Biomechanical characterisation of single-cell mechanics and cell-to materials adhesion using fluidic force microscopy

Abstract

The mechanics of living cells are vital for many of their functions, including mechanotransduction, migration, and differentiation, and changes in cell mechanics are related to disease progression. On the other hand, cell-matrix adhesion is important for the patterning, integrity and homeostasis of tissues, and may provide a target for therapy. Cell mechanics and the adhesion between the cells and matrix are also important for tissue engineering. Therefore, it is important to study cell mechanics and cell-to-material adhesion. Atomic force microscopy (AFM) has been widely adopted for the mechanical characterisation of many cell types. Very recently, fluidic force microscopy has been developed to enable rapid measurements of cell adhesion. However, the simultaneous characterisation of cell-to-material adhesion and the viscoelastic properties of cell is challenging. This study presents a new approach to the simultaneous determination of these properties for single cells, using fluidic force microscopy to investigate cell mechanics and cell-to-material adhesion and the relationship between them. The new method was initially developed to study MCF-7 cells grown on tissue culture-treated polystyrene surfaces, in which case the flat punch contact model was used. It was found that the adhesive force and adhesion energy for each cell are correlated. Well-spread cells tend to have stronger adhesion, which may be due to the greater area of contact between cellular adhesion receptors and the surface. However, the viscoelastic properties of MCF-7 cells cultured on the same surface appear to have little dependence on cell shape. This approach was subsequently adapted to examine how polydimethylsiloxane (PDMS) with different levels of stiffness may affect the cell mechanics and cell-material adhesion of MCF-7 cells and the corresponding cells of the healthy MCF-10A cell line. To further study if cell-material adhesion may be correlated with cell migration rate, the wound healing test (scratching c test) and single-cell tracking were performed. It was found that, as the underlying substrate becomes softer, both types of cells exhibit lower adhesion to the substrate and higher migration speeds. For the MCF-7 cells, the modulus decreased and the relaxation time constant increased while for MCF-10A cells these two parameters did not significantly change. Furthermore, finite element models were developed to examine the reliability of the flat punch contact model in representing the contact between the atomic force microscope’s cantilever and the cell. The simulation results have confirmed that the flat punch contact model provides a reasonably good approximation. In addition, finite element models also reveal the effect of underlying substrate and cell morphology on the apparent cell moduli, which can affect the interpretation of experimental results.