Development of highly biocompatible neuro-electronic interfaces towards monitoring authentic neuronal signaling in the brain

Sammanfattning: Background: To understand how the neuronal circuits in the brain process information there is a need for novelneuro-electronic interfaces that can interact chronically with brain tissue with minimal disturbance of thephysiological conditions in the tissue, in awake and freely moving animals. For this, there is a need for implantableneuro-electronic interfaces that are mechanically compliant with the tissue and that can remain positionally stablewith respect to the neurons, despite the continuous micromotions in the brain. To reach this goal it is alsoimportant to be able to conduct a detailed analysis of the tissue reactions in the juxtapositional tissue around theimplant as well as to incorporate additional strategies such as adding tissue modifying drugs to the implant.Aim: To this end, two different types of implantable neuro-electronic interfaces, addressing the issue ofmechanical compliance with two different approaches, as well as a novel method of sustained drug delivery fromthe neural implants were designed, manufactured and evaluated in vivo.Method: First, arrays of thin gold leads, flexible in 3D, were cut from a 4 μm thin gold sheet, insulated with a thinlayer of Parylene-C (4 μm) and then embedded and structurally locked in a stiff gelatin matrix that dissolves afterimplantation. These arrays were implanted in rats and evaluated electrophysiologically for 3 weeks. Second, anovel tube-like electrode with an opening on the side, comprising a conducting lead embedded in glucoseenveloped by a thin layer of Parylene-C, was developed. After implantation the glucose in this protoelectrodedissolves transforming the protoelectrode into a highly flexible and low density electrode inside the tissue. Suchtube electrodes were implanted in rats and evaluated by means of immunofluorescence microscopy after 6 weeks.Further, minocycline loaded nanoparticles were embedded into a gelatin matrix surrounding neural implants andthe tissue reactions were evaluated in genetically modified mice exhibiting fluorescent microglia by means ofimmunofluorescence microscopy 3 and 7 days after implantation.Results: The developed 3D arrays were found to be implantable with preserved conformation andelectrophysiological recordings showed relatively stable recordings, with spike amplitudes over 400 μV. The tubeelectrode proved to be sliceable in the brain without dislocating, making it possible to analyze the tissue rightoutside the recording site, showing minute glia reactions and no significant loss of neurons as compared tobaseline tissue, even in the inner most zone (0-20 μm). The minocycline loaded nanoparticles where successfullyincorporated in gelatin-coatings of neural implants, and histological analysis showed a significant attenuation ofglia reactions.Conclusion: Two new types of mechanically compliant neuro-electronic interfaces and implantation methods, aswell as a compatible embedding method of local delivery of drug content, has been successfully developed andevaluated, showing very promising biocompatibility and stability in the tissue.