Hydrophilic, heat stable, low protein binding surface modification of PVDF membranes
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Hydrophilic, heat stable, low protein binding surface modification of PVDF membranes John Charkoudian, Alex Xenopoulos and John Lynch Millipore Corporation, Bedford, MA 01730, USA ABSTRACT Protein binding studies reveal increased levels of protein adsorbed to thermally treated polyacrylamide-modified PVDF microporous membranes in comparison to polyacrylate-modified membranes. Application of XPS, ToF-SIMS, densitometry and µthermal AFM indicate polyacryamide-PVDF mixing as the primary cause of the increased protein binding. INTRODUCTION Polyvinylidene fluoride (PVDF) membranes are hydrophobic and bind proteins nonspecifically (NSB) to high levels. Surface modification with hydrophilic polyacrylate and polyacrylamide polymers dramatically reduces their protein binding, while retaining their original porosity. Of the several methods available for PVDF surface modification, one of the most convenient is the in situ polymerization of mono- and polyfunctional unsaturated monomers initiated on the membrane surface by thermal, UV, or electron beam methods. These procedures are amenable to continuous production and have been successfully commercialized. EXPERIMENTAL Samples Commercial 0.22 µm Durapore® membranes were used for these experiments. Membrane properties are described in the Millipore catalog. Typically a PVDF membrane was prewetted with an alcohol, exchanged into water and treated with a solution containing about 10% monofunctional and about 2% polyfunctional monomer in a non-chain transfer solvent. If required, a UV or thermal initiator was included at a level of about 0.2% [1,2]. After polymerization in an inert atmosphere, the membrane was washed to remove unreacted monomers and polymer that was not crosslinked on the membrane. For the acrylate membrane (Example 1), only acrylate monomers were employed; for the acrylamide membrane (Example 2), the formulation contained only acrylamides. The weight % added to the membranes was 3-5%. Compared to the permeability of the unmodified membrane, modification resulted in a loss of about 19%. The acrylate-modified membrane was instantly wetted (less than 0.5 seconds) by a 5% salt solution. After conditioning the membrane in a dry oven at 135°C for two hours, this membrane was completely wetted by the 5% salt solution in 4.7 seconds. In contrast, acrylamide-modified membrane was wet instantly by water, but after conditioning at 135°C for 2 hours, wetting was only accomplished by applying an alcohol-water solution of 59 dynes/cm. Transmission and ATR-FTIR were performed on the membranes. Reversibility of surface states was explored by conditioning membranes in various media and reexamining test performance.
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Characterization techniques Protein binding Protein absorption was evaluated with a radiometric procedure using antibody IgG. The protein solution was prepared in phosphate buffered saline (PBS). Goat gamma globulin, obtained from Sigma (Sigma I-5523) was used at a concentration of 1 mg/ml. 125I-goat anti(rabbit IgG) (NEX155) was purchased from NEN L
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