Originally Published MPMN April 2009

SPECIAL FEATURE: IMPLANTABLES

Lead-ing the Way to Better Implants

Interfacing directly with brain tissue, microelectrodes are critical components of implantable deep brain stimulation devices used to alleviate the symptoms of such conditions as epilepsy and Parkinson’s disease. But they’re not perfect. The parts are plagued by short life spans and high initial impedance owing to their minuscule surface area. A research team from the University of Michigan (Ann Arbor; www.umich.edu), however, may have found the solution to these issues in the form of a nanotech coating.

“Issues are associated with the interface between hard, metallic, engineered electrodes and the soft, electrolytic, living tissue,” explains David Martin, professor of materials science and engineering, macromolecular science and engineering, and biomedical engineering at the university. “Near an implanted probe, you see the loss of the cells you want—neurons—and an increase in the cells you don’t want—astrocytes. This makes the electrodes perform poorly over long periods of time.”
Developed to address this problem, the coating consists of three components that, when combined correctly, can extend lead life and enhance functionality. First, the researchers electrospun biodegradable PLGA nanofibers infused with the antiinflammatory drug dexamethasone to enable controlled drug delivery via a dissolution mechanism. Further facilitating controlled drug release through diffusion control is the application of an alginate hydrogel coating. The hydrogel also multitasks by serving as a scaffold for the growth of cloud-like nanostructures and as a protective buffer between the hard electrode material and soft brain tissue. To improve impedance, the conductive polymer PEDOT was then electrochemically polymerized on the electrode sites, around the nanofibers, and within the hydrogel matrix.
The researchers demonstrated that this surface-modification technique decreased electrode impedance and increased the charge capacity density. Electrochemical polymerization of PEDOT increased the surface area of the interface between the brain tissue and lead. In turn, the capacitance of the electrode site increased while impedance decreased. Furthermore, the drug-incorporated nanofibers aid in thwarting encapsulation—the immune system’s response to foreign bodies—which the team says has a hand in reducing lead efficacy over time.
“Current neural microelectrodes are just able to function for a few months,” explains Mohammad Reza Abidian, a postdoctoral research fellow in the department of biomedical engineering at the university. “We believe that by using our nanotech coatings, we are able to increase the life span [to] years.”
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