**7. Biocompatibility issues related to implantable sensor**

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

3 kHz-300 GHz [55]. Table 3 illustrates the IEEE standard C95.1-1991.

**Magnetic Field Strength, H (A/m)**

**0.003-0.1** 614 163 100 1E6 6 **0.1-3.0** 614 16.3/f 100 10000/f2 6 **3-30** 1842/f 16.3/f 900/f2 10000/f2 6 **30-100** 61.4 16.3/f 1 10000/f2 6 **100-300** 61.4 0.163 1 6 **300-3K** -- -- f/300 6 **3K-15K** -- -- 10 6 **15K-300K** -- -- 10 616000/f1.2

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

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/

*σ*(*r*)|*E*(*r*)|2

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

**Power Density, S (mW/cm2**

**)** Averaging Time |E|2 , |H|2

**E-field H-field** (minutes)

*<sup>ρ</sup>*(*r*) *dr* (5)

**Electric Field Strength, E (V/m)**

kg) or milliwatts per gram (mW/g) and can be expressed as,

*SAR* =*∫*

**Frequency Range (MHz)**

168 Advances in Bioengineering

Body [55].

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

> Since there is direct contact between the implanted device and biological tissue, a compatibility assessment of the sensors needs to be performed prior to being deployed inside the human body. This biocompatibility assessment is defined in [60] as, ''the ability of a material to perform with an appropriate host response in a specific application''. Some of the major characteristics of the bulk and surface materials which can possibly influence host response and some of the important characteristics of host responses are listed in [61]. The biocompat‐ ibility of an implantable sensor depends on parameters such as the part of the human body where the implant is deployed and the surface material of the sensor itself. Also the shape and size of the sensor also have to be optimum to make the sensor compatible with human body. Surface chemistry and composition of the outer material also need to be kept in mind so that it does not react with the tissue and blood that come in contact with the implant. Finally, sterility issues, contact duration and degradation of the material that surrounds the sensor need to be taken into account while designing a biocompatible implantable sensor [62–63]. Several protocols related to the biocompatibility issues that are scrutinized thoroughly before an implantable device could be used in the human body have been developed by the U.S. Food and Drug Administration (FDA). Materials appropriate for long-term reliable implantable devices according to [61] include a) titanium alloys for dental implants, femoral stems, pacemaker cans, heart valves, fracture plates, spinal cages, b) cobalt-chromium alloys for bearing surfaces, heart valves, stents, pacemaker leads, c) platinum group alloys for electrodes, d) nitinol for shape memory applications, e) stainless steel for stents and orthopedic implants, f) alumina for bearing surfaces, g) calcium phosphates for bioactive surfaces, h) polyurethane for pacemaker lead insulation, i) PMMA for bone cement, intraocular lenses, j) silicone for soft tissue augmentation, insulating leads, ophthalmological devices. On May 28, 2014 FDA approved the first implantable wireless device with wireless monitoring feature to measure pulmonary artery pressure for heart patients [64]. With the ongoing research on finding biocompatible materials it can be easily inferred that there will be numerous implantable devices in the global market in near future.
