PROJECT SUMMARY/ABSTRACT The objective of this proposal is to develop, evaluate the potential of Carbon Nanotube (CNT) fibers Microelectrode Arrays (MEAs) and test their performance in-vivo by inserting them in the visual cortex of rats in acute and chronic settings. Its novelty relies on the reduced diameter, super-hydrophilic coating nature of the CNT fibers, and takes advantage of the chemical inertness, flexibility and large surface area of CNTs. Additional feature of these proposal is the hexagonal packing of 7 CNT fibers into ~50 µm strands to provide the required stiffness for insertion, and subsequent unraveling into 7 individual electrodes upon insertion and interaction with water. Proposed approach will allow to pack 112 electrodes into a 4x4 array, and will be able to connect to metal contact board produced with traditional lithography. Currently, most commands emitted from the brain require electrical currents transported through nerves and tissue to elicit cognitive, sensory, visceral, and motor functions. Unfortunately, these connection paths are often perturbed due to traumatic or degenerative diseases causing complete loss of function. Multiple brain diseases like epilepsy and Parkinson's require microelectrode stimulation as part of the treatment and recovery. Most current technology relies on metal-, metal oxide or silicon-based electrodes that have a mechanical mismatch and are considered foreign by cells and neurons causing adverse reactions through inflammatory responses, biofouling and scar tissue formation as they try to encapsulate the electrode. Moreover, metals employed as electrodes have: significantly smaller surface area, larger impedance, and reduced charge injection limit (CIL). To solve these electrode deficiencies currently employed in neural stimulation and recording, this team has developed unidirectional, biocompatible, densely-packed CNT fiber microelectrodes that to this date show impressive CIL (15.6 mC/cm2), fast electron transport, and lower impedance than metals. Surprisingly, these CNT fibers can be assembled up to 16 m/s linear speeds, offering great potential towards scalability. We expect to demonstrate the potential of our fiber for long them stimulation and recording, as well as compare their performance as MEAs to the state-of-the-art carbon fiber, and iridium based MEAs.