Biomedical Engineering Reference
In-Depth Information
16.3 OverviewofVisualProsthesis
Over.the.past.two.decades,.the.engineering.and.science.research.communities.have.wit-
nessed.the.fast.advancement.of.retinal.prosthesis.to.restore.vision.in.blind.patients.with.
retinitis.pigmentosa.(RP).and.age-related.macular.degeneration.(AMD).
9
.In.a.healthy.ret-
ina,.hundreds.of.millions.of.photoreceptors.initiate.neural.responses.to.light.and.contrib-
ute.the.information.used.by.the.visual.system.to.form.a.representation.of.the.visual.world..
Photoreceptors.are.severely.absent.in.blind.patients.suffering.from.RP.and.AMD;.however,.
the.remaining.neuron.circuitries.in.the.signal.path.can.survive.at.high.rates.and.be.used.to.
form.input.for.the.visual.cortex.by.applying.controlled.electrical.stimuli.
10
.Retinal.implant.
is.a.prosthetic.device.that.maps.visual.images.to.control.signals,.based.on.which.it.stimu-
lates.the.surviving.retinal.circuitry..In.early.studies.reported.by.teams.led.by.Humayun.et.
al.
11
.and.Liu.et.al.,
12
.simple.forms,.such.as.an.English.character.or.a.matchbox,.have.been.
perceived. by. human. subjects. with. a. 4. ×. 4. retinal. implant.. Several. approaches. for. reti-
nal.prosthesis
12-21
.target.different.stimulation.regions..This.chapter.focuses.on.epiretinal.
prosthesis,.where.electronics. and.electrodes.are.placed.directly.inside.the.eye,.stimulat-
ing.the.ganglion.and.bipolar.neurons..This.approach.is.preferable.from.surgical.consid-
erations.and.allows.the.vitreous.to.act.as.a.heat.sink.medium.to.dissipate.heat.from.the.
implant.
10,22
An.epiretinal.prosthesis.system.consists.of.external.and.implant.units..The.components.
of.the.external.unit.are.a.camera,.a.battery,.and.power.and.data.transmitters..At.the.exter-
nal. side,. a. digital. camera. captures. images. that. are. encoded. to. bit. stream. as. data. input..
Transmitters.generate.radio.frequency.(RF).electromagnetic.ields.containing.both.energy.
and.the.encoded.image.data.for.the.implant..At.the.implant.side,.the.implant.unit.includes.
power.and.data.receivers,.a.digital.controller,.and.a.stimulator.array..The.power.receiver.
harvests.electromagnetic.energy.and.outputs.DC.supplies.powering.the.remaining.elec-
tronics.. The. data. receiver. demodulates. bit. stream. data,. which. are. further. decoded. and.
then.used.to.command.the.operations.of.the.stimulator.array..In.this.chapter,.the.design.
principles,.optimization.techniques,.and.realization.of.a.retinal.prosthesis.microchip.are.
introduced.
16.4 High-DensityNeuralStimulation
16.4.1 Challenges of High-Density Retinal Stimulation
A.BBC.news
28
.review.of.previous.generations.of.epiretinal.prosthesis.devices.that.included.
16.and.60.electrodes.reported.the.following.experience.from.patients:.“When.I.am.walking.
along.the.street.I.can.avoid.low-hanging.branches—I.can.see.the.edges.of.the.branches,”.
and.“I.can't.recognize.faces.but.I.can.see.them.like.a.dark.shadow.”.There.is.a.clear.moti-
vation.to.improve.vision.quality.by.increasing.the.number.of.pixels..Studies.on.pixelized.
vision.suggest.that.60.pixels.are.needed.for.navigation.through.an.ofice.environment,
29
.
100.pixels.enable.subjects.to.start.recognizing.a.face,
30
.256.pixels.give.the.subject.the.abil-
ity.to.read.slowly,.and.1,000+.pixels.are.required.for.a.normal.reading.speed.
9
.However,.
the.realization.of.a.high-resolution.retinal.implant.is.challenging.from.several.engineer-
ing.perspectives.
