Invited Paper BI-TuM3
Thin Hydrogel Layers on Biomedical Polymers - Biological Responses and Effects on Protein Adsorption Studied by Mass Spectrometry

Tuesday, October 16, 2007, 8:40 am, Room 609

Medical devices used in contact with blood often contain features that are on the order of microns, (e.g. the struts on stents). Application of thin polymeric coatings on stents is feasible for the release of antiproliferative agents. However, the use of hydrogel coatings in this application is limited by a number of factors. Thin hydrogel coatings may be difficult to apply on complex geometries. The large volume of water in the hydrogel also limits the amount of drug that can be delivered from the coating. To address the first issue, we previously synthesized a copolymer of polylysine and polyethylene glycol (PLL-g-PEG) that self-assembles on negatively charged surfaces. We demonstrated that very thin yet stable layers of water-soluble polymer reduce biological responses, both in vitro and in vivo. We followed these experiments with investigations into the uses of layer-by-layer strategies, however, practical utility of these films is limited by time-prohibitive methods of fabrication, and the formed films may be too thin for drug delivery. We have addressed these shortcomings in two ways. Rather than delivering drugs directly, we are incorporating an enzyme into hydrogels. The enzyme produces a biologically active molecule (sphingosine 1-phosphate) from a precursor already present in blood (sphingosine). This molecule causes endothelial cell chemotaxis and inhibits smooth muscle cell migration. Additionally, we are producing multilayer films from nanogels that are formed by crosslinking PEG-vinylsulfone with albumin (average particle sizes 40 - 80 nm). Even a single layer of the nanogels covalently-reacted with RFGD-modified PET greatly reduces cell adhesion. Finally, in characterizing protein adsorption on thin hydrogel films, it is important to know not only the amount of adsorbed protein but also the conformations adopted by the adsorbed proteins. To study this, we have developed a proteomics-based strategy to detect differences in the exposure of lysine residues following adsorption. Our studies demonstrated an increased accessibility of lysine residues in fibrinogen adsorbed from low concentration solutions, which correlated well with the increase in the spread area of fibrinogen as measured at the same solution concentrations by OWLS. Overall, tremendous challenges and opportunities exist for producing thin surface coatings that resist non-specific biological adhesion and deliver drugs to control the biological response.