## 2 Main Blocks for Wirelessly-Powered Cage System
### Coil Design and Optimization
#### 2.1.2 Coil Optimization for Inductive Links Implemented in Wirelessly-Powered Cages  
For the abovementioned two categories of inductive link configurations, the optimization method for each category is different. First, we talk about the optimization method for the inductive link incorporating Tx coil array [11; 12]. Instead of having a single Tx coil, identical Tx coils are repeated and tiled at the bottom of the cage so that wireless power transmission covers the entire cage arena. One of the Tx coils, which is closest to the Rx, is activated, and together with the Rx coil forms a two-coil inductive link. Therefore, the optimization of the Tx coil and the Rx coil can refer to the optimization procedure of the conventional two-coil inductive link. Furthermore, other strategies in terms of the Tx coil array design are implemented to improve the homogeneous distribution of the electromagnetic (EM) field within the entire cage arena. As shown in Figure 7a, the effective area of a single Tx coil, where most of transmitted power are focalized, is located at the center of the Tx coil and has same distances from the edges of the Tx coil. The PTE drops at the boundaries of the adjacent Tx coils. Multi-layer Tx coil array is typically utilized to provide uniform power transmission over large areas. Instead of fully overlapping, one layer is shifted from the other layer, so that the point of intersection of every three adjacent coils is on the center of the coil in the previous layer, as shown in Figure 7a. With the configuration of multi-layer Tx coil array, the effective areas of the Tx coils cover the entire cage arena. Moreover, the Tx coil array should cover larger area than the cage arena so that the edges of the cage arena are still covered by the multi-layer Tx coil array for homogeneous wireless power transmission. As shown in Figure 7b, the overall PTE distribution across the multi-layer Tx coil array has variations within \(\pm 24\%\) of the average PTE. The higher PTE peaks are resulted from the Tx coils on layer 1, while the lower peaks are associated with the Tx coils in layers 2 and 3. On the one hand, the layer 1 is slightly closer to the Rx coil. On the other hand, the Tx coils in layers 2 and 3 are more overlapped and surrounded by other Tx coils. This condition leads to larger parasitic capacitance and resistance, resulting in the lower Q and PTE. Figure 7c shows the PDL distribution when the Rx is swept within the cage at the height of 70 mm. Thanks to the proposed configuration of multi-layer Tx coil array together with the CLPC mechanism, the PDL can be maintained at 20 mW with fluctuations of less than 2 mW. It should be noticed that one of the main disadvantages in this modular system is that one PA is required for each driving coil in the coil array, resulting in the increased complexity and cost of the Tx design. The other type of coil optimization is relevant to the inductive link incorporating Tx and/or Rx resonator, for instance the resonance-based four-coil inductive link implemented in the EnerCage-HC system families [13; 14]. The key factor in determining the Tx resonator geometries in EnerCage-HC system is the compatibility with dimensions of the standard-sized rodent homecage and the maximum overlap with the Tx coil. Instead of having an array of identical Tx coils tiled at the bottom of the cage,  
Figure 8: The configuration of the resonance-based four-coil inductive link implemented in (**a**) the EnerCage-HC system [13] and (**b**) the EnerCage-HC2 system [14]. (**c**) Measured PTE when the headstage is swept inside the homecage at the heights of 4 cm, 8 cm, 12 cm, 16 cm, and 20 cm. in the EnerCage-HC system, multiple Tx resonators wrap around the cage to provide wireless power coverage of the entire cage (see Figure 8).