**1. Introduction**

Implantable medical devices (IMDs) have been developed for behavioral neurosciences which research on small freely moving animal subjects, such as rodents [1–5]. 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.

*Electronics* **2020**, *9*, 1999 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 freelymoving animal. **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.

**2. Main Blocks for Wirelessly-Powered Cage System** Figure 2 shows a simplified block diagram of the wirelessly-powered cage system and the IMD attached to or implanted in the freely-moving animal subject. The wirelessly-powered cage system typically includes a main controller, a power amplifier (PA), a pair of data Tx/Rx, a Tx coil array, and 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.

#### **2. Main Blocks for Wirelessly-Powered Cage System**

Figure 2 shows a simplified block diagram of the wirelessly-powered cage system and the IMD attached to or implanted in the freely-moving animal subject. The wirelessly-powered cage system typically includes a main controller, a power amplifier (PA), a pair of data Tx/Rx, a Tx coil array, and a position sensor to localize the Rx coil in the IMD. The wireless link for the power and data transmission is established between the Tx coil array and the Rx coil through the skin/air while the PA driving the Tx coil array tuned at the power carrier frequency, *fp*. The amount of wireless power driven from the PA is typically controlled by the main controller, namely PC or microcontroller unit (MCU), depending on the amount of received power by the Rx. Unlike the WPT system including Tx and Rx coils with fixed distance, the coupling between the Tx and Rx coils in the wirelessly-powered cage is varying due to the movements of the animal subject, resulted in the received power variations. The CLPC, composed of the received data from the IMD, the main controller, and the PA, increases the Tx power in the PA until enough power is delivered to the Rx while it reduces the Tx power when the Rx receives enough power. *Electronics* **2020**, *9*, x FOR PEER REVIEW 3 of 28 a position sensor to localize the Rx coil in the IMD. The wireless link for the power and data transmission is established between the Tx coil array and the Rx coil through the skin/air while the PA driving the Tx coil array tuned at the power carrier frequency, *fp*. The amount of wireless power driven from the PA is typically controlled by the main controller, namely PC or microcontroller unit (MCU), depending on the amount of received power by the Rx. Unlike the WPT system including Tx and Rx coils with fixed distance, the coupling between the Tx and Rx coils in the wirelessly-powered cage is varying due to the movements of the animal subject, resulted in the received power variations. The CLPC, composed of the received data from the IMD, the main controller, and the PA, increases the Tx power in the PA until enough power is delivered to the Rx while it reduces the Tx power when the Rx receives enough power. 

**Figure 2.** Simplified block diagram of the wirelessly-powered cage system as a stationary unit and the IMD as a mobile unit attached to or implanted in the freely-moving animal subject. **Figure 2.** Simplified block diagram of the wirelessly-powered cage system as a stationary unit and the IMD as a mobile unit attached to or implanted in the freely-moving animal subject.

The Tx coil array is one of the important design considerations in the wirelessly-powered cage system to provide the high and homogeneous PTE and power delivered to the load (PDL) within the cage. The Tx coil array can be optimized for a designated arena while it also can be extended for a larger area with the modular type design. Although the achievable PTE and PDL by the inductive link are important for the Tx coil array, the homogeneity of the PTE across the cage should also be considered due to the animal subject's freely movements. In most of the wirelessly-powered cages with the Tx coil array, the position sensor selects the nearest Tx coil among the Tx coil array to the Rx coil. Since only the selected Tx coil will be activated, this position sensing mechanism helps to reduce power loss significantly. Furthermore, each single Tx coil in the modular design of Tx coil array is optimized with the Rx coil to improve the PTE within the entire wirelessly-powered arena. The nearfield data telemetry between the Tx and Rx coils can be used to control the IMD, monitor the received power, and/or acquire the biomedical data from the IMD. Even though far-field communication has the advantage of longer data transmission distance, near-field communication is regarded as a more suitable method in terms of power saving in IMDs. Given that the IMD is always located within the wirelessly-powered cage or arena, the coupling between Tx and Rx coils is always enough to deliver both power and data [7]. Here are some practical considerations for different types of wirelesslypowered cage used in the freely-moving animal experiments. The Tx coil array is one of the important design considerations in the wirelessly-powered cagesystem to provide the high and homogeneous PTE and power delivered to the load (PDL) within the cage. The Tx coil array can be optimized for a designated arena while it also can be extended for alarger area with the modular type design. Although the achievable PTE and PDL by the inductivelink are important for the Tx coil array, the homogeneity of the PTE across the cage should also be considered due to the animal subject's freely movements. In most of the wirelessly-powered cages with the Tx coil array, the position sensor selects the nearest Tx coil among the Tx coil array to the Rx coil. Since only the selected Tx coil will be activated, this position sensing mechanism helps to reduce power loss significantly. Furthermore, each single Tx coil in the modular design of Tx coil array is optimized with the Rx coil to improve the PTE within the entire wirelessly-powered arena. The near-field data telemetry between the Tx and Rx coils can be used to control the IMD, monitor the received power, and/or acquire the biomedical data from the IMD. Even though far-field communication has the advantage of longer data transmission distance, near-field communication is regarded as a more suitable method in terms of power saving in IMDs. Given that the IMD is always located within the wirelessly-powered cage or arena, the coupling between Tx and Rx coils is always enough to deliver both power and data [7]. Here are some practical considerations for di fferent types of wirelessly-powered cage used in the freely-moving animal experiments.
