Biological effect of SAR on the human head due to variation of dielectric properties at 1800 and 2450 MHz with different antenna substrate materials

1 Introduction

Wireless communication technology has been increasing rapidly in the past 20 years. Because of its expediency, more and more people espouse this technology in mobile phones in worldwide communications. While the usage of mobile phone increases, research on electromagnetic (EM) energy emanating from mobile phones has been extensive. The fundamental safety limits for radio frequency (RF) exposure are defined in terms of immersed power per unit mass, which is expressed as specific absorption rate (SAR) in units of watts per kilogram (W/kg). These protection guidelines are set in terms of maximum mass-normalized rates of EM energy deposition (SARs) for 1 or 10 g of tissue. The two most generally used SAR limits today are those of IEEE [1], 1.6 W/kg for any 1 g of tissue, and ICNIRP [2], 2 W/kg for any 10 g of tissue, excluding the extremities such as ankles, hands, and feet where higher SARs of up to 4 W/kg for any 10 g of tissue are allowed in both of these standards. Dielectric properties (dielectric constant ε and conductivity σ) of the tissue stimulant fluids have been approved in these standards on the basis of the properties measured for the various tissues for humans and other mammals [3]. It also the paved the way for wide usage of mobile phones in modern society, resulting in mounting concerns surrounding its harmful radiation [4–11].

Yet, the majority of the studies discussed only the radiation characteristics and protection from external sources such as wearable medical sensor devices, even though, indeed, more and more implantable or ingestible devices are clinically employed, including pacemakers, implantable defibrillators, capsule endoscopes, and implanted therapeutic devices [12]. In recent times, research on the safety of ingestible or implantable wireless devices (IWDs) in the human body has been studied, mainly due to the perseverance and high local energy declaration of antennas that are embedded in human tissues [13]. Kang and Gandhi [14] analyzed the biological effects and radiation efficiency of IWDs in a realistic human body model in 21 scenarios at frequencies ranging from 430 MHz to 2.4 GHz. This research showed that high values of SAR and temperature rise were localized at the area near the location of the IWD.

Conductivity (σ) and relative permittivity (ε) of human tissues are the influential factors for both optimal RF communication and dosimetry. Recently, the inconsistency of dielectric parameters of human tissues has been discussed in many published papers [14–21].

Utilizing the finite-difference time-domain (FDTD) method, Wang et al. [17] calculated peak SAR in adult and child head models exposed to a mobile phone at 900 MHz, and they found that the dissimilarity of the SAR value with dielectric properties was approximately 10%. Keshvari et al. [13] reported the effect of augmented dielectric values on the SAR in human head tissues and claimed that the 20% increase in dielectric values constantly caused a dissimilarity of SAR <20%.

However, no one has yet analyzed whether the uncertainty of radiation characteristics due to dielectric parameters of body tissues and changing in antenna substrate values vary from person to person.

The main objective of this study was to investigate the effect of augmented dielectric values and altered antenna substrate material on mass-averaged SAR. The human head was exposed to a planar inverted-F antenna at 1800 and 2450 MHz frequencies.

2 Materials and methods

There are several techniques for the computational assessment of RF energy absorption in tissues. The FDTD method was established to be the most efficient technique, particularly for studying the RF energy absorption in highly inhomogeneous bodies. The EM field simulations were carried out using commercial EM simulation software (CST Microwave Studio (CST Microwave Studio, CST AG-Bad Nauheimer, Darmstadt, Germany), which is based on the FDTD method. The densities, conductivities, and dielectric constants of the tissues are given in Table 1 [15]. A nonuniform meshing scheme was assumed so that the major computation endeavor was devoted to regions along the inhomogeneous boundaries for quick and correct analysis. The minimum and maximum mesh sizes were 0.3 and 1.0 mm, respectively. A total of 228,152 mesh cells were produced for the complete model, and the simulation time was 967 s (including mesh generation) for each run on an Intel Core™ 2 Duo E 8400 3.0 GHz CPU with 4 GB RAM system.

3 Results and discussion

The discrepancies in SARs and electric fields were calculated for two human body models at 1800 and 2450 MHz frequency. The calculated SAR values are shown in Figures 1–6.

Figure 1

Deviation of the peak of unaveraged SAR in different dielectric properties of Rogers R03006 (loss free) substrate at 1800 MHz.

Figure 2

Deviation of the peak of unaveraged SAR in different dielectric properties of Rogers R04003 (loss free) substrate at 1800 MHz.

Figure 3

Deviation of the peak of unaveraged SAR in different dielectric properties of FR4 substrate at 1800 MHz.

Figure 4

Deviation of the peak of unaveraged SAR in different dielectric properties of Rogers R03006 (loss free) substrate at 2450 MHz.

Figure 5

Deviation of the peak of unaveraged SAR in different dielectric properties of Rogers R04003 (loss free) substrate at 2450 MHz.

Figure 6

Deviation of the peak of unaveraged SAR in different dielectric properties of FR4 substrate at 2450 MHz.

4 Conclusion

The deviation of SAR with dielectric parameters and substrate material together are reliant on the configuration of the human body, exposure scenario, etc. The most severe case of radiation should consider the influence of the variability of dielectric properties and substrate material of the antenna. It was found that tissue dielectric properties are known to be highly nonuniform and significantly variable between individuals. In conclusion, SAR values will decrease or increase with changes in dielectric values of human tissue. In addition, the choice of antenna substrate material has been more effective in increasing or decreasing SAR values. These assessments might increase confidence in the validity of numerical calculations.