A long straight square microchannel in viscoelastic fluid for focusing submicron-sized particles and bacteria

A viscoelastic flow focusing device is presented that enables simple and robust focusing of submicron-sized particles in the channel center by optimizing operating conditions such as channel length, flow rate and poly(ethylene oxide) (PEO) concentration. Submicron-sized particles (up to 100 nm) can be easily focused to the channel center under viscoelastic fluid flow without any external force via a simply fabricated microchannel with a long channel length and a large square cross-section. The device was fabricated using a common soft lithography technique for the polydimethylsiloxane (PDMS) channel, which has a width of 50 μm, a height of 50 μm and a channel length of 27 cm. The extralong channel enabled submicron-sized particle focusing, even in a channel of a relatively large size with high flow rate, which can realize flow cytometric applications. The focusing performance was first demonstrated using submicron-sized polystyrene (PS) beads ranging from 870 nm to 50 nm and then using biological particles such as E. coli bacteria to demonstrate the biological feasibility of the device. The PS beads, which ranged in diameter from 870 nm to 100 nm, were focused to the center of the channel, achieving over 90% focusing efficiency for beads as small as 510 nm and 62% focusing efficiency for 100-nm beads. The device could also align a bacterial suspension in the center of the channel at flow rates up to 30 µL/min, demonstrating its biological importance. The ability of the developed device to align submicron-sized particles within a narrow flow stream in a highly robust manner is promising for various biological and clinical applications, such as distinguishing pathogenic bacteria and evaluating individual antibiotic responses in a single experiment.