Substrate layer, Integrally-skinned membranes

Immediately below the skin layer of integrally-skinned membranes is found a substrate (22) or transition (23) layer with a density between that of the skin and that of the porous... 

An integrally skinned asymmetric membrane with a porous skin layer (hereafter called substrate membrane) is prepared from a polymer solution by applying the dry-wet phase inversion method and dried according to the method described later, before being dipped into a bath containing a dilute solution of another polymer. When the membrane is taken out of the bath, a thin layer of coating solution is deposited on top of the substrate membrane. The solvent is then removed by evaporation, leaving a thin layer of the latter polymer on top of the substrate membrane. 

In the case of a composite membrane consisting of a skinless porous substrate and a dense film, permeability and permselectivity may be determined solely by the resistance of the denser film. Different membrane polymers may therefore be employed for the thin barrier layer and the thick support structure. This permits a combination of properties which are not available in a single material. Such membranes were initially developed for desalination by reverse osmosis where they are known as thin- or ultrathin-film composites or nonlntegrally-skinned membranes. A second type of composite membrane is utilized for gas separations. It is a composite consisting of an integrally-skinned or asymmetric membrane coated by a second, more permeable skin which is used to fill skin defects. The inventors of the latter have entitled their device a resfstanee model membrane, but the present author prefers the term coated integrally-skinned composites. 

Whereas skin from human cadavers, membranes made out of collagen, cultured corneal fibroblasts [79], and human skin grown in vitro [80] could also be used as substrates in testing the activity of topicals against viruses, they all suffer from a variety of limitations. For example, the viability and barrier integrity of cadaveric skin are compromised [77], layers of cultured cells are too fragile for the... 

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