Analysis of the behaviour of erythrocytes in an optical trapping system

Advances in Nanoarchitectonics: A Review of "Static" and "Dynamic" Particle Assembly Methods

Particle assembly is a promising technique to create functional materials and devices from nanoscale building blocks. However, the control of particle arrangement and orientation is challenging and requires careful design of the assembly methods and conditions. In this study, the static and dynamic methods of particle assembly are reviewed, focusing on their applications in biomaterial sciences. Static methods rely on the equilibrium interactions between particles and substrates, such as electrostatic, magnetic, or capillary forces. Dynamic methods can be associated with the application of external stimuli, such as electric fields, magnetic fields, light, or sound, to manipulate the particles in a non-equilibrium state. This study discusses the advantages and limitations of such methods as well as nanoarchitectonic principles that guide the formation of desired structures and functions. It also highlights some examples of biomaterials and devices that have been fabricated by particle assembly, such as biosensors, drug delivery systems, tissue engineering scaffolds, and artificial organs. It concludes by outlining the future challenges and opportunities of particle assembly for biomaterial sciences. This review stands as a crucial guide for scholars and professionals in the field, fostering further investigation and innovation. It also highlights the necessity for continuous research to refine these methodologies and devise more efficient techniques for nanomaterial synthesis. The potential ramifications on healthcare and technology are substantial, with implications for drug delivery systems, diagnostic tools, disease treatments, energy storage, environmental science, and electronics.

3D integral imaging of acoustically trapped objects

3D imaging provides crucial details about the objects and scenes that may not be obtained via 2D imaging methods. However, there are several applications in which the object to be 3D-imaged requires to be immobilized. The integrated digital holographic microscopy (DHM) and optical trapping (OT) system is a useful solution for such a task, but both DHM and OT are mostly suitable for microscopic specimens. Here, for the first time to the best of our knowledge and as an analogy to the DHM-OT system, we introduce integral imaging (InIm) and acoustic trapping (AT) integrated system for 3D imaging of immobilized mesoscopic and macroscopic objects. Post-processing of InIm data enables reconstructing the scene at any arbitrary plane, therefore, it re-focuses any particular depth of the object, which is a curtail task, especially when the object is trapped by AT. We demonstrate the capability of our system by simultaneous trapping and 3D imaging of single and multiple irregularly shaped objects with mm sizes.

Modelling red blood cell optical trapping by machine learning improved geometrical optics calculations

Optically trapping red blood cells allows for the exploration of their biophysical properties, which are affected in many diseases. However, because of their nonspherical shape, the numerical calculation of the optical forces is slow, limiting the range of situations that can be explored. Here we train a neural network that improves both the accuracy and the speed of the calculation and we employ it to simulate the motion of a red blood cell under different beam configurations. We found that by fixing two beams and controlling the position of a third, it is possible to control the tilting of the cell. We anticipate this work to be a promising approach to study the trapping of complex shaped and inhomogeneous biological materials, where the possible photodamage imposes restrictions in the beam power.

Ray Optics Model for Optical Trapping of Biconcave Red Blood Cells

Red blood cells (RBCs) or erythrocytes are essential for oxygenating the peripherical tissue in the human body. Impairment of their physical properties may lead to severe diseases. Optical tweezers have in experiments been shown to be a powerful tool for assessing the biochemical and biophysical properties of RBCs. Despite this success there has been little theoretical work investigating of the stability of erythrocytes in optical tweezers. In this paper we report a numerical study of the trapping of RBCs in the healthy, native biconcave disk conformation in optical tweezers using the ray optics approximation. We study trapping using both single- and dual-beam optical tweezers and show that the complex biconcave shape of the RBC is a significant factor in determining the optical forces and torques on the cell, and ultimately the equilibrium configuration of the RBC within the trap. We also numerically demonstrate how the addition of a third or even fourth trapping laser beam can be used to control the cell orientation in the optical trap. The present investigation sheds light on the trapping mechanism of healthy erythrocytes and can be exploited by experimentalist to envisage new experiments.

