Ultrafiltration membrane surfaces with grafted polymer ‘tentacles’: preparation, characterization and application for covalent protein binding

Ultrafiltration (UF) membranes from polysulfone (PSf) were functionalized by heterogeneous photo-initiated graft copolymerization of acrylic acid (AA). With radiation susceptible PSf, only proper selection of the UV energy (λ>350 nm; for selective excitation of the photoinitiator) yielded membranes with preserved UF barrier layer. Possibilities for adjusting structure and morphology of the graft polymer (g-PAA) layer by variation of functionalization parameters such as AA concentration and UV irradiation time were investigated. Very long grafted chains (MW>105 g mol-1) at varied grafting density (GD=0.01…1.2 nmol cm-2, relative to the outer surface area) were obtained. Partial penetration of the UF barrier layer by g-PAA was verified. Covalent immobilization of bovine serum albumin (BSA), γ-globulin (γ-Gl) and alkaline phosphatase (APh) was achieved by coupling with a water soluble carbodiimide. Bound BSA and γ-Gl amounts were up to Γ=10 μg cm-2, for membranes accessible only from the outer surface thus not using the entire pore volume. Locally addressed covalent protein immobilization after photo-patterning the PSf surface could be visualized with a fluorescent FITC-BSA conjugate. A strong salt effect onto immobilized APh activity (increase with NaCl concentration) was observed, indicating internal transport/accessibility limitations in the g-PAA layer. Correlations between PAA structure (Mw, GD) and accessibility (from BSA or γ-Gl binding and APh activity) could be established. The ‘tentacle’ g-PAA functionalized PSf UF membranes having preserved UF barrier and, e.g., with surface-bound receptors will find application in cell cultures under diffusion or perfusion conditions.