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29 March 2005 Determination of femto Newton forces and fluid viscosity using optical tweezers: application to Leishmania amazonensis
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The objective of this research is to use the displacements of a polystyrene microsphere trapped by an optical tweezers (OT) as a force transducer in mechanical measurements in life sciences. To do this we compared the theoretical optical and hydrodynamic models with experimental data under a broad variation of parameters such as fluid viscosity, refractive index, drag velocity and wall proximities. The laser power was measured after the objective with an integration sphere because normal power meters do not provide an accurate measurement for beam with high numerical apertures. With this careful laser power determination the plot of the optical force (calculated by the particle displacement) versus hydrodynamic force (calculated by the drag velocity) under very different conditions shows an almost 45 degrees straight line. This means that hydrodynamic models can be used to calibrate optical forces and vice-versa. With this calibration we observed the forces of polystyrene bead attached to the protozoa Leishmania amazonensis, responsible for a serious tropical disease. The force range is from 200 femto Newtons to 4 pico Newtons and these experiments shows that OT can be used for infection mechanism and chemotaxis studies in parasites. The other application was to use the optical force to measure viscosities of few microliters sample. Our result shows 5% accuracy measurements.
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Adriana Fontes, Selma Giorgio, Archimedes B. de Castro Jr., Vivaldo M. Neto, Liliana de Y. Pozzo, Gustavo P. Marques, Luiz C. Barbosa, and Carlos L. Cesar "Determination of femto Newton forces and fluid viscosity using optical tweezers: application to Leishmania amazonensis", Proc. SPIE 5699, Imaging, Manipulation, and Analysis of Biomolecules and Cells: Fundamentals and Applications III, (29 March 2005);

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