Wednesday, March 31, 2010

Optical rotor capable of controlling clockwise and counterclockwise rotation in optical tweezers by displacing the trapping position

Hiroo Ukita and Hiroki Kawashima

A clockwise rotor and a counterclockwise rotor (a clockwise rotor placed upside down) are linked on the optical axis to control the rotation direction in optical tweezers by displacing the trapping (focus) position. The dependence of optical torque on the trapping position of this linked rotor is analyzed using an upward-directed focused beam as illumination, via an objective lens with a numerical aperture of 1.4, using a ray optics model under the condition that laser light is incident to not only the lower surfaces, but also to the side surfaces of both rotors. The rotation rate in water is also simulated for an SU-8 linked rotor with 20 μm diameter at a laser power of 200 mW, with rotor thickness as a parameter, by balancing the optical torque with the drag force evaluated using computational fluid dynamics. It is confirmed that the rotation direction changes from clockwise to counterclockwise with the displacement of the trapping position, that almost the same rotation speed is possible in both directions, and that both speeds increase, reach a maximum at a rotor thickness of 9 μm, and then decrease as the thickness increases.

Tuesday, March 30, 2010

Power spectral analysis for optical trap stiffness calibration from high-speed camera position detection with limited bandwidth

Astrid van der Horst and Nancy R. Forde

The use of camera imaging enables trap calibration for multiple particles simultaneously. For stiff traps, however, blur from image integration time affects the detected particle positions significantly. In this paper we use power spectral analysis to calibrate stiff optical traps, taking the effects of blur, aliasing and position detection error into account, as put forward by Wong and Halvorsen [Opt. Express 14, 12517 (2006)]. We find agreement with simultaneously obtained photodiode data and the expected relation of corner frequency fc with laser power, up to fc = 3.6 kHz for a Nyquist frequency of 1.25 kHz. Spectral analysis enables easy identification of the contribution of noise. We demonstrate the utility of our approach with simultaneous calibration of multiple holographic optical traps.

Shear stress mapping in microfluidic devices by optical tweezers

Jing Wu, Daniel Day, and Min Gu

We present an optical tweezer sensor for shear stress mapping in microfluidic systems of different internal geometries. The sensor is able to measure the shear stress acting on microspheres of different sizes that model cell based biological operations. Without the need for a spatial modulator or a holographic disk, the sensor allows for direct shear stress detection at arbitrary positions in straight and curved microfluidic devices. Analytical calculations are carried out and compared with the experimental results. It is observed that a decrease in the microsphere size results in an increase in the shear stress the particle experiences.

Tight Hydrophobic Contacts with the SecB Chaperone Prevent Folding of Substrate Proteins

Philipp Bechtluft, Alexej Kedrov, Dirk-Jan Slotboom, Nico Nouwen, Sander J. Tans and Arnold J. M. Driessen

The molecular chaperone SecB binds to hydrophobic sections of unfolded secretory proteins and thereby prevents their premature folding prior to secretion by the translocase ofEscherichia coli. Here, we have investigated the effect of the single-residue mutation of leucine 42 to arginine (L42R) centrally positioned in the polypeptide binding pocket of SecB on its chaperonin function. The mutant retains its tetrameric structure and SecA targeting function but is defective in its holdase activity. Isothermal titration calorimetry and single-molecule optical tweezer studies suggest that the SecB(L42R) mutant exhibits a reduced polypeptide binding affinity allowing for partial folding of the bound polypeptide chain rendering it translocation-incompetent.

Monday, March 29, 2010

Effect of pulse temporal shape on optical trapping and impulse transfer using ultrashort pulsed lasers

Janelle C. Shane, Michael Mazilu, Woei Ming Lee, and Kishan Dholakia

We investigate the effects of pulse duration on optical trapping with high repetition rate ultrashort pulsed lasers, through Lorentz-Mie theory, numerical simulation, and experiment. Optical trapping experiments use a 12 femtosecond duration infrared pulsed laser, with the trapping microscope’s temporal dispersive effects measured and corrected using the Multiphoton Intrapulse Interference Phase Scan method. We apply pulse shaping to reproducibly stretch pulse duration by 1.5 orders of magnitude and find no material-independent effects of pulse temporal profile on optical trapping of 780nm silica particles, in agreement with our theory and simulation. Using pulse shaping, we control two-photon fluorescence in trapped fluorescent particles, opening the door to other coherent control applications with trapped particles.

