## 3 Designs of Wirelessly-Powered Cage Systems
### Wirelessly-Powered Cage Systems with Single Tx Coil Configuration  
The single Tx coil configuration can simplify the design of the driver control in the wirelessly-powered cage systems, significantly improving the system robustness. However, the following examples of the wirelessly-powered cage systems with such configuration also suffer from other limitations, which we will discuss in detail [18; 28; 29; 30]. Figure 14(a),b show two wirelessly-powered cage designs, each of which a large Tx coil wraps around the cage to enhance the magnetic field within the volume covered by the Tx coil [28; 29]. In Figure 14(a), the Tx coil comprises two series-connected coaxial rectangular windings, generating an effective operating zone of \(40\times 24\times 4\) (height) cm [28]. The center of such an operation zone is 7 cm height from the bottom of the cage to cover the vertical movement of the mobile device while the animal is walking and standing. A \(\pi\)-capacitor matching circuit, which matches the Rx coil to the load, is also designed to ensure the wireless power delivery maintaining at the maximum PDL condition. Figure 14: Examples of EM simulation of SAR limits for (a) lamb’s head layers [25] and (b) a rat model made entirely out of brain tissue [14]. In Figure 15b, the Tx coil completely encompasses the area in which the animals can roam freely, resulting in an effective space of \(40\times 40\times 20\) (height) cm\({}^{3}\)[29]. These two designs can improve the WPT resilience to lateral and vertical misalignments between the Tx and Rx coils. However, the angular misalignments still can negatively impact the PTE of the inductive link. Moreover, due to huge size difference, the pair of Tx and Rx coils is away from their optimal size matching, resulting in the low PTE of these two designs. For a larger experimental arena, the size of the Tx coil will be enlarged accordingly, which will aggravate the coil size mismatch and then the PTE reduction. For the designs in Figure 15a,b, the Tx coil covers the 3D volume of the experimental arena, which can ensure the strength of the magnetic field within the covered volume. For the wirelessly-powered cage design shown in Figure 15c, the Tx coil is located underneath the circular mouse cage with a diameter of 8 inches, and the power Rx is a 15 mm-long, 2 mm-diameter solenoid coil wrapping around a copper ferrite core [30]. The coil size mismatch is eased in this design, at the cost of smaller wirelessly-powered arena, compared to the designs in Figure 15a,b. The Rx coil continuously receivers a large amount of power of 2 W over distances of several centimeters within the cage. Given that the inductive link is non-optimal, it is challenging to ensure the wireless power transmission safety by not overcoming the SAR limitation in the case of strong power delivery. In this wirelessly-powered cage system, the Tx coil also needs to cover the bottom area of the circular mouse cage. Such a configuration is also not conductive for the system scalability. Compared to the abovementioned wirelessly-powered cage designs which suffer from the size mismatch between the Tx and Rx coils, the wirelessly-powered cage in Figure 15d does not have this issue [18]. The coils in the inductive link are geomecally optimized. Besides, the optimum alignment between the Tx and Rx coils is also achieved. In Figure 15d, the IRPower system uses a servo-controlled Tx coil moving under the cage on the X and Y rails. A permanent magnet in the mobile device and  
Figure 15: Examples of wirelessly-powered cage designs with a single Tx coil configuration presented in (**a**) [28], (**b**) [29], (**c**) [30], and (**d**) [18]. an array of magnetic sensors placed around the Tx coil form the animal tracking system. Given the real-time optimized size and optimized alignment of the Tx and Rx coils, the PTE of this inductive link design is high. The IRPower system in Figure 15d also features a CLPC, which compensates the PDL variations by adjusting the amount of power transmitted into the Tx coil.