Surface quality is a subjective assessment made by manufacturers on the basis of historical experience (2,10). Because the uniformity of the membrane's structure is related to the consistency of solvent evaporation, variations in the process are frequently manifested as artifacts on the air side of the membrane. These artifacts have been described using nonscientific terms such as "stucco" and "orange peel," which, nevertheless, accurately describe the appearance of the membrane.
Ideally, the air side of the membrane is smooth. This ensures the highest probability of uniform deposition of the capture reagents on the membrane. When surface roughness occurs, the structures that contribute to the roughness can cause the reagent line to be irregular. They may alter the uniformity of liquid absorption as the line is applied or cause nonuniform adsorption of the capture reagent. Both problems are manifested in the final test strips as nonuniform signal lines when samples are run.
Another artifact that can affect test-strip performance is powder, which is comprised of small particles of nitrocellulose scattered across the surface of the membrane. These particles arise when a fraction of the nitrocellulose precipitates independently of the mass of nitrocellulose comprising the porous structure. Most nitrocellulose membranes have powder, although at a low level it does not significantly impact test-strip production. When there is a high level of powder, it can be dislodged from the membrane, causing a build-up of powder on production equipment. If capture reagents have been applied, they will be dislodged with the powder. Powder arising from a precipitation artifact should not be confused with powder comprising pieces of membrane that break away from the membrane edges. Cutting processes that are not optimized for nitrocellulose can cause significant edge damage, including breakaway of flecks of membrane.
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