Biocompatibility of Su-8 MEMS-based device

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Homlok József
Department of Control Engineering and Information Technology

Neural implants allow monitoring and/or stimulation of specific brain areas with high spatial and temporal accuracy. Such tools have given rise to revolutionary techniques in the clinics for the treatment of epilepsy, Parkinson’s disease, etc. The utilization of the technology of microelectromechanical systems (MEMS) makes possible the construction of miniature implantable microelectrode arrays (MEAs) with high precision, reproducibility and yield. Polymer-based, flexible implants provide smooth coupling with the soft tissue, since they can follow small motions and pulsations of the brain. SU-8, being a photoresist, is an excellent candidate for insulation material of MEMS neural implants. This involvesdirect contact with the brain cells, therefore the biocompatibility of the material is crucial. Several in vitro and percutaneous in vivoexperimentsfocused on this subject, yet there is a need to thoroughly analyzethe long-term behavior of SU-8 in vivo, in the neural tissue.

I present along-term biocompatibility study of the SU-8 photoresist, performed on rodents. The polymer was used to create 40 µm thick, 300 µm wide, 5 mm long devices, using UV photolithography on 4-inch silicon wafers. Suchprobes were implanted into the neocortex of rats, for 8 weeks.After the implantation period, histological analysis was performed on perfused brain sections surrounding the devices. Neuronal-specific nuclear protein (NeuN) immunostaining showed healthy neurons around the probe track. Furthermore, the glial scar, the product of the hosts’ immune response, was visualized by glial fibrillary acid protein (GFAP)immunostaining. Images of thestained slicesallowed quantitative analysis, and the obtained resultsare comparable to the biocompatibility of other (e.g. silicon-based) implants.

In this thesis first I’m reviewing the literature concerning MEMS devices used as neural implants and the applicability of SU-8. Secondly, I present the materials and methods, which were utilized in this systematic biocompatibility study. In the third part, I present and discuss the obtained results, focusing on the conformation of the neural tissue around the SU-8 implants

In a future study, the analysis of the long-term variation of the recorded neurophysiological signals, obtained by chronically implanted SU-8 based microelectrode arrays will be possible.


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