Harnessing the ability to precisely and reproducibly actuate fluids and manipulate bioparticles such as DNA, cells, and molecules at the microscale, microfluidics is a powerful tool that is currently revolutionizing chemical and biological analysis by replicating laboratory bench-top technology on a miniature chip-scale device, thus allowing assays to be carried out at a fraction of the time and cost while affording portability and field-use capability. Emerging from a decade of research and development in microfluidic technology are a wide range of promising laboratory and consumer biotechnological applications from microscale genetic and proteomic analysis kits, cell culture and manipulation platforms, biosensors, and pathogen detection systems to point-of-care diagnostic devices, high-throughput combinatorial drug screening platforms, schemes for targeted drug delivery and advanced therapeutics, and novel biomaterials synthesis for tissue engineering. The developments associated with these technological advances along with their respective applications to date are reviewed from a broad perspective and possible future directions that could arise from the current state of the art are discussed.

Microfluidics, a technology characterized by the engineered manipulation of fluids at the submillimetre scale, has shown considerable promise for improving diagnostics and biology research. Certain properties of microfluidic technologies, such as rapid sample processing and the precise control of fluids in an assay, have made them attractive candidates to replace traditional experimental approaches. Here we analyse the progress made by lab-on-a-chip microtechnologies in recent years, and discuss the clinical and research areas in which they have made the greatest impact. We also suggest directions that biologists, engineers and clinicians can take to help this technology live up to its potential.

This article reviews existing methods for the isolation, fractionation, orcaptureofrarecellsin microfluidic devices. Rare cell capture devices face the challenge of maintainingtheefficiencystandard of traditional bulk separation methods such as flow cytometersandimmunomagneticseparatorswhile requiring very high purity of the target cell population, whichistypicallyalreadyatverylowstarting concentrations. Two major classifications of rare cell capture approaches arecovered: (1) nonelectrokinetic methods (e.g., immobilization via antibody or aptamer chemistry, size-based sorting, and sheathflowandstreamlinesorting) arediscussedforapplicationsusingbloodcells, cancer cells, and other mammalianc ells, and (2) electrokinetic(primarilydielectrophoretic) methods using both electrode-based and insulative geometries are presented with a view towards pathogen detection, blood fractionation, and cancer cell isolation. The included methods were evaluated based on performance criteria including cell type modeled and used, number of steps/stages, cell viability, and enrichment, efficiency, and/orpurity. Major areas for improvement are increasing viability and capture efficiency/purity of directly processed biological samples, as a majority of currentstudiesonlyprocess spiked cell lines or pre-diluted/lysedsamples. Despite these current challenges, multiple advances have been made in the development of devices for rare cell capture and the subsequent elucidation of new biological phenomena; this article serves to highlight this progress as wellastheelectrokineticand non-electro kinetic methods that can potentially be combined to improve performance in future studies. (C) 2010 Elsevier Ltd.

Substantial evidence shows that the heterogeneity of individual cells within a genetically identical population can be critical to their chance of survival. Methods that use average responses from a population often mask the difference from individual cells. To fully understand cell-to-cell variability, a complete analysis of an individual cell, from its live state to cell lysates, is essential. Highly sensitive detection of multiple components and high throughput analysis of a large number of individual cells remain the key challenges to realise this aim. In this context, microfluidics and lab-on-a-chip technology have emerged as the most promising avenue to address these challenges. In this review, we will focus on the recent development in microfluidics that are aimed at total single cell analysis on chip, that is, from an individual live cell to its gene and proteins. We also discuss the opportunities that microfluidic based single cell analysis can bring into the drug discovery process.

