Adaptable boronate ester hydrogels with tunable viscoelastic spectra to probe timescale dependent mechanotransduction

Cells are capable of sensing the differences in elastic and viscous properties (i.e., the 'viscoelasticity') of their tissue microenvironment and responding accordingly by changing their transcriptional activity and modifying their behaviors. When designing viscoelastic materials to mimic the mechanical properties of native tissue niches, it is important to consider the timescales over which cells probe their microenvironment, as the response of a viscoelastic material to an imposed stress or strain is timescale dependent. Although the timescale of cellular mechano-sensing is currently unknown, hydrogel substrates with tunable viscoelastic spectra can allow one to probe the cellular response to timescale dependent mechanical properties. Here, we report on a cytocompatible and viscoelastic hydrogel culture system with reversible boronate ester cross-links, formed from pendant boronic acid and vicinal diol moieties, where the equilibrium kinetics of esterification were leveraged to tune the viscoelastic spectrum. We found that viscoelasticity increased as a function of the boronic acid and vicinal diol concentration, and also increased with decreasing cross-linker concentration, where the maximal loss tangent achieved with this system was 0.55 at 0.1 rad s^-1. Additionally, we found that the cis-vicinal diols configuration altered the viscoelastic spectra, where a tan δ peak occurred at ~1 rad s^-1 for hydrogels functionalized with boronic acid, while an additional peak formed at ≥10 rad s^-1 for hydrogels functionalized with both boronic acid and cis-vic-diols. In experiments with NIH-3T3 fibroblasts cultured on these hydrogels, the projected cell area and nuclear area, focal adhesion tension, and subcellular localization of YAP/TAZ were all found to be lower for cells cultured on the viscoelastic hydrogels compared to elastic hydrogels with a similar storage modulus. Despite these differences, there was not a statistically significant relationship between the frequency dependent viscoelastic material properties characterized in this study and cellular morphologies, focal adhesion tension, or the subcellular localization of YAP. While these results demonstrate that mechanotransduction pathways are affected by viscoelasticity, they also suggest that these mechanotransduction pathways are not particularly sensitive to the frequency dependent viscoelastic material properties from 0.1 to 10 rad s^-1.