## 4 Power Supply and Wireless Communication Technologies  
The technologies used to supply power to the indwelling module can be broadly divided in single-use non-rechargeable batteries and rechargeable batteries. The former can be commonly found in cardiac pacemakers and deep brain stimulators, whereas the latter are frequently used to power cochlear implants [86]. While single-use batteries require surgical removal to replace them, the rechargeable batteries can be periodically recharged transcutaneously by means of wireless telemetry, which can also be used to continuously powered up battery-less devices (without energy storage). Wireless telemetry is also used to obtain the power status and performance of the non-rechargeable batteries. Most commonly, the power is transmitted from the extracorporeal unit to the indwelling module via an inductive coupling coil, which can be expressed as a lossy transformer. High wireless power transfer efficiency is paramount to ensure minimal heating of the surrounding tissues, minimize the interference with other devices and to reduce the size of the energy source [87, 88]. Only a fraction of alternating magnetic field generated by the coil within the external unit reaches the coil located within the indwelling component, and is converted to alternating voltage [62]. The voltage is then rectified and smoothed, and is fixed at a specific value suitable for the indwelling electronic circuitry. Aside from loss associated with specificities of operating conditions (ambient environment), the power transfer efficiency has been demonstrated to depend on the distance over which the magnetic field is transmitted, _i.e._ the distance between the internal and external coils, the device geometry, and the diameter of the coils [89]. Resonance-based wireless power delivery, where four high-quality (Q) factor coils are employed instead of two, have been investigated for their improved energy transfer efficiency and reduced dependence of the latter on the distance between the primary and secondary coils [90]. The frequency chosen for the transmission is dependent on the type of the living tissues that separate the indwelling module and the external component, specifically the frequency-dependent attenuation by Foucault currents generated within the host tissues vary with the type of tissue [91]. Table 1 shows variations in electrical properties between biological tissues, measured _ex vivo_ at 100 kHz. Furthermore, addition of intermediate physical barriers, such as an encapsulation layer, has been shown to further reduce the strength of the field, with encapsulant conductivity and thickness being key determinants. Typically, using lower frequencies results in less loss compared to employing a higher frequency field, however in real life most commercially available implantable devices use higher frequencies to increase the data transfer rate [86]. The choice of frequencies is also affected by the legislative regulations that specify the radiated power maximum to each frequency band. Less conventional energy harvesting methods that involve internal charging using the energy produced by the physiological environment or natural body motion have also been reported, with several examples presented in Figure 2[21,22]. [PERSON] _et al._ suggested using highly dense electroactive conjugated polymer brushes of poly(thiophene) and poly(phenylene) fabricated by means of a surface-initiated Kumada-type polycondensation reaction to power up implantable devices [95]. [PERSON] _et al._ demonstrated energy extraction from the biologic battery in the inner ear, whereby the electrochemical gradient within the ear is utilized as a power source for an anatomically sized, ultra-low quiescent-power energy harvester chip integrated with a wireless sensor capable of monitoring the ear electrochemical gradient [96].