UC Santa Barbara

This research investigates using transient grating spectroscopy (TGS), a time-resolved optical technique that can generate and detect acoustic wave signatures with micrometer-scale wavelengths, to mechanically characterize hydrogels and other inhomogeneous soft matter in the frequency range of hundreds of MHz. Hydrogels are of interest because they are easily fabricated, tunable, and are compatible with biological tissues. Despite extensive studies of the static mechanical properties of hydrogels, their high-frequency mechanical response remains less explored. I provide a systematic approach across a range of polymer groups, from silicone rubbers to hydrogels in different solvents, to decompose the effects of crosslinking, swelling, and entanglements on their bulk acoustic wave speeds, which can then be related to a material’s elastic modulus. The information obtained from these bulk acoustic wave signals were also analyzed to understand the differences that arise in damping within each material. Through comparison of TGS results to static and/or low-frequency results obtained with traditional characterization methods like indentation and rheometry, the mechanical properties of hydrogels differ significantly in dynamic and static conditions. In particular, higher frequencies lead to similar mechanical properties despite polymer content differences. These differences are attributed to different response time scales of the polymer matrix and the embedded water. These results represent the first step to developing TGS as a noncontact, nondestructive characterization method to monitor hydrogels and related biological systems with potential applications in biomedical engineering. This project also demonstrates how we can model and subsequently develop hydrogels with precisely tunable and dynamic mechanical properties. By mapping broadband behavior of these soft matters from 0 Hz to the hundreds of MHz observed with TGS, this information can then be extrapolated to biological structures with similar properties and compositions and provide more information on less understood dynamic behavior of these gels.