Monday, May 27, 2013

The physics of membrane tubes: soft templates for studying cellular membranes

Aurélien Roux

Lipid membranes under shear or under pulling forces generate surprising cylindrical structures called membrane tubes. Their size varies between a few hundreds of nanometers to a few tens of nanometers. These structures can be formed by multiple ways, and are a clear signature of membrane fluidity and elasticity. Moreover, in vivo, many tubular structures are formed during intracellular transport to exchange material between compartments. The basic principles of their formation are the same than in vitro. Recent studies on the specific physico-chemical properties of membrane involved in membrane tubes shed light onto how similar structures are formed in vivo. As well, in vitro controlled formation of such membrane tubes turned out to be an elegant way to study in vitro many dynamic processes happening in membrane traffic.


Formin mDia1 senses and generates mechanical forces on actin filaments

Antoine Jégou, Marie-France Carlier & Guillaume Romet-Lemonne
Cytoskeleton assembly is instrumental in the regulation of biological functions by physical forces. In a number of key cellular processes, actin filaments elongated by formins such as mDia are subject to mechanical tension, yet how mechanical forces modulate the assembly of actin filaments is an open question. Here, using the viscous drag of a microfluidic flow, we apply calibrated piconewton pulling forces to individual actin filaments that are being elongated at their barbed end by surface-anchored mDia1 proteins. We show that mDia1 is mechanosensitive and that the elongation rate of filaments is increased up to two-fold by the application of a pulling force. We also show that mDia1 is able to track a depolymerizing barbed end in spite of an opposing pulling force, which means that mDia1 can efficiently put actin filaments under mechanical tension. Our findings suggest that formin function in cells is tightly coupled to the mechanical activity of other machineries.

Optical binding of magnetodielectric Rayleigh particles

Patrick C. Chaumet and Adel Rahmani 

We present a theoretical and numerical study of the optical binding and optical torque between two Rayleigh particles with arbitrary, complex, scalar dielectric permittivity and magnetic permeability. We use a computational approach based on the discrete dipole approximation to derive the optical force and torque experienced by the particles when illuminated by a linearly or circularly polarized plane wave. We show that optical binding between magnetodielectic particles is qualitatively different from the traditional case involving dielectric particles only. In particular, we show that for certain configurations, the system of two magnetodielectric particles will experience a long-range optical torque whose amplitude envelope does not decay with the separation between the particles.


Direct imaging of single UvrD helicase dynamics on long single-stranded DNA

Kyung Suk Lee, Hamza Balci, Haifeng Jia, Timothy M. Lohman & Taekjip Ha
Fluorescence imaging of single-protein dynamics on DNA has been largely limited to double-stranded DNA or short single-stranded DNA. We have developed a hybrid approach for observing single proteins moving on laterally stretched kilobase-sized ssDNA. Here we probed the single-stranded DNA translocase activity of Escherichia coli UvrD by single fluorophore tracking, while monitoring DNA unwinding activity with optical tweezers to capture the entire sequence of protein binding, single-stranded DNA translocation and multiple pathways of unwinding initiation. The results directly demonstrate that the UvrD monomer is a highly processive single-stranded DNA translocase that is stopped by a double-stranded DNA, whereas two monomers are required to unwind DNA to a detectable degree. The single-stranded DNA translocation rate does not depend on the force applied and displays a remarkable homogeneity, whereas the unwinding rate shows significant heterogeneity. These findings demonstrate that UvrD assembly state regulates its DNA helicase activity with functional implications for its stepping mechanism, and also reveal a previously unappreciated complexity in the active species during unwinding.

