Determining the Elastic Modulus of Microgel Particles by Nanoindentation

The mechanical properties of materials and cells are key factors for many processes in biofabrication. Nanoindentation using atomic force microscopy represents an important technique to quantify the Young’s elastic modulus in a locally resolved manner or for single microgel particles and cells, respectively. Here, we address the question of the best-suited indenter geometry and continuum model for contact mechanics to describe the nanoindentation of microgels. Two different microgel model systems have been synthesized using microfluidics with a very narrow size distribution of the particles: poly(acrylamide) and ene-functionalized polyoxazoline/thiol-functionalized hyaluronic acid. The respective microgel particles have been characterized by up to six different types of indenter geometries, including spherical, parallel plate, and cone. Moreover, the influence of experimental parameters, such as indentation depth and velocity, on the resulting Young’s modulus has been studied. Compared to the Hertz model, the simplified double contact model (SDC model) provided a better description of the indentation process and a narrower distribution of Young’s moduli with respect to the different indenter geometries. By numerical simulation of the indentation process, we demonstrated that the remaining variation of the Young’s moduli is attributed to adhesion. The adhesive force between a particle and the substrate led to a prestress, which is similar to that of particle deformation by the substrate as accounted for by the SDC model. This effect varied in strength with the indenter geometries and can contribute significantly to the experimentally observed variation of Young’s moduli for different indenter geometries. Importantly, these results can also be extended to the indentation of single living cells.