Considerations on Developmental Aspects of Biocompatible Dialysis Membranes

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Abstract
Modern strategies in developing new polymers for dialysis membranes aim to improve their blood compatibility. To achieve such a goal, two approaches have been successfully applied: existing cellulosic polymers were modified, either by introducing functional groups through ester or ether bonds, by mixing synthetic polymers with bulk additives, or by using copolymerization techniques. As a detailed example, the first synthetically modified cellulose membrane, Hemophan, was prepared by substituting some hydrogen atoms in the cellulosic glucose unit by diethyl‐amino‐ethyl groups with the modification having a considerable impact on the membrane's hemocompatibility. It is further known that the hemo‐compatibility of hydrophobic synthetic membranes is improved by rendering these materials partially hydrophilic. We tested the hypothesis, whether the hemocompatibility of a material, which is hydrophilic per se, such as unmodified cellulose, is changed after the introduction of hydro‐phobic substituents. For this purpose, the number and nature of substituents have been systematically varied in order to alter surface properties, and these variations have been subsequently related to blood compatibility parameters. As expected, thrombin generation as well as complement‐ and cell‐activation depend on the number and nature of the substituents whereby some of the substituents show a very narrow optimum if their hemocompatibility is related to the degree of substitution. Changes in hemocompatibility can be followed by physical methods, such as surface angle analyses and zeta potential determinations. Data show that alterations in the li‐pophilic/hydrophilic balance on the polymer surface may explain substituent‐related changes in polymer hemocompatibility. Molecular modeling of membrane surface and protein structures may be of further help in understanding possible interactions. The recent conclusion is that polymer modification and techniques in membrane surface characterization help us to optimize membrane fabrication for specific applications.