The potential of Brillouin Spectroscopy for investigating the mechanical properties of hydrogels during dehydration

Hydrogels are attractive and versatile gel-based structures consisting of three-dimensional networks of hydrophilic polymers with unique properties that can retain a large amount of water and be tailor-designed according to specific requirements, and thus, suitable for numerous commercial areas (i.e., medicine, agriculture, electronics, cosmetics, etc.). However, since these materials can be very soft and flexible, mechanical characterization is challenging. This study explores the mechanical properties of a cellulose-based hydrogel using the optical, non-contact technique Brillouin light scattering spectroscopy (BLS). Two differently dried hydrogel samples were investigated and pre-characterized for their morphology by scanning electron microscopy (SEM) and their mechanical performance by tensile testing. Then, BLS spectra were recorded to characterize the material's stiffness, indicating that the structural arrangement of the material due to the drying procedure (the density) is the main factor limiting the material's strength. By further analyzing the effect of moisture and the changes during drying, we characterize the mechanical behavior of organic hydrogel films and sponges and their dehydration evolution to understand the liquid-solid transition.