This paper describes the surface modification of macroporous membranes using ATRP (atom transfer radical polymerization) to create cation-exchange adsorbers with high protein binding capacity at high product throughput. The work is motivated by the need for a more economical and rapid capture step in downstream processing of protein therapeutics. Membranes with three reported nominal pore sizes (0.2, 0.45, 1.0 μm) were modified with poly(3-sulfopropyl methacrylate, potassium salt) tentacles, to create a high density of protein binding sites. A special formulation was used in which the monomer was protected by a crown ether to enable surface-initiated ATRP of this cationic polyelectrolyte. Success with modification was supported by chemical analysis using Fourier-transform infrared spectroscopy and indirectly by measurement of pure water flux as a function of polymerization time. Uniformity of modification within the membranes was visualized with confocal laser scanning microscopy. Static and dynamic binding capacities were measured using lysozyme protein to allow comparisons with reported performance data for commercial cation-exchange materials. Dynamic binding capacities were measured for flow rates ranging from 13 to 109 column volumes (CV)/min. Results show that this unique ATRP formulation can be used to fabricate cation-exchange membrane adsorbers with dynamic binding capacities as high as 70 mg/mL at a throughput of 100 CV/min and unprecedented productivity of 300 mg/mL/min.
This contribution describes research on the use of a newly developed multimodal membrane (MMM) adsorber that can be used as a chromatographic stationary phase in bioseparation processes. Compared with commercial cationic multimodal adsorbers, this MMM has superior static binding capacity (SBC = 180 mg IgG/ml), dynamic binding capacity (DBC 10% = 60 mg IgG/ml), and load productivity (>10 mg/ml/min). Furthermore, the incorporation of functional groups that provide orthogonal modes of interactions increases the range of ionic strength for operation of the MMM relative to conventional ion-exchange and hydrophobic interaction chromatography media. The effects of different salt types (kosmotropic, neutral, chaotropic salts) and ionic strength on IgG binding were investigated. To further understand the protein adsorption on the MMM, a thermodynamic model was employed to describe IgG adsorption isotherms on the MMM by providing a unique set of physically meaningful parameters for each salt type. The model was also a precise predictor of the adsorption isotherms under non-test conditions. A breakthrough analysis was used to determine dynamic binding capacities. The MMM maintained 70% DBC as ionic strength increased from 0 to 300 mM
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