Dynamic Interactions of Retinal Prosthesis Electrodes with Neural Tissue and Materials Science in Electrode Design - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 212

electrodes. Because of the unique requirements for high pixel number in a
limited active area, the charge density burden of retinal stimulatory electrodes
is very high. As a result, the potential exists for charge-related tissue trauma in
the area adjacent to implanted neural stimulating electrodes. Interfacial charge
density with protracted stimulation of the central nervous system has been impli-
cated in stimulation-induced neural injury. The mechanism for this damage is
unknown, but is conjectured to be a result of the passage of current through
tissue and neuronal hyperactivity [8]. Mortimer et al. [9] chronically stimulated
the cerebral cortex of cats with platinum (Pt) electrodes and biphasic current
pulses. They observed a relationship between blood-brain barrier damage in the
area surrounding the electrodes and the charge density at the electrode surface.
McCreery et al. [10] found that the threshold for damage to the parietal cortex of
cats during extended stimulation with activated iridium (IrO x ) microelectrodes
was the result of synergistic interaction of charge density and charge per phase
beneath the implanted electrode. In later work, McCreery and his colleagues [11]
were able to link stimulus-induced temporary depression of electrical excitability
of the cat cochlear nucleus with prolonged neuronal activity. Depression was
not correlated with histological changes near the electrodes, suggesting that the
stimulation protocol was above the threshold for reversible neuronal over-activity
but below the threshold for irreversible neuronal damage. Neural damage in
the cortex of the cat was observed by Yuen et al. [12] after continuous stimu-
lation at a charge density of 100C/cm 2 for 6 hours. Less damage resulted
from the application of 40C/cm 2 for 20 hours. Rose and Robblee [13] found
that the safe charge injection limit for cathodic to anodic biphasic 0.2msec
pulsing of Pt electrodes in saline was 100-150C/cm 2 . Based on Pt disso-
lution data in saline, Brummer et al. [14] suggested limiting charge densities to
< 300C/cm 2 for balanced biphasic pulsing. For iridium oxide microelectrodes
in saline, the limits can be extended to 1mC/cm 2 because of the greater charge
carrying capacity of IrO x . [15]. Understanding the electrophysiological inter-
actions between prosthesis electrical components and excitable tissue and the
electrochemical reactions at the electrode-vitreous interface are the key elements
to developing a safe and practical retinal prosthesis insert.
Interfaction Electrochemical Processes
Electrodes charged at a tissue surface interact with ionic species in the fluid
medium bathing the tissue. The voltage, time duration, and conductive properties
of stimulating electrodes determine the electrochemical processes that take place
at the interface. Charged species in the vitreous can realign spatially around
the electrode in an attempt to satisfy electroneutrality at the electrode surface
(non-faradaic or capacitative charging), or they can bind to the electrode surface
and transfer electrons into the vitreous in the form of electrochemical reactions
(Faradaic charging). Faradaic charging is reversible if reaction products remain
bound to the electrode surface. It is irreversible if products leave the electrode
surface and redistribute in the vitreous. Neurostimulatory electrodes can be