## 2 Main Blocks for Wirelessly-Powered Cage System
### Specific Absorption Ratio (SAR) and Safety Issues  
The transmitted power from the Tx coil should be less than allowable specific absorption rate (SAR) limit, determined by the induced electric field intensity, E, in tissue when exposed to a RF EM field. There are mainly two official international safety standard establishment organizations which have deeply impacted each country for their safety standards. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has the series exposure guideline for each E-field and H-field. The recent ICNIRP (2010) used the induced E-Field instead of induced current density metric. Meanwhile, the guideline also provided the exposure limits in central nervous system (CNS) tissues of the head and other tissues of the body. Institute of Electrical and Electronics Engineers (IEEE) also provided [23] for exposure standard. Compared with ICNIRP guideline, IEEE standard is widely used in US, Canada, Japan, Korea, etc. It also has basic restriction and reference restriction. When setting protective limits for localized tissue heating due to the difficulty of SAR measurement, the point SAR is mass averaged, in recognition of the thermal diffusion properties of tissue. The averaging mass in ICNIRP is any 10 g of contiguous tissue. IEEE standard specifies a smaller 1 g averaging mass in the shape of a cube. The IEEE limits differ from the ICNIRP guidelines in both the maximum level specified which are 100 W/mm\({}^{2}\) for IEEE and 50 W/m\({}^{2}\) for ICNIRP, and the frequency at which these peak values reach are 2000 MHz for ICNIRP and 3000 MHz for IEEE [24].  
Figure 13: (**a**) The design of two crossed-dipole coils for Tx-D (Tx1, _x_-axis) and Tx-Q (Tx2, _y_-axis) coils for generating omnidirectional magnetic fields, and the simulated rotating magnetic flux at relative angular frequency of (**b**) 0 (**c**) \(\pi\)/4, and (**d**) \(\pi\)/2 [22].  
SAR can be calculated by \(\sigma|\text{E}|^{2}/\rho\), where \(\sigma\) and \(\rho\) are tissue conductivity and density, respectively. Since the SAR limit allows the less amount of RF exposure for the higher carrier frequency, the designer also needs to select a suitable power carrier frequency for the wirelessly-powered cage system based on the expected maximum power used in the Tx and SAR limit for safety, prior to the PTE or PDL optimization. RF exposure can be evaluated by the simulation using a \"phantom\", which emulates the electrical characteristics of the human head, body, or tissue. In [25], the EM simulator, namely HFSS, is utilized to estimate the peak of average SAR values of tissue layers when a mm-size Rx coil implanted in the tissue receives 1.3 mW power. Under different settings of tissue layers, the SAR values with the WPT configuration of a two-coil link, a three-coil link with loop coils, and a three-coil link with segmented coils are compared, as shown in Figure 13(a). Figure 13(b) shows the SAR simulation results with Poynting vectors (the real component of the power density) generated by the EnerCage-HC2 when the Tx power is 1 W for the rat model. In this simulation setup, the rat model is made entirely out of brain tissue with \(\sigma\) = 105 S/m and \(\rho\) = 1040 kg/m\({}^{3}\), which generates the worst case for SAR simulation [14].  
To ensure the safety of EM exposure, SAR can be simulated by various EM simulators, such as HFSS, in the design stage as shown in Figure 14. Additionally, other measures include monitoring the tissue heating using temperature sensor in the IMD [26; 27]. Based on the tissue heating information sent from the IMD, the wirelessly-powered cage can limit the amount of transmitted power to conform to the SAR limitation and prevent tissue damage from the abnormal heat generation of the IMD.