Physical Adsorption

One of the commonest methods of modifying the surface properties of a membrane is to coat that membrane, which has the desired bulk properties, with an agent that has the desired surface properties. Mostly, coating is based on the physical adsorption of an agent on the membrane surface. Hence, the stability and durability of the coating agent is one of the key factors.

Déjardin et al. (Etheve et al. 2003; Thomas et al. 2000;Valette et al. 1999; Yan et al. 1992) reported a series of studies on modification of the physical adsorption properties of PAN-based membranes. Two types of poly-

Fig. 12. Schematic representation of the structure of the copolymer and terpolymer derived from acrylonitrile, poly(ethyleneoxide) methacrylate and trimethylaminoethyl chloride methacrylate

mers, namely copolymers and terpolymers, were synthesized as depicted in Fig. 12. In the case of preadsorption with the terpolymer, platelet accumulation was considerably reduced. The effect on albumin and fibrinogen adsorption following preadsorption with the copolymer was also investigated. They found that whatever the pH of preadsorption, passivation efficiency was zero for copolymers with a high content of poly(ethyleneoxide) side chains, while a maximum reduction in adsorbance of 60% was attained with a copolymer containing 79 mol% acrylonitrile. They proposed that a higher content of acrylonitrile in the copolymer might improve the anchoring of the copolymer onto the membrane surface and therefore prevent protein adsorption more effectively than in the case of copolymers with a lower content of acrylonitrile, which is more readily displaced from the surface by proteins. Based on this consideration, the terpolymer was also synthesized and physically adsorbed onto the membrane. A reduction of at least 87% in fibrinogen adsorbance was obtained. They have also reported that although this water-soluble terpolymer might be leached out during these experiments, such a leaching should be low enough that it would not affect the prevention of protein adsorption. They also found that thelevelsofactivationofbloodcomponentsbythepreadsorbedPAN-based membrane were lower than those of controls. Based on these results, it can be concluded that this physical adsorption modification did afford protein resistance.

In the study of Thomas et al. (2000), positively-charged poly(ethylen-eimine) was physically adsorbed onto various PAN-based hemodialysis membranes, probably creating a repelling and water-swollen layer for proteins. Participating in the contact-phase activation of the endogenous blood-coagulation cascade, high-molecular-weight kininogen was chosen as a model protein to study the protein resistance of these membranes. It was found that both kininogen adsorption and contact-phase activa tion was greatly reduced, irrespective of the pH value (between 7.0 and 7.8). They (Etheve et al. 2003) also performed lysozyme adsorption on a hemodialysis sulfonated PAN membrane with and without preadsorbed poly(ethyleneimine) on the external faces. They found that over a long time period, the total adsorbed amount of lysozyme adsorbed was unaffected by preadsorption of the membrane with poly(ethyleneimine). However, it was found that the poly(ethyleneimine) layer led to significantly slower kinetics of adsorption at pH 7.2 in Tris buffer. Thus, it can be inferred that surface preadsorption by a high-molecular-weight poly(ethyleneimine), while preventing the activation of contact-phase activation, did not inhibit the high capacity of the membrane to adsorb small proteins.

Both cytochrome C and a-lactalbumin are low-molecular-weight proteins. The former is positively charged in an aqueous solution at pH 7.4 and the latter is negatively charged. Based on the results of the adsorption of these two low-molecular-weight proteins on different hemodialysis membranes, Valette et al. (1999) suggested that the observed differences in the adsorbance of proteins were mostly due to the different microstructure, chemical nature, and surface charge of the membrane.

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