Revealing the base pair stepping dynamics of nucleic acid motor proteins with optical traps

Yann R. Chemla

Nearly all aspects of nucleic acid metabolism involve motor proteins. This diverse group of enzymes, which includes DNA and RNA polymerases, the ribosome, helicases, and other translocases, converts chemical energy in the form of bond hydrolysis into concerted motion along nucleic acid filaments. The direct observation of this motion at its fundamental distance scale of one base pair has required the development of new ultrasensitive techniques. Recent advances in optical traps have now made these length scales, once the exclusive realm of crystallographic techniques, accessible. Several new studies using optical traps have revealed for the first time how motor proteins translocate along their substrates in a stepwise fashion. Though these techniques have only begun to be applied to biological problems, the unprecedented access into nucleic acid motor protein movement has already provided important insights into their mechanism. In this perspective, we review these advances and offer our view on the future of this exciting development.

Friday, March 26, 2010

Observation of Colloidal Particle Dynamics Near a Flat Wall by Using Oscillating Optical Tweezers

Ha, C, Pak, HK, Ou-Yang, D

We study the hydrodynamics of a micron-sized spherical particle positioned nearby a flat wall by using oscillating optical tweezers that. trap and oscillate the colloidal particle. In this situation, the presence of the flat wall complicates the flow field surrounding the particle, so the force felt by the moving particle is quite different from the three dimensional hydrodynamics drag force, known as Stokes' drag. However, in biological processes, this condition is ubiquitous and governs the biological particle hydrodynamics. In this work, we describe observations of confined particles dynamics by using oscillation optical tweezers and compare the results with known theories and other experimental data.


Thursday, March 25, 2010

Monte-Carlo simulation of optical trap stiffness measurement by thermalnoise driven method

Li, J., Yu, Y., Zhang, X.

Optical trap stiffness of the optical tweezers must be accurately calibrated, before it is used to measure the mechanical characteristics of submicron particles or biological macromolecules. It is very important to choose a precise calibration method for exact measurement. With Monte-Carlo method, the signal sequence of displacement varies with time during five seconds for a particle in optical trap is simulated, and the simulative sampling frequency is 105 Hz. The optical trap stiffness is calibrated by three thermal-noise-driven analysis methods based on the experimental data in the condition of different noise levels and optical trap deviations. The results show that the ideal errors are all less than 2.5% for the three methods. The errors introduced by optical trap deviation can be eliminated when we calibrate the trap stiffness with the new coordinate of the particle's displacement sequence, which is the difference between the original coordinate and its average. The mean square displacement method (MSDM) has a better anti-noise ability than the Boltzmann distribution method (BDM) and power spectrum method (PSM).

Manipulation of Suspended Single Cells by Microfluidics and Optical Tweezers

Nathalie Nève, Sean S. Kohles, Shelley R. Winn and Derek C. Tretheway

Chondrocytes and osteoblasts experience multiple stresses in vivo. The optimum mechanical conditions for cell health are not fully understood. This paper describes the optical and microfluidic mechanical manipulation of single suspended cells enabled by the μPIVOT, an integrated micron resolution particle image velocimeter (μPIV) and dual optical tweezers instrument (OT). In this study, we examine the viability and trap stiffness of cartilage cells, identify the maximum fluid-induced stresses possible in uniform and extensional flows, and compare the deformation characteristics of bone and muscle cells. These results indicate cell photodamage of chondrocytes is negligible for at least 20 min for laser powers below 30 mW, a dead cell presents less resistance to internal organelle rearrangement and deforms globally more than a viable cell, the maximum fluid-induced shear stresses are limited to ~15 mPa for uniform flows but may exceed 1 Pa for extensional flows, and osteoblasts show no deformation for shear stresses up to 250 mPa while myoblasts are more easily deformed and exhibit a modulated response to increasing stress. This suggests that global and/or local stresses can be applied to single cells without physical contact. Coupled with microfluidic sensors, these manipulations may provide unique methods to explore single cell biomechanics.