Nematic liquid crystal boojums with handles on colloidal handlebodies

Qingkun Liu, Bohdan Senyuk, Mykola Tasinkevych, and Ivan I. Smalyukh
Topological defects that form on surfaces of ordered media, dubbed boojums, are ubiquitous in superfluids, liquid crystals (LCs), Langmuir monolayers, and Bose–Einstein condensates. They determine supercurrents in superfluids, impinge on electrooptical switching in polymer-dispersed LCs, and mediate chemical response at nematic-isotropic fluid interfaces, but the role of surface topology in the appearance, stability, and core structure of these defects remains poorly understood. Here, we demonstrate robust generation of boojums by controlling surface topology of colloidal particles that impose tangential boundary conditions for the alignment of LC molecules. To do this, we design handlebody-shaped polymer particles with different genus g. When introduced into a nematic LC, these particles distort the nematic molecular alignment field while obeying topological constraints and induce at least 2g − 2 boojums that allow for topological charge conservation. We characterize 3D textures of boojums using polarized nonlinear optical imaging of molecular alignment and explain our findings by invoking symmetry considerations and numerical modeling of experiment-matching director fields, order parameter variations, and nontrivial handle-shaped core structure of defects. Finally, we discuss how this interplay between the topologies of colloidal surfaces and boojums may lead to controlled self-assembly of colloidal particles in nematic and paranematic hosts, which, in turn, may enable reconfigurable topological composites.

Friday, May 24, 2013

Subwavelength particles in an inhomogeneous light field: optical forces associated with the spin and orbital energy flows

A Ya Bekshaev
We analyse the ponderomotive action experienced by a small spherical particle immersed in an optical field, in relation to the internal energy flows (optical currents) and their spin and orbital constituents. The problem is studied analytically, on the basis of the dipole model, and numerically. The three sources of the field mechanical action—the energy density gradient and the orbital and spin parts of the energy flow—differ in their ponderomotive mechanisms, and their physical nature manifests itself in the dependence of the optical force on the particle radius a. If a λ (the radiation wavelength), the optical force behaves as aν, and integer ν can be used to classify the sources of the mechanical action. This classification correlates with the multipole representation of the field–particle interaction: the gradient force and the orbital momentum force appear due to the electric or magnetic dipole moments per se; the spin momentum force emerges due to interaction between the electric and magnetic dipoles or between the dipole and quadrupole moments (if the particle is polarizable electrically but not magnetically or vice versa). In principle, the spin and orbital currents can be measured separately through the probe particle motion, employing a special choice of particles with the necessary magnetic and/or electric properties.


Differential interference contrast microscopy using light-emitting diode illumination in conjunction with dual optical traps

C. Battle, L. Lautscham, and C. F. Schmidt
Differential interference contrast (DIC) microscopy is a common mode of biological light microscopy used to achieve maximal resolution and contrast with label-free, weakly absorbing specimens such as cells. Maintaining the polarization state of the illuminating light is essential for the technique, and this requirement can conflict with optical trapping. We describe how to optimize DIC imaging using a light-emitting diode illumination source in a microscope while integrating a dual optical trap into the set up. Every time a polarized light beam reflects off or transmits through a dichroic mirror in the beam path, its polarization state will change if it is not polarized exactly parallel (p) or perpendicular (s) to the plane of incidence. We observe wavelength-dependent optical rotation and depolarization effects in our illumination light upon reflection from/transmission through dichroic mirrors in the beam path, resulting in significant degradation of image quality. We describe a method to compensate for these effects by introducing quarter-waveplates and a laser clean-up filter into the imaging pathway. We show that this approach achieves a full recovery of image quality.

Computation of radiation pressure force on arbitrary shaped homogenous particles by multilevel fast multipole algorithm

Minglin Yang, Kuan Fang Ren, Mingjiang Gou, and Xinqing Sheng
A full-wave numerical method based on the surface integral equation for computing radiation pressure force (RPF) exerted by a shaped light beam on arbitrary shaped homogenous particles is presented. The multilevel fast multipole algorithm is employed to reduce memory requirement and to improve its capability. The resultant matrix equation is solved by using an iterative solver to obtain equivalent electric and magnetic currents. Then RPF is computed by vector flux of the Maxwell’s stress tensor over a spherical surface tightly enclosing the particle. So the analytical expressions for electromagnetic fields of incident beam in near region are used. Some numerical results are performed to illustrate the validity and capability of the developed method. Good agreements between our method and the Lorenz–Mie theory for spherical and small spheroidal particle are found while our method has powerful capability for computing RPF of any shaped beam on a relatively large particle of complex shape. Tests for ellipsoidal and red blood cell-like particles illuminated by Gaussian beam have shown that the size of the particle can be as large as 50–100 wavelengths, respectively, for the relative refractive of 1.33 and 1.1.

