Biomaterials for MEMS

78 S. Norman and R. Bellamkonda

brain to distal portions of the body. Electrical stimulation of the spinal cord distal

to the injury may restore function or sensation to patients with SCI.

When the spinal cord, nerves, or muscles are stimulated directly, they can elicit

a desired response. In one study, cats were implanted with either microwire arrays

in the lumbar spinal cord or microwires in the hindlimb muscles, and stimulation

was sufficient for standing and balance.

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A review on intraspinal microstimu-

lation states that the technology looks promising for a variety of applications,

including the reestablishment of egestive and sexual function to SCI patients.

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Intraspinal stimulation has been shown to elicit force from and contraction of

the hindlimbs of anesthetized and decerebrate cats;

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the ability to control such

muscular activation is of primary interest for stimulation devices designed to help

SCI patients maintain balance and possibly walk.

Microelectrodes for spinal stimulation are often made from silicon or mi-

crowires. Smaller electrodes presumably result in less damage to neural tissue,

so micromachined devices are important for use in the spinal cord. Investigators

using silicon probes manufactured at the University of Michigan and iridium

microwire arrays implanted into the sacral spinal cord of cats have shown them

to be adequate for modulating bladder pressure.

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In addition to reestablishing lost function, spinal cord stimulation has also

been used to reduce discomfort experienced by chronic pain patients. In an im-

planted spinal cord stimulation study, more than half the patients reported at least

a 50% reduction in pain and some were even able to return to work.

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Continual

refinement of microfabricated spinal cord devices will likely improve the number

of chronic pain patients that achieve relief using spinal cord stimulators.

4.2.9 Brain Computer Interfaces

While a visual or auditory prosthesis translates signals from the external envi-

ronment into electrical stimulation that patients can interpret, brain computer

interfaces (BCIs) are the reciprocal devices; a BCI uses a patient’s brain activity

to interact with the external environment in some way. Patients with extreme

physical trauma or debilitating disease can undergo a major loss of function, such

that they are rendered unable to move and sometimes even unable even to blink;

such patients may have completely intact thought and mental functioning, but

they are unable to interact with the world around them, and these patients are

said to be “locked-in” to their internal environment.

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BCIs offer locked-in patients

clinical hope, as they may one day be able to control mobility devices, computers,

and household appliances simply thinking by about them.

At the highest level, a brain computer interface records signals from the patient

while they perform a task and then gives them feedback (Fig. 4.6). Neuronal

signals for BCIs can be acquired from the patient in a number of ways: classical

electroencephalogram (EEG) has been shown in the literature, as well as the more

invasive electrocorticogram (ECoG), local field potentials (LFPs) and single neuron

action potentials (single unit recordings).

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Generally, more invasive recording

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