**6. Health issues related to wireless power transfer**

While designing wireless power transfer system for biomedical applications, the associated health risks have to be taken into consideration. Since RF energy can quickly heat up the biological tissues due to the thermal effect, the exposure to very high levels of RF radiation can be harmful. Since attenuation increases with frequency, most of the existing work in Wireless Body Area Networks (WBAN) considers only the Medical Implant Communication System (MICS) band (402-405 MHz) or sub-gigahertz bands. Federal Communications Commission (FCC) regulates the time and the amount of exposure of the electromagnetic radiation to human tissues at various frequencies [52]. American National Standard Institute (ANSI) standard C95.1-1982 sets the electromagnetic field strength limits for frequencies between 300 kHz and 100 GHz [53], [54]. For frequencies below 300 MHz, the electric and the magnetic fields have to be accounted for separately. The ANSI standard C95.1-1991 sets the electric and the magnetic field strength limits for the general public for the frequency range of 3 kHz-300 GHz [55]. Table 3 illustrates the IEEE standard C95.1-1991.


**Table 3.** IEEE Standard C95.1-1991: Limit of Maximum Permissible Exposure at Controlled Environment on Human Body [55].

An important parameter that is used to measure the effect of radio frequency exposure on human is SAR (specific absorption rate). SAR is a quantity that is used to measure the amount of energy absorbed by a body which is exposed to radio frequency (RF) electromagnetic field. It is defined as the power absorbed per mass of the tissue with units of watts per kilogram (W/ kg) or milliwatts per gram (mW/g) and can be expressed as,

$$\text{GAR} = \text{J} \frac{\sigma(r) \|E(r)\|^2}{\rho(r)} \, dr \tag{5}$$

Here *σ* is the electrical conductivity of the sample, *E* is the RMS electric field and *ρ* is the sample density. In case of whole-body exposure, a standing human adult can absorb a maximum RF

radiation rate of approximately 80 MHz to 100 MHz. As a result, RF safety standards are often most stringent for these frequencies. SAR has to be within an acceptable range for biological tissues [56], [57]. A whole-body average SAR of 0.4 W/kg has been set as the restriction that provides adequate protection for occupational exposure [58]. An SAR level of 1.6 W/kg has been set as the FCC limit for public exposure from cellular telephones [59]. Two areas of the body, the eyes and the testicles, are particularly vulnerable to RF heating due to the relative lack of available blood flow to dissipate the excessive heat. The detrimental effects of RF exposure which does not cause significant heating (referred to as 'non-thermal' effects) are still unproven.
