Future in vitro life science studies that require in situ functionalities—going beyond the common glass petri dish—can potentially be realized by using microsystems technologies to fabricate bio-microelectromechanical systems (BioMEMS) and laboratory-on-a-chip (LOC). One such material used to construct these microsystems is the commercial photoresist SU-8. Here, the neuronal biocompatibility of SU-8 is examined as a function of its hard-baking thermal treatment. The SU-8 surfaces were non-patterned large uniform thin films—this allowed the biocompatibility and cell adhesion of the SU-8 to be fully tested. In total, 75 SU-8 samples were fabricated for the study using spin-coating and lithography techniques. We observe that a single hard-baking step of 180 °C for 2 h coupled with a coating (poly-d-lysine + laminin) is enough to detoxify SU-8 and promote primary cortical cell adhesion and survival up to 28 days in vitro (DIV). Therefore, the protocol described here makes SU-8 surfaces compatible with the development of neuronal networks from primary neural cells.