The width of the reagent lines is affected by the wettability of the membrane, which is a function of the chemistries in the membrane (2,10,11). If the membrane is completely wettable, the reagent solution should be easily absorbed into the pore structure. If, however, the membrane is not completely wettable, the reagent solution can bead up on the surface of the membrane and be absorbed slowly or not at all. This causes irregularities in the appearance of the line.
Slow wettability can be caused by several factors (10,11). The most significant is insufficient wetting agent in the membrane. In addition to the adverse effect on reagent dispensing, this is likely to affect the lateral-flow properties of the membrane when the test is run. Another cause of slow wettability is high surface tension of the reagent buffer. Buffers with high surface tension tend to be absorbed more slowly, especially if the amount of the wetting agent in the membrane is at the low end of its specified range. A third cause is static charge on the membrane. The charge repels the liquid stream in contact dispensers and can cause deflection of the droplets from an aerosol dispenser. In all cases, the line will not be uniform.
Beyond uniform absorption of the reagent line, chemical interactions affect the adsorption of the capture reagent to the membrane (discussed under Subheading 4). Ideally, the protein will adsorb to the nitrocellulose within the width of the dispensed liquid stream and have a uniform distribution from edge to edge (10,11). Chemical conditions that reduce long-term adsorption usually prevent immediate adsorption at dispensing. When this occurs, the capture reagent will be carried laterally as the buffer is absorbed into the membrane, ultimately resulting in a more diffuse signal. Depending on the interactions among the capture reagent, the buffer chemistry, and the membrane chemistry, the capture reagent may be distributed irregularly (11). Concentration of the capture reagent at the edges gives the line a bipartite appearance. Concentration at the edges coupled with a concentrated central zone produces a tripartite appearance. The edges of the line can also be highly irregular when adsorption is not optimized.
One of the confounding problems with poor line quality is that it is not normally detectable during the dispensing process. The capture reagents normally have no inherent color, and the width and quality of the liquid line are not predictive of capture-reagent distribution. During product development, irregularities in capture-reagent distribution can be detected by staining the membrane for protein (10). In production, however, they usually are not seen until the completed test strips are subjected to quality-control testing. Because of the costs associated with manufacturing completed test strips, capture reagent striping can be useful as an incoming quality-control method.
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