Tuning the Range of Polyacrylamide Gel Stiffness for Mechanobiology Applications

Abstract

Adjusting the acrylamide monomer and cross-linker content in polyacrylamide gels controls the hydrogel stiffness, yet the reported elastic modulus for the same formulations varies widely and these discrepancies are frequently attributed to different measurement methods. Few studies exist that examine stiffness trends across monomer and cross-linker concentrations using the same characterization platform. In this work, we use Atomic Force Microscopy and analyze force-distance curves to derive the elastic modulus of polyacrylamide hydrogels. We find that gel elastic modulus increases with increasing cross-link concentration until an inflection point, after which gel stiffness decreases with increasing cross-linking. This behavior arises because of the formation of highly cross-linked clusters, which add inhomogeneity and heterogeneity to the network structure, causing the global network to soften even under high cross-linking conditions. We identify these inflection points for three different total polymer formulations. When we alter gelation kinetics by using a low polymerization temperature, we find that gels are stiffer when polymerized at 4 °C compared to room temperature, indicating a complex relationship between gel structure, elasticity, and network formation. We also investigate how gel stiffness changes during storage over 10 days and find that specific gel formulations undergo significant stiffening (1.55 ± 0.13), which may be explained by differences in gel swelling resulting from initial polymerization parameters. Taken together, our study emphasizes the importance of polyacrylamide formulation, polymerization temperature, gelation time, and storage duration in defining the structural and mechanical properties of the polyacrylamide hydrogels.