Rapid and simple automatic trapping-force calibration system for optical tweezers

The automation of a rapid and simple trapping-force calibration system is desired for optical tweezers to measure biological forces. One simple calibration method, the water-dragging-force method, is to calibrate the trapping force against a given water dragging force with an image-processing technique. However, the conventional image-processing technique is too slow because of the time it takes for image recording, transferring, storing, and retrieving. The pattern recognition technique of our automated calibration method is rapid and simple, because it directly processes the image signal without recording, storing, and retrieving any image. In an experiment, we have combined the dragging force method with the real-time pattern recognition technique to calibrate the relationship between the trapping force at a given laser power and a displacement of a bead. We demonstrate the calibration results of the trapping forces on two target beads 10 and 6.2 μm in diameter, separately, at two laser powers, 5 and 10 mW. Each calibration procedure is finished in 5 min at a pattern recognition rate of 10 Hz with a spatial resolution of 75 nm. We believe that this technique is reliable and rapid enough to be applied to biological force measurement.