## 3 Designs of Wirelessly-Powered Cage Systems
### Wirelessly-Powered Cage Systems for Omnidirectional Power Transmission  
In the experiments involving small freely-moving animals, such as rodents, the horizontal, distance, and/or angular misalignments between the power Tx and Rx components may frequently happen due to the animal's free movements. The misalignments will result in a significant reduction in the PTE and the PDL. In this section, we will introduce several omnidirectional power transmission solutions that can address robust wireless power delivery in the presence of any position and/or orientation variations of the Rx device. One omnidirectional power transmission solution is a cavity resonator-based wireless cage design, as shown in Figure 18a [34]. On the power Tx side, the H-field produced is rotational around the center of the cavity. The amplitude of the H-field increases radially from the center to the periphery and is not a function of height. On the Rx side, the implantable device has two Rx coils oriented perpendicular to each other. This placement of the two Rx coils can address the device orientation insensitivity within two planes. To achieve truly omnidirectional powering, a third Rx coil oriented perpendicular to the first two Rx coils should be added. Even then, there are still five WPT blind spots, namely the center and four corners of the cavity, where the H-field is very weak. For the wirelessly-power cage system, shown in Figure 11, we call it EnerCage-HC2 system [14]. Based on the EnerCage-HC system shown in Figure 8a, the fifth Tx resonator is added at the top rim of the homecage to enhance the magnetic flux density at top of the cage. More importantly, the Rx resonators in the EnerCage-HC2 system encompass four diagonal planes of the mobile device. Such a novel Rx coil arrangement directs the transmitted EM field, which is already homogenized by the Tx resonators, towards the Rx coil at the bottom of the mobile device at any arbitrary orientations. Together with CLPC, the inductive link achieves omnidirectional power delivery to the mobile device, irrespective of the subject's location or head orientation. When the mobile device is 90\({}^{\circ}\) rotated while locating along the center lines of the homecage, the magnetic fields passing through the two Rx resonators are bent in the opposite direction and cancel with each other. In this case, little magnetic  
Figure 18: Examples of wirelessly-powered cage systems offering omnidirectional power transmission, presented in **(a)**[34], **(b)**[35], **(c)**[36], and **(d)**[22]. field can pass through the Rx coil, resulting in PTE drop. It should be noted that the probability of this case happening is quite low in practice. Therefore, we add a small supercapacitor following the rectifier in the mobile device to prevent any sudden drop in the supply voltage. In addition to the EnerCage-HC2 system, there are three other inductive link configurations that have been proposed to address omnidirectional wireless powering [22; 35; 36]. One possible coil configuration is a set of triple orthogonal Rx coils together with a single Tx coil. For achieving higher uniformity of magnetic flux, a pair of Tx coils facing each other can be used instead of a single Tx coil. On the Rx side, each Rx coil is followed by an AC-DC converter, such as a rectifier. The three rectifiers can be connected in series or in parallel to drive their common load. In the series connection scenario, the three DC output voltages of the rectifiers are stacked for providing supply voltage for the load. With the parallel connection scheme, the highest Rx coil voltage is automatically delivered to the load. In Figure 18b, two pairs of Tx coils facing each other are attached to the vertical sides of the cage and driven by their own class-D PA simultaneously [35]. On the Rx side, three coils are placed at 90\({}^{\circ}\) from each other, which can make the system insensitive to angular misalignments.