FLEXIBLE PROBE INSERTION AND ITS EFFECTS ON PROBE RELIABILITY AND TISSUE RESPONSE
Relevant for Research Area
Dr. Maria Asplund
Dr. Patrick Ruther
Brain machine interfaces (BMI) rely on large-channel count, long-term stable bioelectronic interfaces implanted into neuronal tissue. While silicon-based probe arrays, preferably combined with integrated electronics based on CMOS technology, provide an unprecedented large number of recording sites (cf. Advanced EDC), their long-term recording stability in chronic experiments is adversely affected by the mechanical mismatch between brain tissue and the silicon probe substrate. In contrast, neural probes realized using polymeric materials have demonstrated reliable neuronal recordings over time periods exceeding several months. Their application is accompanied by a drastically reduced tissue response, linked both to their minimal cross-section and high mechanical flexibility. Insertion of flexible probes into brain tissue requests however application specific insertion protocols based on probe shuttles. The importance of surgical methods in this context is exemplified by that the "robotic inserter" system was the technological achievement that received most attention when neuro-tech start-up NeuraLink launched their BMI prototype. For probes targeting superficial brain regions, selectively thinned silicon shuttles have proven an efficient and reliable method for parallel insertion of flexible probes (cf. StiffFlex). For deeper brain regions, these more simplistic methods are nevertheless unsuitable, as the insertion trauma from multiple straight trajectories would be substantial and might affect delicate brain areas. Safe navigation and deployment of flexible probes into deeper brain regions and areas buried within sulci, requires tools by which the surgery can remain minimally invasive.
In FlexInMech we address the insertion of multiple flexible shanks into deeper brain regions. For a minimally invasive approach we will develop a method based on guide tube arrays with a pre-defined curved geometry in combination with wire-based shuttles (bundle arrays). The shuttles will pull the probes through the curved guide tubes and into the brain tissue, so that they laterally spread from the main probe bundle in a pre-defined "funnel" shape. This way, it will be possible to interpenetrate a structure deep within the brain, with a network of slender and flexible probes, thereby generating a three-dimensional interface accessing the entire structure. Technical aspects to be investigated are related to the design of the guide tube bundle (curved geometry, materials), shuttle material (metal, super-elastic alloys) and preparation for probe-shuttle attachment (tip-sharpening vs polyethylene glycole (PEG)). Further, we expect frictional forces of probe shanks inside the curved tubes which need to be analyzed in view of their mechanical stress possibly affecting the slender polymeric probe shanks. FIB-SEM analysis will here be key to validate the integrity of the ultra-thin shanks throughout the assembly and insertion process. The spreading probe bundles will be analyzed in bench tests using transparent brain phantoms followed by in vivo application. In the latter case, we will use micro computer tomography (µCT) to determine the probe distribution in deeper brain areas of the rat brain. Feasibility of this µCT approach to visualize probes and electrodes, has been demonstrated in preliminary tests.
Previous Related Publication
Otte, Vlachos, Asplund, 2022, Cell and Tissue Research, doi.org/10.1007/s00441-021-03567-9