Biomaterials for MEMS

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.

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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.

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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.

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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.

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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

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and

depression.

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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.

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They are inserted into an awake patient, and

electrical recordings can be taken before insertion to guide the physician to proper

placement.

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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.

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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.

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Five years

after implantation, patients still report improved motor function.

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