Biomedical Engineering Reference
In-Depth Information
Electrode
array
Natural
signal path
New signal path
FIGURE 16.5
(See.color.insert.).Illustration.of.epiretinal.prosthesis..An.array.of.electrodes.is.implanted.inside.the.eye.and.
hatched.on.the.surface.of.the.retina..The.natural.signal.path.of.the.photoreceptor.to.the.retinal.ganglion.neuron.
is.blocked..A.new.signal.path.is.created.by.electrically.stimulating.the.surviving.neurons.
V e . is. the. extracellular. voltage. along. the. axon. membrane.. Numeric. simulations. based. on.
cable.equations.with.parameters.extracted.from.retinal.neurons, 36 .as.well.as.stimulation.
experiments.using.retinal.ganglion.neurons, 40,41 .point.out.that.the.compliance.voltage.on.
the.electrode.( V 0 ( t )).required.to.activate.neurons.dramatically.increases.as.the.pulse.width.
or.electrode.size.decreases.
16.4.3 A 256-Channel Epiretinal Implant
Figure  16.6. shows. an. example. of. an. epiretinal. implant. system.. It. includes. a. power. link.
delivering. up. to. 100. mW. to. the. implant,. a. data. link. providing. 2. Mbps. data,. and. 256-
channel.stimulators.performing.parallel.stimulation.at.a.rate.of.50.Hz.or.more..The.power.
regulators,.data.receiver,.digital.blocks,.and.stimulators.are.integrated.onto.a.single.micro-
chip.with.total.area.of.5.1.×.5.3.mm 2 .
16.5 WirelessPowerTelemetry
16.5.1 Coil Model and Coil Design
Power. eficiency,. deined. as. the. ratio. of. power. delivered. to. the. implant. over. the. power.
drained. from. the. battery,. is. an. important. parameter. for. epiretinal. implants.. The. upper.
bound.of.the.eficiency.is.analytically.derived: 42
k Q Q
k Q Q
k Q Q
2
2
η p
=
1
2
1
2
.
(16.2)
+
k Q Q
+
+
k Q Q
4 2
+
2
2
2 1
2
2
.
1
2
1
2
1
2
where.η p .is.the.eficiency.of.the.power.telemetry,. Q 1 .and. Q 2 .are.the. Q .factors.of.coils,.and.
k .is.the.coupling.coeficient.between.coil.pairs..In.the.case.where.coils.are.weakly.coupled.
due.to.a.large.separation.(e.g.,.2%-3%.in.this.system),.the.expression.can.be.simpliied.to.
0.25 k 2 Q 1 Q 2 ,. suggesting. that. coils. are. the. most. critical. components. for. power. telemetry..
Given. the. restrictions. on. coil. size. and. separation,. the. achievable. coupling. coeficient. is.
predetermined..Therefore,.the.main.focus.of.coil.design.is.improving.the.power.eficiency.
by.increasing.the.coil's. Q .factor.
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