CONICET | Buscador de Institutos y Recursos Humanos

Mechanical characterization of hydrogels is a challenging task because this class of solids is much softer than metals, ceramics or polymers. The elastic modulus of hydrogels is within 100-102 kPa range. Because they easily break and slump under their own weight, conventional tensile and bending tests are not suitable configurations to assess elastic modulus. This work reports on thedetermination of elastic modulus of a gelatin hydrogel by means of depth sensing indentation experiments. The indentation configuration is very simple and of technological importance for food, tissue engineering and ballistic applications. It can be applied at different length scales, allowing the determination of global and local material properties with high accuracy. The gelatin hydrogel behavior is first calibrated by conventional uniaxial compression and low strain rheological measurements. The material behaves as a hyperelastic solid with strain hardening capability at large strains and shows no dependence with frequency in the linear viscoelastic range. It can be properly characterized by the First order Ogden material model. Indentation experiments are carried out at macroscale and nanoscale using spherical and flat-ended cylindrical punches. Elastic contact solutions as well as FEM simulations and inverse analysis accounting for hyperelasticity are used to extract the elastic modulus from the experimental force-depth curves. Adhesion between punch and hydrogel influences the indentation response and affects the accuracy of elastic modulus determination in a larger extent than the assumption of elastic behavior. Adhesion leads to overestimation of elastic modulus values. The influence of adhesive forces increases with decreasing the length scale. A markedly decreasing trend of elastic modulus with increasing maximum applied load is observed at the nanoscale. A hybrid model based on Hertz elastic contact solution and Johnson-Kendal-Roberts model for adhesion is used to determine elasticmodulus. This model yields an elastic modulus in good agreement with that obtained from uniaxial compression test.