Nitrocellulose has been used industrially for over a century (1) and for the production of microporous membranes for well over 75 yr (2). The use of
From: Forensic Science and Medicine: Drugs of Abuse: Body Fluid Testing Edited by R. C. Wong and H. Y. Tse © Humana Press Inc., Totowa, NJ
nitrocellulose membranes as a substrate for the formation of biochemical complexes has been commonplace in biochemical applications since the 1970s with the publication of techniques for Southern blotting (3), Northern blotting (4), and Western blotting (5). In the 1980s, the principles of immunodetection were applied in such a way that an immunochemical reaction on the surface of a nitrocellulose membrane could be used as a rapid technique for the detection of an analyte in a liquid sample (6-8). This led to development of a host of convenient and inexpensive on-site or point-of-collection testing (POCT) devices, including testing of drugs of abuse. The immunochemical principles used in a lateral-flow format are similar to those used in other immunoassays. Many tests on the market today require only a single step: application of a liquid sample to the assay device. Their simplicity, however, is sometimes offset by the limited sample volume accommodated within the strip.
Lateral-flow membranes comprise a subset of the nitrocellulose membranes that are currently available. This chapter is intended to provide an assay developer with an understanding of the properties of these membranes and their use in lateral-flow assays. It should be kept in mind that the membrane is but a single material in a lateral-flow test. The integration of the membrane with the other materials and chemistries and the application of appropriate manufacturing techniques are essential to making a functional test (9,10).
The basic design of a lateral-flow test strip (Fig. 1) consists of four porous materials (10). The sample pad usually contains buffering agents, salts, and surfactants that make the sample compatible with the assay. The conjugate pad contains a detector particle, such as a colloidal gold or latex. The membrane is striped with capture reagents that lead to the production of a visual signal through the formation of an immunocomplex. The absorbent pad, located at the distal end of the strip, serves as a sink for the sample as it migrates through the strip. Whereas the membrane is a microporous structure made from nitrocellulose, the pad materials are typically nonwoven materials made from glass fiber or cellulose. Contact points between these materials are included so that there is a continuous flow path from the sample pad to the end of the absorbent pad. During manufacturing (see Chapter 6), the materials are aligned on an adhesive card to hold them in place. The card is cut into individual strips and then placed into a plastic housing. The housing serves to isolate the sample pad as the point for sample application and also contains a viewing window to permit visualization of the test and control lines. The entire assembly is stored under desiccation.
When a sample is applied to the sample port, it enters the sample pad, dissolving any soluble compounds. The sample then migrates into the conjugate pad, simultaneously solubilizing and mixing with the detector particles. The mixture then migrates through the membrane and into the absorbent pad. Once
the absorbent pad is saturated, the test will not accommodate any additional sample volume.
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