Researchers from the Adolphe Merkle Institute’s Mechanoresponsive Materials and Polymer Chemistry and Materials groups have developed an innovative mechanophore that adds stress-sensing capability to existing hydrogels.

Mechanophores are molecular units that respond selectively to mechanical force by undergoing a structural change. Of particular interest are mechanophores that generate optical signals and report deformation, damage, and material failure of polymer materials. Mechanophores have been successfully deployed in a variety of polymers, including elastomers, thermoplastics, fibers, and composites. However, their application to hydrogels – highly hydrated materials that are much softer than other polymers and are used in applications ranging from contact lenses to soft robotics – has proven challenging.

In a project that was part of the NCCR Bio-inspired Materials, the AMI researchers solved this problem with a novel design approach inspired by biological materials, in which weak, reversible bonds are often exploited to create responsive functionality. First, they developed a new mechanophore based on a folded molecular loop held together by weak non-covalent interactions. Even very small forces can pull the loop open, switching on strong fluorescence. Then they attached water-soluble polymer chains to this motif, producing a water-soluble additive that can be simply mixed into a hydrogel of interest.  

“Intriguingly, our additive works without chemically connecting it to the hydrogel matrix,” says Iulia Scarlat, the PhD student who led the study. Instead, the hydrogel is simply produced in the presence of the additive, which becomes topologically trapped within the hydrogel matrix.  The AMI researchers demonstrated that hydrogel deformation efficiently transfers mechanical forces from the matrix to the embedded additive, thereby activating its fluorescence. As a result, the material visibly responds to stretching, compression, and shape changes induced by drying or further swelling, providing a direct optical readout of mechanical stress. “Put simply, the fluorescence turns on when the gel is stressed, giving us a visual way to see when and where it is being deformed. The brighter the fluorescence, the greater the deformation; when the gel relaxes, the fluorescence switches off again,” Scarlat adds.

Compared to previous approaches, this additive-based method offers high modularity, as the same additive works in chemically different gels without requiring custom synthesis for either. The low activation force of the new mechanophore further causes a very large fraction of the additives to be activated, resulting in strong fluorescence contrast. Finally, the design of the mechanophore renders its activation fully reversible, so that the fluorescence is switched off when the mechanical force is released.

The researchers now plan to expand the approach to other supramolecular mechanophores and use the sensors to investigate how features such as polymer concentration, crosslinking, and entanglement determine the transmission of mechanical stress through hydrogels.

The AMI researchers would ultimately like to turn these additives into a toolbox for probing how force is transmitted through very soft and complex materials. Because they can be incorporated without redesigning the underlying hydrogel, they would allow a comparison of very different material architectures and, in the longer term, exploration of mechanically active soft and biological systems.

Reference:

Scarlat, I.; Clough, J. M.; Weder, C. Supramolecular Mechanophore Additives Enable Mechanochromic Hydrogels. Angewandte Chemie International Edition 2026, e7699472. https://doi.org/10.1002/anie.7699472