A Comprehensive Mechanical Testing of Polyacrylamide Hydrogels: The Impact of Crosslink Density

Mechanical properties are one of the most important characteristics of biomaterials for many different applications, including biomedicine. Soft biomaterials, such as hydrogels, are difficult to characterize by conventional mechanical testing, because their mechanical properties are much lower than required by conventional testing machines. In this work, we aimed to systematically study the mechanical behavior of a model soft material, polyacrylamide hydrogels, under different loading modes: tension, torsion, compression, and indentation. This allowed us to develop a comprehensive approach to the mechanical testing of soft materials. To overcome excessive compression and slippage of the hydrogel samples when fixed in the grips during tension, additional 3D-printed grips were designed. Digital image correlation was used to determine the Poisson's ratio of the hydrogels. The Young's modulus values obtained from all types of mechanical tests analyzed were highly correlated. However, for hydrogels with a low crosslinker concentration, 1-2%, tension-compression asymmetry was observed. Moreover, the results of the mechanical tests were verified in indentation tests, including analytical estimation, and full-scale and numerical experiments. We also discuss the limits of using a two-parameter Mooney-Rivlin model for fitting hydrogel uniaxial tension deformation curves, which was unstable for the hydrogels with 4 and 9% crosslinker concentration. The implemented approach provided a comprehensive analysis of the mechanical behavior of biomaterials. The elastic moduli for all hydrogels studied were in the range from 20 to 160 kPa, which corresponds well to human soft tissues, making them a promising material for application as tissue-mimicking phantoms.