# Wireless Power Transfer Approaches for Medical Implants: A Review
## 2 Different Approaches for a Wireless Power Transfer System
### Microwave-Based Wireless Power Transfer  
Another way to efficiently transmit power wirelessly over long distances in the order of meters to kilometers is microwave power transmission. Figure 4 illustrates the external and implanted antennas' behavior. It should be noted that up-link is defined when the implanted antenna acts as a transmitter and the external antenna act as a receiver, whereas down-link is vice versa. Assuming far-field WPT, the budget power link as discussed in [71] can be described as follows:  
\[ \begin{array}{c}\textit{Link margin}\ (\mathrm{dB/Hz})=\ \left(\frac{C}{N_{o}} \right)_{Link}-\ \left(\frac{C}{N_{o}}\right)_{Required}\\ =\ P_{ta}+\ G_{tg}-\ L_{f}+G_{ra}-N_{0}-\frac{E_{t}}{N_{o}}-10\log B_{r}+G_{c} \ -G_{d}\end{array}  \tag{5}\]  
where \(P_{ta}\). is the transmitted power in dBW, \(G_{tg}\) is the transmitting antenna gain in dBi, \(L_{f}\) is the path loss in dB, \(G_{ra}\) is the receiving antenna gain in dBi, \(N_{o}\) is the noise power density in dB/Hz, \(B_{r}\) is the bit rate in kb/s, \(G_{c}\) is the coding gain in dB, and \(G_{d}\) is the fixing deterioration in dB. The path loss can be calculated through the equation below, taking into consideration that the free-space signal strength reduces with the increase in distance between the transmitter and receiver:  
\[L_{f}=20\log\!\left(\frac{4\pi d}{\lambda}\right) \tag{6}\]  
where \(d\) is the distance between the transmitter and the receiver and \(\lambda\) is the wavelength. Considering the impedance mismatch losses,  
\[L_{impedance}=-10\log\!\left(1-r^{2}\right) \tag{7}\]  
where \(r\) is the appropriate reflection coefficient. Both \(L_{f}\) and \(L_{impedance}\) are considered for more accurate evaluation. The received power by the receiver can be calculated as follows:  
\[P_{r}=\ P_{ta}+\ G_{tg}+G_{ra}-\ L_{f}-\ L_{impedance}-\ e_{p} \tag{8}\]  
Figure 4: Microwave wireless power transmission (WPT) principle. where \(e_{p}\) is the polarization mismatch loss between the transmitter and the receiver. Equation (8) can be also described as follows:  
\[P_{r}=\ \frac{G_{tg}G_{n\lambda}\lambda^{2}}{\big{(}4\pi d\big{)}^{2}}\Big{(}1-|S_{ 11}|^{2}\Big{)}\big{(}1-|S_{22}|^{2}\Big{)}e_{p}\times P_{t} \tag{9}\]  
In practice, the received power value for microwave design can be extracted from the value of  
\[|S_{21}|^{2}=\ \frac{P_{r}}{P_{t}} \tag{10}\]  
It should be noted that some amount of transmitted power will be dissipated in tissue due to radiation and coupling into the body [82]. The implant placement depth plays a key role in the amount of lossy power led by the body tissue. The present-day challenges for this technique include the minimization of energy loss, protecting both humans and animals against exposure to excessive microwave radiation, and the reconfiguring of a wireless transmission system resulting from modifications such as a shifting in range between transmitter and receiver [83]. Microwave WPT can transfer a high amount of power between the transmitter and the receiver circuits. However, it is worth mentioning that human tissues cause problems for the propagation of electromagnetic fields and dilute the electrical field. Therefore, the reflection caused by the lossy mediums reduces the overall power transfer efficiency. Pacemaker implantation is a popular method of curing people with cardiac insufficiency. However, the lifetime of the pacemaker is restricted to the lifespan of the battery and the installation of a subcutaneous pocket [13]. [PERSON] et al. [13] built a rectenna-based leadless pacemaker prototype. For energy transmission to the implanted unit, a wearable transmitting antenna range was fabricated to evaluate the system's efficiency through Vivo electrocardiogram (ECG) outcomes.