# Self-Sustainable Biomedical Devices Powered by RF Energy: A Review
## 2 RF Energy Harvesting Principles
### Antenna Design and Characteristics for RF Energy Harvesting
#### 2.1.1 Wearable Antenna Design for RF Energy Harvesting  
The integration of an antenna in wearable medical devices must be conformal and nonobstructive. The mechanical deformations of the wearable antennas affect the bandwidth and center frequency, especially when the antenna operates in a narrow band, where even a small change could detune the frequency of optimization. The wearable antenna must be isolated from the human body loading effect and absorptive losses by the ground plane. Different types of dielectric and conductive materials are used to construct wearable antennas. The selection of these materials is driven by their ability to offer a sufficient number of mechanical deformations (twisting, bending, and wrapping) with little influence from various environmental factors (rain, snow, ice, etc. ), as well as adequate protection from EM radiation. Recently, several types of fabric and nonfabricic materials have been used to fabricate wearable antennas; however, these fabric materials need proper characterization before being used for such purpose [60]. Given that textiles are widely used and available, several wearable antennas for RFEH are fabricated based on textile materials such as felt fabric, polyester cotton, and Cordura [61; 62; 63]. These textile antennas are designed with different topologies such as monopole antenna and patch antenna, operated with a single band, and dual-band. On the other hand, numerous wearable antennas for RFEH used nonfabricic flexible polymer-based materials due to their stable dielectric properties. For example, the work reported by [64] proposed a dual-band dipole antenna printed on Kapton, while in [65], a monopole antenna printed on a polyethylene terephthalate (PET) film. A dualband transparent patch antenna fabricated on polydimethylsiloxane (PDMS) substrate was reported by [66]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Features** & **Resonant Coupling** & **Inductive Coupling** & **Far-Field Transfer** \\ \hline Field & Resonance & Magnetic method & Electromagnetic \\ Scheme & Resonator & Coil & Antenna \\ Efficiency & High & High & Low to high \\ Distance & Medium & Short & Short to long \\ Frequency & KHz to MHz & KHz to MHz & GHz \\ Power & High & High & Low to high \\ Typical load & Fixed impedance & Varying impedance & Fixed impedance \\ Regulation & Under discussion & Under discussion & Radio wave \\ Pros & Medium efficiency in a & High efficiency & Long distance \\  & short distance & Very short distance & Low efficiency and \\ Cons & Difficulties in preserving & & safety issues \\  & high Q & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Power transmission characteristics [58]. Furthermore, wearable antennas with potential use in RFEH must be stretchable to accommodate natural skin deformations or large human motions [67]. The detuning effect of the stretchable antennas significantly reduces their resonant frequency when they are stretched [68], thereby restricting their usage in strain sensing [69; 70; 71]. Additionally, their integration with the commercially available chips is difficult. Besides that, the radiation efficiency of the stretchable antennas degraded significantly due to the lossy human tissues [72; 73]. Consequently, stretchable antennas with high performance and insensitive resonance frequencies are required for RFEH and communication [74; 75; 76; 77; 78; 79]. In this context, [PERSON] et al. [80] designed a stretchable microstrip antenna with different 3D layouts for RFEH. The proposed antenna was fabricated by patterning a meshed patch attached with an elastomeric substrate (Ecoflex00-30) and a meshed ground with an ultraviolet laser. The 3D microstrip antenna achieved better stretchability, strain-insensitive resonance, and improved peak gain when compared to its 2D equivalent. However, further optimizations in the design of asymmetric 3D structures are required to achieve high on-body performance resistant to various mechanical deformations.