Tuesday, March 23, 2010

Optical trapping of colloidal particles and cells by focused evanescent fields using conical lenses

Young Zoon Yoon and Pietro Cicuta

We demonstrate advantages in terms of trapping force distribution and laser efficiency that come from using a telescopic pair of conical lenses (‘axicon’) to generate a ring-like beam, that in conjunction with a high NA objective is used for direct optical trapping with a focused evanescent field near a surface. Various field geometries are considered and compared. First, a Gaussian beam and a laser beam focused on the back focal plane of the objective are compared with each other, and they are scanned across the inlet aperture of the objective. This allows to detect the point of total internal refraction, and to study the trapping power near the surface. We confirm that the hollow beam generated by the conical lenses can generate an evanescent field after a high NA objective lens, and that micron-sized particles can be trapped stably. Finally, we apply the focused evanescent field to erythrocytes under flow, showing that cells are trapped against the flow and are held horizontally against the surface. This is a different equilibrium condition compared to conventional single beam traps, and it is particularly favorable for monitoring the cell membrane. We foresee the integration of this type of trapping with the imaging techniques based on total internal refraction fluoresence (TIRF).

Optofluidic ring resonator switch for optical particle transport

Allen H. J. Yang and David Erickson

In this work, we demonstrate an optofluidic switch using a microring resonator architecture to direct particles trapped in the evanescent field of a solid-core waveguide. When excited at the resonant wavelength, light inserted into the bus waveguide becomes amplified within the ring structure. The resulting high optical intensities in the evanescent field of the ring generate a gradient force that diverts particles trapped on the bus to the ring portion of the device. We show that this increase in optical energy translates to an increase of 250% in the radiation pressure induced steady-state velocity of particles trapped on the ring. We also characterize the switching fraction of the device, showing that 80% of particles are diverted onto the ring when the device is at an on-resonance state. The optofluidic switch we present here demonstrates the versatility in exploiting planar optical devices for integrated particle manipulation applications.

Monday, March 22, 2010

Optical solenoid beams

Sang-Hyuk Lee, Yohai Roichman, and David G. Grier

We introduce optical solenoid beams, diffractionless solutions of the Helmholtz equation whose diffraction-limited in-plane intensity peak spirals around the optical axis, and whose wavefronts carry an independent helical pitch. Unlike other collimated beams of light, appropriately designed solenoid beams have the noteworthy property of being able to exert forces on illuminated objects that are directed opposite to the direction of the light’s propagation. We demonstrate this through video microscopy observations of a colloidal sphere moving upstream along a holographically projected optical solenoid beam.

Bipolar optical forces on dielectric and metallic nanoparticles by evanescent wave

J. J. Xiao, H. H. Zheng, Y. X. Sun, and Y. Yao

We numerically show that both repulsive and attractive (bipolar) optical forces can be exerted on a dielectric or metallic cylindrical nanoparticle by a totally internal refracted wave. This requires that the particles possesses either a whispering gallery (WG) resonance or a localized surface plasmon (LSP) resonance. We further explore the force spectrum that is governed by competition between the separation-dependent resonant Q factor and the coupling strength of the nanoparticle to the evanescent wave. In spite of a much smaller Q of the LSP as compare to the WG resonances, the metallic particle gains much stronger trapping force.

Thursday, March 18, 2010

Characterization of Wet-Heat Inactivation of Single Spores of Bacillus Species by Dual-Trap Raman Spectroscopy and Elastic Light Scattering

