The neural tissue around SU-8 implants: A quantitative in vivo biocompatibility study

The use of SU-8 material in the production of neural sensors has grown recently. Despite its widespread application, a detailed systematic quantitative analysis concerning its biocompatibility in the central nervous system is lacking. In this immunohistochemical study, we quantified the neuronal preservation and the severity of astrogliosis around SU-8 devices implanted in the neocortex of rats, after a 2 months survival. We found that the density of neurons significantly decreased up to a distance of 20 mu m from the implant, with an averaged density decrease to 24 +/- 28% of the control. At 20 to 40 mu m distance from the implant, the majority of the neurons was preserved (74 +/- 39% of the control) and starting from 40 mu m distance from the implant, the neuron density was control-like. The density of synaptic contacts - examined at the electron microscopic level - decreased in the close vicinity of the implant, but it recovered to the control level as close as 24 mu m from the implant track. The intensity of the astroglial staining significantly increased compared to the control region, up to 560 mu m and 480 mu m distance from the track in the superficial and deep layers of the neocortex, respectively. Electron microscopic examination revealed that the thickness of the glial scar was around 5-10 mu m thin, and the ratio of glial processes in the neuropil was not more than 16% up to a distance of 12 mu m from the implant. Our data suggest that neuronal survival is affected only in a very small area around the implant. The glial scar surrounding the implant is thin, and the presence of glial elements is low in the neuropil, although the signs of astrogliosis could be observed up to about 500 mu m from the track. Subsequently, the biocompatibility of the SU-8 material is high. Due to its low cost fabrication and more flexible nature, SU-8 based devices may offer a promising approach to experimental and clinical applications in the future.