Biomedical Engineering Reference
In-Depth Information
nonuniform ''hot-spots''). It was observed that within 26 min of actual stimulation time (because of
the extremely small time step in the FDTD simulations, the actual simulation time was significantly
higher), the thermal elevation profiles in the tissues reached to within 5 to 7% of their final values.
Since this provided a good indication of the approximate thermal rise, all the simulations were
performed for approximately 26 min (physical time).
The maximum temperature increase for both chip positions was observed on the surface of
the insulating layer. In both cases, the maximum thermal increase was approximately 0.82
8
C. In the
first case where the chip was placed in the anterior position, the temperature of the ciliary muscles
rose by 0.36
8
C as compared to 0.19
8
C when the chip was placed in the center position. In
the vitreous cavity, temperature rise was 0.26
8
C for the chip placed in center of the eye while the
anterior chip raised its temperature by 0.16
8
C (Gosalia et al., 2004).
A chip placed in the anterior chamber of the eye raised the temperature of the retina by less than
half the amount that a chip placed in the center did (0.05
8
C by anterior chip as compared to 0.12
8
C
by a center chip) (Gosalia et al., 2004). In these simulations, it was observed that the vitreous cavity
was acting as a heat sink since the rise in temperature of tissues beyond the eyeball is very small.
A graphic comparison of the thermal elevation observed for the anterior and the center placed chips
is provided in Figure 17.6. The anterior position is certainly preferable for the implanted unit in
order to minimize the temperature rise in the vitreous cavity and on the retina.
A similar analysis can be performed to compute the impact of the size of the implant and
dissipated power on the temperature increase in the tissue (Gosalia et al., 2004). It is worth pointing
out, however, that power dissipation of the implanted microchip is probably the most significant
parameter among all to be considered.
Two cases were considered in this example: in the first case, the chip dissipated 12.4 mW
and in the second case, it dissipated 49.6 mW. For both of these cases, the size of the chip was
Influence of POSITION of Implant on tissue heating
0.9
0.8
Insulation of the chip
0.7
0.6
Anterior position of the chip
Mild-vitreous position of the chip
0.5
0.4
0.3
Vitreous cavity
0.2
0.1
Retina
0
4
8
12
16
20
24
Time (min)
Figure 17.6 Thermal rise observed due to different locations of the implanted chip (anterior and center of the
eyeball). (From Gosalia K, Weiland J, Humayun M, and Lazzi G. IEEE Transactions on Biomedical Engineering,
51(8): 1469-1477, 2004. With permission.)
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