AAWSAP DIRD, Biosensors and BioMEMS: A Survey of the Present Field, March 2010

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Over the years, investigators have figured out how to interpret precursor bioelectrical
events in the brain involved in planning of movements. The result is that if enough
recording electrodes are placed in the brain in the motor cortex area, it is possible to
determine planned muscle motion. This works even if the muscles ultimately do not
move, as in the case of a quadriplegic with spinal cord injury.
Thus quite a remarkable possibility presents in that a computer program can be taught
to interpret large numbers of bioelectrodes signals and so can determine a person's
intent to move and then, for example, control a robotic limb that actually performs the
operation.
Crude forms of these thought-controlled devices have been shown in research
laboratories to be feasible, even if they are at present not practical. Among other
things, the electrode interface to the brain is not yet sufficiently developed to provide
large numbers of functioning electrodes that can work over prolonged periods of time.
MICROELECTRODE ARRAY FABRICATION
Neuroengineering applications in the brain can require tens to hundreds of electrodes to
make contact with a similarly large numbers of cells in the brain. Large numbers of
electrodes in arrays inserted into the surface of the brain cortex thus become sensitive
to the timing and patterns of cell firings.
BioMEMS has been a critical part of this development. The electrode array in Figure 20
is made by a electrochemical process of using a long but stiff wire that acts like a
machine tool. Electrical discharge machining (EDM) is a process that makes use of
computer-aided design (CAD), that runs under computer numerical control (CNC), and
that is capable of batch processing.
A block of titanium metal that will ultimately be the array is placed in an electrolyte
solution and the working electrode initiates a submerged electrochemical arc. It finely
removes metal by a chemical process that does not involve a lot of heat. It can
generate intricate features with high aspect ratios and is capable of machining a large
variety of conductive materials.
In this case, the electrode shaft thicknesses are on the order of 50-100 microns and the
entire device is less than a centimeter square with 96 electrode channels. When this
device is encapsulated in a polymer at its base and pressed into the brain cortex, each
electrode then records from a specific set of neurons that are local to its placement.
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