Microrheology of complex systems and living cells using AFM.

Biological cells display viscoelastic mechanical properties that are a key factor in the regulation of cell processes, such as migration, adhesion and deformability. One interesting tool to probe the mechanical properties of living cells and other complex systems is the Atomic Force Microscopy (AFM). Rheological properties of the sample are obtained usually from the force-indentation (F-δ) measurements and by fitting with the Sneddon's modification of the Hertz model [1]. This model describes the relationship between the force applied by a stiff cone, a purely elastic indentation in a flat and soft sample and the Young's modulus E. AFM uses a flexible cantilever with a pyramidal tip (or sphere) at the end that will locally indent the viscoelastic material. Another approach is to superpose low-amplitude sinusoidal oscillations to an initial identation δ0 (polyacrylamide gel or cells). Then one can determine the complex shear modulus G* [2-3] using the following formula: