Continuous Separation of White Blood Cells From Whole Blood Using Viscoelastic Effects

Label-free separation of peripheral blood mononuclear cells from whole blood by gradient acoustic focusing

Efficient techniques for separating target cells from undiluted blood are necessary for various diagnostic and research applications. This paper presents acoustic focusing in dense media containing iodixanol to purify peripheral blood mononuclear cells (PBMCs) from whole blood in a label-free and flow-through format. If the blood is laminated or mixed with iodixanol solutions while passing through the resonant microchannel, all the components (fluids and cells) rearrange according to their acoustic impedances. Red blood cells (RBCs) have higher effective acoustic impedance than PBMCs. Therefore, they relocate to the pressure node despite the dense medium, while PBMCs stay near the channel walls due to their negative contrast factor relative to their surrounding medium. By modifying the medium and thus tuning the contrast factor of the cells, we enriched PBMCs relative to RBCs by a factor of 3600 to 11,000 and with a separation efficiency of 85%. That level of RBC depletion is higher than most other microfluidic methods and similar to that of density gradient centrifugation. The current acoustophoretic chip runs up to 20 µl/min undiluted whole blood and can be integrated with downstream analysis.

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.

Direct separation and enumeration of CTCs in viscous blood based on co-flow microchannel with tunable shear rate: a proof-of-principle study

Circulating tumor cells (CTCs), which have extremely low density in whole blood, are an important indicator of primary tumor metastasis. Isolation and enumeration of these cells are critical for clinical applications. Separation of CTCs from massive blood cells without labeling and addition of synthetic polymers is challenging. Herein, a novel well-defined co-flow microfluidic device is presented and used to separate CTCs in viscous blood by applying both inertial and viscoelastic forces. Diluted blood without any synthetic polymer and buffer solution were used as viscoelastic fluid and Newtonian fluid, respectively, and they were co-flowed in the designed chip to form a sheath flow. The co-flow system provides the function of particle pre-focusing and creates a tunable shear rate region at the interface to adjust the migration of particles or cells from the sample solution to the buffer solution. Successful separation of CTCs from viscous blood was demonstrated and enumeration was also conducted by image recognition after separation. The statistical results indicated that a recovery rate of cancer cells greater than 87% was obtained using the developed method, which proved that the direct separation of CTCs from diluted blood can be achieved without the addition of any synthetic polymer to prepare viscoelastic fluid. This method holds great promise for the separation of cells in viscous biological fluid without either complicated channel structures or the addition of synthetic polymers.

A high-throughput microfluidic device based on controlled incremental filtration to enable centrifugation-free, low extracorporeal volume leukapheresis

Leukapheresis, the extracorporeal separation of white blood cells (WBCs) from red blood cells (RBCs) and platelets (PLTs), is a life-saving procedure used for treating patients with cancer and other conditions, and as the initial step in the manufacturing of cellular and gene-based therapies. Well-tolerated by adults, leukapheresis poses a significant risk to neonates and low-weight infants because the extracorporeal volume (ECV) of standard centrifugation-based machines represents a particularly large fraction of these patients' total blood volume. Here we describe a novel high-throughput microfluidic device (with a void volume of 0.4 mL) based on controlled incremental filtration (CIF) technology that could replace centrifugation for performing leukapheresis. The CIF device was tested extensively using whole blood from healthy volunteers at multiple hematocrits (5-30%) and flow rates (10-30 mL/min). In the flow-through regime, the CIF device separated WBCs with > 85% efficiency and 10-15% loss of RBCs and PLTs while processing whole blood diluted with saline to 10% hematocrit at a flow rate of 10 mL/min. In the recirculation regime, the CIF device demonstrated a similar level of separation performance, virtually depleting WBCs in the recirculating blood (~ 98% reduction) by the end of a 3.5-hour simulated leukapheresis procedure. Importantly, the device operated without clogging or decline in separation performance, with minimal activation of WBCs and PLTs and no measurable damage to RBCs. Compared to the typical parameters of centrifugation-based leukapheresis, the CIF device had a void volume at least 100-fold smaller, removed WBCs about twice as fast, and lost ~ 2-3-fold fewer PLTs, while operating at a flow rate compatible with the current practice. The hematocrit and flow rate at which the CIF device operated were significantly higher than previously published for other microfluidic cell separation methods. Finally, this study is the first to demonstrate a highly efficient separation of cells from recirculating blood using a microfluidic device. Overall, these findings suggest the feasibility of using high-throughput microfluidic cell separation technology to ultimately enable centrifugation-free, low-ECV leukapheresis. Such a capability would be particularly useful in young children, a vulnerable group of patients who are currently underserved.

Optical Dielectrophoretic (DEP) Manipulation of Oil-Immersed Aqueous Droplets on a Plasmonic-Enhanced Photoconductive Surface

We present a plasmonic-enhanced dielectrophoretic (DEP) phenomenon to improve optical DEP performance of a floating electrode optoelectronic tweezers (FEOET) device, where aqueous droplets can be effectively manipulated on a light-patterned photoconductive surface immersed in an oil medium. To offer device simplicity and cost-effectiveness, recent studies have utilized a polymer-based photoconductive material such as titanium oxide phthalocyanine (TiOPc). However, the TiOPc has much poorer photoconductivity than that of semiconductors like amorphous silicon (a-Si), significantly limiting optical DEP applications. The study herein focuses on the FEOET device for which optical DEP performance can be greatly enhanced by utilizing plasmonic nanoparticles as light scattering elements to improve light absorption of the low-quality TiOPc. Numerical simulation studies of both plasmonic light scattering and electric field enhancement were conducted to verify wide-angle scattering light rays and an approximately twofold increase in electric field gradient with the presence of nanoparticles. Similarly, a spectrophotometric study conducted on the absorption spectrum of the TiOPc has shown light absorption improvement (nearly twofold) of the TiOPc layer. Additionally, droplet dynamics study experimentally demonstrated a light-actuated droplet speed of 1.90 mm/s, a more than 11-fold improvement due to plasmonic light scattering. This plasmonic-enhanced FEOET technology can considerably improve optical DEP capability even with poor-quality photoconductive materials, thus providing low-cost, easy-fabrication solutions for various droplet-based microfluidic applications.

