Elastic Modulus Measurement of Hydrogels

Abstract

Hydrogels have been employed for a wide variety of applications, and their mechanical properties need to be modulated based on the applications. In partic- ular, the Young’s modulus, or elastic modulus, of hydrogels is a critical property for understanding their mechanical behaviors. In principle, the Young’s modulus of a hydrogel can be measured by finding a relationship between a force applied to the hydrogel and the resultant deformation of the hydrogel. On a macroscale, Young’s modulus is usually obtained by measuring the stress-strain curves of a hydrogel specimen through the compression method or the tensile method and then finding the slope of the curve. Also, the shear modulus of a hydrogel is measured using a rheometer with parallel plates and then converted into Young’s modulus considering Poisson’s ratio. On a mesoscale, the elastic modulus can be measured by the imaging-based indentation methods which measure the inden- tation depth of a hydrogel sample deformed by a static ball indenter on the gel. The measured indentation depth is converted to the Young’s modulus of the hydrogel via a contact mechanics model. The mesoscale indentation method and pipette aspiration method are also available. On a microscale, the elastic modulus is usually measured using the atomic force microscopy (AFM)-based indentation method. A hydrogel specimen is locally indented by a sharp or colloidal tip of an AFM probe, and the Young’s modulus of the hydrogel is obtained by fitting an appropriate indentation model against the recorded force- distance curves. An appropriate elastic modulus measurement method needs to be chosen depending on the application, length scale and expected elastic property of the hydrogels.