J. Tyson, M. Tran, T. Patrick, G. Slaughter
University of Maryland Baltimore County,
Keywords: neural probes, electrode sites, carbon nanotubes, proteins, cell motility
Summary:The application of micro- or nano-electromechanical systems (MEMs) or (NEMs) to the fabrication of neural probes, have led to the development of minimally-invasive high density probes. Although the majority of neuroMEMs probes have been used in short term implantation studies, chronic implantation has eluded the field of neuroscience and neuroengineering due to the biocompatibility challenges encountered. Tissue damage, scarring and device encapsulation eventually render these device useless. Additionally, traditional neuroMEMs probes are developed to measure electrical or chemical signals separately, however no current methods exist that enable both electrical and chemical signals to be measured and recorded simultaneously. Here we describe the development and characterization of a novel electrical and chemical sensing system that can acquire both electrical and chemical signals simultaneously, while being suitable for chronic implantation. We fabricated multichannel, flexible neural probes using standard microfabrication techniques. Each neural probe comprises of four shanks with 16 electrode sites per shank. Each electrode site is approximately 250 µm2 and is created using gold rectangular pyramidal electrode sandwiched between two polyimide dielectric layers. The electrodes were characterized by impedance spectroscopy in phosphate buffered saline solution. The electrode test impedance values at the physiologically relevant frequency of 1 kHz was observed to be on average 135 kΩ. Multi-walled Carbon Nanotubes (MWCNTs) were deposited on the electrode sites resulting in at least 19-fold reduction in the electrode impedance at 1 kHz to 6.89 kΩ on average. In initial biocompatibility experiments, the polyimde neural probe, polyolefin and common extracellular matrix proteins, collagen and laminin, were incubated with PC-12 cells which formed a monolayer over the probe surface. Cell viability and proliferation results suggest that the polyimide neural probe exhibits desirable characteristics for implant material coatings – high viability (>80%) with low proliferation (< 40%), confirming a lack of cytotoxicity. For chronic studies, cell adhesion and motility are modeled by covalent binding fluorescent-labeled extracellular matrix proteins, collagen and laminin to the probe and electrode surfaces. Protein adhesion was confirmed via fluorescence microscopy and impedance spectroscopy was used to confirm cell attachment and motility. Results suggest that the probe induce little to no cytotoxicity and scarring upon implantation. These polyimide neural probes hold great promise of long term, high-resolution characterization of neurodegenerative disease and psychiatric disorders.