Microrheology of cells with magnetic force modulation atomic force microscopy

We propose a magnetic force modulation method to measure the stiffness and viscosity of living cells using a modified AFM apparatus. An oscillating magnetic field makes a magnetic cantilever oscillate in contact with the sample, producing a small AC indentation. By comparing the amplitude of the free cantilever motion (A(0)) with the motion of the cantilever in contact with the sample (A(1)), we determine the sample stiffness and viscosity. To test the method, the frequency-dependent stiffness of 3T3 fibroblasts was determined as a power law k(s)(f) = alpha + beta(f/f)(gamma) (alpha = 7.6 x 10(-4) N m(-1), beta = 1.0 x 10(-4) N m(-1), f = 1 Hz, gamma = 0.6), where the coefficient gamma = 0.6 is in good agreement with rheological data of actin solutions with concentrations similar to those in cells. The method also allows estimation of the internal friction of the cells. In particular we found an average damping coefficient of 75.1 mu N s m(-1) for indentation depths ranging between 1.0 mu m and 2.0 mu m.