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  • Review Article
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Microfluidics: reframing biological enquiry

Nature Reviews Molecular Cell Biology volume 16, pages 554–567 (2015)Cite this article

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Key Points

  • New biological understanding is emerging from the physical and chemical regimes that are found in microfluidic systems. The small volumes (sub-microlitre), predictable flows (low Reynolds number) and in vivo length scale and timescale matching of microfluidic devices underpin notable recent advances in molecular cell biology research.

  • Microfluidic tools combined with advanced molecular, imaging and bioinformatics techniques form a flexible 'toolbox' that life scientists are actively adopting and adapting to facilitate new lines of biological enquiry.

  • Microfluidic single-cell immunoassays can profile the secretomes and proteomes of thousands of cells in parallel. This has been used, for example, to study the population dynamics of glioma cells in terms of the effects of drug treatment on the PI3K pathway.

  • Emerging platforms for biophysical cytometry — which measure the mechanical properties of cells — offer a label-free cell screening platform for induced pluripotency and cancer diagnostics at a throughput rate that compares with that of fluorescence-based flow cytometry.

  • Water-in-oil droplet emulsions are being used to create discrete reaction vessels for high-throughput screening and directed evolution. Recent examples of the use of this strategy include antibody selection from hundreds of thousands of single hybridoma cells and high-resolution optimization of drug efficacy over a near-continuum of drug concentrations.

  • Precise temporal and spatial flow control in microfluidics make it a powerful platform for studying dynamic biological processes that occur over short timescales. Studies elucidating the dynamics of protein folding, signal transduction and the kinetics of transcription factor binding demonstrate the transient biological phenomena that have been made accessible for research using microfluidic tools.

  • Microfluidics allows researchers to deconstruct complex biological relationships by representing biology in tailored microenvironmental contexts, which can be perturbed in a controlled manner and closely monitored using, for example, real-time high-resolution imaging. In a prominent example, organ-on-a-chip platforms seek to recapitulate the functions of organs to create an in vitro model system for drug screening.

Abstract

The underlying physical properties of microfluidic tools have led to new biological insights through the development of microsystems that can manipulate, mimic and measure biology at a resolution that has not been possible with macroscale tools. Microsystems readily handle sub-microlitre volumes, precisely route predictable laminar fluid flows and match both perturbations and measurements to the length scales and timescales of biological systems. The advent of fabrication techniques that do not require highly specialized engineering facilities is fuelling the broad dissemination of microfluidic systems and their adaptation to specific biological questions. We describe how our understanding of molecular and cell biology is being and will continue to be advanced by precision microfluidic approaches and posit that microfluidic tools — in conjunction with advanced imaging, bioinformatics and molecular biology approaches — will transform biology into a precision science.

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Figure 1: A flow diagram highlighting crucial developments in microfluidics, from fabrication approaches and functional developments in the evolution of the technology platform to some of the applications discussed in this Review.
Figure 2: Physics at the microscale.
Figure 3: Dynamic process analysis.
Figure 4: High-throughput microfluidics.
Figure 5: Biological length scales.

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Acknowledgements

T.A.D. was supported by a US National Science Foundation Graduate Research Fellowship. A.M.T. was supported by a US Department of Homeland Security ORISE Fellowship, a Siebel Scholarship and a California Cancer Coordinating Committee Fellowship. This work was also supported by a US National Institutes of Health (NIH) New Innovator Award (DP2OD007294 to A.E.H.) and a UC Berkeley Bakar Fellowship (to A.E.H.).

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  1. University of California, Berkeley–University of California, San Francisco Graduate Program in Bioengineering, 342 Stanley Hall, Berkeley, 94720, California, USA

    Todd A. Duncombe, Augusto M. Tentori & Amy E. Herr

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  1. Todd A. Duncombe
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Correspondence to Amy E. Herr.

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The authors declare competing financial interests. T.A.D., A.M.T. and A.E.H. are co-inventors on patents related to microfluidic analysis, and A.E.H. has equity interest in a company commercializing a microfluidic analysis tool.

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Glossary

Laminar flow

A flow regime that is observed when the Reynolds number is less than 2,000 in pipe flow; in such cases, fluid moves in parallel stream lines or 'lamina'.

Reynolds number

(Re). A dimensionless number that is defined as the ratio of inertial forces to viscous forces in a system. The value of Re increases with flow velocity and the characteristic length scale (of, for example, the channel diameter) and decreases with viscosity. Microfluidic flows typically have a low Re (<10), which results in laminar flow.

Laminar flow patterning

Laminar patterning uses the slow mixing between multiple microfluidic flows with low Reynolds numbers for spatial patterning or to deliver soluble agents within the channel. The shape and location of patterning are controlled by modulating the relative flow rates of each fluid in the channel.

Multiplexing

The measurement of multiple signals in parallel; that is, carrying out several assays simultaneously.

Barcoding

The use of recognizable labels (tags) to track individual samples throughout an assay or to track the output of distinct assays from a single sample for multiplexing. Examples of such tags include DNA sequences, spectrally-encoded fluorescent beads and spatial patterning of capture reagents.

Strouhal number

A dimensionless number that is defined as the ratio of inertial forces resulting from changes in velocity in the flow field to the inertial forces resulting from unsteady flow oscillation. The Strouhal number increases with flow velocity and decreases with the characteristic length scale of the channel and with oscillation frequency.

Dissociation constant

(Kd). An equilibrium constant that describes the susceptibility of a complex to dissociate into its components. It is often used to describe how strongly molecules interact.

Directed evolution

A method for the engineering of new biomolecules using the principle of natural selection. Typically, several rounds of selection are used.

Taylor–Aris dispersion

A phenomenon that can enhance effective diffusion when there is a non-uniform flow velocity across a channel, as is typically the case in pressure-driven microfluidic flows.

Intravasation

The invasion of cancer cells through the basement membrane into a blood or lymphatic vessel, which is a crucial step in cancer metastasis.

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Duncombe, T., Tentori, A. & Herr, A. Microfluidics: reframing biological enquiry. Nat Rev Mol Cell Biol 16, 554–567 (2015). https://doi.org/10.1038/nrm4041

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