Publications

Pump on a Chip

Electronically manipulated fluids could lead to implantable silicon devices.

Jakub Kedzierski twists a valve on a tiny syringe holding some ordinary water. On a nearby computer screen, an image of a droplet forms, captured by a camera pointed at the syringe. The droplet balances between the needle’s tip and a thin layer of a polymer underneath, a sphere of water. Kedzierski flips a switch and the bottom of the droplet, where it touches the polymer, instantly flattens like a balloon pressed against the table. When he adjusts some knobs and flips the switch again, the droplet flattens even more.

Kedzierski, an electrical engineer in Lincoln Laboratory’s Advanced Silicon group, is demonstrating electrocapillary action. In this process (also known as electrowetting), changing the voltage in a capacitor changes how liquid interacts with a surface. He’s hoping to use the phenomenon to build microfluidic labs-on-a-chip, in which tiny volumes of fluids can be moved around on a silicon chip without external pumps and valves. Such devices, he says, could be used for chemical sensing in environmental monitoring—say, as a safety check on a municipal water supply. They might also be useful for medical applications, perhaps in an implantable device that would provide continued monitoring of blood levels of hormones or insulin, or in rapid bedside diagnostics that would eliminate the wait for lab workups of blood samples.

Without some sort of on-chip pumping system, microfluidics chips need a system of tubes connected to outside pumps that vary pressure in the chip’s fluid channels to move liquid around. That makes the device bulky and power hungry and unsuitable for implantation or for long-term remote use. The electrowetting technique should allow the creation of devices that are much smaller, and therefore don’t use as much expensive reagent for chemical tests. They could also work much faster. Kedzierski says some researchers have attempted to use techniques developed for microelectromechanical systems to build on-chip pumps, but without much success: at the scale of tens of microns, surface tension is too powerful.

“They try to take a classic pump that uses discrete parts and do it on a small scale, but that’s very difficult,” he says.

The concept is fairly straightforward. An electrode is topped with a thin layer of an insulating polymer—Teflon works well—to form a capacitor. Place a droplet of some liquid on top of the polymer, and the droplet will press against the surface, flattening slightly where it touches. The flattening, or contact angle, depends on factors such as surface tension in the droplet and the electrochemical nature of the surface. Apply a voltage, and the droplet deforms in a repeatable, predictable way. “It will seek a different physical shape that will try to minimize its electrical free energy,” Kedzierski says. He compares the action to squeezing a balloon between your hands: apply energy and the balloon flattens; let go and it reverts to its original roundness.

With this deformable droplet, Kedzierski now has a shape-changing material that can act as a miniature piston. One device he designed has a shallow channel to carry liquid, with deeper cavities inscribed in the channel’s floor at regular intervals. Water sits on the bottom of the channel, with oil floating above it; electrodes sit beneath the floor of the channel. As Kedzierski switches the electrodes on and off, droplets of water balloon upward and shrink back, first on one side of the cavity and then on the other, while a flat lake of water in the cavity rises and falls. The result is a set of alternating pistons, creating a pump that propels the oil along. One fluid—in this case, the water—acts as a mechanical system pumping the other fluid—oil—around the chip. But the fluid to be moved could be something else, a blood sample, for instance, or drinking water, a variation that might call for a change in the pumping fluid as well. If the device designers want to keep the fluids from mixing, they can ensure that by using additives.

Alternately expanding and shrinking bubbles create a pumping action to move fluid round on a chip.

Kedzierski and fellow researcher Shaun Berry are studying the properties of various fluid mixes. The best way to control the deformation of droplets, Kedzierski says, is to add a surfactant, which changes surface tension. Plenty of surfactants are cheap and readily available; the researchers found several that work for their purposes, notably sodium dodecyl sulfate, used in many soaps and shampoos. Increasing or decreasing a concentration of salt also provides a way to manipulate the droplet’s properties.

A few other researchers have taken similar approaches to his and have already started companies based on electrowetting. Varioptic, of Lyon, France, is using a similar idea to make tiny liquid lenses for cell phones. These lenses zoom and focus by deforming a liquid—and can do so much more quickly than is possible by physically moving a piece of glass in conventional optics. Advanced Liquid Logic of Research Triangle Park, N.C., is trying to develop handheld medical diagnostic devices that incorporate an electrowetting microfluidics chip developed at Duke University. Kedzierski says his device is much smaller and uses significantly less voltage—4 or 5 volts as compared to 15 to 100 for other devices. The lower power consumption that results means that a device can run for much longer on a small battery, making it suitable for implant or for constant environmental monitoring. His device can be as small as 20 micrometers on a side, compared to a few millimeters, and thus use much smaller samples—as little as 200 femtoliters—and less of the expensive reagents to test them with. Smaller samples can also be moved faster. Having the basic concept in place means the next step will be to come up with a specific project and figure out what design and chemistry works best for that goal. For instance, if Kedzierski wants to make a remote sensor to monitor drinking water, he’ll need to enlist a chemist to figure out what oil or surfactant can be used as the engine to move the water along without mixing with the water or interacting with possible contaminants. Moving blood would pose a similar challenge, but one with a different solution, because of blood’s specific chemistry. He’ll also have to incorporate detection equipment that’s small enough to work in, say, a palm-sized device, perhaps using a tiny photodetector and laser. But he is confident that research dedicated to a specific project will pay off. “You just need to figure out what the different issues are,” he says, “and engineer around them.”