Measuring Cellular Biomechanics with AFM

Cells were detected by a contact mode PicoPlus AFM controlled by Picoview software (Agilent Technologies). Biomechanical properties were calculated from in situ force–distance curve measurements in medium at room temperature. The radius of silicon nitride tips was 20 nm. Its spring constant was calibrated to 0.10–0.11 N m⁻¹ by Thermo K Calibration (Agilent Technologies) and its corresponding deflection sensitivities were 45–50 nm V⁻¹. More than 10 cells were detected, collecting at least 15 force curves on the central area of different cells to avoid spurious detections.^{25 (link),26 (link)} Scanning Probe Image Processor (SPIP) software (Image Metrology) was used to calculate Young’s modulus and adhesion force by fitting the Sneddon variation of Hertz model.^{27 –29 (link)} The half cone-opening angle of tip was 36°, and cellular Poisson’s ratio was 0.5. The detection was accomplished within 2 hours (h) to approximate physiological conditions.
E_cell = 4F_(ΔZ)(1 − η_cell²)/3(ΔZ^1.5)tan θ, where E_cell: Young’s modulus; F: loading force; η_cell: Poisson ratio; ΔZ: indentation; θ: tip half cone opening angle.