Determination of Elastic Modulus of Gelatin Gels by Indentation Experiments

CZERNER, MARINA; SANCHEZ FELLAY, LUCAS; SUAREZ, MARIA P; FRONTINI, PATRICIA MARIA; FASCE, LAURA ALEJANDRA

Congreso; SAM 2013; 2013

Mechanical characterization of hydrogels is a challenging task because this class of solids is much softer than metals, ceramics orpolymers. The elastic modulus of hydrogels is within 10 0 -10 2 kPa range. Because they easily break and slump under their ownweight, 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 indentationconfiguration is very simple and of technological importance for food, tissue engineering and ballistic applications. It can beapplied at different length scales, allowing the determination of global and local material properties with high accuracy. Thegelatin hydrogel behavior is first calibrated by conventional uniaxial compression and low strain rheological measurements. Thematerial behaves as a hyperelastic solid with strain hardening capability at large strains and shows no dependence with frequencyin the linear viscoelastic range. It can be properly characterized by the First order Ogden material model. Indentation experimentsare carried out at macroscale and nanoscale using spherical and flat-ended cylindrical punches. Elastic contact solutions as wellas FEM simulations and inverse analysis accounting for hyperelasticity are used to extract the elastic modulus from theexperimental force-depth curves. Adhesion between punch and hydrogel influences the indentation response and affects theaccuracy of elastic modulus determination in a larger extent than the assumption of elastic behavior. Adhesion leads tooverestimation of elastic modulus values. The influence of adhesive forces increases with decreasing the length scale. Amarkedly decreasing trend of elastic modulus with increasing maximum applied load is observed at the nanoscale. A hybridmodel 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.