To analyse the passive mechanical cell properties one applies small forces that are difficult to calibrate. We studied the stiffness of ventricular myocytes with a 3 mm long and 20 mm thin cell attached glass stylus S whose distance to the patch electrode was controlled by a motorized micro manipulator. The position of S did not fully follow the command, deviations due to bending of S were evaluated from confocal images (resolution of 0.2 mm). One could calculate the applied force form the spring constant of S. S was calibrated against a silicon cantilever comprising a piezosensitive strain gauge employed as a transferable force standard operating in the mN range at an uncertainty of sub-mN. The spring constant of S was estimated by probing S relative to the cantilever mounted to the microscope instead of the biological specimen.

Moving S vertically, we measured apparent elastic moduli that increased with the extent of pressure (0.7 kPa with 5 mm indentation, 2.2 kPa with 10 mm).