Monday, May 20, 2013

Optical trapping map of dielectric spheres

Murat Muradoglu and Tuck Wah Ng
Many applications use a focused Gaussian laser beam to manipulate spherical dielectric particles. The axial trapping efficiency of this process is a function of (i) the particle radius r, (ii) the ratio of the refractive index of particle over the medium, and (iii) the numerical aperture of the delivered light beam. During what we believe is the first comprehensive simulation of its kind, we uncovered optical trapping regions in the three-dimensional (3D) parameter space forming an iso-surface landscape with ridge-like contours. Using specific points in the parameter space, we drew attention to difficulties in using the trapping efficiency and stiffness metrics in defining how well particles are drawn into and held in the trap. We have proposed an alternative calculation based on the maximum forward and restoration values of the trapping efficiency in the axial sense, called the trapping quality. We also discuss the manner in which the ridge regions may be harnessed for effective particle sorting, how the optical trapping blind spots can be used in applications that seek to eschew photothermal damage, and how trapping can proceed when many parameters change, such as when swelling occurs.

The effects of multiple scattering to optical forces on a sphere in an evanescent field

Wei-Ping Zang, Yang Yang, Zhi-Yu Zhao, and Jian-Guo Tian

In this paper we discuss the effects of multiple scattering to the optical forces on a particle by an evanescent field. We show that the iterative method to process the effects of the interaction between the particle and a plane surface is invalid when the radius of particle is large or when the structural resonance of the particle occurs. By using the generalized minimum residual method to solve the set of equations directly, the divergence appears in the iterative method can be removed completely. As an illustrative example, we discussed the effects of multiple scattering to optical forces on a particle in an evanescent field from an incident plane wave. The interpretations of numerical results are presented in detail.

Single-cell and metagenomic analyses indicate a fermentative and saccharolytic lifestyle for members of the OP9 lineage

Jeremy A. Dodsworth, Paul C. Blainey, Senthil K. Murugapiran, Wesley D. Swingley, Christian A. Ross, Susannah G. Tringe, Patrick S. G. Chain, Matthew B. Scholz, Chien-Chi Lo, Jason Raymond, Stephen R. Quake & Brian P. Hedlund
OP9 is a yet-uncultivated bacterial lineage found in geothermal systems, petroleum reservoirs, anaerobic digesters and wastewater treatment facilities. Here we use single-cell and metagenome sequencing to obtain two distinct, nearly complete OP9 genomes, one constructed from single cells sorted from hot spring sediments and the other derived from binned metagenomic contigs from an in situ-enriched cellulolytic, thermophilic community. Phylogenomic analyses support the designation of OP9 as a candidate phylum for which we propose the name ‘Atribacteria’. Although a plurality of predicted proteins is most similar to those from Firmicutes, the presence of key genes suggests a diderm cell envelope. Metabolic reconstruction from the core genome suggests an anaerobic lifestyle based on sugar fermentation by Embden–Meyerhof glycolysis with production of hydrogen, acetate and ethanol. Putative glycohydrolases and an endoglucanase may enable catabolism of (hemi)cellulose in thermal environments. This study lays a foundation for understanding the physiology and ecological role of the ‘Atribacteria’.


Thursday, May 16, 2013

Spatially-resolved rotational microrheology with an optically-trapped sphere

James S. Bennett, Lachlan J. Gibson, Rory M. Kelly, Emmanuel Brousse, Bastian Baudisch, Daryl Preece, Timo A. Nieminen, Timothy Nicholson, Norman R. Heckenberg & Halina Rubinsztein-Dunlop
We have developed a microrheometer, based on optical tweezers, in which hydrodynamic coupling between the probe and fluid boundaries is dramatically reduced relative to existing microrheometers. Rotational Brownian motion of a birefringent microsphere within an angular optical trap is observed by measuring the polarisation shifts of transmitted light. Data gathered in this manner, in the strongly viscoelastic fluid Celluvisc, quantitatively agree with the results of conventional (bulk) rheometry. Our technique will significantly reduce the smallest sample volumes which may be reliably probed, further extending the study of rare, difficult to obtain or highly nonhomogeneous fluids.

