Electroconductive neural interfaces for neural tissue applications
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Creating effective cellular interfaces that can provide specific cellular signals is important for a number of fields ranging from tissue engineering to biosensors. Electroconducting polymers, especially polypyrrole (PPy), have attracted much attention for use in numerous biomedical applications since they provide a potential platform for local delivery of electrical stimuli to target tissues. To effectively modulate cellular functions at neural interfaces, it is essential to incorporate a range of extracellular cues into conducting polymers according to specific applications, such as nerve guidance conduits and implantable neural probes. For nerve regeneration scaffolds, three dimensional forms are desired for control of critical properties, such as porosity, mechanical strength, and topography. However, most researchers have worked on conventional two-dimensional PPy films, which cannot mimic a native three-dimensional architecture. Thus, a portion of my work has focused on introducing three-dimensional nanofibrous features into PPy. I have investigated various coating conditions to obtain uniform and conductive nanofibers. Effectiveness of electrical stimulation through the conducting nanofibers was confirmed by in vitro PC12 cell culture. The effects of different conducting nanofiber topographies (random and aligned) on cell adhesion and neurite outgrowth were examined in conjunction with electrical stimulation. The benefits of immobilized-NGF could be combined with electrical stimuli, which could be an ideal platform for neural tissue engineering scaffolds. Thus, I have modified conducting polymers to display neurotrophic activity. Nerve growth factor (NGF) was chemically immobilized on two dimensional and three dimensional PPy substrates. Specific chemical conjugation was achieved and characterized using diverse techniques. Immobilized NGF was as effective as exogenous NGF in medium in inducing neurite development and extension. NGF immobilized on functionalized PPy substrates was stable in a physiological solution and under electrical stimulation, indicating effective prolonged activity. I also investigated another important application of conducting polymer-based materials for neural interfacing - passivating electrodes with a biocompatible polysaccharide, hyaluronic acid (HA). I synthesized electrically polymerizable HA by chemically conjugating amine-functionalized pyrrole derivatives with HA. This coating was stable under physiological conditions for three months and resistant to enzymatic degradation. In vitro studies have shown the minimal adhesion and migration of astrocytes on the HA-coated electrodes. Implantation of HA-coated commercial probes into rat cortices for three weeks revealed attenuated reactive astrocyte responses from the coated wires, and the importance of glial interaction with non-conducting sites was demonstrated.
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