THE DEVICE: Microfluidic devices offer precise, small-scale methods of delivering fluids to organisms, tissues, and cells. A microfluidic quadrupole, as developed by David Juncker at McGill University and his colleagues, provides a quickly-adjustable concentration gradient in a setup that is designed to minimally disturb cells. Such an apparatus could give researchers a system for exposing cells to signaling molecules, for instance, and watching cells' reaction right in the dish.
The gadget, about the size of a pen and clamped to a microscope, has four apertures: two for spraying the fluid, and two for sucking it back up. The injection and re-aspiration holes are one millimeter apart, arranged in a square. Re-aspirating the fluid is important for keeping the liquid local and not contaminating the entire sample.
The apparatus is designed such that the two injection streams hit each other head-on, creating a “stagnation point” in the middle of the fluid's field of flow in which there is no shear stress against the substrate. This is vital for cellular applications, because cells are sensitive to perturbation. For instance, “even relatively small shears in terms of microfluidic environments can affect how neurons grow,” Juncker said.
WHAT'S NEW: In biological applications, microfluidics has been a bit clunky. Cells, tissues, or organisms are typically grown in the microfluidic device, rather than having the device come to the dish or tube.
“What's cool about this probe is you put the probe where the organism and the cells are, rather than putting the petri dish in the device,” said Joel Voldman, an engineering professor at Massachusetts Institute of Technology, who did not participate in this research. “That lowers the barrier and opens the application areas to using it.”
The quadrupole allows for manipulating the concentration gradient of fluids—carrying drugs or chemicals, for example—in a matter of seconds by changing the level of solute in one of the injection streams. Other approaches that use passive diffusion to alter concentration can take up to a half hour, Juncker said. And those devices that can change the gradient faster often create shear and can disturb cellular behavior.
IMPORTANCE: Stem cell researchers years ago recognized the need to try and recreate the microenvironment of a living organism for cells in culture. But replicating “morphogen gradient patterning”—the distribution of molecules that guide embryonic development—“in vitro to mimic some of the normal signals to stem cells has been relatively hard to achieve,” Marie Csete, a staff anesthesiologist at the University of California, San Diego and the chief scientific officer of iFluidics, told The Scientist in an email.
In this respect, Juncker's work is exciting, “because the technology seems simple...and the gradients are rapidly established and tunable,” Csete said.
Juncker has his sights set on neuronal development. One application, for instance, would be to observe the movement of neurons' growth cones—the tips of axons that establish synapses—in response to gradients of chemical cues.
NEEDS IMPROVEMENT: There's one big caveat to the applicability of the apparatus: “they haven't actually put cells under here and shown the cells are happy,” Voldman said. The researchers used green and red tracer beads to visualize the flow of the solution, but have not yet applied the device to a biological substrate.
“Having interfaced lots of devices to lots of different cell types, I know that some unpredictable problems will arise,” Csete noted. Juncker said his next step is to include cells in the microfluidic set up to see how they react.
One of the upgrades Juncker has in mind is to make the probe transparent for easier imaging, and he'd also like to design a rig that could work within a tissue slice.
Csete said she expects stem cell researchers will be eager to get their hands on the device, but it's currently a custom built apparatus. Though the fabrication is not exceptionally complicated, people would have to be trained to use the setup. But Juncker said if people are interested, he's be willing to help them.
M.A. Qasaimeh et al., “Microfluidic quadrupole and floating concentration gradient,” Nature Communications, DOI: 10.1038/ncomms1471, 2011.