Pengfei Zhang, Lingbo Kong, Peter Setlow, and Yong-qing Li

Dual-trap laser tweezers Raman spectroscopy (LTRS) and elastic light scattering (ELS) were used to investigate dynamic processes during high-temperature treatment of individual spores of Bacillus cereus, Bacillus megaterium, and Bacillus subtilis in water.Major conclusions from these studies included the following. (i) After spores of all three species were added to water at 80 to 90°C, the level of the 1:1 complex of Ca2+ and dipicolinic acid (CaDPA; ~25% of the dry weight of the spore core) in individual spores remained relatively constant during a highly variable lag time (Tlag), and then CaDPA was released within 1 to 2 min. (ii) The Tlag values prior to rapid CaDPA release and thus the times for wet-heat killing of individual spores of all threespecies were very heterogeneous. (iii) The heterogeneity in kinetics of wet-heat killing of individual spores was not due to differences in the microscopic physical environments during heat treatment. (iv) During the wet-heat treatment of spores of all three species, spore protein denaturation largely but not completely accompanied rapid CaDPA release, as some changes in protein structure preceded rapid CaDPA release. (v) Changes in the ELS from individual spores of all three species were strongly correlated with the release of CaDPA. The ELS intensities of B. cereus and B. megaterium spores decreased gradually and reached minima at T1 when ~80% of spore CaDPA was released, then increased rapidly until T2 when full CaDPA release was complete, and then remained nearly constant. The ELS intensity of B. subtilis spores showed similar features, although the intensity changed minimally, if at all, prior to T1. (vi) Carotenoids in B. megaterium spores' inner membranes exhibited two changes during heat treatment. First, the carotenoid's two Raman bands at 1,155 and 1,516 cm–1 decreased rapidly to a low value and to zero, respectively,well before Tlag, and then the residual 1,155-cm–1 band disappeared, in parallel with the rapid CaDPA release beginningat Tlag.

Trapping and Sensing 10 nm Metal Nanoparticles Using Plasmonic Dipole Antennas

Weihua Zhang, Lina Huang, Christian Santschi and Olivier J. F. Martin

The optical trapping of Au nanoparticles with dimensions as small as 10 nm in the gap of plasmonic dipole antennas is demonstrated. Single nanoparticle trapping events are recorded in real time by monitoring the Rayleigh scattering spectra of individual plasmonic antennas. Numerical simulations are also performed to interpret the experimental results, indicating the possibility to trap nanoparticles only a few nanometers in size. This work unveils the potential associated with the integration of plasmonic trapping with localized surface plasmon resonance based sensing techniques, in order to deliver analyte to specific, highly sensitive regions (“hot spots”).

Friday, March 12, 2010

Model Accounting for the Effects of Pulling-Device Stiffness in the Analyses of Single-Molecule Force Measurements

Arijit Maitra and Gaurav Arya

Single-molecule force spectroscopy provides a powerful approach for investigating molecular transitions along specific reaction coordinates. Here, we present a general analytical model for extracting the intrinsic rates and activation free energies from force measurements on single molecules that is applicable to a broad range of pulling speeds and device stiffnesses. This model relaxes existing limitations to perform force measurements with soft pulling devices for proper theoretical analyses and, in fact, allows experiments to specifically exploit device stiffness as a control parameter in addition to pulling speed for a reliable estimation of energetic and kinetic parameters.

Visualizing protein–DNA interactions at the single-molecule level

Jovencio Hilario and Stephen C Kowalczykowski

Recent advancements in single-molecule methods have allowed researchers to directly observe proteins acting on their DNA targets in real-time. Single-molecule imaging of protein–DNA interactions permits detection of the dynamic behavior of individual complexes that otherwise would be obscured in ensemble experiments. The kinetics of these processes can be monitored directly, permitting identification of unique subpopulations or novel reaction intermediates. Innovative techniques have been developed to isolate and manipulate individual DNA or protein molecules, and to visualize their interactions. The actions of proteins that have been visualized include: duplex DNA unwinding, DNA degradation, DNA packaging, translocation on DNA, sliding, superhelical twisting, and DNA bending, extension, and condensation. These single-molecule studies have provided new insights into nearly all aspects of DNA metabolism. Here we focus primarily on recent advances in fluorescence imaging and mechanical detection of individual protein–DNA complexes, with emphasis on selected proteins involved in DNA recombination: DNA helicases, DNA translocases, and DNA strand exchange proteins.

Polarization gradient: exploring an original route for optical trapping and manipulation

Gabriella Cipparrone, Ibis Ricardez-Vargas, Pasquale Pagliusi, and Clementina Provenzano

We report a study of the capabilities of an optical tweezer based on polarization gradient. We use a light polarization pattern that is able to simultaneously exert forces and torques in opposite directions depending on the particle’s position. It allows to perform oscillatory displacements and control the sense of rotation of several particles inside a uniformly illuminated region. Unconventional trapping of spinning particles in circularly polarized fringes has been observed, which suggests the involvement of hydrodynamic forces.

