Species and density of implant surface chemistry affect the extent of foreign body reactions.

Implant-associated fibrotic capsule formation presents a major challenge for the development of long-term drug release microspheres and implantable sensors. Since material properties have been shown to affect in vitro cellular responses and also to influence short-term in vivo tissue responses, we have thus assumed that the type and density of surface chemical groups would affect the degree of tissue responses to microsphere implants. To test this hypothesis, polypropylene particles with different surface densities of -OH and -COOH groups, along with the polypropylene control (-CH2 groups) were utilized. The influence of functional groups and their surface densities on fibrotic reactions were analyzed using a mice subcutaneous implantation model. Our comparative studies included determination and correlation of the extents of fibrotic capsule formation, cell infiltration into the particles, and recruitment of CD11b+ inflammatory cells for all of the substrates employed. We have observed major differences among microspheres coated with different surface functionalities. Surfaces with -OH surface groups trigger the strongest responses, while -COOH-rich surfaces prompt the least tissue reactions. However, variation of the surface density of either functional group has a relatively minor influence on the extent of fibrotic tissue reactions. The present results show that surface functionality can be used as a powerful tool to alter implant-associated fibrotic reactions and, potentially, to improve the efficacy and function of drug-delivery microspheres, implantable sensors, and tissue-engineering scaffolds.