Cellulosic Dialysis Membranes

The monomeric subunit of cellulosic membranes is cellobiose, a naturally occurring saccharide found in plants. Chemically, cellobiose is a ringed structure richly endowed with hydroxyl groups. The interaction of complement cascade products with these hy-droxyl groups is felt to be responsible, at least partly, for the relatively pronounced complement activation observed when unsubstituted cellulosic membranes contact blood. For the past several years, a major objective among manufacturers has been the development of modified (substituted) cellulosic membranes in which a certain fraction of these hydroxyl groups are replaced with other moieties. The substitution groups diminish the degree of complement activation by at least three different mechanisms. One mechanism is the replacement of a large percentage of the hydroxyl groups with acetate groups. In the Rrst substituted cellulosic membrane, cellulose (di)acet-ate, approximately 70-80% of the hydroxyl groups on the cellulosic backbone were replaced with an acetate group. Most likely because this modification eliminates a large fraction of the active surface sites for interaction with complement components, an attenuation of the intense complement activation seen with unmodiRed cellulosics was achieved. This membrane modiRcation also resulted in a moderate increase in pore size, yielding a slightly higher water permeability and broader solute removal spectrum for cellulose acetate in comparison to unsubstituted cellulosic membranes of similar surface area. Extrapolation of this process to total replacement of the hydroxyl groups resulted in the cellulose triacetate fibre characterized by further attenuation of complement activation and higher water permeability.

A second cellulosic substitution mechanism is the replacement of a relatively small percentage (less than 5%) of the hydroxyl groups with a bulky chemical group, which sterically reduces the degree of interaction between complement activation products and the membrane. Examples for which this strategy is employed are Hemophan® (tertiary amine substitution) and synthetically modified cellulose (SMC; benzyl substitution group).

The evolution in cellulosic membranes has resulted in a wide spectrum of biocompatibility and flux profiles. If complement activation and neutropenia are used as the major biocompatibility criteria, regenerated cellulose is the least biocompatible while cellulose triacetate is the most biocompatible, with the other modified cellulosic membranes having intermediate profiles. However, characterization of the flux properties of these membranes is not as straightforward. For dialysers of comparable surface area, a simplistic approach is to report KUF values in the following ascending order: regenerated cellulose < Hemophan®, synthetically modified cellulose < cellulose acetate < cellulose triacetate. In this simplistic scheme, a 1.5 m2 dialyser having a regenerated cellulose, Hemophan®, or SMC membrane generally falls in the low flux category (KUF < 8 mL h_1 mmHg) while comparably sized dialysers having cellulose acetate and cellulose triacetate membranes fall in the midflux (Kuf 10-20 mL h_1 mmHg) and high flux (Kuf > 20 mLh-1 mmHg) categories, respectively. However, this simplistic categorization scheme breaks down in several respects. High flux cellulose acetate membranes have now been produced and cellulose triacetate dialysers of low water permeability (Kuf 9.5 mL h_1 mmHg) are also available. Finally, the recent development of unmodified cellu-losic and cellulose acetate membranes having relatively low water permeability but solute removal capabilities that include ^2M further confounds this classification scheme and provides additional examples of a dissociation between water and solute flux.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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