Thursday, March 11, 2010

Theory of Optical Trapping by an Optical Vortex Beam

Jack Ng, Zhifang Lin, and C. T. Chan

We propose a theory to explain optical trapping by optical vortices (OVs), which are emerging as important tools to trap mesoscopic particles. The common perception is that the trapping is solely due to the gradient force and that it may be characterized by three real force constants. However, we show that the OV trap can exhibit complex force constants, implying that the trapping must be stabilized by ambient damping. At different damping levels, particles exhibit remarkably different dynamics, such as stable trapping and periodic and aperiodic orbital motions.

Measuring storage and loss moduli using optical tweezers: Broadband microrheology

Manlio Tassieri, Graham M. Gibson, R. M. L. Evans, Alison M. Yao, Rebecca Warren, Miles J. Padgett, and Jonathan M. Cooper

We present an experimental procedure to perform broadband microrheological measurements with optical tweezers. A generalized Langevin equation is adopted to relate the time-dependent trajectory of a particle in an imposed flow to the frequency-dependent moduli of the complex fluid. This procedure allows us to measure the material linear viscoelastic properties across the widest frequency range achievable with optical tweezers.

Wednesday, March 10, 2010

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

L. A. Ambrosio and H. E. Hernández-Figueroa

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.

Investigating the thermodynamics of small biosystems with optical tweezers

Alessandro Mossa, Josep Maria Huguet and Felix Ritort

We present two examples of how single-molecule experimental techniques applied to biological systems can give insight into problems within the scope of equilibrium and nonequilibrium mesoscopic thermodynamics. The first example is the mapping of the free energy landscape of a macromolecule, the second the experimental verification of Crooks’ fluctuation theorem. In both cases the experimental setup comprises optical tweezers and DNA molecules.

Tuesday, March 9, 2010

Quasi 3-dimensional optical trapping by two counter-propagating beams in nano-fiber

Lihua Zhao, Yudong Li, Jiwei Qi, Jingjun Xu, and Qian Sun

Optical forces on a nanoparticle around an absorptive dielectric nano-fiber illuminated by two linear polarized counter-propagating beams were investigated. The results show the scattering force of the two beams causes the steady trapping along the fiber and the gradient force makes the trapping in the transverse plane of the nano-fiber. By altering the intensity ratio between the two incident beams and the polarization direction of the beams, manipulation along the nano-fiber and in the transverse direction can be realized, respectively. The numerical results present a new promising method to realize quasi 3-dimensional optical manipulation.

Thursday, March 4, 2010

Experimental analysis of Hb oxy–deoxy transition in single optically stretched red blood cells

G. Rusciano

Raman confocal microscopy, combined with an optical stretcher, is used to study the spatial distribution and the oxidation state of hemoglobin in erythrocytes under stretching condition. In particular, a near infrared laser (λ = 1064 nm) is used to generate multiple time-sharing Optical Tweezers to trap and stretch a single erythrocyte, while a second laser (λ = 532 nm) acts as Raman probe. Our study demonstrates that stretching induces hemoglobin transition to the deoxygenated state. Moreover, by using Principal Component Analysis we prove the reversibility of the oxydeoxy hemoglobin transition after application of the optically induced mechanical stress.

A microfluidic device for reversible environmental changes around single cells using optical tweezers for cell selection and positioning

Emma Eriksson, Kristin Sott, Fredrik Lundqvist, Martin Sveningsson, Jan Scrimgeour, Dag Hanstorp, Mattias Goksör and Annette Granéli

Cells naturally exist in a dynamic chemical environment, and therefore it is necessary to study cell behaviour under dynamic stimulation conditions in order to understand the signalling transduction pathways regulating the cellular response. However, until recently, experiments looking at the cellular response to chemical stimuli have mainly been performed by adding a stress substance to a population of cells and thus only varying the magnitude of the stress. In this paper we demonstrate an experimental method enabling acquisition of data on the behaviour of single cells upon reversible environmental perturbations, where microfluidics is combined with optical tweezers and fluorescence microscopy. The cells are individually selected and positioned in the measurement region on the bottom surface of the microfluidic device using optical tweezers. The optical tweezers thus enable precise control of the cell density as well as the total number of cells within the measurement region. Consequently, the number of cells in each experiment can be optimized while clusters of cells, that render subsequent image analysis more difficult, can be avoided. The microfluidic device is modelled and demonstrated to enable reliable changes between two different media in less than 2 s. The experimental method is tested by following the cycling of GFP-tagged proteins (Mig1 and Msn2, respectively) between the cytosol and the nucleus in Saccharomyces cerevisiae upon changes in glucose availability.

