# Self-Sustainable Biomedical Devices Powered by RF Energy: A Review
## 2 RF Energy Harvesting Principles  
RFEH is enabled by ambient, dedicated, or unidentified RF signal sources. The amount of energy harvested using RFEH is dependent on the transmission power, the RF signal's wavelength, and the distance between the RF power source and the harvesting node. The harvested power at the receiving antenna can be computed using the Friis equation as given in (1) for a transmitter and receiver antenna with a line of sight propagation [49]. \[P_{r}=\frac{P_{t}G_{t}G_{r}\lambda^{2}}{(4\pi d)^{2}L} \tag{1}\]  
where \(P_{r}\) signifies received power and \(P_{t}\) indicates transmitted power, \(G_{t}\) denotes the transmitter of the antenna and \(G_{r}\) represents receiver gain of the antenna, \(\lambda\) refers to transmitted wave wavelength, \(L\) denotes the path loss factor, and \(d\) defines the distance between transmission and receiving antenna. Free space loss represents the loss in the signal strength and is computed by determining the distance between the transmitter and receiver, the transmitting frequency, and the antenna gain [50]. The equation to compute the free space path loss is given in (2-3):  
\[PL=\frac{(4\pi fd)^{2}}{G_{t}G_{r}\lambda^{2}}=\frac{(4\pi fd)^{2}}{G_{t}G_{R}c^ {2}} \tag{2}\]  
\[PL(dB)=32.44+20 log_{10}(f)+20 log_{10}(d)-G_{t}-G_{R} \tag{3}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Energy source** & **Power density** & **Efficiency (\%)** & **Advantage** & **Bottleneck** \\ \hline
[30; 31] & Perovskite solar cells & 35.0 \(\mu\)W/cm\({}^{2}\) & 25.2 &
\begin{tabular}{c} Flexible and \\ lightweight, suitable \\ for wearable \\ \end{tabular} & Require light \\  & & & & applications & \\ \hline
[32; 33; 34; 35] & Thermoelectric & \
\begin{tabular}{c} Human: 100 \(\mu\)W/cm\({}^{3}\) \\ Industrial: 100 mW/cm\({}^{3}\) \\ \end{tabular} & 10–15 &
\begin{tabular}{c} Cost-effective \\ technology; does not \\ require body motion \\ or light \\ \end{tabular} & Low power source \\  & & & & & or light \\ \hline
[36; 37] & Acoustic & 1.436 mW/cm\({}^{2}\) at 123 dB & 0.012 & \
\begin{tabular}{c} Require minimum \\ maintenance; suitable \\ to be used in remote \\ or inaccessible \\ locations \\ \end{tabular} &
\begin{tabular}{c} Hard to capture \\ energy from the \\ sounds wave source \\ \end{tabular} \\ \hline
[35; 38; 39; 40] & Pyroelectric & \
\begin{tabular}{c} 3.5 \(\mu\)W/cm\({}^{3}\) \\ at the temperature of 85 \({}^{\circ}\)C/s \\ @0.11 Hz \\ \end{tabular} & 1–3.5 &
\begin{tabular}{c} Cost-effective \\ technology; \\ ubiquitous and \\ serves as a low-grade \\ waste \\ \end{tabular} & Low output power \\ \hline
[41; 42; 43] & Piezoelectric & 29.2 \(\mu\)W/mm\({}^{3}\) & 83.3 & \
\begin{tabular}{c} Does not require RF \\ waves or light \\ \end{tabular} &
\begin{tabular}{c} low power source; \\ require body activity \\ \end{tabular} \\ \hline
[44; 45; 46] & Biofuel cells & 3.7 mW cm\({}^{-2}\) & 86 & \
\begin{tabular}{c} The integration of the \\ power module and \\ sensing module \\ results in better \\ compactness ; does \\ not require RF waves, \\ body activity, or light \\ \end{tabular} &
\begin{tabular}{c} The analytic \\ concentration \\ influences the power \\ density \\ \end{tabular} \\ \hline
[47; 48] & Triboelectric & 2.5 W/m\({}^{2}\) & — & \
\begin{tabular}{c} Simple fabrication \\ process and low cost \\ \end{tabular} &
\begin{tabular}{c} low power source; \\ require body activity \\ \end{tabular} \\ \hline
[38] & RFEH & GSM: 0.1 \(\mu\)W/cm\({}^{2}\) & 50–70 &
\begin{tabular}{c} Does not require light \\ or body motion and \\ is continuously \\ available \\ \end{tabular} & Low output power; \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of various sources for energy harvesting. where _PL_ stands for the free space path loss, \(c\) signifies the speed of light, and \(f\) denotes the frequency of the transmitted wave.