By: Malithi Wickramathilaka, Ph.D. (Mali), Mar.11, 2020
Wicking, or capillarity is the ability of liquids to flow through very narrow openings. The flow motion occurs without the assistance of external forces, and is generally against gravity. If the diameter of the opening is extremely small, the surface tension of the opening’s wall, and the adhesive forces between the liquid and the wall are sufficient enough to drive the liquid forward. A famous example of capillarity is how water and nutrients flow all the way to the top of a tree. Lateral Flow Assay (LFA) is a simple diagnostic method used to detect the presence or absence of a target analyte such as a pathogen, contaminants, or biomarkers in a human or animal body. Blood glucose tests, HIV tests, and pregnancy tests are examples of commonly used LFAs. It is nearly impossible to explain how an LFA functions, without using the words “wicking”, or “capillarity”. In an LFA, when a sample is placed at one end of the test strip, the liquid passively flowing to the other end is termed the “capillary flow rate.” This passive transport of liquid within the micro porous structure can be manipulated by tweaking design parameters of the porous strip, which allows for point of care (POC) devices to be low-cost, convenient to use, and quick to provide results. The capillary flow properties of the porous material used in the test device (whether it is sintered particles, bonded fiber, or other porous media) dictate the assay sensitivity, specificity, and test line consistency. The test could easily yield faulty results due to poor wicking, which in return leads to misdiagnosis and eventually improper medical treatment routes. So, it is needless to explicitly state how wicking is an important phenomena in devices utilized in the diagnostics and healthcare fields.