MIT researchers have developed hardware that uses electric fields to move droplets of chemical or biological solutions around a surface, mixing them in ways that could be used to test thousands of reactions in parallel. The researchers view their system as an efficient and cost-effective alternative to the microfluidic devices now commonly used in biological research.
"Traditional microfluidic systems use tubes, valves, and pumps, which are mechanical and break down all the time," says Udayan Umapathi, a researcher at the MIT Media Lab, who led the development of the new system, which is described in detail in the January issue of MRS Advances.
"Biology is moving toward more and more complex processes, and we need technologies to manipulate smaller- and smaller-volume droplets," Umapathi says. "Pumps, valves, and tubes quickly become complicated." With his new system, Umapathi explains, thousands of droplets could be deposited on the surface of his device, and they would automatically move around to carry out biological experiments.
The prototype uses a printed circuit board, which helps keep costs down. The chief technical challenge was to design a coating for the surface of the circuit board that would reduce friction, enabling droplets to slide across it, and that would prevent biological or chemical molecules from sticking to it, so that they won't contaminate future experiments. The circuit board is patterned with an array of electrodes. In the prototype, the researchers coat the board with a much denser array of tiny spheres, only a micrometer high, made from a hydrophobic material. Droplets skate across the tops of the spheres.
Because the device's surface is hydrophobic, droplets deposited atop it naturally try to assume a spherical shape. Charging an electrode pulls the droplet downward, flattening it out. If the electrode below a flattened droplet is gradually turned off, while the electrode next to it is gradually turned on, the hydrophobic material will drive the droplet toward the charged electrode.
If the droplet isn't moving rapidly enough, the system will automatically boost the voltage of the low-frequency signal. From the sensor signal, the system can also estimate a droplet's volume, which, together with location information, allows it to track a reaction's progress.