Showing papers on "Optical stretcher published in 2010"

Femtosecond laser fabricated monolithic chip for optical trapping and stretching of single cells.

Abstract: We report on the fabrication by a femtosecond laser of an optofluidic device for optical trapping and stretching of single cells. Versatility and three-dimensional capabilities of this fabrication technology provide straightforward and extremely accurate alignment between the optical and fluidic components. Optical trapping and stretching of single red blood cells are demonstrated, thus proving the effectiveness of the proposed device as a monolithic optical stretcher. Our results pave the way for a new class of optofluidic devices for single cell analysis, in which, taking advantage of the flexibility of femtosecond laser micromachining, it is possible to further integrate sensing and sorting functions.

Detection of Plasmodium falciparum-infected red blood cells by optical stretching

Abstract: We present the application of a microfluidic optical cell stretcher to measure the elasticity of malaria-infected red blood cells. The measurements confirm an increase in host cell rigidity during the maturation of the parasite Plasmodium falciparum. The device combines the selectivity and sensitivity of single-cell elasticity measurements with a throughput that is higher than conventional single-cell techniques. The method has potential to detect early stages of infection with excellent sensitivity and high speed.

Dynamic ray tracing for modeling optical cell manipulation

Abstract: Current methods for predicting stress distribution on a cell surface due to optical trapping forces are based on a traditional ray optics scheme for fixed geometries. Cells are typically modeled as solid spheres as this facilitates optical force calculation. Under such applied forces however, real and non-rigid cells can deform, so assumptions inherent in traditional ray optics methods begin to break down. In this work, we implement a dynamic ray tracing technique to calculate the stress distribution on a deformable cell induced by optical trapping. Here, cells are modeled as three-dimensional elastic capsules with a discretized surface with associated hydrodynamic forces calculated using the Immersed Boundary Method. We use this approach to simulate the transient deformation of spherical, ellipsoidal and biconcave capsules due to external optical forces induced by a single diode bar optical trap for a range of optical powers.

Inversion of gradient forces for high refractive index particles in optical trapping

Abstract: The unexpected fact that a spherical dielectric particle with refractive index higher than the surrounding medium will not always be attracted towards high intensity regions of the trapping beam is fully demonstrated here using a simple ray optics approach. This unusual situation may happen due to the inversion of gradient forces, as shown here. Therefore, conventional schemes, such the one based on the use of two counter-propagating beams to cancel the scattering forces, will fail to trap the particle. However, effective trapping still can be obtained by adopting suitable incident laser beams.

Cellular deformation and intracellular stress propagation during optical stretching.

Abstract: Experiments have shown that mechanical stress can regulate many cellular processes. However, in most cases, the exact regulatory mechanisms are still not well understood. One approach in improving our understanding of such mechanically induced regulation is the quantitative study of cell deformation under an externally applied stress. In this paper, an axisymmetric finite-element model is developed and used to study the deformation of single, suspended fibroblasts in an optical stretcher in which a stretching force is applied onto the surface of the cell. A feature of our physical model is a viscoelastic material equation whose parameters vary spatially to mimic the experimentally observed spatial heterogeneity of cellular material properties. Our model suggests that cell size is a more important factor in determining the maximal strain of the optically stretched fibroblasts compared to the thickness of the actin cortical region. This result could explain the higher deformability observed experimentally for malignant fibroblasts in the optical stretcher. Our model also shows that maximal stress propagates into the nuclear region for malignant fibroblasts whereas for normal fibroblasts, it does not. We discuss how this may impact the transduction of cancer signaling pathways.

Dual-beam laser traps in biology and medicine: When one beam is not enough

Abstract: 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.

An enhanced far-field-diffracted optical trap for cold atoms or molecules on an optical chip

Abstract: An enhanced or improved far-field-diffracted optical dipole trap with a red or blue detuning for cold atoms or molecules on an optical chip is proposed, and our trap scheme is formed by an optical system composed of a binary phase plate and a circular aperture illuminated by a plane light wave. The relative radial and axial intensity distributions of the far-field optical trap and its optical potentials for 87Rb atoms are calculated, and the dependences of the relative intensity of the far-field optical trap and its well depth on the modulated phase φ of the phase plate are studied. Also, the feasibility of our enhanced far-field optical trap is analyzed, and some potential applications of our optical trap are briefly discussed. Our research shows that some geometric and optical parameters of the far-field optical trap can be greatly improved, and the evolution of the far-field optical trap from a red-detuned optical trap to a blue-detuned one can be realized by using our proposed binary phase plate. Particularly, with the change of the modulated phase φ from 0 to −π, the maximum intensity (i.e., the well depth) of the far-field optical trap and its trapping volume will be increased by about four and eight times, respectively. While the modulated phase φ of the phase plate is changed from 0 to π, a red-detuned optical trap will be evolved as a blue-detuned one, and the corresponding trapping intensity and volume will be enhanced by about 4 times. So such an enhanced far-field optical trap scheme has some important applications in the fields of laser cooling and trapping; atomic, molecular, and optical physics; integrated atom or molecule optics; quantum information science; and so on.

Analysis of transverse and longitudinal optical trap stiffness and its application in a novel measurement method of micro-particle refractive index

Abstract: Based on the model of the single TEM00 Gaussian beam optical tweezers, the transverse and longitudinal optical trapping forces of micro-particles are calculated by ray-optics theory. The similarities and differences of the longitudinal optical trap shapes, which are computed by two different methods, are analyzed. We studied the relation between the optical trap stiffness and the particle parameters which include the size and refractive index (RI), and found that the ratio of the transverse and longitudinal stiffness is a constant when the refractive indices of the particles are the same. Through the analysis, the results show that there is an important relationship between the optical stiffness and the product of particle RI and radius. With this relationship, we can obtain the particle RI by measuring the optical stiffness and the particle radius. This novel micro-particle RI measurement method is of great usefulness in atmospheric environmental science, polymer chemistry science, identification of mineral science and bio-medical science.

Optical Torque Analysis of Double-Negative Optical Trapping with Focused Gaussian Beams

Abstract: Preliminary results of the optical torques exerted on negative refractive index spherical particles trapped by focused Gaussian beams are shown, based on the generalized Lorenz-Mie theory and the integral localized approximation, revealing new trapping behaviors.

Dual-beam optical trapping of cells in an optofluidic device fabricated by femtosecond lasers

Abstract: We present design and optimization of an optofluidic monolithic chip, able to provide optical trapping and controlled stretching of single cells. The chip is fabricated in a fused silica glass substrate by femtosecond laser micromachining, which can produce both optical waveguides and microfluidic channels with great accuracy. Versatility and three-dimensional capabilities of this fabrication technology provide the possibility to fabricate circular cross-section channels with enlarged access holes for an easy connection with an external fluidic circuit. Moreover, a new fabrication procedure adopted allows the demonstration of microchannels with a square cross-section, thus guaranteeing an improved quality of the trapped cell images. Optical trapping and stretching of single red blood cells are demonstrated, thus proving the effectiveness of the proposed device as a monolithic optical stretcher. We believe that femtosecond laser micromachining represents a promising technique for the development of multifunctional integrated biophotonic devices that can be easily coupled to a microscope platform, thus enabling a complete characterization of the cells under test.