Viscoelastic relaxation of fibroblasts over stiff polyacrylamide gels by atomic force microscopy

Cell viscoelasticity provides mechanistic insights into fundamental biological functions and may be used in many applications. Using atomic force microscopy in time and frequency domains, we find a peculiar behavior in the viscoelastic relaxation of L929 mouse fibroblasts that may help understand how cells perceive and adapt to distinct extracellular environments. They are stiffer when cultured over polyacrylamide gels (20-350 kPa) than over glass-bottom Petri dishes. The stiffness enhancement of cells over gels is attributed to a significant increase in the low-frequency storage shear moduli compared to the loss moduli, indicating that gels induce a remodeling of cytoskeleton components that store elastic energy. Morphological alterations are then expressed by the fractal dimension measured on confocal images of the f-actin cytoskeleton. We show a direct scaling between the fractal dimension and the substrate's rigidity.