## 4 Designs of Wirelessly-Powered Cage for mm-Sized Implants
At high frequency, it also increases the risk of unsafe personnel exposure to EM radiation and interference with nearby instruments [40]. The microwave chamber delivers power at low efficiency through the animal body. Its extension to larger animals and eventually humans without surpassing SAR limits at this frequency does not seem to be feasible, even though it might be a reasonable solution for ultra-low-power applications. To address this issue, Figure 20 shows the wireless cage for powering tiny implants using inductive coupling at 13.56 MHz frequency with near-field communication hardware [41]. The wireless cage equips a double-loop coil with turns at heights of 4 and 11 cm from the bottom of the animal enclosure as shown in Figure 20a. This technique is similar with the design in Figure 15a, which improves the inductive coupling at further distance from the bottom of the cage resulting in the extension of wireless coverage to the higher side. The wireless cage covers \(30\times 30\times 11\) cm and the power is relatively uniform across the region of interest as shown in Figure 20b although the normalized received power in the implant decreases to 0-0.9 for 80\({}^{\circ}\) angular misalignment. The height for wireless coverage and its uniformity in coverage can be optimized for a given experimental area based on the separation of double-loop coil. While the wireless cage with high frequency is typically suffer from the SAR in the tissue, the wireless cage in the near-field regime has a difficulty in the reduction of Rx size due to the poor EM field focusing. In Figure 20b, the two-coil inductive link configuration results in the Rx coil with a diameter of 9.8 mm, which becomes the main limiting factor in the device miniaturization. In addition, the single Tx coil configuration, which can simplify the design of the power control circuitry, suffers from low PTE at the center of the cage. Therefore, they transmitted a large amount of power to energize the entire cage. This will result in excessive heat dissipation and large EM interference between the cage and other lab instruments. In Figure 21a, the dual-band EnerCage-HC system, built upon the EnerCage-HC2 system (Figure 11), uses a two-stage inductive power transmission technique to wirelessly deliver sufficient PDL to the tiny implant within the cage [42]. Given the mismatch between the optimal operating frequencies of the Tx and Rx coils due to their size difference, the dual-band EnerCage-HC system includes two inductive links operating simultaneously at their optimal frequencies. To do so, this system also includes an intermediate unit (a headstage device) in addition to the power Tx and Rx. This intermediate unit acts as a power converter, which receives power from the cage via a four-coil inductive link at 13.56 MHz and delivers it to the tiny implant via a three-coil inductive link at 60 MHz. This intermediate unit is also used as  
Figure 20: (**a**) Simulated normalized output power inside the wireless cage without angular misalignment, and (**b**) wireless operation of 13 implants, placed at height of 3 cm from the bottom [41]. a power buffer that uses its stored energy to support the continuous operation of the tiny implants in the presence of misalignments in the four-coil inductive link. A dual-loop sequential CLPC mechanism, which includes two loops, further stabilizes the amount of power delivered to the tiny implant despite any misalignments by adjusting PDLs of the four-coil inductive link and the three-coil inductive link at designated levels, respectively. Figure 21b shows the final in vitro measurement setup for enabling optical stimulation in the dual-band EnerCage-HC system. The experiment was performed on \(6\times 6\times 7\) cm cube from a fresh sheep head, including brain, skull, fat, and skin, and placed the cube in the center of the cage. The headstage was mounted on top of the tissue cube to receive 13.56 MHz carrier from the EnerCage-HC coils while the mm-sized implant was placed \(\sim\)2 mm under the skull.