Determination by Relaxation Tests of the Mechanical Properties of Soft Polyacrylamide Gels Made for Mechanobiology Studies

Following the general aim of recapitulating the native mechanical properties of tissues and organs in vitro, the field of materials science and engineering has benefited from recent progress in developing compliant substrates with similar physical and chemical properties. In particular, in the field of mechanobiology, soft hydrogels can now reproduce the precise range of stiffnesses of healthy and pathological tissues to study the mechanisms behind cell response to mechanics. However, it was shown that biological tissues are not only elastic but also relax at different timescales. Cells can indeed perceive and actually need this dissipation because it is a critical signal integrated with other signals to define adhesion, spreading and even more complicated functions. The mechanical definition of hydrogels used in mechanobiology is however commonly limited to the elastic stiffness (Young’s modulus) and this value is known to depend greatly on the measurement conditions that are rarely reported. Here, we report that a simple relaxation test performed under well defined conditions can provide all the necessary information to characterize soft materials mechanically, by fitting the dissipation behavior with a generalized Maxwell model (GMM). The method was validated using soft polyacrylamide hydrogels and proved to be very useful to unveil precise mechanical properties of gels that cells can sense and offer a set of characteristic values that can be compared with what is typically reported from microindentation tests.