Optical traps are nowadays quite ubiquitous in biophysical and biological studies. The term is often used synonymously
with optical tweezers, one particular incarnation of optical traps. However, there is another kind of optical trap consisting
of two non-focused, counter-propagating laser beams. This dual-beam trap predates optical tweezers by almost two
decades and currently experiences a renaissance. The advantages of dual-beam traps include lower intensities on the
trapped object, decoupling from imaging optics, and the possibility to trap cells and cell clusters up to 100 microns in
diameter. When used for deforming cells this trap is referred to as an optical stretcher. I will review several applications
of such traps in biology and medicine for the detection of cancer cells, sorting stem cells, testing light guiding properties
of retinal cells and the controlled rotation of cells for single cell tomography.
Inverse light scattering methods have been applied by several groups as a means to probe cellular structure in both
clinical and scientific applications with sub-wavelength accuracy. These methods determine the geometric properties
of tissue scatterers based on far field scattering patterns. Generally, structure is determined by measuring scattering
over some range of angles, wavelengths, or polarizations and then fitting the observed data to a database of
simulated scattering selected from a range of probable geometries. We have developed new light scattering software
based on the T-matrix method that creates databases of scattering from spheroidal objects, representing a substantial
improvement over Mie theory, a method limited to simulating scattering from spheres. The computational cost of
the T-matrix method is addressed through a simple but massively parallel program that concurrently simulates
scattering across hundreds of PCs. We are exploring the use of these T-matrix databases in inverting interferometric
measurements of angle-resolved scattering from spheroidal cell nuclei using a technique called angle-resolved low
coherence interferometry (a/LCI). With a/LCI, we have previously distinguished between healthy and dysplastic
tissue in both cell cultures and in ex vivo rat and hamster tissue using Mie theory to measure nuclear diameter. We
now present nuclear volume and spheroidal aspect ratio measurements of unstained, living MCF7 cells using the
improved T-matrix database to analyze a/LCI data. We achieve measurement accuracy equivalent to conventional
image analysis of stained samples. We will further validate the approach by comparing experimental measurements
of scattering from polystyrene microspheroids, and show that the T-matrix is a suitable replacement for Mie theory
in ex vivo tissue samples.
A well-established method of assessing structure is inverse light scattering analysis. With inverse
light scattering analysis, the measured scattering properties of a scatterer(s) are associated with the most
probable scattering distribution predicted by an appropriate light scattering model. One commonly used
light scattering model is Mie theory, the electromagnetic theory of spherical scattering. Although Mie
theory is a spherical scattering model, it has been used for deducing the geometry of spheroidal scatterers,
which are important for studies of biological cell structure. The angle-resolved low coherence
interferometry (a/LCI) technique is one method of Mie theory - based inverse light scattering analysis that
has been used to evaluate biological structure both ex vivo and in vitro. In the present study, we examine
the ability of a/LCI to assess structure, geometry, and cellular organization in ways that will further enable
the study of function in biological materials.
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