Comparison of in vivo biocompatibilities between parylene-C and polydimethylsiloxane for implantable microelectronic devices

Implantable devices are often composed of or coated with different biologically compatible materials based on their requirements. Selecting a surface material for an implantable device is not an easy task, and it is necessary to compare the biocompatibilities of the available surface materials. In this study, we perform a comparison of the in vivo biocompatibilities of polydimethylsiloxane (PDMS) and para-xylyene polymer (parylene-C) as they are considered to be candidates for a coating material for implantable microelectronic devices. For in vivo biocompatibility testing, fifty four male Sprague-Dawley rats were used for testing, and they were divided into three groups (PDMS, parylene-C and a positive control). At one, four and twelve weeks after implantation of the test object, the density of inflammatory cells and the granulation layer thickness were recorded for each group and compared with other groups using visible light and fluorescence microscopy. The thickness of the granulation layer tended to decrease over time for all of the experimental groups, whereas the granulation layer thickness remained constant in the positive control group. The thinnest capsular layer was observed for the parylene-C group and fewest inflammatory cells were observed in this group during the entire experimental period. Macrophage infiltration was minimal, even at one week, and was not observed thereafter. The parylene-C group showed better biocompatibility than the PDMS groups, both for acute and chronic implantation. Thus, parylene-C is the best candidate of the tested materials for applications involving permanent implantable micro-devices.