Tuesday, May 14, 2013

Simultaneous Analysis of the Equilibrium Hygroscopicity and Water Transport Kinetics of Liquid Aerosol

James F. Davies , Allen E. Haddrell , Andrew MJ Rickards , and Jonathan Philip Reid
We demonstrate that the equilibrium hygroscopic response of an aerosol droplet and the kinetics of water condensation and evaporation can be retrieved with high accuracy, even close to saturation, through comparative measurements of probe and sample aerosol droplets. The experimental methodology is described and is based on an electrodynamic balance with a newly designed trapping chamber. Through use of a probe aerosol, composed of either pure water or a sodium chloride solution of known concentration, the gas phase relative humidity (RH) can be accurately measured with an uncertainty of typically <0.005. By fast manipulation of the airflows into the chamber, a step-change in RH over a timescale <0.5 s can be achieved. Using this approach, the kinetics of mass transfer are studied using the comparative procedure, and results are compared to theoretical mass flux predictions. The time-dependent measured mass fluxes for sodium chloride, ammonium sulfate, sorbitol and galactose are used to calculate droplet water activities as a function of the droplet growth factor, allowing retrieval of a hygroscopic growth curve in a matter of seconds. Comparisons with both new and established thermodynamic predictions of hygroscopicity, as well as to optical tweezers measurements, are presented, demonstrating good agreement within the experimental uncertainties.

Measurements of Light Extinction by Single Aerosol Particles

Jim S. Walker , Antonia E. Carruthers , Andrew J. Orr-Ewing, and Jonathan P. Reid
A Bessel beam optical trap is combined with continuous wave cavity ringdown spectroscopy to measure the extinction cross section of individual aerosol particles. Particles, 1 μm in size, can be captured indefinitely and processes that transform size or refractive index studied. The measured light extinction induced by the particle is shown to depend on the position of the particle in the cavity, allowing accurate measurements of the mode structure of a high finesse optical cavity without significant perturbation. The variation in extinction efficiency of a sodium chloride droplet with relative humidity is shown to agree well with predictions from Mie scattering theory.

Direct formation of giant unilamellar vesicles from microparticles of polyion complexes and investigation of their properties using a microfluidic chamber

Hidehiro Oana, Mutsuki Morinaga, Akihiro Kishimura, Kazunori Kataok and Masao Washizu

Although hollow microscopic capsules have a variety of potential biomedical applications, reports of organic-solvent-free methods for their preparation are rather limited. Herein, a novel approach is demonstrated for organic-solvent-free preparation of giant unilamellar vesicles utilizing the unique response of polyion complexes (PICs) to changes in additive salt concentration. A microfluidic device consisting of a main channel bearing side pockets that work as microscale reaction chambers is designed for facilitating the preparation process under an optical microscope. With this device, real-time observation of morphological transformation of individual PIC microparticles is carried out during rapid reduction of the additive salt concentration and direct formation of giant vesicles from PIC microparticles is shown. There is a quasilinear relationship between the surface areas of the formed vesicles and the volumes of the PIC microparticles, and the thickness of the vesicle membrane estimated by the relationship is indicative of the formation of a uniform unilamellar structure of the PIC membrane. Furthermore, detailed properties of the formed PIC vesicles with regard to salt response, loading of guest molecules, and permeability of the PIC membrane with/without modification of the PIC membrane by cross-linking are investigated using the microfluidic chamber. Thus, the usefulness of the microfluidic chamber for visualization and investigation of dynamic responses of microscale soft materials during changes in surrounding conditions is also demonstrated.

