Bioactive properties of nanostructured porous silicon for enhancing electrode to neuron interfaces

Many different types of microelectrodes have been developed for use as a direct brain-machine interface (BMI) to chronically recording single-neuron action potentials from ensembles of neurons. Unfortunately, the recordings from these microelectrode devices are not consistent and often last for only on the order of months. For most microelectrode types, the loss of these recordings is not due to failure of the electrodes, but most likely due to damage to surrounding tissue that results in the formation of non-conductive glial scar. Since the extracellular matrix consists of nanostructured fibrous protein assemblies, we have postulated that neurons may prefer a more complex surface structure than the smooth surface typical of thin-film microelectrodes. This porous structure could then act as a drug-delivery reservoir to deliver bioactive agents to aid in the repair or survival of cells around the microelectrode, further reducing the glial scar. We, therefore, investigated the suitability of a nanoporous silicon surface layer to increase the biocompatibility of our thin film ceramic-insulated multisite electrodes. In vitro testing demonstrated increased extension of neurites from PC12 pheochromocytoma cells on porous silicon surfaces compared to smooth silicon surfaces. Moreover, the size of the pores and the pore coverage did not interfere with this bioactive surface property, suggesting that large highly porous nanostructured surfaces can be used for drug delivery. The most porous nanoporous surfaces were then tested in vivo and found to be more biocompatible than smooth surface, producing less glial activation and allowing more neurons to remain close to the device. Collectively, these results support our hypothesis that nanoporous silicon may be an ideal material to improve biocompatibility of chronically implanted microelectrodes. The next step in this work will be to apply these surfaces to active microelectrodes, use them to deliver bioactive agents, and test that they do improve neural recordings.