No abstract

Pure separation and sorting of microparticles from complex fluids are essential for biochemical analyses and clinical diagnostics. However, conventional techniques require highly complex and expensive labeling processes for high purity separation. In this study, we present a simple and label-free method for separating microparticles with high purity using the elasto-inertial characteristic of a non-Newtonian fluid in microchannel flow. At the inlet, particle-containing sample flow was pushed toward the side walls by introducing sheath fluid from the center inlet. Particles of 1 μm and 5 μm in diameter, which were suspended in viscoelastic fluid, were successfully separated in the outlet channels: larger particles were notably focused on the centerline of the channel at the outlet, while smaller particles continued flowing along the side walls with minimal lateral migration towards the centerline. The same technique was further applied to separate platelets from diluted whole blood. Through cytometric analysis, we obtained a purity of collected platelets of close to 99.9%. Conclusively, our microparticle separation technique using elasto-inertial forces in non-Newtonian fluid is an effective method for separating and collecting microparticles on the basis of size differences with high purity.

Platelet separation from blood is essential for biochemical analyses and clinical diagnosis. In this article, we propose a method to separate platelets from undiluted whole blood using standing surface acoustic waves (SSAWs) in a microfluidic device. A polydimethylsiloxane (PDMS) microfluidic channel was fabricated and integrated with interdigitated transducer (IDT) electrodes patterned on a piezoelectric substrate. To avoid shear-induced activation of platelets, the blood sample flow was hydrodynamically focused by introducing sheath flow from two side-inlets and pressure nodes were designed to locate at side walls. By means of flow cytometric analysis, the RBC clearance ratio from whole blood was found to be over 99% and the purity of platelets was close to 98%. Conclusively, the present technique using SSAWs can directly separate platelets from undiluted whole blood with higher purity than other methods.

A uniform deposition of the suspended particles in an evaporating droplet is necessary in many research fields. Such deposition is difficult to achieve, because the coffee-ring effect dominates the internal flow in a droplet. The present study adopts a biocompatible, surfactant-like polymer (Polyethylene glycol, PEG) to break the coffee-ring effect and obtain a relatively uniform deposition of the microparticles with yielding multi-ring pattern over a droplet area. Movements of the suspended particles in evaporating droplets and deposition patterns of them on a glass substrate were analyzed with microscopic images and video files. The PEG in the droplets successfully altered the coffee-ring effect because of the surface tension variation, which induced a centripetal Marangoni flow. Balancing these two phenomena apparently generated the Marangoni vortex. For PEG solution droplets, the pinning–depinning process during evaporation was periodically repeated and multiple rings were regularly formed. In conclusion, adding a surfactant-like viscous polymer in a droplet could provide a uniform coating of suspended particles, such as cells and various biomaterials, which would be essentially required for droplet assays of biomedical applications.

Malaria is a serious disease that threatens the public health, especially in developing countries. Various methods have been developed to separate malaria-infected red blood cells (i-RBCs) from blood samples for clinical diagnosis and biological and epidemiological research. In this study, we propose a simple and label-free method for separating not only late-stage but also early-stage i-RBCs on the basis of their paramagnetic characteristics due to the malaria byproduct, hemozoin, by using a magnetic field gradient. A polydimethylsiloxane (PDMS) microfluidic channel was fabricated and integrated with a ferromagnetic wire fixed on a glass slide. To evaluate the performance of the microfluidic device containing the ferromagnetic wire, lateral displacement of NaNO2-treated RBCs, which also have paramagnetic characteristics, was observed at various flow rates. The results showed excellent agreement with theoretically predicted values. The same device was applied to separate i-RBCs. Late-stage i-RBCs (trophozoites and schizonts), which contain optically visible black dots, were separated with a recovery rate of approximately 98.3%. In addition, using an optimal flow rate, early-stage (ring-stage) i-RBCs, which had been difficult to separate because of their low paramagnetic characteristics, were successfully separated with a recovery rate of 73%. The present technique, using permanent magnets and ferromagnetic wire in a microchannel, can effectively separate i-RBCs in various developmental stages so that it could provide a potential tool for studying the invasion mechanism of the malarial parasite, as well as performing antimalarial drug assays.