Neural activities of free-moving animals provide valuable insights into behavior, memory formation and cognitive function of the hippocampus. Unitary activities simultaneously recorded from multiple sub-regions of the hippocampus enable detailed study of hippocampal neural circuits, but require high fidelity recordings with high temporal and spatial resolution. In this work, we explored the possibility of using Parylene-C as the structural material for a penetrating, multi-electrode array designed to record from multiple sub-region of the rat hippocampus. A 64-channel Parylene-based flexible electrode array was designed and fabricated. The layout of the electrode array was arranged to conform to the shape of cell body layers of the rat hippocampus. An insertion technique of temporarily reduce the effective length of the probe with polyethylene glycol (PEG) was developed and tested in vivo. The multi-electrode array was implanted into a rat hippocampus for chronic experimentation and unitary activities were collected both during the implantation and after recovery while the animal ran freely in an open field. Unitary activities with an average signal to noise ratios (SNR) of 3 to 4 were recorded with the Parylene probe over the period of one month after implantation.
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