Biomedical Engineering Reference
In-Depth Information
has been demonstrated that reducing IOP in individuals with higher IOP may
delay or prevent the onset of glaucoma [54, 55]. It is believed that high pressure
interferes with axoplasmic flow inside the nerve fibers, leading to the death of
ganglion cells and optic nerve atrophy. In addition, it is thought that high IOP
may compromise optic nerve head circulation and cause nerve fiber layer loss.
Because peripheral nerve fiber layers are more sensitive to the effects of pressure,
the disease starts with the loss of peripheral vision; if left untreated for months
or years, the macular fibers may also be affected and eventually total blindness
may ensue. Because of the loss of ganglion cells, retinal prosthesis does not play
any role in the treatment of this condition. Compared to glaucoma (in which
pressure is applied to all parts of the retina) pressure is only exerted to some
areas of the retina underneath the array with an epiretinal prosthesis. Because
of the curvature of the inner eye, the point of maximum pressure is likely to be
around the tack and at the edges of the array. Other factors affecting the amount
of pressure exerted on the retina include the shape of the tack, the stiffness of
the array, and improper alignment of the array in relation to the scleral incision
during surgery, which may cause rotation of the cable and tilting of the array.
Since pressure from a device will be localized, and since elevated IOP applies
pressure globally, research into the effect of localized pressure on the retina may
reveal a different type of damage other than that caused by raised IOP.
Bioadhesives for Attaching the Array to the Retina
Given the potential problems with the use of a tack, tissue glue may provide
a safe alternative for attaching the array to the retina. In addition to being
biocompatible, ideal glue should create an attachment which is strong enough
to keep the array close to the retina, but not too strong to pull the retina off
the RPE layer. Moreover, the attachment should be reversible to allow for the
removal of the array, in case of improper functioning, or any other complication.
Tissue glues are usually used for creating an adhesion between different parts
of a tissue or between two different tissues and usually disintegrate with time,
whereas glues used for retinal prosthesis should have an adhesive effect on both
the retina and the array, and should remain stable for years.
A study using dissected rabbit retina demonstrated that hydrogels have from
2 to 39 times greater adhesive force than commercial fibrin sealant, autologous
fibrin, Cell-Tak, or photocurable glues [56]. Hydrogels liquefied at body temper-
ature anywhere from 3 days to a few months. In terms of safety, the relative
strength of adhesion and consistency, the specific hydrogel SS-PEG appeared to
be an excellent substance for intraocular use; however, it was short lasting and
liquefied after 72 hours under wet conditions.
Implantation of a Microelectronic Device in the Eye (All-Intraocular
Retinal Prosthesis)
In this approach, all of the components of a retinal prosthesis are inserted into the
eye (Figure 1.19). The inductive transmitter can be accommodated in a spectacle
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