Novel injection molding technique for coating soft-siloxanes on neural microelectrodes for stable pO2 sensing using MR imaging

There is a critical need for creating an implantable microscale neural interface that can chronically monitor neural activity and oxygenation. These are key aspects for understating the development of impaired neural circuits and their functions. A technology with such capability would foster new insights in the studies of brain diseases and disorders. The propose is that MR-PISTOL (Proton imaging of Siloxane to Map Tissue Oxygenation Levels) imaging technique can be used for direct measurements of oxygen partial pressure at microelectrode-tissue interface. The strategy consists of coating microelectrodes with soft-silicone, a ultra-soft conductive PDMS (polydimethylsiloxane), as a carrier for liquid siloxanes MR-PISTOL contrast agents. This work presents a proof-of-concept of an injection molding technique for batch fabricate microelectrodes with such coating. Also, reports stability studies of soft-silicone loaded with liquid polydimethylsiloxane (PDMSO) in rodent brains. A batch of thirty coated carbon electrodes was achieved using candy molds. Coating uniformity was evaluated in twelve probes. They were randomly chosen and imaged with a custom image setup that allows 90o rotation of the probes. The total average coating thickness before and after rotation were 0.397 millimeters with standard deviation of 0.070 millimeters and 0.442 millimeters with standard deviation of 0.062 millimeters. Therefore, data confirms that this technique yields uniform coating. Stability of fabricated coated carbon electrodes unloaded (n= 3) and loaded with PDMSO (n= 3) was assessed. 3D X-ray imaging using Zeiss Xradia 520 machine was chosen for studying coatings mechanical stability in ex-vivo rat brain. Transmission electron microscopy (TEM) and scanning electron microscope (SEM) with an energy dispersive x-ray microanalysis (EDS) detector were used to investigate their chemical stability in in vivo mouse brain. Initial EDS analysis from TEM and SEM of acute (6 hours) and chronic (2 weeks) brain slices suggest that PDMSO does not leach into brain. More experiments should be done to confirm and endorse this finding. The mechanical study shows that coating loaded with PDMSO delaminated during insertion. This was not observed with electrodes used in the chemical stability studies. Further experiments need to be done to identify possible causes of mechanical failures.