Plasma is a complex substance that contains proteins and circulating nucleic acids and viruses that can be utilised for clinical diagnostics, albeit a precise analysis depends on the plasma being totally free of cells. We proposed the use of a dielectrophoresis (DEP)-active hydrophoretic method to isolate plasma from blood in a high-throughput manner. This microfluidic device consists of anisotropic microstructures embedded on the top of the channel which generate lateral pressure gradients while interdigitised electrodes lay on the bottom of the channel which can push particles or cells into a higher level using a negative DEP force. Large and small particles or cells (3 mu m and 10 mu m particles, and red blood cells, white blood cells, and platelets) can be focused at the same time in our DEP-active hydrophoretic device at an appropriate flow rate and applied voltage. Based on this principle, all the blood cells were filtrated from whole blood and then the plasma was extracted with a purity of 94.2% and a yield of 16.5% at a flow rate of 10 mu L min(-1). This solved the challenging problem caused by the relatively low throughput of the DEP based device. Our DEP-active hydrophoretic device is a flexible and tunable system that can control the lateral positions of particles by modulating the external voltages without redesigning and fabricating a new channel, and because it is easy to operate, it is easily compatible with other microfluidic platforms that are used for further detection.

Microfluidics has experienced massive growth in the past two decades, and especially with advances in rapid prototyping researchers have explored a multitude of channel structures, fluid and particle mixtures, and integration with electrical and optical systems towards solving problems in healthcare, biological and chemical analysis, materials synthesis, and other emerging areas that can benefit from the scale, automation, or the unique physics of these systems. Inertial microfluidics, which relies on the unconventional use of fluid inertia in microfluidic platforms, is one of the emerging fields that make use of unique physical phenomena that are accessible in microscale patterned channels. Channel shapes that focus, concentrate, order, separate, transfer, and mix particles and fluids have been demonstrated, however physical underpinnings guiding these channel designs have been limited and much of the development has been based on experimentally-derived intuition. Here we aim to provide a deeper understanding of mechanisms and underlying physics in these systems which can lead to more effective and reliable designs with less iteration. To place the inertial effects into context we also discuss related fluid-induced forces present in particulate flows including forces due to non-Newtonian fluids, particle asymmetry, and particle deformability. We then highlight the inverse situation and describe the effect of the suspended particles acting on the fluid in a channel flow. Finally, we discuss the importance of structured channels, i.e. channels with boundary conditions that vary in the streamwise direction, and their potential as a means to achieve unprecedented three-dimensional control over fluid and particles in microchannels. Ultimately, we hope that an improved fundamental and quantitative understanding of inertial fluid dynamic effects can lead to unprecedented capabilities to program fluid and particle flow towards automation of biomedicine, materials synthesis, and chemical process control.

Microfluidics has experienced massive growth in the past two decades, and especially with advances in rapid prototyping researchers have explored a multitude of channel structures, fluid and particle mixtures, and integration with electrical and optical systems towards solving problems in healthcare, biological and chemical analysis, materials synthesis, and other emerging areas that can benefit from the scale, automation, or the unique physics of these systems. Inertial microfluidics, which relies on the unconventional use of fluid inertia in microfluidic platforms, is one of the emerging fields that make use of unique physical phenomena that are accessible in microscale patterned channels. Channel shapes that focus, concentrate, order, separate, transfer, and mix particles and fluids have been demonstrated, however physical underpinnings guiding these channel designs have been limited and much of the development has been based on experimentally-derived intuition. Here we aim to provide a deeper understanding of mechanisms and underlying physics in these systems which can lead to more effective and reliable designs with less iteration. To place the inertial effects into context we also discuss related fluid-induced forces present in particulate flows including forces due to non-Newtonian fluids, particle asymmetry, and particle deformability. We then highlight the inverse situation and describe the effect of the suspended particles acting on the fluid in a channel flow. Finally, we discuss the importance of structured channels, i.e. channels with boundary conditions that vary in the streamwise direction, and their potential as a means to achieve unprecedented three-dimensional control over fluid and particles in microchannels. Ultimately, we hope that an improved fundamental and quantitative understanding of inertial fluid dynamic effects can lead to unprecedented capabilities to program fluid and particle flow towards automation of biomedicine, materials synthesis, and chemical process control.

