Hydrodynamic particle focusing enhanced by femtosecond laser deep grooving at low Reynolds numbers

Passive microfluidic devices for cell separation

The separation of specific cell populations is instrumental in gaining insights into cellular processes, elucidating disease mechanisms, and advancing applications in tissue engineering, regenerative medicine, diagnostics, and cell therapies. Microfluidic methods for cell separation have propelled the field forward, benefitting from miniaturization, advanced fabrication technologies, a profound understanding of fluid dynamics governing particle separation mechanisms, and a surge in interdisciplinary investigations focused on diverse applications. Cell separation methodologies can be categorized according to their underlying separation mechanisms. Passive microfluidic separation systems rely on channel structures and fluidic rheology, obviating the necessity for external force fields to facilitate label-free cell separation. These passive approaches offer a compelling combination of cost-effectiveness and scalability when compared to active methods that depend on external fields to manipulate cells. This review delves into the extensive utilization of passive microfluidic techniques for cell separation, encompassing various strategies such as filtration, sedimentation, adhesion-based techniques, pinched flow fractionation (PFF), deterministic lateral displacement (DLD), inertial microfluidics, hydrophoresis, viscoelastic microfluidics, and hybrid microfluidics. Besides, the review provides an in-depth discussion concerning cell types, separation markers, and the commercialization of these technologies. Subsequently, it outlines the current challenges faced in the field and presents a forward-looking perspective on potential future developments. This work hopes to aid in facilitating the dissemination of knowledge in cell separation, guiding future research, and informing practical applications across diverse scientific disciplines.

Simplified 3D hydrodynamic flow focusing for lab-on-chip single particle study

Accurately control of the position of a fluid and particle within lab-on-a-chip platform is a critical prerequisite for many downstream analysis processes, such as detection, trapping and separation, moving the sensing at the single-particle level. With the development of microfluidic fabrication technology, particle/cell focusing has shifted from two to three dimensions. 3D hydrodynamic focusing, which sorts and aligns the incoming cloud of particles so that they pass through the interrogation area one by one, enables new possibilities and breakthroughs in the single-cell analysis system. Despite the excellent results shown in literature, there is still a lack of a device that can simultaneously fulfilling the requirements of high throughput, compactness, high integrability, and ease of use operation to become a widely accepted work center for biomedical research and clinical applications. Here, we proposed a unique 3D flow focusing microfluidic device buried in fused silica substrate that potentially combines all this advantages. By designing a sample channel suspended inside a larger buffer channel, manufactured by exploiting the laser-assisted micromachine technique, a not size-dependent focusing capability is shown. A spatially and temporally stable central flow of a mixture of 15 μm and 6 μm PS particles to a 1 μm PS microsphere solution has been obtained with high accuracy. Finally, to test the achievable focusing resolution, the chip was tested for the detection of Escherichia Coli bacteria in water solution as proof of concept of biological application.

Microfluidic Separation and Enrichment of Escherichia coli by Size Using Viscoelastic Flows

Here, we achieve the separation and enrichment of Escherichia coli clusters from its singlets in a viscoelastic microfluidic device. E. coli, an important prokaryotic model organism and a widely used microbial factory, can aggregate in clusters, leading to biofilm development that can be detrimental to human health and industrial processes. The ability to obtain high-purity populations of E. coli clusters is of significance for biological, biomedical, and industrial applications. In this study, polystyrene particles of two different sizes, 1 and 4.8 μm, are used to mimic E. coli singlets and clusters, respectively. Experimental results show that particles migrate toward the channel center in a size-dependent manner, due to the combined effects of inertial and elastic forces; 4.8 and 1 μm particles are found to have lateral equilibrium positions closer to the channel centerline and sidewalls, respectively. The size-dependent separation performance of the microdevice is demonstrated to be affected by three main factors: channel length, the ratio of sheath to sample flow rate, and poly(ethylene oxide) (PEO) concentration. Further, the separation of E. coli singlets and clusters is achieved at the outlets, and the separation efficiency is evaluated in terms of purity and enrichment factor.

Focusing of Particles in a Microchannel with Laser Engraved Groove Arrays

Continuous microfluidic focusing of particles, both synthetic and biological, is significant for a wide range of applications in industry, biology and biomedicine. In this study, we demonstrate the focusing of particles in a microchannel embedded with glass grooves engraved by femtosecond pulse (fs) laser. Results showed that the laser-engraved microstructures were capable of directing polystyrene particles and mouse myoblast cells (C2C12) towards the center of the microchannel at low Reynolds numbers (Re < 1). Numerical simulation revealed that localized side-to-center secondary flows induced by grooves at the channel bottom play an essential role in particle lateral displacement. Additionally, the focusing performance proved to be dependent on the angle of grooves and the middle open space between the grooves based on both experiments and simulation. Particle sedimentation rate was found to critically influence the focusing of particles of different sizes. Taking advantage of the size-dependent particle lateral displacement, selective focusing of micrometer particles was demonstrated. This study systematically investigated continuous particle focusing in a groove-embedded microchannel. We expect that this device will be used for further applications, such as cell sensing and nanoparticle separation in biological and biomedical areas.