Biomedical Engineering Reference
In-Depth Information
charge injection capability, corrosion resistance, and could be packaged with
biocompatible materials [1]. The most commonly used electrodes are made by
four methods based on their fabrication processes: direct-wiring of conductive
wires or fibers, electroplating, vacuum-deposited thin films, and micromachining.
While more and more prosthetic devices are in development, there are increasing
demands for high-performance and high-density electrodes. The electrode array
in contact with the living tissue forms an interface between the electronic device
and the biological tissues. The size of charge-injection electrodes becomes
smaller in order to increase selectivity and accommodate more electrodes on the
arrays to achieve high resolutions for neural recording and stimulation. For such
high-density microelectrode arrays, the design of electrode arrays and choice of
electrode materials become increasingly important. An electrode must be able
to deliver higher charge density without generating irreversible electrochemical
reactions such as metal corrosion or dissolution, gas evolution, or introduction
of toxic chemical reaction products.
Using metal nanoparticles and microfabrication in an innovative nanopowder
molding process, 2D and 3D patternable structures were produced with a height
to width ratio greater than 10:1. The unique fabrication process of this molding
method is the ability to fuse the entire nerual stimulation circuit, including stimu-
lating electrode, connection trace, and contact pad, into one integrated continuous
structure in which the different sections of structure might have a different height,
width, and shape. The fabricated electrode is robust, flexible and is also suitable
for mass production with high reproducibility. More importantly for biomedical
applications, the entire fabricated structure can be packed at room temperature
into bio-compatible flexible substrates, such as poly-dimethylsiloxane (PDMS),
parylene and polyimide, and other temperature-sensitive or vacuum-sensitive
materials. The molded electrodes and wires not only have the same electrical
resistivities as their bulk materials, but also have desirable electrochemical
properties, including low electrochemical impedance.
For many biomedical applications, it is necessary for any electrode array
to be a patternable, flexible, and biocompatible package. Traditionally, noble
metals such as gold or platinum (Pt) are the preferred conductive materials
used for coating electrodes and forming conducting circuits. Vacuum-deposited
thin films are the most widely used method for making various neural stimu-
lation devices [2, 3]. There are many materials that could be used to make thin
conductive films on both hard surfaces (i.e. silicon, glass) and flexible substrates
[i.e. PDMS, parylene, polyimide or other polymers]. The most frequently used
conductive materials are Ti, Ti alloys, TiN, Au, Pt, Pt alloys, Pt/W, Pt/Ir, Ir, IrOx,
etc. Biocompatibility is one of the main concerns for material selection [4, 5].
Vacuum deposition is a good technique for making patternable conducting
electrodes and conductive lines on various 2D surfaces, but microscopic obser-
vation reveals that the deposited metal thin film is actually a collection of
nanometer scale metal particles stacked on top of each other. Generally, the
bonding forces among those particles are not strong enough to support the
structure itself. As a direct result, the vacuum-deposited metal films retain
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