Increasingly researchers are developing lab-on-a-chip devices that can do the work of an entire lab on the area the size of a microchip. Pumps are needed to move tiny amounts of fluid from one chamber to another within these microfluidic devices, but integrating useful pumps into such devices is often difficult. Now scientists reveal droplets of liquid metal could enable a novel type of pump without rigid moving parts for microfluidic devices. The findings are detailed in the Proceedings of the National Academy of Sciences.
Microfluidic devices enable scientists to conduct thousands of experiments simultaneously at a fraction of the time, space, materials, cost and effort of what it might ordinarily have taken. They essentially shrink beakers, flasks and other lab equipment into components such as channels and valves. Similar to how modern electronic devices route electrons, these mechanical systems route fluids.
Although researchers would like to incorporate pumps into microfluidic devices, they often have limited rates of flow compared to how much power they consume and they are typically complicated to fabricate. In addition, conventional mechanical pumps require intricate moving parts that are vulnerable to breakdown and lose energy due to heat generated by friction.
Instead of rigid moving parts, Khashayar Khoshmanesh and Shiyang Tang at the Royal Melbourne Institute of Technology University in Australia and their colleagues suggest using liquid metal droplets in microfluidic pumps.
“We find liquid metals fascinating — the ability for something that has such a strong effect both electronically and chemically on its surroundings to be able to change its shape in response to these strong interactions creates all sorts of exciting possibilities for unusual structures,” Khoshmanesh says.
Originally, the researchers were studying liquid metal as a component for reconfigurable electromagnetic structures. “We then observed that the metal would move around rather dramatically in response to an applied voltage,” Khoshmanesh says. “From there, the idea of using the liquid metal droplet to move fluid around in a channel was generated.”
The droplets are made of the alloy Galinstan, which is made of gallium, indium and tin. The liquid metal droplets are not actually moving around when the pumping is occurring. “There is something more subtle going on,” Tang says.
When an electrical potential or voltage is applied to the metal droplet, the liquid in the microfluidic system “wets” or sticks to the droplet’s surface. “Because the potential is applied to only one end of the device, the liquid sticks more to one end of the metal droplet than the other,” Tang says. “This is where the fact that the metal is liquid becomes important.”
The liquid in the microfluidic system — an ionic solution — gets attracted to the end of the Galinstan droplet that is more sticky, but the metal in the Galinstan droplet is also attracted to this sticky end. Since the metal is liquid, the droplet’s surface can flow toward this end. This drags the ionic solution along with it like a conveyor belt, making the droplet work like a pump, Tang said.
The system generates high flow rates of more than 5,000 microliters per minute at exceptionally low powers of less than 15 milliwatts.
“We think the most surprising thing about the results is just how simple the structure is and how well it works,” Khoshmanesh says. “The exciting prospect here is that it should be possible to make really complex systems using this pump using truly scalable technologies, so it should be possible to design systems with thousands or more active fluidic components and then just print them up.”
The surface of the Galinstan droplet can oxidize and solidify, keeping its surface from moving. “Every so often it is necessary to reverse the voltage to reverse the chemical reaction causing this oxidation to turn the surface back into liquid metal again,” Tang says.
The researchers caution a lot of work still needs to be done to practically apply this research. “One concern people might have is that the liquid metal might contaminate the fluid that they are manipulating,” Tang says. “That is a valid concern, but we have in mind some techniques that could be used using an intermediate fluid to stop the liquid metal droplet from touching the fluid being processed.”
“The next most important step is to really use this pump for some of our lab-on-a-chip applications,” Khoshmanesh says. “We do a lot of work with blood-clotting research and also with manipulation of cells and simple organisms such as yeast to understand how they communicate and so, if we can show that this pump can be used to study such complex biological systems, then I would expect the community to really adopt this new technology.”