(32b) Atom Transfer Radical Polymerization Grafting for Surface Modification - High Grafting Density and Controlled Molecular Weight

Biocompatibility is an essential requirement for all biomaterials regardless of applications. However, biocompatibility is elusive and very difficult to achieve since the body responds defensively to the presence of foreign materials. It is well known that protein adsorption is the first event following the implantation of biomaterials and that the adsorbed protein layer controls further biological reactions. There is thus considerable interest in surfaces that might inhibit or prevent protein adsorption.

In this work, 2-methacryloyloxyethyl phosphorylcholine (MPC), a biomimetic monomer, and oligo(ethylene glycol) methyl ether methacrylate (OEGMA, MW 300 g/mol, PEO side chains of average length n = 4.5), were successfully grafted from silicon wafer surfaces at room temperature by combining self-assembly of initiator and surface-initiated atom transfer radical polymerization (ATRP), which is a recently developed controlled/living radical polymerization technique for surface grafting of end-tethered polymer chains with controlled pattern, chain length, density, and functionality. The graft density and chain length of poly(MPC) and poly(OEGMA) were successfully controlled via the surface density of the initiator and the ratio of monomer/sacrificial initiator, respectively. The resulting surfaces were characterized by ellipsometry, water contact angle, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), neutron reflectivity (NR), protein adsorption and platelet adhesion.

It was found that protein adsorption decreased dramatically with increasing graft density and chain length of poly(MPC) and poly(OEGMA). When the graft density is ?d 0.29 chains/nm2 and chain length is ?d 100 monomer units, the adsorbed fibrinogen or lysozyme were less than 10 ng/cm2, which is comparable with the best results from other groups. Meanwhile, binary protein adsorption experiments indicated poly(MPC) and poly(OEGMA) chains can strongly prevent non-specific protein adsorption. It was found, somewhat surprisingly, that for a given chain length and graft density, adsorption from TBS buffer on both polymer surface types was basically the same.

NR results showed that both poly(MPC) and poly(OEGMA) grafted chains were highly stretched and that the water fraction in poly(MPC) and poly(OEGMA) layers having similar graft density were comparable. It was therefore concluded that the water content of poly(MPC) and poly(OEGMA) layers is strongly correlated to their protein resistance. Platelet adhesion from flowing whole blood indicated that both the poly(MPC) and poly(OEGMA) surfaces are highly platelet resistant at high graft density and may thus possess good hemocompatibility.