Optical Tweezers in Viscoelastic Media – Niels Bohr Institute - University of Copenhagen

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Optical Tweezers in Viscoelastic Media

PhD defence by Mario Fischer 

With optical tweezers, a tracer particle of nano- to micrometer size is caught in the focus of a laser trap. Within the last years optical tweezers have become an important tool in biophysics and are nowadays used in many experiments. A recent application of optical tweezers are studies in viscoelastic media like the cytoplasm of a living cell or a filament solution. So far, in none of these studies, optical tweezers could be used for quantifying the applied forces. The reason is that there is no method for measuring forces in viscoelastic media <em>in situ </em>, as <em>ex situ </em> calibrations cannot be applied. Indeed, there is considerable interest in such force measurements. A force calibration is in particular important for investigating how forces are generated inside the cell.

 In my dissertation, I present a novel force calibration method for optical tweezers inside viscoelastic media. The method is based on the validity of the fluctuation-dissipation theorem. Knowledge about the trapped particle or the medium is not required for the application of the method, except for the medium's temperature. In the thesis, the complete theoretical framework of the method is derived. Simulations and experiments are presented, which have been performed in order to test the method.

Furthermore, within the dissertation, two investigations on the viscoelasticity of biological systems on the subcellular scale are presented, which employ microrheological and simulation techniques.