Abstract
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 physical and chemical properties similar to those of biological materials. 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 responses to mechanics. However, it was shown that biological tissues are not only elastic but also relax at different timescales. Cells can, indeed, perceive this dissipation and actually need it because it is a critical signal integrated with other signals to define adhesion, spreading and even more complicated functions. The mechanical characterization 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 in great detail. Here, we report that a simple relaxation test performed under well-defined conditions can provide all the necessary information for characterizing soft materials mechanically, by fitting the dissipation behavior with a generalized Maxwell model (GMM). The simple method was validated using soft polyacrylamide hydrogels and proved to be very useful to readily 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.
Highlights
Mechanical characterization has become a fundamental tool for understanding the behavior and organization of living systems [1]
The generalized Maxwell model (GMM) is advantageous over the simpler viscoelastic linear models, such as the standard linear solid (SLS) model since it considers the nonhomogeneous disorder at microscale: the material relaxation occurs according to a time distribution rather than at a single time
A significant difference between the value of the elastic modulus was obtained with the Hertz model between the data of the microindentation performed in detergent and the results computed from relaxation tests
Summary
Mechanical characterization has become a fundamental tool for understanding the behavior and organization of living systems [1]. In the case of model, artificial systems, such as soft porous hydrogels, these differences have been reported to arise from experimental manipulations at small length scales and misinterpretations in the models, because the contact area or surface interactions between the tip of the interrogation probe and the sample surface are underestimated It was shown, for instance, that the true mechanical properties of soft polyacrylamide hydrogels (poroelasticity and viscoelasticity) can be properly accounted for using indentation at small or large scales (indentation depth and tip diameter) by using indentation-relaxation tests and adjusting the duration of the experiment [33]. Because gels’ dissipative properties are critical in mechanobiology as recently shown [20], the relaxation tests provide more complete information than microindentation about the mechanics of soft gels as perceived by biological cells This simplifies the characterization of dissipative soft materials by employing a single force-time test to define the material’s behavior in a broad frequency domain without the need for tedious and complex analysis demanding dynamic tests.
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