Magnetic bead induces surface tension change at the gas-solid-liquid interface to manipulate the reciprocating flow of a droplet for rapid immunoassay of viruses.

Droplet manipulation provides tools for medical diagnostics, cell culturing, and drug screening. Among droplet manipulation techniques, magnetic manipulation enables transportation, mixing and separation of droplets, and thus has garnered broad attention for rapid immunoassay. Typically, magnetic manipulation requires a ferrofluid or micron-sized magnetic particles as droplet actuators. However, ferrofluids may contaminate the droplet, and magnetic particles may trap unbound molecules (e.g., viruses, antigens) within their aggregation gaps. These disadvantages decrease the accuracy of immunoassay. Here, we use a millimeter-sized magnetic bead to manipulate a droplet by inducing a surface tension change at the gas-solid-liquid interface. The manipulation of each droplet solely requires a single magnetic bead that releases no solvent or contaminant, eliminating the trapping of unbound molecules in the gaps between aggregated magnetic beads. We further design a chip and its complementary miniaturized analyzer to manipulate the reciprocating flow of droplets, and develop a rapid immunoassay method to detect Sendai virus and pneumonia virus of mice within 10 min. This method can detect target viruses in serum with a true positive rate of 100% (6/6) and a true negative rate of 93.3% (28/30). This work demonstrates great performance in droplet manipulation and the rapid immunoassay of viruses, with good potential to serve as a universal platform for in vitro detection and medical diagnosis.