Magnetic liquid frees microfluidics from friction

A microfluidic system with channels lined with magnetic liquids can ease the flow of samples and safely transport delicate cells, possibly helping to miniaturize microfluidic devices (Nature 2020, DOI: 10.1038/s41586-020-2254-4). “I think this will open up a lot of microfluidic applications where we can use low pressure in very small tubes,” says Thomas M. Hermans at the University of Strasbourg, who led the work.

Friction can cause big problems in tiny microfluidic systems. In channels just a few micrometers wide, a large proportion of a liquid sample is in contact with the channel wall, creating friction that limits flow rates and increases the risk of fouling. It also produces shear forces that can break apart fragile cells, proteins, and antibodies.

The new magnetic system overcomes this by dispensing with solid walls altogether, and instead wraps flowing aqueous samples inside a sheath of magnetic liquid. The researchers tested various magnetic liquids, including commercial ferrofluids that contain nanoparticles of magnetite. The magnetic liquid is contained in a plastic channel with four neodymium-iron-boron bar magnets mounted lengthwise, which create a region of zero magnetic field along the center of the magnetic liquid. This opens a thin pipe inside the magnetic liquid that an aqueous sample can flow through. These samples experience far less friction than in conventional microfluidic systems and require much less pressure to achieve comparable flows.

This is partly because of a helpful counter-flow within the magnetic liquid. As the aqueous core flows, it pulls the inner surface of the magnetic liquid sheath along with it. But when the aqueous sample drips out of the end of the pipe, the magnetic liquid is reined in by the bar magnets and pulled back up the outer surface of the sheath. “It makes a kind of fluidic conveyor belt,” Hermans says.

“It’s really surprising that they can use a magnetic fluid as a boundary layer like this,” says Leidong Mao at the University of Georgia, who has previously used ferrofluids to manipulate cells. The idea of using magnetic liquids as a lubricating layer has been mooted before, he adds, “but I’ve never seen a study that applies the principle in a microfluidic application—that’s very unique.”

Inside the magnetic device, a 1 mm wide aqueous core can produce a flow of about 40 mL per minute. At that diameter, Hermans says, most microfluidic systems would max out at a few mL per minute. The system can also handle highly viscous liquids—honey can drain through the device about 70 times as fast as through a plastic tube of the same diameter, for example. So far, the team has created aqueous cores as narrow as 14 µm, but calculate that it should be possible to shrink that diameter below 1 µm.

The team can use additional magnets to pinch the magnetic liquid, which acts as a valve. Repeating this valving over and over creates a pumping effect that is much gentler on samples than conventional peristaltic pumping, which relies on a roller to squeeze a plastic tube. The researchers found that their device transported human blood cells with 11 times less damage than a peristaltic system. Although tiny amounts of magnetite nanoparticles did transfer from the magnetic liquid to the sample, these parts-per-million quantities did not affect any of the standard measurements routinely carried out on blood samples.

Various other approaches have been used to reduce friction within microfluidic systems, including hydrophobic coatings and nano-patterned surfaces. “The benefit of our approach is that it’s dirt cheap,” Hermans says. “At the microfluidic scale, the magnets are just one or two euros.”

The magnetic sheath can also flex around obstacles, such as glass beads, which avoids any clogging. Hermans’s team is now using the magnetic device to carry out in-flow chemistry, such as nanoparticle synthesis. He has also cofounded a company called Qfluidics to develop the system for the biotech market.