Biomedical Engineering Reference
In-Depth Information
map the position of each electrode to its visuotopic map, and these will be dynamically
updatable via telemetry as the structure of the image changes. These design issues are
already being addressed with prototypes having been implemented on very-large-scale
integration (VLSI) chips.
7.9.3.5 Neural Interface (Electrode Array)
This should be thought of as an active element of the system that transforms the currents
generated by the stimulator electronics into ionic currents that flow in the body. Most
materials can achieve this to some degree; however, some, including silver, are toxic to
neurons in their vicinity, and platinum, though biocompatible, is not particularly effective.
To date, the most effective material that is both biocompatible and an effective transducer
from electric to ionic currents is oxidized iridium, and most implantable neural interfaces
are made from that material (Robblee and Rose, 1990).
These interfaces are particularly difficult to develop because the human body has
developed numerous defenses against intrusions of nonbiological materials. After time
most are rendered inert or sealed off by a fibrous tissue capsule, so the challenge is to find
materials that the body recognizes as benign in all ways.
The implant must be chemically benign to avoid reaction, and it must also have a
very similar density to that of the surrounding tissue so that it is not differently affected
by gravity or kinematic accelerations. Incompatibilities can generate shear forces, which
result in micromovements that both displace the electrode from its intended site and also
cause chronic inflammation due to the irritation. These are particularly difficult to solve in
retinal implants because saccadic eye movements produce large accelerations (Finn and
LoPresti, 2003).
Another problem is that because all human cells exchange, for example, nutrients,
with extracellular fluid and need to remain in equilibrium, the introduction of a large
impermeable structure adjacent to any cells, even if they are biologically invisible, can
result in disruption of their equilibrium. Once again, this is a major issue that has not been
completely addressed, particularly for retinal implants, where the cell nutrition comes
from either the vitreous humor on the one side and the retinal pigment epithelium on the
other.
Other issues include the flexibility of the implant. Ideally it should be mechanically
compliant to subtle movements of the surrounding tissue caused by blood pressure changes
during the cardiac cycle. Typically, polymers of various kinds are superior to more rigid
materials in this situation.
The final problem that needs to be addressed is that of tethering. Lead wires to the
implant must be conductors and are therefore generally made of metal, which needs to be
both insulated and very flexible. As array sizes increase, the numbers of wires must also
increase, and the tethering problem will become more acute. Ultimately, flexible image
processing electronics integrated directly onto the array will be required to minimize the
number of connections to the stimulator.
As discussed earlier in this chapter, four main areas are suitable for the electrode
array. The two main sites in the retina are the subretinal and epiretinal regions (below and
above the retina) used by most researchers. There are also two main sites within the brain,
one around the optic nerve and the second in the visual cortex, as shown in Figure 7-41.
Alternatives include the supra-choroidal space behind the eyeball being the site of choice
by researchers of the Australian Vision Prosthesis Group (AVPG), and possibly the LGN
in the brain, though access to the latter involves a particularly invasive procedure.
