74 S. Norman and R. Bellamkonda
parylene to complete the probe; the photoresist is then dissolved in a solvent and
the hollow channel remains.
44
Another method of parylene channel formation
involves depositing parylene on a glass substrate and bonding this parylene to
parylene deposited in a silicon mold lined with aluminum; while the process
is slightly more complicated, the seal between bonded parylene layers appears
to make a reliable channel.
45
Because parylene is softer than silicon, it is more
susceptible to bending. To prevent buckling, one group filled the microfluidic
electrode with polyethylene glycol (PEG) before insertion, then dissolved the PEG
using saline.
44
Polyamide channels have also been fabricated with a technique
where partially cured polyamide is bonded onto another polyamide sheet to create
a seal; electrodes are first patterned on the polyamide pieces.
46
Silicon microfluidic
channels have been made to deliver bio-active molecules or drugs to the area
around insertion.
47
The ability to sample the brain environment in response to behavioral triggers
or after trauma is an exciting prospect to researchers. One group has used
capillary liquid chromatography tandem mass spectrometry to monitor the neural
environment of rat striatum using small dialysis probes.
48
Using the microdialysis
technique and a slightly different analysis system, they have been able to identify
glutamate, aspartate, and serine in fluid taken from mouse striatum.
49
Push-pull
methods for drug delivery and fluid sampling have also been developed, in which
fluid is pulled from the region of interest and an equal volume of fluid, such as
saline or drug, is pushed into its place.
50
This assures that there is no volumetric
displacement in the test area, as this could cause excess trauma to the tissue
involved.
4.2.4 Deep Brain Stimulation Electrodes
Deep brain stimulation (DBS) is now approved as a therapy for advanced Parkin-
son’s disease, and has also been used to treat the symptoms of epilepsy
51
and
depression.
52
Electrodes used for DBS are approximately 1 mm in diameter and
usually have four stimulation electrodes (not all of which are used); they are
commercially available from Medtronic, and the Medtronic models used in clinical
practice include 3387 and 3389.
53
They are inserted into an awake patient, and
electrical recordings can be taken before insertion to guide the physician to proper
placement.
54
To alleviate Parkinson’s symptoms, including tremor, the electrode
is usually placed in one of three brain regions: the internal segment of the globus
pallidus (iGP), the subthalamic nucleus (STN) or the motor thalamus.
55
Movement disorders associated with Parkinson’s disease, such as tremor and
rigidity, appear to respond well to stimulation treatments. The clinical standard
in the past was to lesion areas in the subthalamic nucleus or globus pallidus,
but lesioning too much of the brain can have adverse consequences. Stimulation
appears to have a similar effect to lesioning, and can more readily be done
bilaterally. Patients that receive DBS electrodes have fewer motor deficiencies and
more time per day when they are not affected by motor deficiencies.
53
Five years
after implantation, patients still report improved motor function.
56
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