Development of an optrode for characterization of tissue optical properties at the neural tissue-electrode interface
Segura, Carlos Alejandro
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The use of implantable neural probes has become common, both for stimulation and recording, and their applications range from chronic pain treatment to implementation of brain machine interfaces (BMI). Studies have shown that signal quality of implanted electrodes decays over time mainly due to the immune response. Characterization of the tissue-electrode interface is critical for better understanding of the physiological dynamics and potential performance improvement of the electrode itself and its task. This work describes the fabrication of an implantable electrode with optical measurement capabilities for providing means to characterize the tissue-electrode interface using optical spectroscopy. The electrode has a set of waveguides embedded in its shanks, which are used to inject white light into tissue and then collect the light reflected from the tissue surrounding the shanks. The collected light was analyzed with a spectrometer and the spectra processed to detect changes in optical properties, information that allows to track physiological changes. It is believed that the immune response can be correlated to changes in scattering as more cells are recruited to the injury site. The increased cell density in local injury/implantation sites increases the amount of scattering due to the increased number of cell nuclei and membranes that light encounters in its path. Investigation of scattering and absorption coefficients in such interface and their change over time can provide useful data for modeling and determining physiological parameters like blood oxygenation while the actual shape of the acquired spectra might highlight particular phenomena that can be indicative of scaring process or hemorrhaging. Validation of this system was done using optical phantoms based on polystyrene spheres and solutions with various concentrations of fat emulsion, which yielded scattering coefficients similar to those of brain tissue. Results suggest that the developed optrodes are able to detect differences between samples with different scattering coefficients. Improvements of fabrication process are discussed based on experimental results and future work includes attempting to perform fluorescence measurements of voltage reporters for optogenetic applications. The ultimate goal of this project was to create a novel device that is capable of satisfying the unmet need of tissue characterization at the implanted electrode interface as well as a tool for the optogenetics field suitable for greater depths than those a microscope can achieve.
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