A smart hydrogel coating from Georgia Institute of Technology creates “stick-slip” control of capillary action. By placing the material inside of glass microtubes, capillary forces are altered and draw water differently. The researchers' discovery could provide a new way to control microfluidic systems, including lab-on-a-chip devices.
Water entering a tube coated on the inside with the dry hydrogel must first wet the film and allow it to swell before it can proceed farther. The wetting and swelling take place with discrete steps in which the water meniscus first sticks and its motion remains arrested while the polymer layer locally deforms. The meniscus then rapidly slides for a short distance before the process repeats. The “stick-slip” process forces the water to move into the tube in a step-by-step motion.
The flow rate measured by the researchers in the coated tube is three orders of magnitude less than the flow rate in an uncoated tube. Instead of filling the capillary in a hundredth of a second, the hydrogel may cause the same capillary to be filled in tens of seconds.
The ability to turn the stick-slip behavior on and off with temperature may provide a new way to control the flow of water-based liquid in microfluidic devices, including labs-on-a-chip. The transition temperature can be controlled by varying the chemical composition of the hydrogel.
By locally heating or cooling the polymer inside a microfluidic chamber, one can either speed up the filling process or slow it down. The heating or cooling, done locally with lasers, tiny heaters, or thermoelectric devices, may allow precise timing of reactions in microfluidic devices and control the rate of drug release and product removal.
The team also wants to explore other “smart” polymers which change the flow rate in response to different stimuli, including the changing pH of the liquid, exposure to electromagnetic radiation, or the induction of mechanical stress.
