Publications Cell Physics Division

The interaction of a Gaussian laser beam with a particle that is located off axis is a fundamental problem encountered across many scientific fields, including biological physics, chemistry, and medicine. For spherical geometries, generalized Lorenz-Mie theory affords a solution of Maxwell's equations for the scattering from such a particle. The solution can be obtained by expanding the laser fields in terms of vector spherical harmonics (VSHs). However, the computation of the VSH expansion coefficients for off-axis beams has proven challenging. In the present study, we provide a very viable, theoretical framework to efficiently compute the sought-after expansion coefficients with high numerical accuracy. We use the existing theory for the expansion of an on-axis laser beam and employ Cruzan's translation theorems [Q. Appl. Math. 20, 33 (1962)] for the VSHs to obtain a description for more general off-axis beams. The expansion coefficients for the off-axis laser beam are presented in an analytical form in terms of an infinite series over the underlying translation coefficients. A direct comparison of the electromagnetic fields of such a beam expansion with the original laser fields and with results obtained using numerical quadratures shows excellent agreement (relative errors are on the order of less than or similar to 10(-3)). In practice, the analytical approach presented in this study has numerous applications, reaching from multiparticle scattering problems in atmospheric physics and climatology to optical trapping, sorting, and sizing techniques. (C) 2011 Optical Society of America

Screening drugs for their specific impact on cell mechanics, in addition to targeting adhesion and proteolysis, will be important for successfully moderating migration in infiltrative disorders including cancer metastasis. We present 3D inverted colloidal crystals made of hydrogel as a realistic cell migration assay, where the geometry and stiffness can be set independently to mimic the tissue requirements in question. We show the utility of this 3D assay for drug screening purposes, specifically in contrast to conventional 2D migration studies, by surveying the effects of commonly used cytoskeletal toxins that impact cell mechanics. This assay allows studying large cell numbers for good statistics but at single-cell resolution.

The retina is an active soft material, in which mechanosensitive cells are thought to respond to the local mechanical heterogeneity they encounter during development and adult physiological functioning. The retina is also constantly exposed to mechanical stress with shear and traction forces acting at its inner surface. Consequences of these forces depend on the tissue's resistance to deformation, which is characterized by its stiffness. However, currently there is a lack of high-resolution data on retinal mechanical properties. Here, we used scanning force microscopy to determine the apparent elastic modulus K of the retinal inner surface along the length of the eye with sub-millimetre resolution, and compared characteristic K values of the retinal quadrants. We found that the inner retina is a mechanically inhomogeneous tissue. Most elastic moduli were in the range of 940 to 1800 Pa; significant differences were found between areas less than 50 mu m apart. To identify the origin of this mechanical inhomogeneity, we investigated the size and distribution of structures comprising the retinal surface: large cell bodies in the ganglion cell layer, nerve fibers, inner limiting membrane, and Muller cell endfeet. Our data suggest that the distribution of compliant nerve fiber bundles and stiff neuronal cell bodies contributes most to the mechanical properties of the inner retina. These data offer a basis for understanding cellular mechanoresistivity and -sensitivity in the retina as a mechanically active tissue, and they may help to understand mechanisms and consequences of a variety of retino-pathological processes and their surgical treatment.

Many scientific fields-including astronomy, climatology, and biology, among others-require the calculation of the scattered optical fields from multiparticle distributions. In the present study, we combine the established results for the scattering from clusters of homogeneous spheres and from single core-shell particles into a computationally tractable solution that is valid for irregular configurations of nonidentical, coated particles. The presented multiparticle scattering (MPS) model is based on a generalized Lorenz-Mie theory framework and the vector translation theorems for the vector spherical harmonics. We provide the MPS model in both the near and far fields, and for plane-wave and Gaussian beam illumination. A message-passing-interface protocol is used for the computational implementation of the model in a parallel computer program. The computer model is validated by verifying the accuracy of the vector translation theorems utilized in our theoretical methods and by qualitative comparison to existing multiparticle scattering data. We conclude by presenting the scattering profiles from several examples of particle distributions. This MPS model is a practicable method of calculating the optical fields arising in the scattering from particle aggregates and is straightforwardly extensible to arbitrary illumination and to more complex internal-particle structures, such as stratified spheres. Vital applications of this model include the exact computation of forces exerted on irregular objects in optical traps and the simulation of light propagation through biological tissues.