Monolithic cell counter based on 3D hydrodynamic focusing in microfluidic channels

Abstract

Hydrodynamic focusing is a powerful technique frequently used in microfluidics that presents a wide range of applications since it allows focusing the sample flowing in the device to a narrow region in the center of the microchannel. In fact thanks to the laminarity of the fluxes in microchannels it is possible to confine the sample solution with a low flow rate by using a sheath flow with a higher flow rate. This in turn allows the flowing of one sample element at a time in the detection region, thus enabling analysis on single particles. Femtosecond laser micromachining is ideally suited to fabricate device integrating full hydrodynamic focusing functionalities thanks to the intrinsic 3D nature of this technique, especially if compared to expensive and complicated lithographic multi-step fabrication processes. Furthermore, because of the possibility to fabricate optical waveguides with the same technology, it is possible to obtain compact optofluidic devices to perform optical analysis of the sample even at the single cell level, as is the case for optical cell stretchers and sorters. In this work we show the fabrication and the fluidic characterization of extremely compact devices having only two inlets for 2D (both in vertical and horizontal planes) as well as full 3D symmetric hydrodynamic focusing. In addition we prove one of the possible application of the hydrodynamic focusing module, by fabricating and validating (both with polystyrene beads and erythrocytes) a monolithic cell counter obtained by integrating optical waveguides in the 3D hydrodynamic focusing device.