Signaling when a material is about to fail

NCCR Bio-Inspired Materials researchers at the University of Fribourg’s Adolphe Merkle Institute have created a force-sensing probe, called a mechanophore, that rapidly and reversibly changes its fluorescence color when activated by a strain.

Molecules that change color or fluorescence characteristics in response to mechanical stress can warn of damage to materials such as polymers used in construction, potentially avoiding catastrophic failures. A new mechanophore could help researchers study the effects of mechanical stress on materials. In the future, it may also serve as an early warning for material failure, says NCCR Principal Investigator Prof. Christoph Weder, who co-led the study. Several physiological processes, including hearing, rely on a phenomenon called mechanotransduction, which translates mechanical stimuli into a chemical response. Inspired by such behavior of biological systems, Weder’s team has been exploring ways of making stress sensors out of force-sensitive molecules.

Past efforts resulted in molecules that did not return to their original state or were extremely difficult to produce. This time around, the researchers connected two fluorescent molecules, called perylenes, with a spacer. Then, they incorporated the perylenes in the center of polymer chains that make up a rubbery material known as an elastomer. The team tested their probe by subjecting the elastomer to different levels of mechanical stress. Perylenes act like molecular magnets, so in the absence of stress, the attractive interactions between perylene molecules leads to the formation of loops and the production of orange fluorescence.

However, when the researchers applied force to stretch the elastomer, the perylene loops untangled, and their fluorescence changed to green. “If the loops experience a sufficiently high force, the perylene magnets are pulled apart,” Weder explains. The color change took place instantaneously and reflected how much stress the elastomer had experienced. The effect was reversible, with the fluorescence reverting to orange as soon as the force was removed. Producing the new mechanophore only involves three steps and the probe can be incorporated into several polymers, including rigid materials such as Plexiglas, Weder says. The findings were published in the journal Angewandte Chemie. Next, Weder’s team plans to look at the response of other polymers to investigate failure mechanisms. “If you look at rigid polymers, they typically fail because they develop a microscopic defect — a crack,” he adds. “Our question is, how does the crack start in the first place, and can we predict where it will develop?”

Although the new mechanophore is just a research tool for now, in the future it may have practical applications in all sorts of materials. For example, understanding how failures happen at the molecular level may inform the creation of more resistant polymers. “We are also interested in healable polymers, so in principle one could equip polymers with the ability to indicate failure and render the polymers healable,” Weder says. The new mechanophore may also enable biologists to measure forces on molecular scales and understand how cells react to mechanical stress. “Biologists also use force probes, but so far there has been little interaction between polymer science and biology with respect to such probes,” Weder points out. “In the next phase of the NCCR Bio-Inspired Materials, we would like to bridge this gap.”

Reference: Traeger, H.; Sagara, Y.; Kiebala, D. J.; Schrettl, S.; Weder, C. Folded Perylene Diimide Loops as Mechanoresponsive Motifs. Angewandte Chemie International Edition 2021, 60 (29), 16191–16199.https://doi.org/10.1002/anie.202105219

Author: Giorgia Guglielmi