Rapid pregnancy and COVID-19 tests have a great advantage over other medical analyses: they are so simple that anyone can perform the test themselves, virtually anywhere. This is due to the robust principle behind these microfluidic methods, whereby aqueous solutions diffuse through a paper test strip with the aid of capillary forces. During this process, antibodies capture the target substances, such as virus particles or pregnancy hormones, and concentrate them at a desired location. A staining system then slowly makes the increasingly concentrated target substance visible as a stripe.
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Source: Phys.org