Single laser trapping for optical folding and rotation of red blood cells in sickle cell disease in response to hydroxyurea treatment

Optical folding and rotation behavior of red blood cells (RBCs) in polarized laser tweezers are considerably important for understanding the biophysical and biomechanical properties using the fast probing method. Here, a dual-mode polarized single-laser tweezers technique with distinct principal axes exhibiting different polarization states is presented and designed to investigate the deformation, optical folding, and rotation of single living cells with one measurement. RBC optical folding and rotation speed are measured in patients with sickle cell disease (SCD), including follow up of patients after hydroxyurea (HU) treatment for at least three months. Folding angle and rotation speed are significantly lower in patients with SCD and do not significantly differ in patients treated by HU compared with the healthy control group. The RBC folding angle and rotation speed in patients treated with HU drug increase linearly at lower laser powers and rapidly at higher powers, and increase much slowly in patients not treated with HU. The difference in the folding angle and rotation speed of RBCs could be useful for drug response in SCD or predicting pain crisis in SCD.

Multimodal Diagnostics of Microrheologic Alterations in Blood of Coronary Heart Disease and Diabetic Patients

Coronary heart disease (CHD) has serious implications for human health and needs to be diagnosed as early as possible. In this article in vivo and in vitro optical methods are used to study blood properties related to the aggregation of red blood cells in patients with CHD and comorbidities such as type 2 diabetes mellitus (T2DM). The results show not only a significant difference of the aggregation in patients compared to healthy people, but also a correspondence between in vivo and in vitro parameters. Red blood cells aggregate in CHD patients faster and more numerously; in particular the aggregation index increases by 20 ± 7%. The presence of T2DM also significantly elevates aggregation in CHD patients. This work demonstrates multimodal diagnostics and monitoring of patients with socially significant pathologies.

An optofluidic "tweeze-and-drag" cell stretcher in a microfluidic channel

The mechanical properties of biological cells are utilized as an inherent, label-free biomarker to indicate physiological and pathological changes of cells. Although various optical and microfluidic techniques have been developed for cell mechanical characterization, there is still a strong demand for non-contact and continuous methods. Here, by combining optical and microfluidic techniques in a single desktop platform, we demonstrate an optofluidic cell stretcher based on a "tweeze-and-drag" mechanism using a periodically chopped, tightly focused laser beam as an optical tweezer to trap a cell temporarily and a flow-induced drag force to stretch the cell in a microfluidic channel transverse to the tweezer. Our method leverages the advantages of non-contact optical forces and a microfluidic flow for both cell stretching and continuous cell delivery. We demonstrate the stretcher for mechanical characterization of rabbit red blood cells (RBCs), with a throughput of ∼1 cell per s at a flow rate of 2.5 μl h^-1 at a continuous-wave laser power of ∼25 mW at a wavelength of 1064 nm (chopped at 2 Hz). We estimate the spring constant of RBCs to be ∼14.9 μN m^-1. Using the stretcher, we distinguish healthy RBCs and RBCs treated with glutaraldehyde at concentrations of 5 × 10^-4% to 2.5 × 10^-3%, with a strain-to-concentration sensitivity of ∼-1529. By increasing the optical power to ∼45 mW, we demonstrate cell-stretching under a higher flow rate of 4 μl h^-1, with a higher throughput of ∼1.5 cells per s and a higher sensitivity of ∼-2457. Our technique shows promise for applications in the fields of healthcare monitoring and biomechanical studies.

Peculiarities of control of erythrocytes moving in an evanescent field

An investigation of the influence of an evanescent wave on the dynamics of motion of erythrocytes in blood plasma is presented. Computer simulation of erythrocytes moving in an evanescent field and experimental demonstration of the forecasted motion substantiate the possibility for control of position of red blood cells in a solution. The range of velocities of transversal motion of erythrocytes due to the action of the optical force of the generated evanescent field is determined as a function of the angle of illumination of a cell by a linearly polarized wave with the azimuth of polarization 45 deg.

