Cytometry and velocimetry on a microfluidic chip using polyelectrolytic salt bridges

This paper reports a polyelectrolytic salt bridge-based electrode (PSBE), which is a key embedded unit in a microchip device that can size-selectively count microparticles and measure their velocities. The construction of salt bridges at specific locations within a microfluidic chip enables de-driven electrical detection to be performed successfully. This is expected to be a competitive alternative to the optical methods currently used in conventional cell sorters. The PSBEs were fabricated by irradiating ultraviolet light over a patterned mask on the parts of interest, which were filled with an aqueous monomer solution containing diallyldimethylammonium chloride. A pair of such PSBEs was easily formed at the two lateral branches perpendicular to the main microchannel and was found to be very useful for dc impedometry. The human blood cells as well as the fluorescent microbeads passing between the two PSBEs produced impedance signals in proportional to their size. The information about the velocity of a microparticle was extracted from a doublet of the dc impedance signals, which were generated when cells or microbeads sequentially passed through two PSBE pairs separated from each other by a fixed distance. The plot of peak amplitude versus velocity of the moving microbeads and cells indicated only a slight correlation between the size and the velocity, which means that the peak amplitude of the dc impedance signals alone can provide information about the size of the cells in a mixture. The experimental results showed a screening rate of over 1000 cells s^-1 and a velocity of the cells of over 100 mm s^-1. Compared with the previously suggested electrical detection system based on metal electrodes, the sensitivity and selectivity in cell detection were remarkably improved. In addition, the detection unit including the operating circuit became innovatively simple and the whole device could be miniaturized.