Optical deformability : micromechanics from cell research to biomedicine
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When a laser beam is incident on the surface of a transparent object, the optical surface forces, generated by the interaction of the light with the material, are directed away from the denser material and normal to its surface. This physical phenomenon can be used to probe the mechanical properties of dielectric materials. This optical deformability was exploited for the measurement of cellular elasticities with a novel micromanipulation device based on a two-beam fiber-based laser trap, the optical stretcher, which can generate surface forces from 1 pN–1 nN at frequencies ranging from static experiments to several MHz. The feasibility of accurately determining the elastic properties of biological cells with the optical stretcher was demonstrated by stretching human erythrocytes. A simple ray-optics treatment was used to explain and quantify the stresses on the surface of the cell, while their resulting deformation was recorded by video viii microscopy. Subsequent analysis of these data, modeling the erythrocyte as a thin shell, resulted in a cortical shear modulus of (1.3 ± 0.5) × 10–5 Nm–1, which is in excellent agreement with literature values. This result validates the ray-optics treatment. Higher developed, eukaryotic cells have an extensive threedimensional cytoskeleton throughout their cytoplasm, which renders them much more resistant to deformation. Using the optical stretcher, human neutrophils, normal and malignantly transformed mouse fibroblasts, and rat precursor cells were successfully stretched. These cells could be distinguished based on their optical deformability, with less differentiated cells showing lower elastic strength. The viability of the cells stretched was not jeopardized even when irradiated with 1.4 W of 780 nm laser light in both beams. By incorporation into a microfluidic flow chamber, the optical stretcher has the potential to measure up to several cells per minute, taking the speed of cell elasticity measurements to a new level and predisposing it for applications in biomedical diagnostic applications. As examples, the implications for cancer diagnosis and stem cell sorting are discussed in detail. Other applications of optical surface forces are envisioned.