Biomedical Engineering Reference
In-Depth Information
radio frequency transmission from implantable devices inside the human body [
7
,
8
,
22
]. Some of them report the SAR variations caused by implantable devices
operating at low frequency bands such as MICS and ISM bands [
7
,
8
]. The work
reported in [
22
] illustrates the SAR variation caused by an IR-UWB source inside
the human stomach. The latter uses the Finite Difference Time Domain (FDTD)
computation method, which discretises the derivatives in Maxwell's curl equations
[
23
] using finite differences. However, [
22
] has not considered an antenna model
in the simulations; additionally, the FCC regulations that govern the UWB indoor
propagation are not taken into account. Many studies have analysed path-loss as
the sole indicator of electromagnetic power absorption [
24
-
27
]. However, SAR
and SA measurements can be associated easily with the temperature increase in the
human tissue.
This chapter presents the electromagnetic effects and thermal effects of IR-
UWB implant communication, which are important safety measures in the
applications of body area network applications. IR-UWB based WCE is used as
the application of focus in this chapter. A human anatomical body model devel-
oped by CST Studio [
28
], which has been recognised by the Federal Communi-
cations Commission (FCC) [
29
] as a suitable simulation tool for SAR calculations,
is used to simulate the human tissue properties at UWB frequencies. An
implantable antenna working at UWB frequencies is used as the source of the
UWB signals. The 4-Cole Cole model [
30
] is used in order to consider the dis-
persive nature of the tissue materials at these high frequencies. In addition, the
effect of the human age is considered when calculating the tissue properties such as
relative permittivity. An IR-UWB pulse operating at a centre frequency of 4 GHz
and a bandwidth of 1 GHz has been chosen as the excitation to the antenna. This
range is selected so that the UWB spectrum has minimum interference from other
wireless technologies, such as 5 GHz Wi-Fi.
7.2 Simulation Models and Methods
This section givens a brief introduction to the simulation models and methods used
for calculation of SAR and temperature increase of human tissue that is exposed to
IR-UWB signals.
7.2.1 Effect of Human Tissue Properties on SAR
A compilation of experimental and theoretical results on human tissue property
variation depending on the incident frequency provided in [
30
,
31
] proves that the
former behaves differently at different incident frequencies. This is due to a
property known as dielectric dispersion. The SAR value for a certain material
subjected to an electromagnetic field can be calculated by (
7.1
)[
32
].
