array developed for a retinal prosthesis device. The electrodes
are embedded in silicone-based substrate polydimethylsiloxane
(PDMS). PDMS is a promising material for the microelectrode
array, providing flexibility, robustness, and biocompatibility
for long-term implantation.
VISION involves a complex process
requiring numerous components of the human eye and brain to work
together. When light enters the eye, nearly 127 million rods and
cones, which are the photoreceptors in the retina, initiate a series
of electrical signals so rapid that the images the eye receives
appear to be continuously updated in a seamless process. A breakdown
in this light-conversion process can lead to vision impairment
or loss of sight.
of Lawrence Livermore engineers and scientists is participating
in a national effort to develop a technology that would help restore
sight to those who are legally blind from the loss of photoreceptor
function. Attached to and functioning as the eye’s retina,
the retinal prosthesis device promises hope for those with age-related
macular degeneration, retinitis pigmentosa, or related diseases
where photoreceptors are damaged but the optic nerve and its connections
to the brain are still intact.
Department of Energy’s Office of Science has committed $9
million over three years to retina research as part of the department’s
medical applications technology program. Courtney Davidson, electrical
engineer and Livermore’s lead on the project, is collaborating
with colleagues from Oak Ridge, Argonne, Sandia, and Los Alamos
national laboratories; the University of California (UC) at Santa
Cruz; University of Southern California’s (USC’s) Doheny
Eye Institute; North Carolina State University; and Second Sight,
a private company that plans to commercialize the prosthetic device.
The project is in its second year of funding.
and team members Satinderpall Pannu, Julie Hamilton, and Terri
DeLima are part of Livermore’s Center for Micro and Nanotechnology.
The center is applying its expertise in the area of microelectromechanical
systems (MEMS), which integrates millimeter-size mechanical elements,
sensors, actuators, and electronics through microfabrication technology.
The center’s recent successes include developments in microfluidic
filtration devices, microsensor technology with increased sensitivity,
and micro fuel cells.
continues the work begun by his predecessors, engineers Peter Krulevitch
and Mariam Maghribi, to develop an electrode array for the retinal
prosthesis. The array will serve as the interface between an electronic
imaging system and the eye, providing electrical stimulation normally
generated by the photoreceptors that convert visual signals to
electrical signals transmitted to the optic nerves. The goal is
to develop a 4- by 4-millimeter array with 1,000 electrodes attached
to a microchip system that powers them.
of a prototype polydimethylsiloxane (PDMS) array used in testing.
(b) Cross-section of an eight-electrode PDMS device shows
conductive lead and electrode metallization contained between
two layers of PDMS. Reinforcement ribs facilitate handling
of the thin PDMS device. A tack hole is used to pin the device
to the retina.
Designing Biocompatible Electronics
electrode array is embedded in a silicone-based substrate, polydimethylsiloxane
(PDMS). Livermore researchers previously used PDMS as a substrate
for microfluidic tools in devices that collect and identify biological
pathogens such as proteins, viruses, and bacteria. Additionally,
Livermore efforts have focused on developing processes for embedding
metal electrodes within PDMS for use in biomedical applications.
notes that PDMS is a biocompatible material, making it suitable
for implants. While PDMS is somewhat permeable to oxygen, it is
highly impermeable to water. This feature is expected to enable
long-term implants where the electrodes must be isolated from corrosive
and electrically conductive body fluids. The flexible nature of
PDMS also allows the embedded electrodes to conform to the shape
of the retina. Sandia and Livermore are each developing microfabrication
processes and prototype electrode arrays in an effort to determine
the best interface with the retina.