Quantitative phase microscopy of red blood cells during planar trapping and propulsion

Red blood cells (RBCs) have the ability to undergo morphological deformations during microcirculation, such as changes in surface area, volume and sphericity. Optical waveguide trapping is suitable for trapping, propelling and deforming large cell populations along the length of the waveguide. Bright field microscopy employed with waveguide trapping does not provide quantitative information about structural changes. Here, we have combined quantitative phase microscopy and waveguide trapping techniques to study changes in RBC morphology during planar trapping and transportation. By using interference microscopy, time-lapsed interferometric images of trapped RBCs were recorded in real-time and subsequently utilized to reconstruct optical phase maps. Quantification of the phase differences before and after trapping enabled study of the mechanical effects during planar trapping. During planar trapping, a decrease in the maximum phase values, an increase in the surface area and a decrease in the volume and sphericity of RBCs were observed. QPM was used to analyze the phase values for two specific regions within RBCs: the annular rim and the central donut. The phase value of the annular rim decreases whereas it increases for the central donut during planar trapping. These changes correspond to a redistribution of cytosol inside the RBC during planar trapping and transportation.

Squeezing red blood cells on an optical waveguide to monitor cell deformability during blood storage

Red blood cells squeeze through micro-capillaries as part of blood circulation in the body. The deformability of red blood cells is thus critical for blood circulation. In this work, we report a method to optically squeeze red blood cells using the evanescent field present on top of a planar waveguide chip. The optical forces from a narrow waveguide are used to squeeze red blood cells to a size comparable to the waveguide width. Optical forces and pressure distributions on the cells are numerically computed to explain the squeezing process. The proposed technique is used to quantify the loss of blood deformability that occurs during blood storage lesion. Squeezing red blood cells using waveguides is a sensitive technique and works simultaneously on several cells, making the method suitable for monitoring stored blood.

Optically driven oscillations of ellipsoidal particles. Part I: experimental observations

We report experimental observations of the mechanical effects of light on ellipsoidal micrometre-sized dielectric particles, in water as the continuous medium. The particles, made of polystyrene, have shapes varying between near disk-like (aspect ratio k = 0.2) to very elongated needle-like (k = 8). Rather than the very tightly focused beam geometry of optical tweezers, we use a moderately focused laser beam to manipulate particles individually by optical levitation. The geometry allows us varying the longitudinal position of the particle, and to capture images perpendicular to the beam axis. Experiments show that moderate-k particles are radially trapped with their long axis lying parallel to the beam. Conversely, elongated (k > 3) or flattened (k < 0.3) ellipsoids never come to rest, and permanently "dance" around the beam, through coupled translation-rotation motions. The oscillations are shown to occur in general, be the particle in bulk water or close to a solid boundary, and may be periodic or irregular. We provide evidence for two bifurcations between static and oscillating states, at k ≈ 0.33 and k ≈ 3 for oblate and prolate ellipsoids, respectively. Based on a recently developed 2-dimensional ray-optics simulation (Mihiretie et al., EPL 100, 48005 (2012)), we propose a simple model that allows understanding the physical origin of the oscillations.

Nanoplasmonically-induced defects in lipid membrane monitored by ion current: transient nanopores versus membrane rupture

We have developed a nanoplasmonic-based approach to induce nanometer-sized local defects in the phospholipid membranes. Here, gold nanorods and nanoparticles having plasmon resonances in the near-infrared (NIR) spectral range are used as optical absorption centers in the lipid membrane. Defects optically induced by NIR-laser irradiation of gold nanoparticles are continuously monitored by high-precision ion conductance measurement. Localized laser-mediated heating of nanorods and nanoparticle aggregates cause either (a) transient nanopores in lipid membranes or (b) irreversible rupture of the membrane. To monitor transient opening and closing, an electrophysiological setup is assembled wherein a giant liposome is spread over a micrometer hole in a glass slide forming a single bilayer of high Ohmic resistance (so-called gigaseal), while laser light is coupled in and focused on the membrane. The energy associated with the localized heating is discussed and compared with typical elastic parameters in the lipid membranes. The method presented here provides a novel methodology for better understanding of transport across artificial or natural biological membranes.

Digital holographic microscopy with coupled optical fiber trap for cell measurement and manipulation

We present an integrated optical system for three-dimensional (3D) imaging of micrometer-sized samples, while immobilizing and manipulating the samples by means of an optical fiber trap. Optical traps allow us to apply and measure pico-Newton-sized forces, and perform detailed measurements of micrometer-sized dielectric systems in the field of biology. The integrated 3D system can be used as a major tool in the field of biophysics. The trap is built using a tapered optical fiber to enhance the effective numerical aperture of the fiber. The trapping system is mounted on a conventional microscope, in which the two eyepieces' output ports are used as the paths of an off-axis self-referencing digital holographic microscopy (DHM) setup. The trap is calibrated using a high-speed camera, and trap stiffness is determined through the power spectrum method. The compact setup provides an elegant apparatus for temporally stable DHM for 3D imaging of optically controlled samples. Three-dimensional information and quantitative phase contrast images of the trapped samples are obtained by postprocessing the recorded digital holograms. Experiments were performed on lipids and red blood cells. Quantitative phase contrast images and temporal evolution of optical thickness of trapped samples are presented.

