We developed a novel method to measure the rheological properties of single red blood cells (RBC) using the atomic force microscope (AFM). A spherical bead at the AFM cantilever tip compressed and relaxed the RBC. The force and displacement were converted into effective stress and strain. The impulse viscoelastic technique was used to compute the effective storage (E') and loss (E") moduli and phase lag (δ). Unfixed and fixed red blood cells were tested. Both cells were on glass coated with poly-l-lysine and then kept in phosphate buffered saline (PBS) until the experiment was finished. Measurements were done with height change and force up to 451nm and 64nN. The cells were found to be quite elastic, with phase lag on the order of 10-2 to 10-1 rad. Stepped changes in oscillation rate from 0.5Hz to 2.5Hz did not result in significant change in the measured results. To improve accuracy, we also design a bimaterial cantilever which consists of a gold layer on silicon with controlled thermal stresses such that the cantilever is curved. The curvature allows the root to fits the angle of the AFM head and the tip to be parallel to the substrate so that the RBC is squeezed between two parallel surfaces.
