Microfluidic hydrodynamic focusing for high-throughput applications

Microfluidic hydrodynamic focusing is critical for chip-based bioanalytical systems to increase throughput and sensitivity, especially for microflow cytometers, enabling a sample flow to be confined to the center of a microchannel with a narrow cross-section. Current microfluidic hydrodynamic focusing designs are usually unable to maintain stable focusing in high flow velocity conditions, resulting in a large cross-section or even failed focusing. To overcome this challenge, this paper aims to develop a design that can achieve effective microfluidic hydrodynamic focusing at high velocity with favorable performance. For this purpose, specially designed structures and arc-shaped channels are used. Two focusing regions are modeled and optimized mathematically, and flow behavior is investigated using numerical simulations. The functional relationship between flow rates and the cross-sectional dimensions of the focused sample flow is explored, and a measurement method for testing the dimensions is developed. The design is implemented in glass chips and characterized experimentally. In a rectangular channel with a cross-section of 300 mu m x 150 mu m the sample flow can be focused down to 5-11 mu m horizontally and 10-21 mu m vertically at a roughly constant velocity of 4.4 m s(-1) when the sample flow rate varies between 10 and 60 mu l min(-1). Effective focusing is accessible within a wide velocity range from 0.7 to 10 m s(-1). The experimental results validate that the focusing design performs better than existing microfluidic designs at high velocities, while its performance is close to that of the designs used in conventional flow cytometers with much less volume and a simpler structure. The focusing design can serve as the basis for microflow cytometers or it can be integrated into various microfluidic systems where complete focusing is needed.