Separation and sorting of micron-sized particles has great importance in diagnostics, chemical and biological analyses, food and chemical processing and environmental assessment. By employing the unique characteristics of microscale flow phenomena, various techniques have been established for fast and accurate separation and sorting of microparticles in a continuous manner. The advancements in microfluidics enable sorting technologies that combine the benefits of continuous operation with small-sized scale suitable for manipulation and probing of individual particles or cells. Microfluidic sorting platforms require smaller sample volume, which has several benefits in terms of reduced cost of reagents, analysis time and less invasiveness to patients for sample extraction. Additionally, smaller size of device together with lower fabrication cost allows massive parallelization, which makes high-throughput sorting possible. Both passive and active separation and sorting techniques have been reported in literature. Passive techniques utilize the interaction between particles, flow field and the channel structure and do not require external fields. On the other hand, active techniques make use of external fields in various forms but offer better performance. This paper provides an extensive review of various passive and active separation techniques including basic theories and experimental details. The working principles are explained in detail, and performances of the devices are discussed.

Focusing particles (both biological and synthetic) into a tight stream is usually a necessary step prior to counting, detecting, and sorting them. The various particle focusing approaches in microfluidic devices may be conveniently classified as sheath flow focusing and sheathless focusing. Sheath flow focusers use one or more sheath fluids to pinch the particle suspension and thus focus the suspended particles. Sheathless focusers typically rely on a force to manipulate particles laterally to their equilibrium positions. This force can be either externally applied or internally induced by channel topology. Therefore, the sheathless particle focusing methods may be further classified as active or passive by the nature of the forces involved. The aim of this article is to introduce and discuss the recent developments in both sheath flow and sheathless particle focusing approaches in microfluidic devices.

Solid-liquid disperse systems are complex fluids, extensively used for a variety of applications. The flow behavior of such materials is strongly dependent on the matrix rheological properties: when the suspending fluid shows viscoelastic behavior, isolated suspended particles in confined flow fields tend to migrate across the streamlines. In this work, we carry out a detailed experimental investigation of such migration phenomenon for a rigid, spherical particle in planar shear flow, and compare the experimental findings with 3D numerical simulations based on finite elements. A highly elastic non-Newtonian fluid is chosen as the suspending medium. The migration is studied by varying the confinement, the external shear rate and the fluid normal stresses. The particle is always found to move towards the closest wall, regardless of its initial position. For a fixed set of geometrical and flow parameters, the trajectories from different initial positions can be shifted in time to overlap on a single curve. Migration dynamics is governed by an exponential law as the particle is in the region around the midgap. Stronger confinements, higher shear rates and higher levels of normal stresses all enhance the migration speed. We found qualitative agreement between predictions from numerical simulations and experimental results.

Particle focusing in planar geometries is essentially required in order to develop cost-effective lab-on-a-chips, such as cell counting and point-of-care (POC) devices. In this study, a novel method for sheathless particle focusing, called "Elasto-Inertial Particle Focusing", was demonstrated in a straight microchannel. The particles were notably aligned along the centerline of the straight channel under a pressure-driven flow without any additional external force or apparatus after the addition of an elasticity enhancer: PEO (poly(ethylene oxide)) (similar to O(100) ppm). As theoretically predicted (elasticity number: El approximate to O(100)), multiple equilibrium positions (centerline and corners) were observed for the viscoelastic flow without inertia, whereas three-dimensional particle focusing only occurred when neither the elasticity nor the inertia was negligible. Therefore, the three-dimensional particle focusing mechanism was attributed to the synergetic combination of the elasticity and the inertia (elasticity number: El approximate to O(1-10)). Furthermore, from the size dependence of the elastic force upon particles, we demonstrated that a mixture of 5.9 and 2.4 mu m particles was separated at the exit of the channel in viscoelastic flows. We expect that this method can contribute to develop the miniaturized flow cytometry and microdevices for cell and particle manipulation.

We perform 3D numerical simulations, heuristic modeling and microfluidic experiments to demonstrate, for the first time, the presence of a bistability scenario for transversal migration of particles suspended in a viscoelastic liquid flowing in a pipe. Our results show that particle migration, either at the centerline or at the wall, can be controlled by the rheological properties of the suspending liquid and by the relative dimensions of the particle and tube. Proper selection of these parameters can promote strict aligning of particles on a line, i.e., 3-D focusing. Simple design rules are given to rationally control particle focusing under flow in micropipes.

