# Self-Sustainable Biomedical Devices Powered by RF Energy: A Review
## 3 RF Energy Harvesting Application in Medical Devices
### RF Energy Harvesting for Implantable Medical Devices
#### 3.2.4 Discussion  
In the former section of this paper, the IMDs whose power needs meet with RFEH technology were reviewed. Table 6 summarizes the characteristics of the recently reported devices. The transmitting method, test model, size, frequency, depth, and transfer distance are presented, along with the harvested power and PTE. In this subsection, we will compare the wireless power transfer methods and discuss the safety regulations and packaging materials for IMDs. **Wireless Power Transfer Methods**: The near-field inductive coupling WPT method transfers a large amount of power to IMDs with a depth of approximately 10-30 mm. However, the performance of the link in this method deteriorates significantly due to high-frequency losses of the human tissues [204]. Furthermore, the distance between the Tx power source and the Rx implant in this method cannot surpass a few centimeters, and proper alignment is needed between the internal coils and external coils, which is difficult to implement in some cases [205]. On the other hand, the near-field magnetic resonant coupling method provides high PTE. However, the frequency employed in this technology is quite low, hence limiting the channel capacity and the potential data rate that are required for numerous applications. Additionally, this method requires precise alignment and an extra component to be stuck in the skin to overcome the short communication range. \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Reference** & **Implantable Device** & **Method** & **Test Model** & **Size** & **Frequency (GHz)** & **Depth** & **Transfer Distance** & **Output Power** & **PTE (\%)** \\ \hline
[200] & Wireless charging system for an implanted capsule robot & Near field & Human torso & Diameter: 1 cm length: 2 cm length: 10 & 0.003 & — & — & 1 W & 10 \\ \hline
[84] & A Dual-Band Implantable Rectenna & Near field & Mixed Fork & 16 x 14 x 1.27 mm & 0.915 & 1 & 50 cm & 25 \(\mu\)W & 0.006 \\ \hline
[201] & Implantable wireless optogenetic device & Near field & Mouse tissue & 10 mm\({}^{3}\) tissue & 1.5 & 3 & — & 15.7 mW & 0.5 \\ \hline
[202] & Implantable loop antennas & Near field & Minced pork & 20 \(\times\) 10.5 mm & 0.433 & 3 & — & 1 mW & 0.1 \\ \hline
[85] & WPT system for multipurpose biomedical implants & Mid field & Minced pork & 9.4 mm\({}^{3}\) pork & 1.47 & 5.5 & 60 mm & 10.7 mW & 1.07 \\ \hline
[165] & Bioelectronic microdevices & Mid field & Pocine animal & 12 mm\({}^{3}\) animal & 1.6 & 4 & 4.5 & 0.45 mW & 0.06 \\ \hline
[191] & Broadband high-efficiency rectifier & Mid field & Pocine animal & 16 \(\times\) 11 mm\({}^{2}\) animal & 1.5 & 5.5 & 55 mm & 0.9 mW & 0.56 \\ \hline
[183] & Midfield WPT for Deep-Tissue Biomedical Implants & Mid field & Minced pork & 93.6 mm\({}^{3}\) pork & 1.5 & 4.5 & 55 mm & 5.6 mW & 0.56 \\ \hline
[203] & Multichannel passive neuronsing system & Mid field & Pig Skin & 1632 mm\({}^{3}\) & 2.4 & 0.25 & 2.5 mm & 0.6 mW & 15 \\ \hline
[194] & Implantable rectenna & Far field & Minced pork & 4 \(\times\) 8 mm & 2.45 & 0.3 & 50 mm & 1 W & 0.007 \\ \hline
[195] & Implantable miniaturized optoelectronic systems & Far field & Mouse tissue & 2 mm & 2.34 & 0.5 & — & 0.1 mW & — \\ \hline
[65] & Epidermal RF power harvester & Far field & Skin & 2160 mm\({}^{2}\) & 1 & — & 1.5 m & 32 mW & 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of implantable RF energy harvester devices. Midfield regime power transfer methods have a wider transmission range, but their efficiency is less than that of inductive coupling methods [165]. However, these devices allow greater implant depth, compared to the previous types of devices, at the expense of the device efficiency. They also require careful alignments between the equipment; the transmitting component of the system must also be equipped with an external power source, and this can affect the mobility of the user.