**3.2 Patient safely**

Due to the propagation of electromagnetic field causes rise in temperature in human tissue, to evaluate this heat issue SAR is used. Generally, issues related to patient safety limit maximum allowable power incident on the implantable antenna. The rate of energy deposited per unit mass of tissue is called a Specific Absorption Rate (SAR). SAR is an internationally accepted FCC (Federal Communication Commission) guideline. For example, IEEE C95.1-1999 patient safety standard restricts the specific average of over 1 g of tissue in the shape of a cube to less than 1.6 W/kg ((SAR1g, max≤ 1.6 W/kg), IEEE C95.1-1999 is found to restrict transmission power up to 5.186 mW [82] and ANSI/IEEE C95.1-2005 standard restricts the Specific Absorption Rate averaged over any 10 g of tissue in the shape of cube less than 2 W/kg (SAR10g, max≤ 2 W/kg), IEEE C95.1-2005 is found to restrict transmission power up to 30.17 mW [83]. To attenuate electromagnetic interference, MedRadio regulations restrict effective radiated power of implantable antenna to 25 μW [84], power transmission is restricted to 50 mW. In [61] CSRR reduces the electric far-field of antenna this power absorption and SAR also reduced. As a result, radiation power increases, antenna radiation, and gain are increased.

SAR can be defined with the following equation,

*Antenna Systems*

$$SAR = \frac{\sigma |E|^2}{2\rho} \tag{1}$$

where, ρ (Kg/m<sup>3</sup> ) is mass density, σ (S/m) is conductivity and |E| is electric field intensity.

## **3.3 Biocompatibility**

Biocompatibility is one of the necessary conditions while designing an implantable antenna to preserve patients'safety. Human tissues are conductive, if they were allowed direct contact with metallization then there is a chance of short circuit. For long-term implantation, it's crucial to handle biocompatibility and prevention from short circuits. Most of the materials from **Table 4** are not biocompatible materials. There are different biocompatible materials reported in literature like macor [7], alumina [65], PDMS, Parylene C film, polyimide, PEEK (polyetheretherketone), polyethylene, silastic MDX4-4210 [46], etc. For thickness of encased biocompatible coating material can also affect the antenna performance [85].

#### **3.4 Wireless communication ability**

In the current scenario, an implantable antenna acts as a transmitting device, and an external device acts as receiving device as shown in **Figure 2**. Assuming far-field communication, the link power budget can be described as in terms of [43, 86, 87],

$$\text{Link Margin} \ (dB) = \text{Link} \frac{\text{C}}{\text{NO}} - \text{Required} \frac{\text{C}}{\text{NO}} \tag{2}$$

$$\text{Link Margin} \ (\text{dB}) = \text{Pt} + \text{Gt} - \text{L}f + \text{Gr} - \text{No} - \frac{\text{Eb}}{\text{No}} - 10 \log 10 \text{Br} + \text{Gc} - \text{Gd} \tag{3}$$

Where Pt is transmitted power, Gt is transmitted antenna gain, Lf is path loss in free space, Gr gain of receiving antenna, and N0 is the noise power density. Also, Path loss can be given as,

$$Lf(dB) = 20\log\left(\frac{4\Pi d}{\lambda}\right) \tag{4}$$

Where d is the distance between transmitter and receiver. Impedance Mismatch loss is given as,

*Limp dB* ð Þ¼�10 log 1 � j j <sup>Γ</sup> <sup>2</sup> (5)

**Figure 2.** *Wireless communication link between IMD and external device.*

*Microwave Antennas Suggested for Biomedical Implantation DOI: http://dx.doi.org/10.5772/intechopen.101060*

Where Γ is the reflection coefficient.

For, wireless communication, Link C/N0 must exceed than required C/No, in uplink transmission input power of the transmitter antenna is limited for safety purposes. Received power can be given as,

$$p\_r = p\_t + G\_t + G\_r - L\_f - L\_{imp} - \mathbf{e}\_p \tag{6}$$

Where *ep* is polarization mismatch loss between transmitter and receiver.