Much difficulty has been encountered in manipulating small-scale materials, such as submicron colloidal particles and macromolecules (e.g., DNA and proteins), in microfluidic devices since diffusion processes due to thermal (Brownian) motion become more pronounced with decreasing particle size. Here, we present a novel approach for the continuous focusing of such small-scale materials. First, we successfully focused fluorescent submicron polystyrene (PS) beads along equilibrium positions in microchannels through the addition of a small amount water-soluble polymer [500 ppm poly(ethylene oxide) (PEO)]. Lateral migration velocity significantly depends upon the viscoelastic effect (Weissenberg number: Wi) and the aspect ratio of particle size to channel height (a/h). Interestingly, focusing using viscoelastic flows was also observed for flexible DNA molecules (lambda-DNA and T4-DNA), which have radii of gyration (R-g) of approximately 0.69 mu m and 1.5 mu m, respectively. This small-scale material manipulation using medium viscoelasticity will contribute to the design of nanoparticle separation and genomic mapping devices.

Centrifugal damage has been known to alter bacterial cell surface properties and interior structures, including DNA. Very few studies exist on bacterial damage caused by centrifugation because of the difficulty in relating centrifugation speed and container geometry to the damage caused. Here, we provide a simple, versatile method of analysis for describing the compaction of bacteria during centrifugation based on a proposed centrifugation coefficient, C. Values of C can be related to different bacterial cell surface properties. Changing the geometry of the centrifugation container or centrifugation speeds changed the value of C significantly. Initial deposition rates of Staphylococcus aureus ATCC 12600 to a glass surface decayed exponentially from 4,217 to 1,478 cm(-2) s(-1) with increasing C, while the proportion of staphylococci with a zeta potential of around -15 mV decreased from 97 to 58%. These surface-sensitive parameters were used independently to derive a critical centrifugation coefficient (0.040), above which centrifugation was considered to impact the outcome of surface-sensitive experiments due to cell surface damage. The critical centrifugation coefficient could successfully predict staphylococcal cell surface damage, i.e., a significant change in initial deposition rate or zeta potential distribution, in 84% of all cases included here, whereas the centrifugation speed could predict damage in only 58% of all cases. Moreover, controlling the centrifugation coefficient within narrow limits over a series of experiments yielded 43% smaller standard deviations in initial staphylococcal deposition rates than with centrifugation at fixed speeds for replicate experiments.

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 mu m and 5 mu m 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.

In this work, we investigated the lateral migration of microparticles suspended in two different viscoelastic fluids with or without the second normal stress difference. For the viscoelastic fluid without the second normal stress difference, competing forces existed between microfluidic inertia and the first normal stress difference (N (1)), which resulted in a synergetic effect of particle focusing. For the fluid with the second normal stress difference (N (2)), particles were greatly affected by a N (2)-induced secondary flow, and the competition among the inertia, N (1), and N (2) determined the lateral migration trajectories of the particles. The obtained results were delineated with the blockage ratio, which showed good agreement with the results of a recent numerical study (Villone et al. in J Non Newton Fluid Mech 195:1-8, 2013). The present study also examined the possibility of particle separation in a size-dependent manner using the N (2)-induced secondary flow in microchannel flow.

We present an efficient method for measuring cell stretching based on three-dimensional viscoelastic particle focusing. We suspended cells in a biocompatible viscoelastic medium [poly(vinylpyrrolidone) solution in phosphate-buffered saline]. The medium viscoelasticity significantly homogenized the trajectories of cells along the centerline of a simple straight channel, which could not be achieved in conventional Newtonian media. More than 95% of red blood cells (RBCs) were successfully delivered to the stagnation point of a cross-slot microchannel and stretched by extensional flow. By computational simulations, we proved that this method prevents inaccuracies due to random lateral distributions of cells and, further, guarantees rotational-free cell stretching along the shear-free channel centerline. As a demonstration, we characterized the differences in RBC deformabilities among various heat treatments. Furthermore, we monitored the decrease of deformability due to nutrient starvation in human mesenchymal stem cells. We envisage that our novel method can be extended to versatile applications such as the detection of pathophysiological evolution in impaired RBCs due to malaria or diabetes and the monitoring of cell quality in stem cell therapeutics.

