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 219

itance [26]. They must be mechanically and biologically compatible with their
implant location and transfer enough charge to depolarize excitable tissue without
triggering significant interfacial faradaic oxidation-reduction reactions. Future
demands for higher resolution, high pixel, and small diameter electrode arrays
will require the consideration of a variety of materials for electrode construction,
including Pt and other precious metals, metal composites, andmetal alloys. Micron
diameter Pt, Ir, Au, and PtIr electrodes can perform well as high density, low
impedance cortical stimulatory electrodes. Pt and its more durable alloy PtIr have a
long track record as neural stimulatory electrodes, chiefly because of their biocom-
patibility, relative resistance to corrosion, and high charge carrying capacities [22].
Iridium can be activated by biphasic cycling of the electrochemical potential in
electrolyte solutions to form iridium oxide IrO x on the surface of precious metal
substrates [27]. Charge is transferred in IrO x electrodes by stimulus-generated
valence changes in the surface oxide layer [15]. IrO x electrodes can deliver a
net safe charge of 10mC/cm 2 [28] with 0.2ms biphasic pulses in bicarbonate-
buffered saline, and the charge storage capacity is much greater than that of Pt or
PtIr [29]. Moreover, it is hard and durable, resistant to fowling and dissolution in
physiological solutions. In vivo comparisons of the performance of IrO x and PtIr
electrodes [30] demonstrated that they could both safely evoke action potentials
with extended continuous microstimulation in cat cortex without notable histo-
logical damage. Both electrodes performed well for extended periods at charge
densities of 800C/cm 2 , but poorly at charge densities above 3200C/cm 2 .
Weiland et al. [31] examined the histology and the electrical properties of the
guinea pig cortex implanted with thin-film iridium oxide electrodes. The cortex
was energized for two hours with 5-100A, 100msec biphasic pulses. Electro-
chemical impedance spectroscopy and cyclic voltammetry measurements showed
that the cortex and electrodes were both affected by a temporary impedance shift
but, histologically, the tissue was unaffected.
Titanium nitride (TiN) is a material for electrode construction that offers good
potential for extending the safe stimulation limits beyond iridium oxide or Pt [32].
The electrochemical performance of titanium nitride deposited on silicon bioelec-
trodes was compared with similarly constructed iridium oxide electrodes by
Weiland et al. [33]. Titanium nitride had similar charge injection capability to Pt
087mC/cm 2 , significantly below the capacity of iridium oxide 4mC/cm 2 .
Operational Stability of Pt Retinal Prosthesis Electrodes
The development of a retinal prosthesis for artificial sight includes a study of the
factors affecting the structural and functional stability of chronically implanted
Pt microelectrode arrays. The maintenance of electrical integrity between the
electrode and soft tissue depends, in part, on initial electrode placement in close
proximity to the retina and insulation of prosthesis components from intraocular
fluids. Only the electrode surface is in direct contact with the retina and the
corrosive salt solution bathing the ocular cavity. The 16-pixel microelectrode
retinal prosthesis now implanted in the eye of a retinitis pigmentosa patient [3]