Defense Intelligence Reference Document Biosensors And A Survey Of The Present Field

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Only a few BioMEMS applications are well known for implanted devices and these are
used for positioning and repositioning of sensors. A specific example is application to
brain-electrode systems.
BRAIN-IMPLANTED BIOMEMS MICROMACHINE NEUROELECTRODES
There are applications in biomedicine and research where very small electrodes are
implanted into the brain. These microelectrodes are used in the cortex (surface layer)
to detect electrical activity associated with the volitional desire to move some part of
the body. Electrical activity is recorded as a very small change in voltage within the
tissue.
Near the top of the human head and about 2 cm beneath the scalp lie the parts of the
brain where nerve cells (neurons) are found which control the muscles of the body.
Specific locations in the cortex are associated with specific parts of the body. When a
person moves a limb, there can be detected a corresponding electrical activity of these
neurons. Neuroscientists have recorded these signals and developed a kind of map of
the brain that defines what brain cells actuate certain muscles of the body.
An interesting phenomenon occurs in which some of these brain cells become active
even when there is intent to move a limb but no actual movement occurs. In a healthy
person the intent to move can be detected by measurement of microvolt signals from
the brain cells about 120 milliseconds before any muscle movement occurs. In a person
who has lost a limb or has become paralyzed, the intent to move can still be detected in
the brain even though there is no limb movement.
Thus in principle there is an ability to electrically record from the brain and determine a
person's intent to move. Monitoring of the brain is accomplished by using an array of
implanted microelectrodes whose signals anticipate movement of specific limbs. These
signals can in turn be used to control machines. This is the idea behind advanced
devices that allow quadriplegics to interact with their environment. Signals from
implanted electrodes are used by computers to control robotic actuators.
A problem recording the signals occurs because the body naturally tends to encapsulate
the electrodes with scar tissue, meaning the electrodes lose electrical contact with the
neurons. A solution to this problem incorporates a very small electromechanical
actuator attached to the electrode. This device allows the implanted electrodes to
change their position occasionally in order to continue to monitor neural events. By
moving the thin rod-like electrode up or down a small distance after several months of
implantation, the useful lifetime of the electrode array can be greatly extended.
The BioMEMS device described here is meant to be implanted under the skull and on
top of the brain cortex. It allows fine adjustment of less than a millimeter in order to
make sure the electrode system, once implanted, is able to contact the desired brain
cells even after the electrode has been encapsulated by scar tissue over time.
Figure 2 shows a photomicrograph of a gear-driven micromachine that was made
through collaboration between ASU and Sandia National Laboratories. The device is
driven by an electrostatic vibrating comb motor. Under a microscope it can be seen that
the combs move in an oscillatory fashion at about 40 Hz when energized. Electrostatic
forces between two blade-like combs a few tens of microns apart are operative with
about 15 volts as the electric field source.
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