for pacemakers. The design of the sandwiching form of Tx coil, which is demonstrated in Figure 2, accomplishes the feasibility level and flexibility status of the spacing distance for various types of micro-level medical robotics and provides a power of up to 5 W and a PTE of utmost 88%. Moreover, to address the issue of poor efficiency and less transmission coverage area of WPT in implantable devices, ref. [21] presented a system with a switchable pi-match network, where a new two-coil system was used. In that two-co \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Approach** & **Operating Frequency** & **Performance** & **Remarkable Observation** \\ \hline \multirow{4}{*}{[19]} & System with multi-coil transmitting array technique & & & (i) & 16.4 dB power gain attained; \\  & & & 0.66\% efficiency & (ii) & Covering distance of WPT \\  & & & (3 \(\times\) 3 array) & & is 200 mm; \\  & & & & (iii) & The higher the array level, \\  & & & & & the more the gain improves. \\ \hline \multirow{4}{*}{[20]} & System with sandwiched transmission coil technique & & & Load power up to 5 W; 8\% PTE; high flexibility & (i) & Specifically for cardiac pacemakers; \\  & & & & 88\% PTE; high flexibility & (ii) & Low-resonant frequency \\  & & & & & is advantageous \\ \hline \multirow{4}{*}{[21]} & WPT system with switchable pi-match network & & & Tr distance— up to 180 mm; & \\  & & & & Constant output voltage— 16.8 V; & \\  & & & & Efficiency: 78\% & \\ \hline \multirow{4}{*}{[22]} & The tissue channel technology & & & PTE: 0.39\% for 1 mm implant with depth of 5 cm in pork tissue & Uses body tissue as an energy transfer channel but implant should be in contact with tissue. \\ \hline \multirow{4}{*}{[23]} & Transmitting coil with optimal radius ratio & & & \\  & & & & Up to 77\% PTE & No need to utilize multi-coil adoption \\ \hline \multirow{4}{*}{[24]} & Capacitor-segmented coils & & & Up to 31\% PTE; & SAR: 5 times lower than \\  & & & & Coverage: 20 mm & non-segmented coils \\ \hline \multirow{4}{*}{[25]} & \multirow{4}{*}{3-coil approach} & & & Up to 48\% PTE; & \\  & & & & Power: 80 mW; & \\  & & & & Coverage: 15 cm & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: WPT systems based on different transmitting and receiving coil technique. Figure 2: Sandwiching arrangement of coils on transmitter and receiver of WPT system. 90 mm external diameter and Secondary has a 65 mm external diameter.