Generate gradient in a microfluidic chip

The use of microfluidic chemical gradient in cell biology

Microfluidic gradients have been successfully used to study the chemotaxis of neutrophil [2-4], chemotaxis of bacteria [5], chemotaxis of cancer cells [7], neural stem cells growth and differentiation [1], endothelial cells migration [6], cellular response to virus [8], and yeast gene expression under gradients of pheromones [9].

For more information, you can also read Keenan and Folch review  [10].

Which kind of gradient can we generate using microfluidic ?

Using microfluidic, researchers can generate large numbers of gradient type: time invariant gradients, dynamic gradient, gradient with subcellular resolution, both continuous or discrete gradients. All those gradient generators can be done using simple microfluidics technology, e.g. with glass/PDMS chips.

Which method can be used to generate gradient on a microfluidic chip ?

The second type of microfluidic gradient generator is not based on laminar flow properties. The flow resistive microfluidic gradient generator uses flow resistive elements to eliminate convection around cells. This kind of device enables passive diffusion of biomolecules through a flow barrier to generate gradients. The flow barrier can be an hydrogel [14-16], a nanopore membrane [4] or microchannel [15].

These devices can generate steady-state gradients, eliminate shear stress generated by flow, and preserve the autocrine/paracrine signals secreted by cells. In addition, they use less reagents than laminar flow gradient generator. They also enable experiments with non adherent cells like yeast or bacteria.

Major drawbacks of these microfluidic gradient generator are their inability to create complex profiles. They are slower than laminar flow microfluidic gradient generator and for the hydrogel-based microfluidic gradient generator, they are harder to fabricate than laminar flow gradient generator based on single-layer PDMS device.

Which setup and which microfluidic chip to generate gradient ?

There are several device designs that enable to create microfluidic gradients. The choice of the design will depend on your particular experiment.

In term of setup, you can generate gradient using classical syringe pump or pressure generator. These latter are preferable if you want to be able to quickly change gradient concentration or if you need to avoid flow oscillation.

You can find more information about gradient generation in microfluidic in the section 3 of our review of microfluidic for cell biology.