## 4 Designs of Wirelessly-Powered Cage for mm-Sized Implants  
Instead of a single, large, and centralized mobile device [37; 38], distributed neural interfaces made of a large number of tiny implants are poised to play a key role in future brain-computer interfaces (BCI) because of their less damage to the surrounding tissue. However, it is challenging to wirelessly operate the tiny implants in a cage. As a result of the significant size difference, the coupling between the Tx and Rx coils is quite weak. The optimal operating frequency for a mm-sized coil to maximize its quality factor is within several tens to hundreds of megahertz range. At such high operating frequencies, it also increases the risk of unsafe exposure to EM radiation and interference, which, in turn, imposes new challenges on SAR compliance. The misalignments caused by free movements of the animal subject and/or the arbitrary placement of the tiny implants, which combine with the other challenges, make it challenging to maintain sufficient power delivery to the tiny implants within a cage. Thus, it is important to develop WPT systems that can overcome these challenges. A wireless cage based on the resonant cavity was introduced in [39] to power and control tiny implanted devices. An aluminum resonant cavity, which has 21 cm diameter and 15 cm height with a surface lattice of hexagons (2.5 cm diameter), is utilized to couple EM energy with 1.5 GHz carrier frequency to the tissue of a mouse as shown in Figure 19a. Compared to the conventional inductive power transfer system through direct coupling between two coils, the energy in this system is confined to the mouse placed on the grid due to the resonance excitation of the limited EM field pattern. This system can provide self-tracking wireless power to the tiny implant within the mouse resulting in no needs of tracking algorithms over the experimental area. Figure 19b shows a schematic of wireless implant for optogenetic stimulation, designed in the size of 10-25 mm\({}^{3}\). This light-emitting implants can receive 5.6-15.7 mW with 3.2 W of averaged input power. Although the wireless operation in RF bands are typically susceptible to signal reflection, interference, and absorption by obstructions, the proposed system in [39] has been successfully demonstrated by behavioral testing environments and has shown the possibility of using RF bands for wireless cage applications. Figure 19: (**a**) A diagram of wirelessly-powered cage system, and (**b**) a schematic of wireless implant customized for the brain in [39]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Publications** & [34] & [14] & [35] & [36] & [22] \\ \hline Mobile device size, length \(\times\) width \(\times\) height & \(\pi\times(7)^{2}\times 25\) mm\({}^{3}\) & \(20\times 22\times 11\) mm\({}^{3}\) & \(-64\times 64\times 64\) mm\({}^{3}\) & N/A & \(20\times 20\times 0.5\) cm\({}^{3}\) \\ PTE & 14.32\% & 23.6–33.3\% & 7.9\% & N/A & 33.6\% \\ PDL & 6.1–13 mW & 42 mW & 1.4 W & N/A & 10 W \\ CLPC & \(\times\) & \(\surd\) & \(\times\) & \(\times\) & \(\times\) \\ Scalability & \(\times\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) \\ Tx control simplicity & \(\surd\) & \(\surd\) & \(\surd\) & \(\times\) & \(\times\) \\ Compatible to cracks & \(\surd\) & \(\surd\) & \(\surd\) & \(\times\) & \(\times\) \\ Blind spots & Center + 4 corners & Center lines & 2 corners & None & None \\ No view-blocking & \(\times\) & \(\surd\) & \(\surd\) & \(\times\) & \(\surd\) \\ No Rx access blocking & \(\surd\) & \(\surd\) & \(\times\) & \(\surd\) & \(\surd\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._EM WPT at high frequency in the GHz range facilitate the Rx size reduction due to shorter wavelength, but they suffer from difficulties in creating homogeneous WPT in a large experimental arena [40]. The SAR of EM field in the tissue, which mainly consists of water, at high frequencies is rather high [23].