Modeling of hemodynamics arising from malaria infection

Yohsuke Imai, Hitoshi Kondo, Takuji Ishikawa, Chwee Teck Lim and Takami Yamaguchi

We propose a numerical model of hemodynamics arising from malaria infection. This model is based on a particle method, where all the components of blood are represented by the finite number of particles. A two-dimensional spring network of membrane particles is employed for expressing the deformation of malaria infected red blood cells (IRBCs). Malaria parasite within the IRBC is modeled as a rigid object. This model is applied to the stretching of IRBCs by optical tweezers, the deformation of IRBCs in shear flow, and the occlusion of narrow channels by IRBCs. We also investigate the effects of IRBCs on the rheological property of blood in micro-channels. Our results indicate that apparent viscosity is drastically increased for the period from the ring stage and the trophozoite stage, whereas it is not altered in the early stage of infection.

Using a slightly tapered optical fiber to attract and transport microparticles

Fang-Wen Sheu, Hong-Yu Wu, and Sy-Hann Chen

We exploit a fiber puller to transform a telecom single-mode optical fiber with a 125 µm diameter into a symmetric and unbroken slightly tapered optical fiber with a 50 µm diameter at the minimum waist. When the laser light is launched into the optical fiber, we can observe that, due to the evanescent wave of the slightly tapered fiber, the nearby polystyrene microparticles with 10 µm diameters will be attracted onto the fiber surface and roll separately in the direction of light propagation. We have also simulated and compared the optical propulsion effects on the microparticles when the laser light is launched into a slightly tapered fiber and a heavily tapered (subwavelength) fiber, respectively.

Monday, March 1, 2010

Online Fluorescence Suppression in Modulated Raman Spectroscopy

Anna Chiara De Luca, Michael Mazilu, Andrew Riches, C. Simon Herrington and Kishan Dholakia

Label-free chemical characterization of single cells is an important aim for biomedical research. Standard Raman spectroscopy provides intrinsic biochemical markers for noninvasive analysis of biological samples but is often hindered by the presence of fluorescence background. In this paper, we present an innovative modulated Raman spectroscopy technique to filter out the Raman spectra from the fluorescence background. The method is based on the principle that the fluorescence background does not change whereas the Raman scattering is shifted by the periodical modulation of the laser wavelength. Exploiting this physical property and importantly the multichannel lock-in detection of the Raman signal, the modulation technique fulfills the requirements of an effective fluorescence subtraction method. Indeed, once the synchronization and calibration procedure is performed, minimal user intervention is required, making the method online and less time-consuming than the other fluorescent suppression methods. We analyze the modulated Raman signal and shifted excitation Raman difference spectroscopy (SERDS) signal of 2 μm-sized polystyrene beads suspended in a solution of fluorescent dye as a function of modulation rate. We show that the signal-to-noise ratio of the modulated Raman spectra at the highest modulation rate is 3 times higher than the SERDS one. To finally evaluate the real benefits of the modulated Raman spectroscopy, we apply our technique to Chinese hamster ovary cells (CHO). Specifically, by analyzing separate spectra from the membrane, cytoplasm, and nucleus of CHO cells, we demonstrate the ability of this method to obtain localized sensitive chemical information from cells, away from the interfering fluorescence background. In particular, statistical analysis of the Raman data and classification using PCA (principal component analysis) indicate that our method allows us to distinguish between different cell locations with higher sensitivity and specificity, avoiding potential misinterpretation of the data obtained using standard background procedures.

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

N. Bellini, K. C. Vishnubhatla, F. Bragheri, L. Ferrara, P. Minzioni, R. Ramponi, I. Cristiani, and R. Osellame

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.

Note: Toward multiple addressable optical trapping

Alexei R. Faustov, Michael R. Webb, and David R. Walt

We describe a setup for addressable optical trapping in which a laser source is focused on a digital micromirror device and generates an optical trap in a microfluidic cell. In this paper, we report a proof-of-principle single beam/single micromirror/single three-dimensional traparrangement that should serve as the basis for a multiple-trap instrument.