Throughout BioPharm International's 25 th anniversary year, we will be looking back at articles published in the first volume of the journal. This month, Jerold Martin of Pall Life Sciences takes a look at protein recovery through direct-flow microporous membrane filters over the past 25 years.
Protein binding to membrane filters was a "hot topic" back in 1988. While biologicals were routinely sterile filtered through Nylon, cellulose ester, or PVDF membranes without notable losses, concerns were raised over reports of reduced cell culture response in multi-well plates filled with sequentially filter-sterilized culture media using 25-mm membrane syringe filters. Although inhibition from initial filter leachables was not ruled out, the primary cause of poor cell growth in initial filled wells was attributed to binding and depletion of nutrient protein growth factors. Consequently, process developers began focusing on initial binding of proteins to membranes in large-scale protein filtration processes. Bench-scale data were often misleading because it was frequently misunderstood that protein binding to microporous membranes is non-specific and will vary with protein type, concentration, pH, and other formulation components. Membrane filters do not have a finite "protein binding capacity" independent of formulation and process conditions.
Today, the dynamics of protein binding are better understood, and saturation dynamics are routinely measured for specific target protein formulations. New membrane filters used today for process-scale protein filtration, both for serum-free culture media and dilute-protein biotherapeutics, feature extremely low non-specific protein binding surfaces. Most of these surfaces, either on polyvinylidene fluoride or polyethersulfone membranes, are covalently grafted from a mixture of acrylate monomers to form a hydrophilic, non-ionic polyacrylate surface on the internal pores of the membrane. These surfaces, comparable to size exclusion chromatography media and low protein binding contact lens materials, serve to minimize non-specific protein binding to membranes even on initial throughputs, which can be important when filling containers directly through membrane filters without batch pooling.