From microfluidic applications to nanofluidic phenomena

Over the past 2 decades, research on microfluidic phenomena, devices and applications has grown from non-existing (or at least no identified as such) to a large community producing today over 2500 publications per year. Initiated originally in the early 90's by activities in microfabrication techniques as spin-off from MicroElectroMechanical Systems (MEMS) or Micro System Technology (MST) research, in the beginning most of the focus was on analytical chemistry applications, with a major role for capillary electrophoresis (CE) on a chip. A large variety of microfluidic phenomena has been explored since then based upon fast thermal and species diffusion, large surface-to-volume ratio or large field gradients (electrical, acoustic, optical), resulting from the small structural dimensions. Besides these new phenomena, what has been perhaps the most important driving force for microfluidics is the ability to control minute amounts ((sub)nl) of liquid in a very precise way. This aspect has been brought within the reach of the much broader research area of life sciences just before the 00's by the introduction of PDMS-based microfluidics devices. PDMS devices can be easily replicated from photolithographically fabricated masters, and enable a wide variety of applications. The excellent fit between typical microchannel sizes realizable in PDMS (10–100 μm) and the size of a human cell (typically 10 μm) is probably the main reason for the great interest from biologists. So, gradually the field of microfluidics moved from the exploration of new principles and mechanisms towards the realization of new applications in the life-sciences and (bio)medical domain.

Meanwhile, with the upcoming of nanotechnology, researchers have also looked at fluidics at an even smaller scale, down to several nanometres, and the new field of nanofluidics was born. One special feature is that the structural dimensions are in the order of the size of large biomolecules such as DNA or proteins. New analysis techniques have emerged since then using nanoscale geometries. Another very important length scale is that the size of the electrical double layer, present on each surface immersed in an aqueous solution, is also in the order of 1–100 nm. Thus, in a sense, nanofluidics is present in fluidic systems of all scales, although its effects are largest when the channel dimensions are also below 100 nanometres.

The rapid growth in scientific activities in the micro- and nanofluidic domains is illustrated by the number of ISI publications: for both topics a region of exponential growth in number of publications (in the range between 10 and 1000 publications/yr) can be observed, with publication doubling times of 15 months and 20 months respectively (see Fig. 1). From this figure one can conclude that nanofluidics follows a similar trend as microfluidics, but with a slower growth and a 5–10 year time delay.

Fig. 1 Growth of number of ISI publications found with “microfluidic*” and “nanofluidic*” over the past 20 years. The inset shows the same graph in a logarithmic fashion, indicating a 15 month publication doubling time for “microfluidic*” and a 20 month doubling time for “nanofluidic*” (light-blue and yellow lines respectively).

 

In this themed issue micro- and nanofluidic contributions are arranged in three areas:

∘ General principles and theory

∘ Fabrication aspects and applications

∘ Bio-oriented aspects: DNA-proteins-cells

General principles and theory

Daiguji (DOI: 10.1039/b820556f) discusses the possibility to model nanofluidic systems with continuum models and stochastic and/or molecular dynamics. Lemay (DOI: 10.1039/b902072c) introduces the concepts necessary for understanding the generation of forces on charged objects in solution by externally applied electric fields. Santiago (DOI: 10.1039/b902074h) summarizes the current state of theoretical and experimental approaches to describing concentration polarization (CP) in hybrid microfluidic-nanofluidic systems. Bocquet and Charlaix (DOI: 10.1039/b909366b) examine the interplay between bulk and interface phenomena, from hydrodynamic slippage to the various electro-kinetic phenomena originating from the couplings between hydrodynamics and electrostatics. Eijkel and van den Berg (DOI: 10.1039/b913776a) make an attempt to introduce the chemical potential as a tool to describe equilibrium and transport properties of nanofluidic systems. Taset al. (DOI: 10.1039/b909101g) present studies of capillary behavior of fluids in nanochannels, results of which may lead to new principles for chemical separation.

Fabrication aspects and applications

Zengerle and von Stetten (DOI: 10.1039/b820557b) summarize different microfluidic platforms in the context of various potential applications, with a focus on lateral flow tests, linear actuated devices, pressure driven laminar flow, microfluidic large scale integration, segmented flow microfluidics, centrifugal microfluidics, electrokinetics, electrowetting, surface acoustic waves, and dedicated systems for massively parallel analysis. Jensen (DOI: 10.1039/b821324k) presents an overview of the current progress in synthesis of micro and nanostructures by using microfluidics techniques. Guo (DOI: 10.1039/b822554k) reviews recent advances in the experimental and theoretical studies of ion current rectification in nanofluidic diodes. Kitamori (DOI: 10.1039/b822557p) surveys several fundamental technologies and unique liquid properties in nanofluidic environments. Bohn (DOI: 10.1039/b900409m) examines chemical manipulations in nanopores that include nanopore-mediated separations, microsensors, especially resistive-pulse sensing of biomacromolecules, fluidic circuit analogs and single molecule measurements. Siwy (DOI: 10.1039/b909105j) reviews engineered solid-state and protein nanopores with voltage-responsive properties. These engineered systems show nonlinear current–voltage curves, and/or voltage-dependent switching between discrete conductance states.

Bio-oriented aspects: DNA-proteins-cells

Craighead (DOI: 10.1039/b820266b) reviews recent experiments utilizing nanofluidic systems for DNA manipulation, sorting and mapping. Han (DOI: 10.1039/b822556g) discusses trapping and concentration of biomolecules by utilizing ion concentration polarization near nanofluidic structures. Kaji (DOI: 10.1039/b900410f) reviews fabrication technologies for nano-scaled structures inside microchannels and how these precisely designed structures contribute to better performances in DNA separations. Beebe (DOI: 10.1039/b909900j) discusses fundamental principles from cell biology and local microenvironments with cell culture techniques and concepts in microfluidics. Tegenfeldt (DOI: 10.1039/b912918a) reviews their efforts on the stretching of DNA in nanoscale channels for coarse-grained mapping of DNA. Laurell (DOI: 10.1039/b915999c) outlines the most recent developments in microfluidic technology for cell and particle separation in continuous flow based systems. Ismagilov (DOI: 10.1039/b917851a) introduces the concept of microfluidic stochastic confinement for use in detection and analysis of rare cells. Austinet al. (DOI: 10.1039/b911230h) present the use of microfluidic technologies to study changes in evolutionary fitness of bacteria upon exposing them to stressful micro-ecology environments. From the results insights will be obtained to study and predict behavior of more complex biological systems.

 

We hope you enjoy reading this collection of reviews which we believe represents the state of the art in micro- and nanofluidic research.

Footnote

† Part of the themed issue: From microfluidic application to nanofluidic phenomena.

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