## 1 Introduction  
Implantable medical devices (IMDs) have been developed for behavioral neurosciences which research on small freely moving animal subjects, such as rodents [1; 2; 3; 4; 5]. Conventional hardwired IMD in Figure 1a, which is restricting experiments for freely behaving animal subjects [1; 2], have been replaced with battery-powered IMDs. However, the battery needs be replaced after 2-4 h animal experiments [3; 4], resulting in the interruption for continuous and smooth flow of the experiments, as shown in Figure 1b. When the IMD is fully implanted inside the animal body, the risk for replacing the battery dramatically increases due to the potential infection in the animal body during the surgery.  
In an attempt to overcome the limitations imposed by the replacement of battery in IMDs, the wirelessly-powered cage systems have been developed to recharge the batteries without detaching the IMD from the animal during the experiment. However, since these systems are designed to provide the wireless charging in the fixed distance, as shown in Figure 1c, similar to the mobile phone chargers, they are still not suitable for longitudinal animal studies over the span of several days, weeks, or months [5]. Therefore, several concepts of the wirelessly-powered cage have been proposed to extend the wireless coverage and also provide the homogenous power transfer efficiency (PTE) while the small animal is freely behaving inside the cage, as shown in Figure 1d. The key techniques for the wirelessly-powered cage include: (1) the optimized transmitter (Tx) coil design for the extended area; (2) the closed-loop power control (CLPC) for safe wireless power transmission (WPT) [6]; (3) the compensation technique for spatial/angular misalignments of receiver (Rx) coil; (4) the animal tracking technique for the scalability of wireless coverage; and (5) near-field data transmission, while other technologies can also be considered depending on the intended medical applications. Despite the exciting current art, all the requirements of an application involving high-performance or mm-scaled IMDs cannot be addressed by a current existing wirelessly-power cage design. It is important for designers to select suitable wirelessly-powered platforms considering their practical limitations with respect to the IMD design. This review article focuses on the overview of related technologies in recent wirelessly-powered cage platforms including the fundamental principles and practical considerations and provides the guidelines for designers to customize the appropriate wirelessly-powered cages with respect to their IMD applications. The optimized wirelessly-powered cage for the target application based on this article will enable the automated, high throughput, and long-term experiments in a large number of parallel standard cages or in a cage with specific shape for single or multiple animal subjects.  
In this article, these key technologies for wirelessly-powered cages are categorized and discussed with the practical design considerations that include the CLPC, coil design/optimization, scalability for wireless coverage, special/angular misalignment, near-field data telemetry, and safety issues. In Section 2, the main blocks for wirelessly-powered cage systems are introduced with the practical considerations related to the key technologies. Section 3 provides the different designs of wirelessly-powered cages with the performance comparison. Section 4 introduces the state-of-the-art wirelessly-powered cages for mm-sized IMDs, followed by a conclusion.  
Figure 1: Neural interfaces categorized by power sources: (**a**) hard-wired, (**b**) battery-powered, (**c**) wirelessly-powered at a fixed distance, and (**d**) wirelessly-powered neural interfaces for freely-moving animal.