In vitro and in vivo characterization of SU-8 flexible neuroprobe: From mechanical properties to electrophysiological recording

• We designed and fabricated SU-8 flexible neuroprobe which provides improved tissue compatibility.
• We performed the qualitative studies from in vitro cytotoxicity tests and in vivo immunohistological staining show that SU-8 neuroprobe owns suitable properties for long-term implant applications.
• We demonstrated the electrophysiological recordings from in vitro cortical neuronal network and in vivo implantation which proven the functionality of flexible SU-8 neuroprobe.

Flexible neuroprobe with better tissue compliance exhibits unique mechanical characteristics in maintaining stability of neural implant in vivo. In this study, a flexible neuroprobe using SU-8 was designed and fabricated for in vitro and in vivo electrical sensing to show the improved tissue compatibility compared to that of the traditional rigid neuroprobe. The validation of neuroprobe was achieved by in vitro mechanical and cytotoxicity tests as well as in vivo neural recording and immunohistological staining. The fabrication process consisted of the creation of a backbone structure using photolithography, photopatterning of evaporated metal, and insulating of the electrode trace. The results of mechanical test of our fabricated SU-8 neuroprobe showed four times of physical stress (18.77 mN) than the insertion force (4.69 mN) to sustain resistance from brain tissue during implantation. The in vitro cytotoxicity assay showed well neuronal survival and proved the sufficient surface biocompatibility of the SU-8 neuroprobe. Further in vivo immunohistological staining showed no obvious glia aggregation around the implantation site indicating suitable biocompatibility compared with that of a rigid neuroprobe. Our in vitro and in vivo studies showed SU-8 neuroprobe possessed enough stress to complete the implantation in brain tissue and remained flexibility to comply micromovement of soft tissue with minor immune responses to achieve in vivo electrophysiological recordings at a signal-noise-ratio of greater than 7.