Measurement of the trapping efficiency of an elliptical optical trap with rigid and elastic objects

Optical tweezers and their various modifications offer a sophisticated way to perform noncontact cell manipulation. In this paper, we quantify forces existing in an elliptical trap formed by two cylindrical lenses and compare the results with a point optical trap case. The trapping efficiency of point and elliptical traps was analyzed by measuring the Q values of both traps. Polystyrene microspheres and red blood cells (RBCs) were used as samples. Stretching of the RBC was taken into account in the Q value measurements. Although the Q value of a point optical trap is larger than that of an elliptical trap when measured for a single RBC, we can manipulate the orientation of an RBC in a point trap with the elliptical trap and can also trap several RBCs simultaneously in the elliptical trap far from the cuvette surfaces by using a long-working-distance water immersion objective. This opens new possibilities for studying light-matter interactions at the cellular level.

Nanoplasmonics for dual-molecule release through nanopores in the membrane of red blood cells

A nanoplasmonics-based opto-nanoporation method of creating nanopores upon laser illumination is applied for inducing diffusion and triggered release of small and large molecules from red blood cells (RBCs). The method is implemented using absorbing gold nanoparticle (Au-NP) aggregates on the membrane of loaded RBCs, which, upon near-IR laser light absorption, induce release of encapsulated molecules from selected cells. The binding of Au-NPs to RBCs is characterized by Raman spectroscopy. The process of release is driven by heating localized at nanoparticles, which impacts the permeability of the membrane by affecting the lipid bilayer and/or trans-membrane proteins. Localized heating and temperature rise around Au-NP aggregates is simulated and discussed. Research reported in this work is relevant for generating nanopores for biomolecule trafficking through polymeric and lipid membranes as well as cell membranes, while dual- and multi-molecule release is relevant for theragnostics and a wide range of therapies.

The self-orientation of mammalian cells in optical tweezers--the importance of the nucleus

Here we present the first evidence showing that eukaryotic cells can be stably trapped in a single focused Gaussian beam with an orientation that is defined by the nucleus. A mammalian eukaryotic cell (in suspension) is trapped and is re-oriented in the focus of a linearly polarized Gaussian beam with a waist of dimension smaller than the radius of the nucleus. The cell reaches a position relative to the focus that is dictated by the nucleus and nuclear components. Our studies illustrate that the force exerted by the optical tweezers at locations within the cell can be predicted theoretically; the data obtained in this way is consistent with the experimental observations.

Lysophosphatidic acid induced red blood cell aggregation in vitro

Under physiological conditions healthy RBCs do not adhere to each other. There are indications that RBCs display an intercellular adhesion under certain (pathophysiological) conditions. Therefore we investigated signaling steps starting with transmembrane calcium transport by means of calcium imaging. We found a lysophosphatidic acid (LPA) concentration dependent calcium influx with an EC(50) of 5 μM LPA. Downstream signaling was investigated by flow cytometry as well as by video-imaging comparing LPA induced with "pure" calcium mediated phosphatidylserine exposure and concluded the coexistence of two branches of the signaling pathway. Finally we performed force measurements with holographic optical tweezers (HOT): The intercellular adhesion of RBCs (aggregation) exceeds a force of 25 pN. These results support (i) earlier data of a RBC associated component in thrombotic events under certain pathophysiological conditions and (ii) the concept to use RBCs in studies of cellular adhesion behavior, especially in combination with HOT. The latter paves the way to use RBCs as model cells to investigate molecular regulation of cellular adhesion processes.

