Biomedical Engineering Reference
In-Depth Information
planar Pt foil. Pure Pt has been the favored material for retinal prosthesis micro-
electrode arrays. The surface area of smooth (planar) Pt wire is approximately
equal to its geometric surface area. Other forms of Pt, however, have rougher
surfaces and higher effective surface areas that compensate for small diameters
of microelectrodes in the charge density equation. Rough Pt (HiQ) made by
electrochemical treatment of standard Pt electrodes [37] is being tested in the
auditory system as a possible alternative to planar Pt in cochlear implants. HiQ
electrodes [38, 39] have lower impedances and 70-75 times the real surface area
of conventional cochlear electrodes. High-intensity cochlear implant stimulation
can result in decreased auditory brainstem response, due, in part, to DC along
the implant during stimulation. Stimulation of mammalian auditory nerves with
HiQ Pt electrodes did not change the amplitude and threshold of the brainstem
response, as did conventional electrodes. Histology of the electrode implant area
showed no significant damage from stimulation with either HiQ or standard Pt
electrodes.
Another form of rough Pt, equally interesting for prosthesis implant arrays,
is Pt gray. Pt gray has a surface area as high as Pt black, 5 times higher than
planar Pt. It is a durable material, not subject to stress cracking like Pt black.
Zhou [40] has patented a method for electroplating Pt gray onto the surface
of microelectrodes. The Pt gray surface increases the effective surface area,
retaining desirable Pt characteristics of low impedance, high conductivity and
high threshold for onset of faradaic interfacial redox chemistry. Small diameter
76m, planar Pt electrodes were coated with Pt gray by Zhou and tested for
dissolution and electrolysis during chronic charging in synthetic vitreous humor.
Figure 11.8. SEM image of Pt gray before pulsing.
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