(141d) Overcoming Diffusional Limitations for Protein Binding in Polymer Brush Grafted Nonwovens to Reduce Equilibrium Binding Times

Authors: 
Heller, M., North Carolina State University
Gurgel, P. V., ProMetic Life Sciences and NC State University
Carbonell, R. G., Kenan Institute for Engineering, Technology & Science, NC State University

Polymer brushes grafted to nonwoven membranes create a 3-dimensional matrix capable of achieving very high equilibrium protein binding capacities. Polybutylene terephthalate (PBT) nonwovens can readily be grafted with glycidyl methacrylate (GMA) via UV induced radical polymerization creating polymer brushes that can further be functionalized to become ion exchangers.  The capacities achieved by these materials when used as an ion exchange platform are many times that of monolayer coverage according to the specific surface area of the material. However, very long residence times are required to reach equilibrium due to the diffusion of protein through a dense protein-polyGMA matrix. Increasing the specific surface area of the material can effectively reduce the polyGMA graft thickness while still observing a high equilibrium binding capacity, resulting in shorter times to reach equilibrium. A novel islands-in-the-sea (I/S) PBT nonwoven having a specific surface area of 2.5 m2/g was compared to a commercially available PBT nonwoven having a specific surface area of 0.9 m2/g, as a base material for polyGMA grafting and ion exchange capture of target proteins. Islands-in-the-sea nonwoven PBT was provided by Dr. Behnam Pourdeyhimi at the Nonwovens Cooperative Research Center.  I/S PBT nonwovens are made of bicomponent fibers containing a removable polymer (“the sea”) that encapsulates discreet fiber entities of a permanent polymer (“the islands”) that are substantially smaller in diameter.  Both materials were successfully grafted to achieve conformal uniform polyGMA graft layers of varying degrees of grafting.  Following grafting, the membranes were either derivatized with diethyl amine or phosphoric acid to become weak anion exchangers or strong cation exchangers respectively. It was observed for both PBT nonwovens in both ion exchange formats that equilibrium binding capacity increased linearly with the degree of polyGMA grafting, and it was shown that the initial surface area of the PBT material did not have an impact on the final equilibrium binding capacity. Capacities of 800-900 mg/g of protein were observed for all systems when grafted to 18-20% polyGMA weight gain.  At these weight gains, equilibrium binding was reached on an order of minutes for the I/S PBT nonwoven compared to hours required for the commercially available PBT nonwoven.  The enhanced binding performance of the I/S PBT nonwoven is due to the lower graft thickness of the polyGMA layer at specific degrees of grafting. The diffusion of protein through a dense matrix of protein bound to functionalized polyGMA was modeled using a shrinking core model. Using this model a relative diffusion coefficient can be estimated for the transport of protein through the polymer/protein layer. the diffusion of protein through this system was found to be several orders of magnitude slower than bulk diffusion of protein in an aqueous solution.
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