Nitrocellulose membranes are inherently brittle and break easily under tension. Tensile strengths for lateral-flow membranes are usually <2 lb/in, making them very difficult to handle, especially in automated processes. To compensate for the weakness, the membrane can be cast directly onto a polyester film (10). This virtually eliminates all of the handling issues associated with breakage of the membrane, making it easier to manufacture and then process into test strips. The nitrocellulose adheres to the polyester without any additional components being added to the lacquer or the surface of the film. Another advantage of casting on a film is that the composite structure is much less flammable than the nitrocellulose alone.
Membranes are typically cast on polyester films that are either 2 mil or 4 mil thick. This is how the market has developed over the past 15 yr, as opposed to limiting membrane manufacturing. Also, most of the films that are used are either transparent or semi-transparent. The choice of a 2-mil or 4-mil backing is dictated primarily by the design of the test strip and the housing in which it will be placed. The internal compression points of the housing have to be matched to the thickness of the strip at all contact points so that the flow path is maintained without crushing any of the porous components. In theory, the thickness and opacity of the film can be modified as needed for a particular design. Custom-made membranes, however, may be significantly more expensive than standard products.
A disadvantage of backed membranes is that reagents can be applied to only one side of the membrane (10). The air side of the membrane (the surface from which the solvents are evaporated during casting) may have structural inconsistencies that affect the consistency with which capture reagents are laid down and the uniformity of sample flow when the test is run. These inconsistencies arise during the casting process as a result of variation in the precipitation of nitrocellulose at the membrane's surface when compared with the bulk of nitrocellulose through the depth of the membrane. Many of these inconsistencies are visible macroscopically and can be culled by visually checking the surface quality and uniformity (discussed later). On an unbacked membrane, the belt side, which was in contact with the moving belt during casting, is free of these defects. Obviously, during the development process, a decision has to be made as to whether the handling difficulties of an unbacked membrane outweigh the advantage of surface uniformity. Most test manufacturers use backed membranes.
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