Mechanical Strains Induced in Osteoblasts by Use of Point Femtosecond Laser Targeting

A study demonstrating how ultrafast laser radiation stimulates osteoblasts is presented. The study employed a custom made optical system that allowed for simultaneous confocal cell imaging and targeted femtosecond pulse laser irradiation. When femtosecond laser light was focused onto a single cell, a rise in intracellular Ca2+ levels was observed followed by contraction of the targeted cell. This contraction caused deformation of neighbouring cells leading to a heterogeneous strain field throughout the monolayer. Quantification of the strain fields in the monolayer using digital image correlation revealed local strains much higher than threshold values typically reported to stimulate extracellular bone matrix production in vitro. This use of point targeting with femtosecond pulse lasers could provide a new method for stimulating cell activity in orthopaedic tissue engineering

Angular momentum in tightly focused beams with circular polarization

Abstract not available

The optical Hall effect in tightly focused light beams

We show that topological charge is not conserved in tightly focused beams. Conversion of spin angular momentum into orbital angular momentum occurs through a geometrical phase. We demonstrate a connection between the shape of the focal spot, and the recently discovered optical Hall effect

Space-variant geometrical phases in focused cylindrical light beams

We show that the depolarization caused when light is focused with a high-numerical-perture lens is accompanied by a space-variant geometrical phase. This phase results in the formation of modes with helicities and phase singularities that differ from those of the original beam. We show that this effect can be explained as a transverse shift of the rays, which is reminiscent of the recently discovered optical Hall-Magnus effect. Our results show that the asymmetric focal spot associated with the focus of linearly polarized light can be explained through geometrical effects

Angular momentum and geometrical phases in tight-focused circularly polarized plane waves

Calculations for the field at the focal plane of a high numerical aperture lens focusing a circularly polarized plane wave are presented. The calculations show that the polarization of the wave front in the focal plane is space varying, and that a geometrical phase is added to the wave front. Calculation of the angular momentum at the focal plane reveals that it depends on the numerical aperture of the lens. It is shown that this dependence is directly connected to the lens acting as a spatial filter

Probing mechanical heterogeneity in chondrocytes using passive microrheology

Characterising chondrocytes mechanics is important for understanding mechanotransduction. The bulk viscoelastic response of chondrocytes have been measured. However, their mechanical properties have not been resolved on a subcellular scale. Microrheolgy is a technique in which the mechanical properties of a material are found by analysing the Mean Square Displacement (MSD) of tracerparticles. Microrheology can resolve mechanical properties with subcellular resolution. However, a limitation of microrheology is that large sequences of images of the particles are required for accurate measurements. These are not always available due to particles moving out of focus and photobleaching. This paper presents a microrheology-based study on mechanical heterogeneity in chondrocytes using short sequences of images. Bovine Articular chondrocytes were seeded into agarose constructs [2]. The mitochondria were fluorescently labeled and imaged every 30 seconds for 15 minutes with a confocal microscope. Digital Image Correlation was used to quantify the motion of the mitochondria and their MSDs were calculated. An average MSD was found for every cell and the variability in mitochondrial motion was obtained by comparing the distribution of measured MSDs to the distribution of MSDs obtained from Monte-Carlo simulations of particles embedded within heterogeneous media. Measured mitochondrial motion was consistent with directed diffusion. The diffusion coefficient of the mitochondria varied by about 50% within single cells. Calculations based on statistical mechanics showed that directed diffusion can only occur if the cytoplasm behaves like a fluid on large time-scales. It is probable that this viscous behavior is connected to the non-equilibrium nature of the cytoskeleton

Combining optical tweezing and confocal microscopy for the study of cell mechanics

We have developed a system combining an optical tweezers and a confocal microscope for the study of cell mechanics. The system enables us to measure how mechanical forces are distributed within cells. This provides an important insight into the mechanisms by which how cells sense and respond to mechanical forces