Microfluidic Blood Separation: Key Technologies and Critical Figures of Merit

Abstract

Blood is a complex sample comprised mostly of plasma, red blood cells (RBCs), and other cells whose concentrations correlate to physiological or pathological health conditions. There are also many blood-circulating biomarkers, such as circulating tumor cells (CTCs) and various pathogens, that can be used as measurands to diagnose certain diseases. Microfluidic devices are attractive analytical tools for separating blood components in point-of-care (POC) applications. These platforms have the potential advantage of, among other features, being compact and portable. These features can eventually be exploited in clinics and rapid tests performed in households and low-income scenarios. Microfluidic systems have the added benefit of only needing small volumes of blood drawn from patients (from nanoliters to milliliters) while integrating (within the devices) the steps required before detecting analytes. Hence, these systems will reduce the associated costs of purifying blood components of interest (e.g., specific groups of cells or blood biomarkers) for studying and quantifying collected blood fractions. The microfluidic blood separation field has grown since the 2000s, and important advances have been reported in the last few years. Nonetheless, real POC microfluidic blood separation platforms are still elusive. A widespread consensus on what key figures of merit should be reported to assess the quality and yield of these platforms has not been achieved. Knowing what parameters should be reported for microfluidic blood separations will help achieve that consensus and establish a clear road map to promote further commercialization of these devices and attain real POC applications. This review provides an overview of the separation techniques currently used to separate blood components for higher throughput separations (number of cells or particles per minute). We present a summary of the critical parameters that should be considered when designing such devices and the figures of merit that should be explicitly reported when presenting a device's separation capabilities. Ultimately, reporting the relevant figures of merit will benefit this growing community and help pave the road toward commercialization of these microfluidic systems.

Keywords: blood contents; blood sorting; figures of merit; lab-on-a-chip; microfluidic separations; pathogenic bacteria; red blood cells; separation of blood.

Figure 1

Human blood composition (figure adapted from [18]).

Figure 2

(a) Expansion-contraction channels for the separation of lung cancer cells (NCI-H1299) from RBCs (figure adapted from [67]). (b) A bimodal stretchable straight channel for separating breast cancer cells (T47D) from WBCs and T47D from RBCs (stretched) (figure adapted from [69]). (c) Sinusoidal (curved) inertial separation of T47D cancer cells, RBCs, and WBCs (reproduced from [76] with permission from the Royal Society of Chemistry). (d) Multi-spiral separation of WBCs and RBCs (reproduced from [71] with permission from the Royal Society of Chemistry). (e) Deterministic lateral displacement (DLD) separation of WBCs, RBCs, and blood parasites (reproduced from [73] with permission from the Royal Society of Chemistry). (f) Integrated device for CTC, WBC, and RBC separation from whole blood with fluorescence flow cytometry (reprinted with permission from [82], copyright (2019) American Chemical Society).

Figure 6

Magnetophoretic separations of pathogens in blood. (a) Schematic of the configuration of a magnetophoretic particle separator with two fluidic channels (reprinted from [164] with permission from Elsevier, copyright (2021)). (b) Device schematic about the extracorporeal blood detoxification and bead magnetophoresis (reproduced from [165] with permission from the Royal Society of Chemistry). (c) An illustration of the hemocompatible magnetic particles (reprinted from [162] with permission from Elsevier, copyright (2020)). (d) Process to separate and pre-concentrate bacteria from blood and later purify and detect the bacterial genomic DNA (reproduced from [163] with permission from the Multidisciplinary Digital Publishing Institute).

Figure 7

Dielectrophoretic separations of pathogens in blood. (a) Diagram of interdigitated ITO microelectrodes used for the universality testing (reproduced from [166], with permission from the Royal Society of Chemistry). (b) A microfluidic device for continuous cell separation applying DEP (reproduced from [167] with permission from the Royal Society of Chemistry). (c) Prototype for detecting E. coli, using DEP to separate the bacteria (reprinted from [169] with permission from Elsevier, copyright (2015)). (d) Device with an H filter and pDEP capture (reprinted from [171] with permission from AIP Publishing, copyright (2018)). (e) Prototype to apply eDEP and isolate B. abortus (reprinted from [172] with permission from AIP Publishing, copyright (2019)).

Figure 3

Passive and inertial microfluidic devices used to isolate pathogens from human blood. (a) Microfluidic device that can be coupled with a smartphone camera (reprinted from [83] with permission from Elsevier, copyright (2014)). (b) Template of a spiral microchip (reprinted from [85] with permission from Oxford University Press US, copyright (2021)). (c) Drawing in AutoCAD with microchannel dimensions and photographs of the device (reprinted from [86] with permission from Wiley Online Library, copyright (2021)). (d) A 3D illustration of the microRAAD for HIV testing (reproduced from [87] with permission from the Royal Society of Chemistry). (e) Schematic of the working principle of the plasma separation (reprinted from [88] with permission from Elsevier, copyright (2020)).

Figure 4

(a) Here, (i) and (ii) represent the high electric field points in orange (left side) and the particle of interest in red and yellow. It shows the particle displacement vector. The pDEP displacement vector is represented in (i) in blue, and nDEP is represented (ii) in red. In (b), the magnetophoresis concept is represented, showing the particle displacing toward (blue displacement vector) or away from the magnetic field (red displacement vector), depending on the magnetic susceptibility contrast (X_c) between the X_p (particle) and X_f (surrounding fluid) response. (c) Positive acoustophoresis with a particle displacement toward the pressure node (red particles) (i) and negative acoustophoresis (ii) with a particle displacement toward the pressure anti-node (yellow particles), based on the particle acoustic contrast response.

Figure 5

(a) DEP separation of RBCs from fixed RBCs (reproduced from [107] with permission from the Royal Society of Chemistry). (b) DEP separation of RBCs from plasma with integrated prostate-specific antigen detection (reprinted from [29] with permission from Elsevier, copyright (2022). (c) Inertial and magnetophoretic separation of RBCs, WBCs, and CTCs (reproduced from [156] with permission from the Multidisciplinary Digital Publishing Institute). (d) SAW acoustophoretic separation of glioma tumor cells from RBCs (reprinted from [161] with permission from Elsevier, copyright (2018)).

Figure 8

Acoustophoretic separations of pathogens in blood. (a) Process of acoustic separation of bacteria from blood (reproduced from ([176]) with permission from the Royal Society of Chemistry). (b) Illustration of acoustofluidic separation of a pathogen from human blood using taSSAW (reprinted from [177] with permission from IOP Publishing, copyright (2016)). (c) Illustration of the separation of large food debris particles or red blood cells from smaller intrinsic microflora (reprinted from [178] with permission from Elsevier, copyright (2016)).

Figure 9

Labeled or label-free classification of (a) passive and hybrid as well as (b) active, microfluidic blood separation identifying the separated blood components.

Figure 10

Key reported blood separation parameters. (a) Blood content separations and (b) blood separation of blood pathogens.