Biomedical Engineering Reference
In-Depth Information
There have been studies on the use of commercially available compounds for their suitability as
intraocular adhesives in rabbits. One type of adhesive (SS-PEG hydrogel, Shearwater Polymers,
Inc.) proved to be strongly adherent and nontoxic to the retina (Margalit et al., 2000). Other groups
have done similar experiments (Lowenstein et al., 1999).
The preferable fixation site for the intracortical microstimulation arrays is the cortex itself; skull
will not be a good site due to the brain's constant movement in relation to the skull. These arrays are
currently inserted either manually in an individual fashion or in a group of 2 to 3 electrodes normal
to the cortical surface to a depth of 2 mm or by a pneumatic system that inserts 100-electrode arrays
into the cortex in about 200 msec.
17.3.4
Hermetic Sealing of the Electronics
Prostheses will be composed of electronic parts within the eye. These components will be exposed
to the chemical environment in the eye. These implanted parts will have to be sealed, such that they
are not exposed to corrosion of the ocular fluids. Also, this protective coat will have to last for some
years or decades for the continued functioning of the implant. The requirement of hermetically
sealing a circuit in the case of neural stimulating devices is complicated by the demand that
multiple conductors (feedthroughs) must penetrate the hermetic package so that the stimulation
circuit can be electrically connected to each electrode site in the array. These connections are the
most vulnerable leakage points in the system (Margalit et al., 2004).
17.4
ELECTRICAL CONSIDERATIONS IN RETINAL PROSTHETIC DEVICES
The effectiveness of an electrical stimulation for an intraocular retinal prosthesis, whether epiretinal
or subretinal, is governed by a number of parameters characteristic of the electrode array, including
shape and size of the electrodes, spacing between electrodes, electrode materials, current return
positions, and stimulating current waveform, to name a few. Optimal electrode array type and
characteristics must also take into account other factors that can influence the one or more
parameters, including thermal or electrical safety or ease of surgical implantation.
17.4.1 Stimulating Electrodes: General Considerations with Regard to Electrical
Stimulation of the Retina
The characteristics of the stimulating electrode array are often of competing nature: for example, it
might be desirable to mechanically position the electrodes as close as possible to the ganglion and
bipolar cells, but that would then result in penetrating electrodes that could harm the fragile
structure of the retina. Similarly, it may appear natural to develop small electrodes to achieve
high-resolution electrical stimulation of the retina; however, current densities needed to elicit
phosphenes may exceed safety limits and potentially cause damage to the retina. Further, it is not
completely clear, to say the least, the relation between size of the electrode and size of the visual
spot induced by that electrode.
The problem is phenomenally complex, as it simultaneously involves neural activation at the
microscopic level and control of the spread of the current in retinal tissue at the macroscopic level.
Both problems are strongly coupled and involve very different scales and methods of analysis,
which increases the complexity of solving the problem of optimal stimulation of retinal tissue and,
indirectly, the problem of optimal physical characteristics of the stimulating electrode arrays.
Besides geometrical considerations that can affect the effectiveness of the electrical stimulation
of the retinal tissue, other aspects of the system design can have a significant impact on the induced
stimulation. Among the challenges that must be considered to achieve optimal electrical stimula-
tion, in the sense of an electrical stimulation which uses as little current as possible to elicit visual
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