A novel integrated scheme for modeling incompressible polymer viscoelastic fluid flows is proposed. Lattice Boltzmann method (LBM) is incorporated into finite volume method (FVM) to solve the incompressible Navier-Stokes equations and the constitutive equation respectively, and is implemented using open source CFD toolkits to predict nonlinear dynamics of polymer viscoelastic fluid flows. The hybrid numerical scheme inherits the efficiency and scalability of LBM and maintains the accuracy and generality of FVM. It has been critically validated using the Oldroyd-B model and linear PTT model under Poiseuille flow, Taylor-Green vortex flow and 4: 1 abrupt planar contraction flow, respectively. The results from the integrated scheme have good agreement with the analytical solutions and the numerical results of other FVM schemes in previous publications. (C) 2014 Elsevier B.V.

In this paper, the fluid dynamics induced by a rigid lamina undergoing harmonic oscillations in a non-Newtonian calm fluid is investigated. The fluid is modelled through the lattice Boltzmann method and the flow is assumed to be nearly incompressible. An iterative viscosity-correction based procedure is proposed to properly account for the non-Newtonian fluid feature and its accuracy is evaluated. In order to handle the mutual interaction between the lamina and the encompassing fluid, the Immersed Boundary method is adopted. A numerical campaign is performed. In particular, the effect of the non-Newtonian feature is highlighted by investigating the fluid forces acting on a harmonically oscillating lamina for different values of the Reynolds number. The findings prove that the non-Newtonian feature can drastically influence the behaviour of the fluid and, as a consequence, the forces acting upon the lamina. Several considerations are carried out on the time history of the drag coefficient and the results are used to compute the added mass through the hydrodynamic function. Moreover, the computational cost involved in the numerical simulations is discussed. Finally, two applications concerning water resources are investigated: the flow through an obstructed channel and the particle sedimentation. Present findings highlight a strong coupling between the body shape, the Reynolds number, and the flow behaviour index. (C) 2014 Elsevier Ltd.

Hemodynamics is a complex problem with several distinct characteristics; fluid is non-Newtonian, flow is pulsatile in nature, flow is three-dimensional due to cholesterol/plague built up, and blood vessel wall is elastic. In order to simulate this type of flows accurately, any proposed numerical scheme has to be able to replicate these characteristics correctly, efficiently, as well as individually and collectively. Since the equations of the finite difference lattice Boltzmann method (FDLBM) are hyperbolic, and can be solved using Cartesian grids locally, explicitly and efficiently on parallel computers, a program of study to develop a viable FDLBM numerical scheme that can mimic these characteristics individually in any model blood flow problem was initiated. The present objective is to first develop a steady FDLBM with an immersed boundary (IB) method to model blood flow in stenoic artery over a range of Reynolds numbers. The resulting equations in the FDLBM/IB numerical scheme can still be solved using Cartesian grids; thus, changing complex artery geometry can be treated without resorting to grid generation. The FDLBM/IB numerical scheme is validated against known data and is then used to study Newtonian and non-Newtonian fluid flow through constricted tubes. The investigation aims to gain insight into the constricted flow behavior and the non-Newtonian fluid effect on this behavior.

格子玻尔兹曼方法（lattice Boltzmann method，LBM）能够直接计算局部剪切速率并可以达到二次精度，因此在非牛顿流动数值模拟中展现出一定优势。尽管已证实LBM 对于非牛顿流动的适用性，但是LBM 需要通过即时调节BGK（Bhatnagar-Gross-Krook）碰撞项中的松弛时间来实时反映黏度改变，当松弛时间接近1/2 时，迭代会出现数值不稳定现象。该文对LBM 在非牛顿流体研究中的进展进行了总结，介绍了增加数值稳定性的方法并对结果的精度进行了比较，在此基础上对LBM 在非牛顿研究中的进一步发展进行了展望。