A. Farré, E. Martín-Badosa, M. Montes-Usategui
After an intense development of optical tweezers as a biophysical tool during the last decades, quantitative experiments in living cells have not found in this technique its best ally, due, in part, to the lack of a standard method to measure forces in complex environments. The existent alternatives either require complicated in situ calibrations, which make their use impossible in the study of dynamic processes, or they lack accuracy. Using an approach completely different from the most extended options, Steven Smith at Carlos Bustamante’s Lab at the University of Berkeley developed method based on the direct measurement of the momentum change of the trapping beam, which has the potential to become the standard for measuring forces in all kind of experiments. Measurements are performed regardless of the physical properties of the sample or the trapping laser, and they only depend on some parameters of the instrument design. As a consequence, the method does not need a continuous calibration and can be used in a wider range of experiments. However, its diffusion has been modest mainly because it requires a counter-propagating optical trapping system, which is difficult to implement and combine with other techniques. Here, we show how this method can be implemented in the more extended trapping configuration, optical tweezers, and how this relates to the well-known position detection technique, back-focal-plane interferometry. We finally discuss the potential of this method to perform experiments inside cells and also for commercial purposes, to make optical trapping available to non-experts.
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