Biomedical Engineering Reference
In-Depth Information
7.4 Case Study II: Electromagnetic Effects Caused
by IR-UWB Signals Used in Head Implant Applications
This section presents the SAR/SA variation and temperature increase caused by
IR-UWB signals used in head implant applications. This is compiled from the
findings presented in [
39
,
51
].
7.4.1 Head Implantable Antenna Model
and Impedance Matching
A head implantable version of the antenna mentioned in above section is used for
these simulations. The antenna is tuned to operate at around 4 GHz with a
bandwidth of 1 GHz, which fulfils the bandwidth requirement imposed by the FCC
for UWB communication. The antenna was inserted inside a capsule shaped casing
with a negligible thickness compared to the antenna dimensions, in order to pre-
vent direct contact between antenna radiating elements and the neighboring tissue.
The radiating element of the antenna used in the simulations, which occupies the
lower half of the antenna, is inserted in glycerin for the purpose of impedance
matching. Glycerin has a relative permittivity of 50, which is close to the relative
permittivity of the surrounding tissue material; hence allows minimal reflections of
the electromagnetic wave near the transitional boundaries between the tissue
medium and the capsule. The antenna is placed at 6 mm distance from the surface
of the head. Figure
7.9
depicts how this antenna model is used alongside the CST
voxel head model in order to perform present simulations. Through simulation-
based optimization, the optimum value for the radius of the capsule ('D') was
obtained to be 5 mm for the head implanted antenna, which is indicated in
Fig.
7.9
.
S-parameter characteristics and antenna gain/directivity far field characteristics
are presented in Figs.
7.10
and
7.11
, respectively. It should be noted that the near
field characteristic of the antenna predominantly affects the SAR and the thermal
behavior of the body tissues. The near field simulation characteristics are shown in
the results section. Although the far field stretches outside of the brain and
therefore does not influence the SAR, the far field characteristics are shown in
order to compare the antenna performance in different scenarios as well as to get
an idea about the direction in which the maximum power transfer is radiated by the
antenna.
The three-dimensional far field antenna gain is calculated using the following
equation:
