# Wireless Power Transfer Approaches for Medical Implants: A Review
## 2 Different Approaches for a Wireless Power Transfer System  
The lifespan of IMDs is limited to battery capabilities. Patient pain and the danger of infection are the major development concerns in implantable medical systems because using implanted batteries can cause diseases [4]. Therefore, the WPT link is a safer option to power biomedical implants [4]. Improving WPT techniques and efficiency will enable rechargeable batteries to be employed for IMDs rather than non-rechargeable batteries, which usually have a greater weight and volume and a shorter period of effectiveness compared to rechargeable batteries. Medical implants like implanted spinal cord stimulators can use a rechargeable battery to improve their capability and reduce overall costs [5]. Lately, there has been a great interest in the usage of WPT for medical applications. The development of implantable electronic devices in biological systems has made it easier to use this technology for powering various IMDs, such as biological sensors, pacemakers, and neurostimulator, working in a range of power from a few microwatts to a few watts. In Figure 1, the power ranges of common IMDs are illustrated [1, 6]. The WPT systems for the neurostimulator and the pacemaker are discussed in detail in [7, 8, 9, 10, 11, 12, 13]. There are reliability problems with the classic wireless power links. An option that facilitates the growth of a number of bio-implants is the use of CMOS processes. In this procedure, the standard CMOS is included with the implanted receiver. This reduces the cost, improves productivity, and provides compatibility and reliability of prototypes [14, 15]. The usage of CMOS for WPT systems is presented in [15, 16, 17, 18, 19, 20, 21, 22, 23]. A backward communication unit transmits the information to an external data communicator using modulation. Typically, FSK [24], PSK [25], ASK [26], OOK [27], LSK [28], PPSK [29],  
Figure 1: Power ranges of implantable medical devices (IMDs). QMPM [30], QPSK [31], and impedance modulation [32] have been used for data communication units in medical applications. Long-term RF and microwave exposure are dangerous. The device layout must comply with the associated safety regulations to protect patients from electromagnetic radiation damage. It is possible to evaluate maximum permissible exposure (MPE) in environments for electromagnetic field intensity by assessing the specific absorption rate (SAR). IEEE Standard Basis C95.1 expresses SAR limitation. According to IEEE 1992 standard, the maximum SAR value must be below 1.6 W/kg for any 1 g of the body tissue and below 0.08 W/kg for the whole body. Nevertheless, the maximum SAR limitation is 4 W/kg for every 10 g of tissue of body parts such as hands, feet, ankles, and wrists, as per IEEE 2005 standard. The authors proposed an arrangement to decrease the SAR and improve safety for inductive WPT systems [33]. They designed a multi-transmitter configuration consisting of an array of symmetric resonant elements. The array will significantly decrease the amount of electric field generated by the fed loop and thus reduce the electromagnetic exposure of the biological tissues. Mainly, determining SAR can be achieved by using numerical techniques and empirical models using fabricated tissue phantoms [4; 34; 35; 36; 37; 38]. In [39], the body tissue was used as the power transfer channel. According to this technique, medical electrodes are attached on the body surface to supply power to a miniaturized implant with a differential input. The maximum SAR value was studied in [40; 41]. The empirical results can be obtained in vivo [8; 42; 43], using a living organism, or in vitro [44; 45; 46], outside of a living organism. To mimic the biological effects of human body tissue, the phantom is very popular among researchers in this field. The material type and amount needed for muscle phantom fabrication is summarized and measured in [47]. The tissue's electromagnetic properties play an important role in the design of implantable devices.