Biomedical Engineering Reference
In-Depth Information
wires through fragile branches of the brain's vascular system can be solved
using new conductive polymers nanowires, which change the shape in response
to variable electrical fields, which may allow the steering of probes. Polymer
nanowires are 20-30 times smaller than the platinum ones and they can be
made biodegradable for short-term brain implantation.
Soft, fuzzy biomaterials and electroactive conductive polymers—
poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT), biologically
conjugated melanin, etc.—can also be deposited on any kind of electrodes to
improve their biocompatibility.
57
However, polymer materials are problematic,
especially in long-term biocompatibility, as they are not sealed completely and
become conductive. Sputtered iridium oxide film significantly improves the
neural interface devices. The film exhibits better electrochemical characteristics
in comparison to platinum. Its charge injection capacity is an order of
magnitude higher than that of Pt, while impedance (at 1 kHz) is ten times lower
than that of platinum, making it an attractive stimulation and recording
electrode material.
48
Carbon nanotubes are a promising solution for implantable
electrodes. They express metallic or semiconductive properties depending on the
folding modes of the nanotube walls and represent a novel class of nanowires.
Different methods to separate semiconductive from conductive carbon
nanotubes have been developed and synthetic strategies to chemically modify the
side walls or tube ends by molecular or biomolecular components have been
reported. Tailoring such hybrid systems consisting of carbon nanotubes and
biomolecules (proteins and DNA) has expanded rapidly and attracted
substantial research effort. The same integration of biomaterials enables the use
of the hybrid systems as active field-effect transistors (FETs) or biosensor
devices, and allows the generation of complex nanostructures and nanocircuitry
of controlled properties and functions, with potential applications in nanobioe-
lectronics and nanobiotechnology.
58,59
Optical neural interface might be an alternative possibility for solving the
debilitating side effects of DBS or the lack of specificity in targeting excitatory
or inhibitory neurons.
60
Integrated fiber optic and laser diode optobionic
systems may also constitute a more natural interface for retinal prostheses,
based on the photostimulation of retinal neurons. Typically 100mWcm
2
in
instantaneous light intensity is necessary to stimulate action potentials in
neurons. This can be reduced to safe levels in order to negate thermal and
photochromic damage to the eye.
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Neural implants have progressed from wires to micro- and nanowires,
to silicon arrays and to fiber optic interfaces. The next generation will be
represented by flexible structures with engineered surfaces and bioactive
coatings for a better integration with the neural tissue. To insert them,
dissolvable coatings can be applied to stiffen the electrode. The coating can
rapidly disintegrate after insertion. Such electrodes can be equipped with
amplifiers and circuits for magnetic or thermal stimulation. Micromachined
channels could deliver drugs at specific sites in the brain. For long-term
applications such implants must use advanced biomaterials to prevent the
foreign body effect expressed by local
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