Inertial microfluidic cube for automatic and fast extraction of white blood cells from whole blood

We report here a novel inertial microfluidic (IM) cube integrated with lysis, storage and extraction modules for extracting white blood cells (WBCs) from whole blood automatically, harmlessly and quickly. Lysis, storage, and extraction modules are designed to realize the purposes of complete mixing of whole blood and lysing buffer, thorough lysis of red blood cells (RBCs), and automatic extraction of WBCs from the lysed background RBCs, respectively. After demonstrating its conceptual design, we characterize the performances of the lysis and extraction modules. The results show that a high mixing efficiency of 94.2% can be achieved using our lysis modules for complete mixing of whole blood and lysing buffer. In the extraction module, an extraction efficiency of 88.1% can be achieved for the extraction of WBCs. Finally, we successfully apply our IM cube for the high throughput extraction of WBCs from human whole blood with an extraction efficiency of 83.9% and a cell viability of 96.6%, which are comparable to those using centrifugation and even better in some aspects. Our IM cube is based on passive secondary-flow mixing and inertial sorting, offers the advantages of small footprint, high stability and simple fabrication, and is a promising alternative method for extracting WBCs from human blood.

Manipulation of micro- and nanoparticles in viscoelastic fluid flows within microfluid systems

Manipulation of micro- and nanoparticles in complex biofluids is highly demanded in most biological and biomedical applications. A significant number of microfluidic platforms have been developed for inexpensive, rapid, accurate, and efficient particle manipulation. Due to the enormous potential of viscoelastic fluids (VEFs) for particle manipulation, various emerging microfluidic-based VEFs techniques have been presented over the last decade. This review provides an intuitive understanding of VEF physics for particle separation in different microchannel geometries. Besides, active and passive VEF methods are critically reviewed, highlighting the potential and practical challenges of each technique for particle/cell focusing, sorting, and separation. The outcome of this study could enable recognizing deliverable VEF technology with the promising prospect in the manipulation of submicron biological samples (e.g., exosomes, DNA, and proteins).

Size-dependent enrichment of leukocytes from undiluted whole blood using shear-induced diffusion

Little work has been done in microfluidics with separation of cells directly from whole blood, and the handful of microfluidic systems reported the literature offer only limited throughput. Yet high throughput is highly desirable to avoid degradation of samples, which can result in loss of information critical to disease diagnosis or monitoring. In this work, we investigated particle migration dynamics in whole blood flow at a single-particle level and subsequently successfully demonstrated the preferential enrichment of white blood cells (WBCs) in unprocessed whole blood flows flanking a buffer flow. Our in-depth investigation reveals a counter-intuitive, size-based migration of cells in whole blood flow and their tendency to accumulate in the regions near flow interfaces, which is employed for inherent enrichment of WBCs. More importantly, we found the strong size-dependent migration in blood flow stemming from the differentiated downstream velocity of particles, which inversely scales with particle size. Our new insights improve understanding of this counterintuitive microfluidics field, offering guidance for new device design to directly handle whole blood and to expand the applications to meet the real-world need for ultra-fast cell separation.

Viscoelastic Separation and Concentration of Fungi from Blood for Highly Sensitive Molecular Diagnostics

Isolation and concentration of fungi in the blood improves sensitivity of the polymerase chain reaction (PCR) method to detect fungi in blood. This study demonstrates a sheathless, continuous separation and concentration method of candida cells using a viscoelastic fluid that enables rapid detection of rare candida cells by PCR analysis. To validate device performance using a viscoelastic fluid, flow characteristics of 2 μm particles were estimated at different flow rates. Additionally, a mixture of 2 μm and 13 μm particles was successfully separated based on size difference at 100 μl/min. Candida cells were successfully separated from the white blood cells (WBCs) with a separation efficiency of 99.1% and concentrated approximately 9.9-fold at the center outlet compared to the initial concentration (~2.5 × 10⁷ cells/ml). Sequential 1st and 2nd concentration processes were used to increase the final number of candida cells to ~2.3 × 10⁹ cells/ml, which was concentrated ~92-fold. Finally, despite the undetectable initial concentration of 10¹ CFU/ml, removal of WBCs and the additional buffer solution enabled the quantitative reverse transcription (RT)-PCR detection of candida cells after the 1st concentration (Ct = 31.43) and the 2nd concentration process (Ct = 29.30).

Two-level submicron high porosity membranes (2LHPM) for the capture and release of white blood cells (WBCs)

A method modifying a vacuum-assisted UV micro-molding (VAUM) process is proposed for the fabrication of polymer two-level submicron high porosity membranes (2LHPM). The modified process allows for the fabrication of robust, large-area membranes over 5 × 5 cm2 with a hierarchical architecture made from a 200 nm-thick layer having submicron level pores (as small as 500 nm) supported by a 20 μm-thick layer forming a microporous structure with 10-15 μm diameter pores. The fabricated freestanding membranes are flexible and mechanically robust enough for post manipulation and filtration of cell samples. Very high white blood cell (WBC) capture efficiencies (≈97%) from healthy blood samples after red blood cell (RBC) lysis are demonstrated using a 3D-printed filter cartridge incorporated within these 2LHPM. A high release efficiency of ≈95% is also proved using the same setup. Finally, on-filter multistep immunostaining of captured cells is also shown.