Effect of the size and shape of a red blood cell on elastic light scattering properties at the single-cell level

We demonstrate the use of a double-beam optical tweezers system to stabilize red blood cell (RBC) orientation in the optical tweezers during measurements of elastic light scattering from the trapped cells in an angle range of 5-30 degrees. Another laser (He-Ne) was used to illuminate the cell and elastic light scattering distribution from the single cell was measured with a goniometer and a photomultiplier tube. Moreover, CCD camera images of RBCs with and without laser illumination are presented as complementary information. Light scattering from a RBC was measured in different fixed orientations. Light scattering from cells was also measured when the length of the cell was changed in two different orientations. Light scattering measurements from spherical and crenate RBCs are described and the results are compared with other cell orientations. Analysis shows that the measured elastic light scattering distributions reveal changes in the RBC's orientation and shape. The effect of stretching on the changes in scattering is larger in the case of face-on incidence of He-Ne laser light than in rim-on incidence. The scattering patterns from RBCs in different orientations as well as from a spherical RBC were compared with numerical results found in literature. Good correlation was found.

Optical orientation and rotation of trapped red blood cells with Laguerre-Gaussian mode

We report the use of Laguerre-Gaussian (LG) modes for controlled orientation and rotation of optically trapped red blood cells (RBCs). For LG modes with increasing topological charge the resulting increase in size of the intensity annulas led to trapping of the cells at larger tilt angle with respect to the beam axis and thus provided additional control on the stable orientation of the cells under trap. Further, the RBCs could also be driven as micro-rotors by a transfer of orbital angular momentum from the LG trapping beam having large topological charge or by rotating the profile of LG mode having fractional topological charge.

Optical trapping and propulsion of red blood cells on waveguide surfaces

We have studied optical trapping and propulsion of red blood cells in the evanescent field of optical waveguides. Cell propulsion is found to be highly dependent on the biological medium and serum proteins the cells are submerged in. Waveguides made of tantalum pentoxide are shown to be efficient for cell propulsion. An optical propulsion velocity of up to 
6 µm/s on a waveguide with a width of ~6 µm is reported. Stable optical trapping and propulsion of cells during transverse flow is also reported.

Hemoglobin degradation in human erythrocytes with long-duration near-infrared laser exposure in Raman optical tweezers

Near-infrared laser (785-nm)-excited Raman spectra from a red blood cell, optically trapped using the same laser beam, show significant changes as a function of trapping duration even at trapping power level of a few milliwatts. These changes in the Raman spectra and the bright-field images of the trapped cell, which show a gradual accumulation of the cell mass at the trap focus, suggest photoinduced aggregation of intracellular heme. The possible role of photoinduced protein denaturation and hemichrome formation in the observed aggregation of heme is discussed.

Polymeric microcapsules with light responsive properties for encapsulation and release

This review is dedicated to recent developments on the topic of light sensitive polymer-based microcapsules. The microcapsules discussed are constructed using the layer-by-layer self-assembly method, which consists in absorbing oppositely charged polyelectrolytes onto charged sacrificial particles. Microcapsules display a broad spectrum of qualities over other existing microdelivery systems such as high stability, longevity, versatile construction and a variety of methods to encapsulate and release substances. Release and encapsulation of materials by light is a particularly interesting topic. Microcapsules can be made sensitive to light by incorporation of light sensitive polymers, functional dyes and metal nanoparticles. Optically active substances can be inserted into the shell during their assembly as a polymer complex or following the shell preparation. Ultraviolet-addressable microcapsules were shown to allow for remote encapsulation and release of materials. Visible- and infrared- addressable microcapsules offer a large array of release strategies for capsules, from destructive to highly sensitive reversible approaches. Besides the Introduction and Conclusions, this review contains in four sections reviewing the effects of light 1) on polymer-based microcapsules, 2) microcapsules containing metal nanoparticles and 3) functional dyes, as well as a fourth section that revisits the implications of light addressable polymeric microcapsules as a microdelivery system for biological applications.

Cell deformation cytometry using diode-bar optical stretchers

The measurement of cell elastic parameters using optical forces has great potential as a reagent-free method for cell classification, identification of phenotype, and detection of disease; however, the low throughput associated with the sequential isolation and probing of individual cells has significantly limited its utility and application. We demonstrate a single-beam, high-throughput method where optical forces are applied anisotropically to stretch swollen erythrocytes in microfluidic flow. We also present numerical simulations of model spherical elastic cells subjected to optical forces and show that dual, opposing optical traps are not required and that even a single linear trap can induce cell stretching, greatly simplifying experimental implementation. Last, we demonstrate how the elastic modulus of the cell can be determined from experimental measurements of the equilibrium deformation. This new optical approach has the potential to be readily integrated with other cytometric technologies and, with the capability of measuring cell populations, enabling true mechanical-property-based cell cytometry.