Modular Aspects of Kinesin Force Generation Machinery

William R. Hesse, Miriam Steiner, Matthew L. Wohlever, Roger D. Kamm, Wonmuk Hwang, Matthew J. Lang
The motor head of kinesin carries out microtubule binding, ATP hydrolysis, and force generation. Despite a high level of sequence and structural conservation, subtle variations in subdomains of the motor head determine family-specific properties. In particular, both Kinesin-1 (Kin-1) and Kinesin-5 (Kin-5) walk processively to the microtubule plus-end, yet show distinct motility characteristics suitable for their functions. We studied chimeric Kin-1/Kin-5 constructs with a combination of single molecule motility assays and molecular dynamics simulations to demonstrate that Kin-5 possesses a force-generating element similar to Kin-1, i.e., the cover-neck bundle. Furthermore, the Kin-5 neck linker makes additional contacts with the core of the motor head via loop L13, which putatively compensates for the shorter cover-neck bundle of Kin-5. Our results indicate that Kin-1 is mechanically optimized for individual cargo transport, whereas Kin-5 does not necessarily maximize its mechanical performance. Its biochemical rates and enhanced force sensitivity may instead be beneficial for operation in a group of motors. Such variations in subdomains would be a strategy for achieving diversity in motility with the conserved motor head.

Spherical vortex beams of high radial degree for enhanced single-beam tweezers

Diego Baresch, Jean-Louis Thomas, and Régis Marchiano

We demonstrate gradient optical forces in metal-dielectric hybrid plasmonic waveguides (HPWG) for the first time. The magnitude of optical force is quantified through excitation of the nanomechanical vibration of the suspended waveguides. Integrated Mach-Zehnder interferometry is utilized to transduce the mechanical motion and characterize the propagation loss of the HPWG. Compared with theory, the experimental results have confirmed the optical force enhancement, but also suggested a significantly higher optical loss in HPWG. The excessive loss is attributed to metal surface roughness and other non-idealities in the device fabrication process.


Three-dimensional light-scattering and deformation of individual biconcave human blood cells in optical tweezers

Lingyao Yu, Yunlong Sheng, and Arthur Chiou
For studying the elastic properties of a biconcave red blood cell using the dual-trap optical tweezers without attaching microbeads to the cell, we implemented a three-dimensional finite element simulation of the light scattering and cell’s deformation using the coupled electromagnetic and continuum mechanics modules. We built the vector field of the trapping beams, the cell structure layout, the hyperelastic and viscoelastic cell materials, and we reinforced the constraints on the cell constant volume in the simulation. This computation model can be useful for studying the scattering and the other mechanical properties of the biological cells.

Thursday, May 9, 2013

Brownian motion at short time scales

Tongcang Li, Mark G. Raizen
Brownian motion has played important roles in many different fields of science since its origin was first explained by Albert Einstein in 1905. Einstein's theory of Brownian motion, however, is only applicable at long time scales. At short time scales, Brownian motion of a suspended particle is not completely random, due to the inertia of the particle and the surrounding fluid. Moreover, the thermal force exerted on a particle suspended in a liquid is not a white noise, but is colored. Recent experimental developments in optical trapping and detection have made this new regime of Brownian motion accessible. This review summarizes related theories and recent experiments on Brownian motion at short time scales, with a focus on the measurement of the instantaneous velocity of a Brownian particle in a gas and the observation of the transition from ballistic to diffusive Brownian motion in a liquid.

Dye lasing in optically manipulated liquid aerosols

Y. Karadag, M. Aas, A. Jonáš, S. Anand, D. McGloin, and A. Kiraz
We report lasing in airborne, rhodamine B-doped glycerol–water droplets with diameters ranging between 7.7 and 11.0 μm, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser, the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of the intensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. The average wavelength of the lasing WGMs could be tuned between 600 and 630 nm by changing the droplet size. These results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium.

