Optimizing cell-surface interactions by photografting of poly(ethylene glycol)

A new general approach for improving polymer substratum biocompatibility is proposed. In a first example, polysulfone (PSf) film was modified by coval ent end-on grafting of poly(ethylene glycol) (PEG) (2, 5, and 10 kDa) using well-defined, photoreactive alpha-4-azidobenzoyl beta-methoxy PEG conjugat es (ABMPEG). After adsorption from aqueous solution, ABMPEG was photografte d under wet conditions onto PSf, where the degree of surface functionalizat ion could be controlled through the applied ABMPEG concentration during ads orption. Attained surface characteristics, after changing systematically AB MPEG concentration, molecular weight, and the ratio of binary ABMPEG mixtur es. were monitored by air-water contact angles (CA, captive bubble method) and partially also by X-ray photon spectroscopy (XPS). For ABMPEG 10 kDa ad sorption kinetics and grafting efficiency as a function of applied concentr ation were evaluated by both CAs and fibronectin (FN) adsorption (in situ e llipsometry) to surfaces modified at different degrees of functionalization . CAs attained equilibrium values only after about 1-2 h, suggesting that s urface organization processes retard ABMPEC adsorption. FN adsorption decre ased monotonically as the degree of surface functionalization increased. Hu man skin fibroblast interaction with ABMPEG 10 kDa functionalized PSf films was studied, and a clear optimum of fibroblast-material intel action on mi ldly modified surfaces could be found based on the number of adhering cells , but also on morphological criteria including overall cell morphology, cel l spreading, and formation of focal adhesion contacts, visualized by fluore scent staining of vinculin. The results suggest that adhesive proteins such as FN are adsorbed in a biologically active state yielding enhanced cell-s ubstratum interaction when a hydrophobic substratum is surface modified at an intermediate degree with hydrophilic, flexible, sterically demanding, an d possibly "self-assembled" macromolecules, e.g., PEG. Presumably, those ma cromolecules exert a lateral pressure upon neighboring adsorbed adhesive pr oteins, yielding surface bound but in their active conformation stabilized proteins with high biological activity.