Automated stiffness characterization of living tobacco BY2 cells using the Cellular Force Microscope

Understanding the process of cellular morphogenesis requires the characterization of local mechanical properties of living cells in situ. For this purpose, an automated microrobotic system, the Cellular Force Microscope (CFM) has been developed. Its suitability for single cell characterization has been reported in previous work and extensive use on characterization of both single cells and tissues has been demonstrated. A simple experimental method for the determination of the CFM´s accuracy, based on the comparison of the measured stiffness with the true stiffness of a calibrated SI-traceable reference artifact, is demonstrated. This method is a practical alternative to the complex complete uncertainty analysis of all the sensors involved in the stiffness measurement based on an SI-traceable reference artifact. We then use the CFM to characterize the stiffness of living Tobacco BY2 cells. The apparent stiffness is determined from the force indentation curves collected from living cells at forces ranging from 10 microNewtons to forces in which the cell slips away or ruptures. The results show a nonlinear increase in the apparent cell stiffness as the applied load increases. This observation combined with the reference measurements done on the calibrated stiffness standard dictate that the stiffness variation stems from the biological organism and not from the CFM.