Making ‘Smarter’ Use of ‘Smart’ Gels

dolbowzauscher.jpgOnce considered something of a laboratory novelty, ‘smart’ gels–— synthesized from polymers that can undergo dramatic transformations in response to changes in their surroundings–— are now poised to become integral mechanical components and sensors in the increasingly tiny devices of the future. Through a combination of computational and experimental efforts, a team of researchers at Duke's Pratt School of Engineering aims to make the process of smart gel engineering even smarter.

“These materials exhibit dramatic volume changes in response to small changes in external stimuli, such as temperature, solvent concentration, and light,” said John Dolbow, a professor of civil and environmental engineering at Duke’s Pratt School and a member of the Duke Computational Mechanics Lab.

“Smart gels can have a remarkable range of motion and a special ‘memory’ that allows them to return to their original state,” added Stefan Zauscher, professor of mechanical engineering and material science, also at Duke. “The idea is that these gels could be used as robotic fingers, as mechanical actuators, as drug carriers--the possibilities are vast.”

Capturing Shape-Shifters

Already, the shape-shifting gels have been successfully integrated by others into molecular recognition sensor surfaces, optical switches, drug carriers and microfluidic devices, according to the researchers. Yet, there is still a lot about the gels’ complex behaviors and abilities that remains mysterious, leaving their designers today to rely on trial and error.

The fluid and elastic gels--consisting of a web of cross-linked molecular chains that can capture and hold water--can easily be synthesized into any shape, Dolbow said. But, he added, the way in which they swell and collapse can be difficult to predict and control. Therefore, improvements in the gels’ design and optimal use will require a better understanding of their properties--and that’s where tools often applied by civil engineers can help.

“Historically, civil engineers in particular have worked with concrete and steel,” Dolbow said. “But a lot of methods that have been developed for that class of materials can be applied to a broader class of soft and wet materials.”

The activation of the stimulus-responsive gels proceeds via motion of the interface between swelled and collapsed regions, he explained. “Our computational efforts are focused on developing numerical methods to robustly simulate the evolution of this interface over time.”

Back to Basics

Accurate models of smart gels’ behavior will require information obtained through careful experimental measurements of the material in action. Last year, Dolbow and his colleagues reported a study in The International Journal of Solids and Structures on the kinetics of a smart gel as it swelled in response to changes in temperature. More recently, Dolbow and Zauscher conducted experiments to describe how friction between two gels changes as they transition from one phase to another–— a proposition that proved challenging.

“It’s a simple problem,” Zauscher said. “But imagine trying to make measurements of friction on slippery Jello.” The situation is made all the more difficult by measurement tools designed for more traditional, larger-scale materials.

Nonetheless, the Duke team reported earlier this year in Langmuir results revealing that the gels exhibit reversible and possibly tunable changes in friction. Gels in the collapsed conformation showed significantly more friction than swollen gels, a change they traced back to differences in the roughness, adhesive interactions and chain entanglements at the surface. Those properties, triggered by environmental stimuli, “may have a significant impact on the design of coatings for biosensors and for actuation devices that exploit the unique mechanochemical properties of stimulus-responsive hydrogels,” they concluded.

“It shows you could build a device with a large range of frictional properties,” Dolbow said. “You can tune that transition from slippery to sticky with changes in temperature, for example.”

Many challenges lie ahead. Ultimately, researchers will need to create models that incorporate the interrelated phenomena that occur within the gels, accounting for not only their mechanics but also their chemistry. And while traditional mechanical models offer great potential for insight into the gels, “we always have to be careful to consider what makes these materials different from concrete and steel,” Dolbow said.