Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Peclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world.

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Microfluidics: Fluid physics at the nanoliter scale

基礎講座 電気泳動(Electrophoresis)

Microsystems create new opportunities for the spatial and temporal control of cell growth and stimuli by combining surfaces that mimic complex biochemistries and geometries of the extracellular matrix with microfluidic channels that regulate transport of fluids and soluble factors. Further integration with bioanalytic microsystems results in multifunctional platforms for basic biological insights into cells and tissues, as well as for cell-based sensors with biochemical, biomedical and environmental functions. Highly integrated microdevices show great promise for basic biomedical and pharmaceutical research, and robust and portable point-of-care devices could be used in clinical settings, in both the developed and the developing world.

Cells on chips

Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects?rather than a unique one?are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices.

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Electrowetting: from basics to applications

The developing world does not have access to many of the best medical diagnostic technologies; they were designed for air-conditioned laboratories, refrigerated storage of chemicals, a constant supply of calibrators and reagents, stable electrical power, highly trained personnel and rapid transportation of samples. Microfluidic systems allow miniaturization and integration of complex functions, which could move sophisticated diagnostic tools out of the developed-world laboratory. These systems must be inexpensive, but also accurate, reliable, rugged and well suited to the medical and social contexts of the developing world.

Microfluidic diagnostic technologies for global public health

Electrorotation measurements were used to demonstrate that the dielectric properties of the metastatic human breast cancer cell line MDA231 were significantly different from those of erythrocytes and T lymphocytes. These dielectric differences were exploited to separate the cancer cells from normal blood cells by appropriately balancing the hydrodynamic and dielectrophoretic forces acting on the cells within a dielectric affinity column containing a microelectrode array. The operational criteria for successful particle separation in such a column are analyzed and our findings indicate that the dielectric affinity technique may prove useful in a wide variety of cell separation and characterization applications.

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Separation of Human Breast Cancer Cells From Blood by Differential Dielectric Affinity

Dielectrophoretic field-flow-fractionation (DEP-FFF) was applied to several clinically relevant cell separation problems, including the purging of human breast cancer cells from normal T-lymphocytes and from CD34+ hematopoietic stem cells, the separation of the major leukocyte subpopulations, and the enrichment of leukocytes from blood. Cell separations were achieved in a thin chamber equipped with a microfabricated, interdigitated electrode array on its bottom wall that was energized with AC electric signals. Cells were levitated by the balance between DEP and sedimentation forces to different equilibrium heights and were transported at differing velocities and thereby separated when a velocity profile was established in the chamber. This bulk-separation technique adds cell intrinsic dielectric properties to the catalog of physical characteristics that can be applied to cell discrimination. The separation process and performance can be controlled through electronic means. Cell labeling is unnecessary, an...

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Cell Separation by Dielectrophoretic Field-flow-fractionation

The application of dielectrophoretic field-flow fractionation (depFFF) to the isolation of circulating tumor cells (CTCs) from clinical blood specimens was studied using simulated cell mixtures of three different cultured tumor cell types with peripheral blood. The depFFF method can not only exploit intrinsic tumor cell properties so that labeling is unnecessary but can also deliver unmodified, viable tumor cells for culture and/or all types of molecular analysis. We investigated tumor cell recovery efficiency as a function of cell loading for a 25 mm wide x 300 mm long depFFF chamber. More than 90% of tumor cells were recovered for small samples but a larger chamber will be required if similarly high recovery efficiencies are to be realized for 10 mL blood specimens used CTC analysis in clinics. We show that the factor limiting isolation efficiency is cell-cell dielectric interactions and that isolation protocols should be completed within approximately 15 min in order to avoid changes in cell dielectric properties associated with ion leakage.

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Isolation of rare cells from cell mixtures by dielectrophoresis

Measurements have demonstrated that the dielectric properties of cells depend on their type and physiological status. For example, MDB-MB-231 human breast cancer cells were found to have a mean plasma membrane specific capacitance value of 26 nF/m/sup 2/, more than double the value observed for resting T-lymphocytes. When an inhomogeneous DC electric field is applied to a particle, a dielectrophoretic (DEP) force arises that depends on the particle dielectric properties. Therefore, cell having different dielectric characteristics will experience differential DEP forces when subjected to such a field. In this article we demonstrate the use of these differential DEP forces for the reparation of several different cancerous cell types from blood. These separations were accomplished using thin, flat chambers having microelectrode arrays on the bottom wall. DEP forces generated by the application of AC fields to the electrodes were used to influence the rate of elution of cells from the chamber by hydrodynamic forcer from a parabolic fluid flow profile. Electrorotation measurements were first made on the various cell types found within cell mixtures to be separated and theoretical modelling was used to derive the cell dielectric parameters. Optimum separation conditions were then predicted from the frequency and suspension conductivity dependencies of cell DEP responses defined by these dielectric parameters. Cell separations were then undertaken for various ratios of cancerous to normal cells at different concentrations. Fluted cells were characterized in terms of separation efficiency, cell viability, and separation speed. For example, after the separation of mixtures of MDA-MB-231 cells and normal whole human blood (ratio 1:3), the eluted normal blood cell fraction contained less than 0.01% of the starting concentration of cancerous cells, cell viability was not compromised, and separation speeds of at least 10/sup 3/ cells/sec were achieved. Theoretical and experimental criteria for the design and operation of such separators are presented.

Dielectrophoretic separation of cancer cells from blood

Dielectrophoretic/gravitational field-flow fractionation (DEP/G-FFF) was used to separate cultured human breast cancer MDA-435 cells from normal blood cells mixed together in a sucrose/dextrose medium. An array of microfabricated, interdigitated electrodes of 50 μm widths and spacings, and lining the bottom surface of a thin chamber (0.42 mm H × 25 mm W × 300 mm L), was used to generate DEP forces that levitated the cells. A 10-μL cell mixture sample containing ∼50 000 cells was introduced into the chamber, and cancerous and normal blood cells were levitated to different heights according to the balance of DEP and gravitational forces. The cells at different heights were transported at different velocities under the influence of a parabolic flow profile that was established in the chamber and were thereby separated. Separation performance depended on the frequency and voltage of the applied DEP field and the fluid-flow rate. It took as little as 5 min to achieve cell separation. An analysis of the DEP/G-F...

Frederick F. Becker

Papers

Separation of Human Breast Cancer Cells From Blood by Differential Dielectric Affinity

Cell Separation by Dielectrophoretic Field-flow-fractionation

Isolation of rare cells from cell mixtures by dielectrophoresis

Dielectrophoretic separation of cancer cells from blood

Cell separation on microfabricated electrodes using dielectrophoretic/gravitational field-flow fractionation.