Cell deformation cytometry using diode-bar optical stretchers

The measurement of cell elastic parameters using optical forces has great potential as a reagent-free method for cell classification, identification of phenotype, and detection of disease; however, the low throughput associated with the sequential isolation and probing of individual cells has significantly limited its utility and application. We demonstrate a single-beam, high-throughput method where optical forces are applied anisotropically to stretch swollen erythrocytes in microfluidic flow. We also present numerical simulations of model spherical elastic cells subjected to optical forces and show that dual, opposing optical traps are not required and that even a single linear trap can induce cell stretching, greatly simplifying experimental implementation. Last, we demonstrate how the elastic modulus of the cell can be determined from experimental measurements of the equilibrium deformation. This new optical approach has the potential to be readily integrated with other cytometric technologies and, with the capability of measuring cell populations, enabling true mechanical-property-based cell cytometry.

Optical tweezers for single cells

Optical tweezers (OT) have emerged as an essential tool for manipulating single biological cells and performing sophisticated biophysical/biomechanical characterizations. Distinct advantages of using tweezers for these characterizations include non-contact force for cell manipulation, force resolution as accurate as 100aN and amiability to liquid medium environments. Their wide range of applications, such as transporting foreign materials into single cells, delivering cells to specific locations and sorting cells in microfluidic systems, are reviewed in this article. Recent developments of OT for nanomechanical characterization of various biological cells are discussed in terms of both their theoretical and experimental advancements. The future trends of employing OT in single cells, especially in stem cell delivery, tissue engineering and regenerative medicine, are prospected. More importantly, current limitations and future challenges of OT for these new paradigms are also highlighted in this review.

Passive optical separation within a 'nondiffracting' light beam

A passive, optical cell sorter is created using the light pattern of a 'nondiffracting' beam-the Bessel beam. As a precursor to cell sorting studies, microspheres are used to test the resolution of the sorter on the basis of particle size and refractive index. Variations in size and, more noticeably, refractive index, lead to a marked difference in the migration time of spheres in the Bessel beam. Intrinsic differences (size, refractive index) between native (unlabeled) cell populations are utilized for cell sorting. The large difference in size between erythrocytes and lymphocytes results in their successful separation in this beam pattern. The intrinsic differences in size and refractive index of other cells in the study (HL60 human promyelocytic leukaemic cells, murine bone marrow, and murine stem/progenitor cells) are not large enough to induce passive optical separation. Silica microsphere tags are attached to cells of interest to modify their size and refractive index, resulting in the separation of labeled cells. Cells collected after separation are viable, as evidenced by trypan blue dye exclusion, their ability to clone in vitro, continued growth in culture, and lack of expression of Caspase 3, a marker of apoptosis.

A single beam near-field laser trap for optical stretching, folding and rotation of erythrocytes

To understand the fundamental mechanical and viscoelastic properties of RBCs, one needs laser tweezers in which cells can not only be trapped, but also be stretched, folded, and rotated. Stretching, folding and rotating an RBC is particularly important in order to reveal the shear elasticity of the RBC membrane. Here we show a single beam near-field laser trapping technique under focused evanescent wave illumination for optical stretching, folding and rotation of a single RBC. This multifunctional manipulation method will provide a new platform for measuring cell properties such as the membrane elasticity, viscoelasticity and deformability.

Cellular and Colloidal Separation Using Optical Forces

The separation or sorting of cellular and colloidal particles is currently a central topics of research. In this chapter, we give an overview of the range of optical methods for cell sorting. We begin with an overview of fluorescence and magnetically activated cell sorting. We progress to describing methods at the microfluidic scale level particularly those exploiting optical forces. We distinguish between what we term passive and active schemes for sorting. Optical forces pertinent to the sorting schemes are described, notably the gradient force and the optical radiation pressure (or scattering force). We discuss some of the most recent advances. This includes techniques without fluid flow where we have either stationary or moving light patterns to initiate separation. Further methods have shown how using an externally driven flow either counter-propagating against a light field (optical chromatography) or over a periodic light pattern (an optical potential energy landscape) may result in the selection of particles and cells based on physical attributes such as size and refractive index. We contrast these schemes with the field of dielectrophoresis where electric field gradients may separate cells and also briefly mention the upcoming area of light-induced dielectrophoresis which marries the reconfigurability of optical fields with the power of dielectrophoresis.

