A weak shear stress microfluidic device based on Viscoelastic Stagnant Region (VSR) for biosensitive particle capture

Particle manipulation in microfluidic devices is of great significance in biological research. However, currently available inertial capture methods require relatively high flow rates, which will cause damage to biological particles, especially for single-celled organisms that are sensitive to environments. Herein, we demonstrate a label-free, size-based, low shear stress manipulation method using the Viscoelastic Stagnant Region (VSR) to capture sensitive bioparticles. This method uses the deformation of molecular chains in the polymer solution which can generate elastic stresses to form vortices which is called VSR because of the extremely low velocity in the contraction-expansion array (CEA) microchannel. Formation and evolution of VSR was observed experimentally using the Micro-PIV system in polyethylene oxide (PEO) solutions with different concentrations. On this basis, 20 μm and 5 μm polystyrene (PS) particles were confined to a certain area in the microchamber and the trajectory of particles motion in VSR was observed. Both the inertial lift force and the viscoelastic force are affected by the particle size, so the method also presented size selectivity. By quantitatively studying the velocity distribution of the particles on the orbit of motion, it was found that the maximum velocity of the particles in the VSR was about 0.02 m s^-1, which is only about 1/100 of that in inertial vortices. Moreover, the spiral motion of particles captured by VSR with variable trajectories was first observed, which is different from the equilibrium orbit in inertial vortices and can provide more motion paths for the particles. This method was further applied to the research of Crypthecodinium cohnii (C. cohnii), which is vulnerable but has high nutritious value. We found that the activity of the C. cohnii decreased slightly after being processed by VSR, but was lost after being processed by the inertial vortices. This research, as a simple and low shear stress particle manipulation method, will provide useful guidance for the manipulation, capture and separation of sensitive biological cells under higher biocompatibility, which is of great significance for the study of cellular and molecular biology.