Biomedical Engineering Reference
In-Depth Information
10000
1000
100
10
1
0.01
0.1
1
Frequency (kHz)
10
100
50 µm
100 µm
200 µm
10000
1000
100
10
1
0.01
0.1
1
Frequency (kHz)
10
100
50 µm
100 µm
200 µm
FIGURE 16.3
Electrode.impedance.vs..electrode.size.in.the.vitreous.(left).and.on.the.retina.(right)..The.error.bars.represent.1.
standard.deviation..The.X.axis.is.1/(pulse.duration).in.Hz,.and.the.Y.axis.is.the.measured.impedance.in.ohms..
(From. Shah. S,. Hines. A,. Zhou. D,. Greenberg. RJ,. Humayun. MS,. Weiland. JD.. Electrical. properties. of. retinal-
electrode.interface.. J Neural Eng, 2007;.4(1):.S24-S29.)
First,.the.size.of.the.electrode. array.is.limited. by.a.central.6.mm.diameter.area.of.the.
human.retina. 22 .The.increase.of.the.number.of.pixels.reduces.the.area.available.for.an.elec-
trode.and.its.corresponding.stimulator.circuitry..As.shown.in.Figure 16.3,.the.reduction.
of.electrode.size.results.in.higher.electrode.impedance..Given.the.same.stimulation.cur-
rent,.the.required.compliance.voltage.to.stimulate.neurons.is.proportional.to.the.electrode.
impedance..This.leads.to.the.adoption.of.high-voltage.semiconductor.technology,.which.
has.transistors.much.larger.in.size.due.to.the.isolation.techniques.involved..Since.the.size.
available.for.each.stimulator.pixel.is.reduced.due.to.the.increased.number.of.pixels,.area-
eficient.circuit.architecture.is.critical.to.accomplish.high-density.stimulator.design.
Second,.because.the.time.to.accomplish.stimulation.of.the.entire.electrode.array.is.ixed,.
the.increase.in.the.number.of.pixels.inevitably.reduces.the.pulse.width.available.for.stimu-
lating.each.pixel..As.shown.in.Figure 16.4,.a.reduced.pulse.width.signiicantly.increases.
the.current.threshold.and,.consequently,.the.required.compliance.voltage..The.high.com-
pliance.voltage.resulting.from.the.smaller.electrode.size.and.shorter.pulse.width.demands.
a.high.power.supply.voltage..This.increases.implant.power.dissipation.and.causes.tissue.
heating..Consider.a.simpliied.case.of.a.disk-shaped.implant.with.diameter. D .surrounded.
by.liquid.with.a.thermal.conductivity.¸.and.dissipated.power P heat ..The.temperature.incre-
ment.of.the.implant.is.Δ T temp .=. P heat /(4 π λ D ),.where. T temp .is.the.temperature.elevation.at.the.
implant.surface..Assuming. D .=.6.mm.and.¸.λ =.0.58.W.m -1 .K -1 ,.50.mW.power.dissipation.
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