Evaluation of trapping efficiency of optical tweezers by dielectrophoresis

A relatively new method for measuring optically induced forces on microparticles and cells, different from the conventional Brownian motion and viscous drag force calibration methods widely used, is introduced. It makes use of the phenomenon of dielectrophoresis for the calibration of optical tweezers through the dielectrophoretic force calculations. A pair of microelectrodes is fabricated by photolithography on a microscope slide and it is connected to a high-frequency generator. The calibration of the optical tweezers setup is performed by the manipulation of polystyrene beads and yeast cells. Calibration diagrams of the transverse forces versus power are deduced for different cell radii and numerical apertures of the objective lenses. The optical system and the related technique provide a fast and easy method for optical tweezers calibration.

Surface stress on the erythrocyte under laser irradiation with finite-difference time-domain calculation

The surface stress on the real shape (biconcave disklike) of an erythrocyte under laser irradiation is theoretically studied according to the finite-difference time-domain (FDTD) method. The distribution of the surface stresses depends on the orientation of erythrocytes in the laser beam. Typically when the erythrocyte was irradiated from the side direction (the laser beam was perpendicular to the normal of the erythrocyte plane), the surface stresses were so asymmetrical and nonuniform that the magnitude of the surface stress on the back surface was three times higher than that on the front surface, and the highest-to-lowest ratio of the stress reached 16 times. For comparison, the surface stress was also calculated according to the ray optics (RO) method. The tendency of the stress distribution from the RO calculation was roughly similar to that of the FDTD method. However the RO calculation produced some unphysical results, such as the infinite stress on some surface region and the zero stress on the most parts of the erythrocyte surface, which is due to the neglecting of light diffraction. The results obtained from the FDTD calculation are believed quantitatively reliable, because the FDTD method automatically takes into account of the diffraction and interference effects of the light wave. Thus, the FDTD method is more suitable than the RO method for the stress study of erythrocytes.

Raman imaging of floating cells

Raman imaging can yield spatially resolved biochemical information from living cells. To date there have been no Raman images published of cells in suspension because of the problem of immobilizing them suitably to acquire space-resolved spectra. In this paper in order to overcome this problem the use of holographic optical tweezers is proposed and implemented, and data is shown for spatially resolved Raman spectroscopy of a live cell in suspension.

Real-Time Detection of Hyperosmotic Stress Response in Optically Trapped Single Yeast Cells Using Raman Microspectroscopy

Living cells survive environmentally stressful conditions by initiating a stress response. We monitored changes in the Raman spectra of optically trapped Saccharomyces cerevisiae yeast cell under normal, heat-treated, and hyperosmotic stress conditions. It is shown that when glucose was used to exert hyperosmotic stress, two chemical substances-glycerol and ethanol-can be monitored in real time in a single cell.

Self-centering of a ball lens by laser trapping: fiber-ball-fiber coupling analysis

Fiber-to-fiber coupling through use of a laser-trapped microball lens is examined. A model based on radiation pressure predicts that the ball lens will align axially between the fiber endfaces. Laser manipulation of the ball lens axial position results in a configuration in which the ball lens optically bridges the gap between the fibers. Experimental results are presented for several fiber endface separations, and it is found that the presence of the microball lens can increase the coupling by a factor of 2 above the level expected by direct fiber-to-fiber coupling for the same fiber endface separation.

Laser-trapping properties of dual-component spheres

The optical trapping properties of dual-component spheres consisting of a cocentered outer transparent dielectric spherical shell and internal solid sphere are examined on the basis of the enhanced ray optics model. It is shown that stable trapping can occur on axis, off axis, or at multiple axial positions and depends on the dual-sphere and laser beam parameters. Computation results are also presented for an internal reflecting sphere surrounded by an outer dielectric spherical shell.