Mechanical characterization of soft biomaterials: which time and spatial scale to choose?

The mechanical properties of soft gels hold significant relevance in biomedicine and biomaterial design, including the development of tissue engineering constructs and bioequivalents. It is important to adequately characterize the gel's mechanical properties since they play a role both in the overall structural properties of the construct and the physiological responses of cells. The question remains which approach for the mechanical characterization is most suitable for specific biomaterials. Our investigation is centered on the comparison of three types of gels and four distinct mechanical testing techniques: shear rheology, compression, microindentation, and nanoindentation by atomic force microscopy. While analyzing an elastic homogeneous synthetic hydrogel (a polyacrylamide gel), we observed close mechanical results across the different testing techniques. However, our findings revealed more distinct outcomes when assessing a highly viscoelastic gel (Ecoflex) and a heterogeneous biopolymer hydrogel (enzymatically crosslinked gelatin). To ensure precise data interpretation, we introduced correction factors to account for the boundary conditions inherent in many of the testing methods. The results of this study underscore the critical significance of considering both the temporal and spatial scales in mechanical measurements of biomaterials. Furthermore, they encourage the employment of a combination of diverse testing techniques, particularly in the characterization of heterogeneous viscoelastic materials such as biological samples. The obtained results will contribute to the refinement of mechanical testing protocols and advance the development of soft gels for tissue engineering.