# Approach-Based Analysis on Wireless Power Transmission for Bio-Implantable Devices
## 3 Analysis of Various WPT Approaches
### WPT Systems Based on Different Transmitting and Receiving Coils Technique
The system at 6.78 MHz frequency allows a transmission distance of up to 180 mm, achieving the output voltage of about 16.8 V constantly with a transmission efficiency of up to 78%. This is achieved using the specified switchable pi-match network. This is a remarkable attainment of efficiency. Along with these approaches for implantable devices, quite new techniques have been discussed. Study [22] uses the tissue of the body as an energy transfer medium, in which two medical electrodes were placed on the bodily skin surface to provide power to a miniature implant without contravening the IEEE-SAR Standard. This tissue channel technology for a 1 mm implant achieved a PTE of 0.39% in a declination of 5 cm inside the tissue. Another approach is shown in [23], where by changing the radius of the transmit coil to almost the equivalent level of the coil radius of the receiver and by adjusting the space between both coils to about 1 cm without changing the radius of the receiver coil, a transmission efficiency of up to 77% can be reached without any need of adopting multi-coils for transmission. A WPT system [24] in which spiral coils lying in a planar form with the integration of capacitances, called capacitance-segmented coils, was applied to work in the WPT interface. By this capacitive-coil segmentation and at the operating frequency, dielectric losses, along with some current distributions (non-uniform), are concealed by repealing the inductive role of every coil segment. In this work, wireless power connection with the transmitter coil of 30 mm and receiver coil of 10 mm were designed and validated. Through this, with a 31% of efficiency in tissue that covers a space of 20 mm, a PTE 10 times greater and a SAR rate five times smaller, when compared to non-segmented models, can be attained. Additionally, operating at 150 MHz in the air, it is up to 1.5 times more efficient than other systems of the same size. [PERSON] et al. gave a primary coil configuration on the basis of the Helmholtz principle to introduce a magnetic field inside the human body in the range of intermediate frequencies [26]. While analyzing coil-based WPT approaches, various techniques have been proposed in various research papers, where magnetic resonance WPT using a 3D dual-coil approach [27] is one of them. It works with 3D coils. Several optimal designs of primary, as well as secondary WPT coils have also been exposed to improve WPT performance and avoid potential safety issues due to electromagnetic fields (EMF) [28]. Moreover, a three-coil system for power transmission with one receiver coil and two transmitter coils is proposed in [25], where its structural design is optimized via a memetic algorithm. Actually, this algorithm unites the characteristics of the method of artificial bee colonies along with the covariance matrix method, which is an adaptive evolutionary strategy type. Hence, this system helps to increase the reception of power at the receiver by about 48%, and also the power transfer coverage distance increased by up to 15 cm. The model circuit for such a three-coil (2 Tr and 1 Rr) approach is given in Figure 3.  
Figure 3: The model of three coil power transfer system.