## 1 Introduction  
Implantable devices with wireless connectivity has led to many advances in the field of biomedical applications, as they allow long term powering of devices within the human body [1; 2; 3; 4; 5; 6]. The principal power required by the medical implants such as cochlear implants, retinal prostheses, pacemakers, and neural recordings are different [7; 8; 9] and therefore powering these devices without damaging the tissues remain an arduous challenge. The conventional techniques used physical wirings and batteries to provide power to the implantable medical devices (IMDs). However, the probability of wires getting tangled is high and the batteries are larger in size and have a shorter span [10; 11]. These drawbacks of the conventional techniques result in surgical complications and infections. By employing wireless power transfer (WPT) technology in IMDs, we can potentially reduce the risk of infection and patient's discomfort.  
The 2-coil inductive link with transmitter and receiver coils is the most popular conventional method for wirelessly transferring power to biomedical implants [1; 12]. This system can transfer power only over a relatively short distance. In the recent past [2], the 4-coil resonance-based link operating in the midfield region was considered to be the efficient alternative to 2-coil inductive link as using the 4-coil resonance-based link increases the overall power transfer efficiency (PTE) of the system. However, the main drawback of the 4-coil resonance-based link is that the method increases the PTE by reducing the power delivered to the load (PDL). Instead, a 3-coil inductive link proposed in [13; 14; 15], provides a higher PTE without reducing the PDL. Figure 1a-c show the block diagrams of 2-coil, 3-coil and 4-coil links respectively.  
One of the key challenges in designing a WPT system for biomedical applications is to attain a maximum PTE without contravening the specific absorption rate (SAR) limits established by the Federal Communications Commission (FCC) [16]. A significant amount of research has been dedicated to the goal of designing a small-sized implant in optimal frequency range. The size of the implantable coil operating in the sub-gigahertz to low-gigahertz frequencies is typically between 1 to 2 mm [17; 18] and the size of the implantable coil can be increased by curtailing the frequency [4; 19]. The commonly suggested optimal operating frequency (\(f_{0}\)) for designing mm-sized coils to achieve higher PTE is above 100 MHz [20]. Despite the fact that the system achieves a higher PTE with mm-sized coil at \(f_{0}\geq 100\) MHz, a frequency higher than 20 MHz can adversely affect the tissues of human due to high electromagnetic radiations [10; 21]. So, in this paper, a 3-coil inductive link with a small-sized circular implant at a distance of 10 mm from the circular shaped external coil is designed to investigate the effects of low and high industrial, scientific, and medical (ISM) frequencies on SAR in tissue medium. 13.56 MHz is considered as a low ISM frequency in the near-field domain as it is FCC approved for industrial and medical applications and 40.68 MHz is considered as a high ISM frequency as it is more than the prescribed FCC limit. Additionally, the simulated SAR is analyzed with respect to different tissue composition, input power, and coil shape at 13.56 MHz and 40.68 MHz.  
The remaining paper is organized as follows: the equivalent structure of a 3-coil inductive link and orientation of the coils are discussed in Section 2. Design procedure and simulation results are presented in Section 3. Section 4 shows the measurement results, followed by the conclusion in Section 5.