Tissues in the human body are predominantly made of cells and the extracellular matrix (ECM). The elastic modulus (Young’s Modulus E) of these tissues varies over many orders of magnitude. Epithelial and other cells in soft tissues adhere to a microenvironment whose stiffness typically falls in the kilopascal range. For example, the elastic modulus of brain is several hundred pascal, whereas that of muscle is more than 10 kilo pascals and that of cartilage is in the range of megapascals. Flexible substrates such as polyacrylamide and silicone gels have proven to be excellent biomimetic substrates for cell culture in vitro. Several methods have been used to measure the stiffness of flexible substrates, including atomic force microscopy, macroscopic deformation of whole samples upon stretching, rheology and indentation using spheres and spherically tipped micro-indentors. While each technique has its own advantages and disadvantages, indentation with a sphere is an especially simple yet fairly accurate method that only requires access to a widefield fluorescence microscope. Recently, confocal microscopy has also been used for an elegant characterization of the indentor method. To characterize the Young’s moduli of isotropic linear elastic substrates, we present here a simple method that only employs a widefield fluorescence microscope for the actual stiffness testing. Common availability of this equipment, use of suitable indentors and methods to restrict fluorescent marker beads to the substrate top surface, enables this method potentially widely accessible.https://digitalcommons.odu.edu/engineering_batten/1009/thumbnail.jp