Humayan, a retinal surgeon and biomedical engineer at USC’s
Doheny Eye Institute, is testing prototypes of the Livermore implants
to determine how well the materials work and how long they are
likely to last after implantation. Davidson says, “We’ve
shown we can build electrode sets on PDMS, and we’re looking
at concepts that will increase the number of electrodes on a small
area and allow a surgeon to test the device for conformability
past spring, surgeons at USC successfully implanted a prosthetic
device in a dog’s eye. The objectives were to determine how
well the device conformed to the retina, the mechanical effects of
the device on the retina, and any biocompatibility issues. Scanned
images using optical coherence tomography showed the conformity of
the implanted array on the retina. Surgeons were pleased with the
device is designed to be epiretinal; that is, it will be placed
on the surface of the retina inside the eye. The implant will overlap
the center of the eye’s visual field, which is the area affected
in macular degeneration. Once implanted, a small camera attached
to eyeglasses will capture a video signal that will be processed
and transmitted inside the eye using a radio-frequency (rf) link.
The rf link is composed of an external rf coil that will either
be part of the eyeglass apparatus or will rest on the eyeball like
a contact lens. Another rf coil inside the eye will pick up the
signal and transmit it to electronics that will format the signal
for stimulating the electrode array.
coherence tomography scans taken one week after the prosthetic
device is implanted in a dog’s eye show it is conforming
to the retina.
Powering Electrodes Inside the Eye
power for the circuitry, or microchip system, will be provided
inductively through transcutaneous coupling. That is, a coil attached
to a battery on the side of the eyeglasses will inductively generate
power in a coil parallel to it under the skin. Wen Tai Liu, an
engineer at UC Santa Cruz, is researching the requirements for
the external camera and transmitter to determine the amount of
power required by the implanted device and how best to supply the
Carolina State University researcher Gianluca Lazzi is modeling
the biological effects of retinal stimulation, notably, thermal
dissipation. Davidson explains, “The electrodes must be
stimulated in a very controlled manner. The amount of time you
stimulate them and the amount of time given between the pulsing
electrodes are critical. It’s not certain what stimulation
might be required to artificially generate normal vision. This
is an extremely interesting area for both basic and clinical
research.” Los Alamos researcher John George is developing
optical imaging techniques to observe the visual neural system
and to better understand electrical stimulation of the retina.
electrical stimulation hardware within the fragile biological environment
of the human eye poses challenges. Charged metal electrodes produce
gases such as hydrogen and generate toxins that can damage tissues.
Eli Greenbaum, project manager at Oak Ridge, is performing electrochemical
tests of the electrodes to determine the limits before tissue damage.
Second Sight is conducting experiments to determine the robustness
of the device and producing prototypes. Argonne is developing an
encapsulating package that will insulate the electronics to help
assure that the implant will last a lifetime.
challenge for the team is determining the best electrode metal
for the array. Gold was a useful material for preliminary studies,
but platinum has proven to be a better choice for biocompatibility.
The questions now are what is the best design for the array and
what method should be used to attach platinum to the PDMS substrate.
challenge is determining the correct density of electrodes. While
a small number of electrodes may provide favorable results,
the optimum density of electrodes is still to be established. The
current operational goal is to produce 1,000 electrodes. An
additional challenge is finding the best method to connect the
microchip system to the electrode array.
available retinal prosthesis is at least a few years away. While
the retinal project continues, Davidson is working on other potential
applications of this technology. “Many parties are interested
in collaborating with the Laboratory on other applications for
the microarrays,” he says. “For example, with just
over a dozen electrodes in a prosthesis for hearing, you can get
amazing results.” In addition to a cochlear (hearing) implant,
possibilities include a deep brain stimulation device for treating
diseases such as Parkinson’s and a spinal cord stimulation
device for treating chronic pain.
the retinal device will rely on an external camera transmitter,
but researchers hope to develop a complete implantable system.
The Livermore team is encouraged with the results of the research
that may help to restore eyesight to blind persons and may revolutionize
the treatment for many neurologically based illnesses.
Key Words: epiretinal, microarray, microfabrication,
photoreceptor, polydimethylsiloxane (PDMS), retinal prosthesis.
For further information Courtney Davidson (925) 423-7168
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