There is a great need to improve synthetic membrane filtration performance (increased permeation flow rates and higher selectivity) for pressure-driven membrane systems for water, energy, pharmaceutical, and biotechnology applications like desalination, fuel cells, small molecule purification, and protein separation, respectively. Recently, we have developed a set of chiral membranes that allow for selective protein separation and reduced fouling. Two chiral surfaces of opposite chirality (S and R) were synthesized in order to determine if they had an effect on fouling and protein separations, since almost all amino acids of proteins are of one specific stereoisomeric form, i.e.,the (S) form. Two proteins that have been tested, BSA and ovalbumin, resulted in the same preliminary qualitative result. That is, when the surface of the membrane was the same chirality as the protein feed, (S), minimal fouling occurred; however, when the chirality of the surface was the opposite to that of the foulant, (R), the membrane fouled significantly. These represent a new class of chiral membranes using covalently attached chiral monomers to the surface of poly(ether sulfone) (PES) membranes. These membranes will allow for low fouling separation of proteins and small molecules based on chirality, in addition to the classical separation parameters of membranes such as pore size, charge, trans-membrane pressure and surface wettability.
Using our high-throughput platform, a vast number of model surfaces can be produced using a library of chiral monomers and grafted from PES membrane surfaces. A chiral library will be synthesized using the 20 natural amino acids and a common linker containing a methacrylate and various different repeated PEG units (n = 1-6) to graft the amino acids to the PES surface. We utilize a high-throughput platform with an Atmospheric Pressure Plasma (APP) polymerization system to activate irradiation-sensitive commercial PES membranes. The synthesized chiral monomers can then be grafted from the membrane surface and evaluated. These chiral surfaces will be tested using various proteins foulants of different molecular weights, such as bovine serum albumin (67 kDa) and ovalbumin (45 kDa). The selected winners are determined by measuring fouling index, selectivity, and permeation flux or inverse resistance. We believe these results could lead to a fifth fundamental rule to use as a guideline when choosing anti-protein fouling surfaces. That is, chirality can be important when separating chiral solutions. Developing a chiral library will allow us to better understand what is necessary for the chiral effects to occur. Steric effects in addition to chain length and functionality should all be critical when the chiral surface interacts with the foulant. We speculate that chiral membranes could have drastic implications in the pharmaceutical industry, when only one stereoisomer is desired from a racemic mixture.