May 21st, 2015Research highlightsJuan Sebastian Totero Gongora 0 Comments


In recent years, the interest in the study of the interaction between light and biological matter has largely increased. While optics has historically been mainly used for imaging applications, light has now become a tool of manipulation and direct interaction with biological samples. It is well known that when light impinges on an object it exerts a small force on it, whose values are in the range of tens of pN for micro-sized specimens. Such optical force has two fundamental components: the gradient force, which is related to the electrostatic interaction between the polarization charges in the object and the field distribution itself, and the scattering force, due to the photon momentum transfer. The scattering part of the optical force depends on the wavenumber distribution of both the incident and scattered fields. Application examples of optical forces are found in optical tweezers, where a focused light beam is exploited to trap and transport micro/nano-particles. In biology, optical tweezers are usually employed as a non-invasive technique to manipulate cells and their sub-cellular components. While optical tweezers usually employ gradient forces, scattering forces have been recently investigated and a series of counterintuitive dynamics has been demonstrated, including fully optical tractor beams. These techniques, including more pioneering idea such as e.g., cell optical sorting and in-vivo manipulation that are still under investigation, require a precise understanding on how the optical pressure affects biological organisms. This calls for the development of precise numerical methods, whose development will be crucial for applications of the aforementioned techniques. In this respect, ab-initio techniques represent a very important tool that can provide quantitative answers to the problem. Among the different approaches, the Finite Differences Time Domain (FDTD) method, yields a very flexible technique to study any experimental condition, as it is based on the numerical solution of Maxwell equations with no approximation. Despite FDTD techniques have been applied in biology to simulate scattering from tissues and cells, nothing has been done in the direct computation of optical forces on bio samples.

In this work, we computed the optical forces acting on biological samples using FDTD simulations. In our FDTD approach, which explicitly considers the dispersive properties of the sample and of the environment, optical forces are expressed in terms of the Maxwell Stress Tensor formulation. As an interesting case study, we measured the variation of optical force on a Red Blood cell when the morphology and the internal structure are changed, as in the case of RBC infected by Plasmodium falciparum. Our results show that the optical force changes significantly with the morphology of the cell, allowing to sort RBC according to their disease progression.

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