Wednesday, May 8, 2013

Dynamic optical tweezers based assay for monitoring early drug resistance

Xiaojing Wu, Yuquan Zhang, Changjun Min, Siwei Zhu, Jie Feng and X-C Yuan
In this letter, a dynamic optical tweezers based assay is proposed and investigated for monitoring early drug resistance with Pemetrexed-resistant non-small cell lung cancer (NSCLC) cell lines. The validity and stability of the method are verified experimentally in terms of the physical parameters of the optical tweezers system. The results demonstrate that the proposed technique is more convenient and faster than traditional techniques when the capability of detecting small variations of the response of cells to a drug is maintained.

Brownian nanoimaging of interface dynamics and ligand–receptor binding at cell surfaces in 3-D

Igor R. Kuznetsov, Evan A. Evans
We describe a method for nanoimaging interfacial dynamics and ligand–receptor binding at surfaces of live cells in 3-D. The imaging probe is a 1-μm diameter glass bead confined by a soft laser trap to create a “cloud” of fluctuating states. Using a facile on-line method of video image analysis, the probe displacements are reported at ∼10 ms intervals with bare precisions (±SD) of 4–6 nm along the optical axis (elevation) and 2 nm in the transverse directions. We demonstrate how the Brownian distributions are analyzed to characterize the free energy potential of each small probe in 3-D taking into account the blur effect of its motions during CCD image capture. Then, using the approach to image interactions of a labeled probe with lamellae of leukocytic cells spreading on cover-glass substrates, we show that deformations of the soft distribution in probe elevations provide both a sensitive long-range sensor for defining the steric topography of a cell lamella and a fast telemetry for reporting rare events of probe binding with its surface receptors. Invoking established principles of Brownian physics and statistical thermodynamics, we describe an off-line method of super resolution that improves precision of probe separations from a non-reactive steric boundary to ∼1 nm.

Monday, May 6, 2013

Active Shape-Morphing Elastomeric Colloids in Short-Pitch Cholesteric Liquid Crystals

Julian S. Evans, Yaoran Sun, Bohdan Senyuk, Patrick Keller, Victor M. Pergamenshchik, Taewoo Lee, and Ivan I. Smalyukh
Active elastomeric liquid crystal particles with initial cylindrical shapes are obtained by means of soft lithography and polymerization in a strong magnetic field. Gold nanocrystals infiltrated into these particles mediate energy transfer from laser light to heat, so that the inherent coupling between the temperature-dependent order and shape allows for dynamic morphing of these particles and well-controlled stable shapes. Continuous changes of particle shapes are followed by their spontaneous realignment and transformations of director structures in the surrounding cholesteric host, as well as locomotion in the case of a nonreciprocal shape morphing. These findings bridge the fields of liquid crystal solids and active colloids, may enable shape-controlled self-assembly of adaptive composites and light-driven micromachines, and can be understood by employing simple symmetry considerations along with electrostatic analogies.

Rapid internal contraction boosts DNA friction

Oliver Otto, Sebastian Sturm, Nadanai Laohakunakorn, Ulrich F. Keyser & Klaus Kroy
Macroscopic objects are usually manipulated by force and observed with light. On the nanoscale, however, this is often done oppositely: individual macromolecules are manipulated by light and monitored with force. This procedure, which is the basis of single-molecule force spectroscopy, has led to much of our quantitative understanding of how DNA works, and is now routinely applied to explore molecular structure and interactions, DNA–protein reactions and protein folding. Here we develop the technique further by introducing a dynamic force spectroscopy set-up for a non-invasive inspection of the tension dynamics in a taut strand of DNA. The internal contraction after a sudden release of the molecule is shown to give rise to a drastically enhanced viscous friction, as revealed by the slow relaxation of an attached colloidal tracer. Our systematic theory explains the data quantitatively and provides a powerful tool for the rational design of new dynamic force spectroscopy assays.

Using GPUs for Realtime Prediction of Optical Forces on Microsphere Ensembles

Sujal Bista, Sagar Chowdhury, Satyandra K. Gupta and Amitabh Varshney
Laser beams can be used to create optical traps that can hold and transport small particles. Optical trapping has been used in a number of applications ranging from prototyping at the microscale to biological cell manipulation. Successfully using optical tweezers requires predicting optical forces on the particle being trapped and transported. Reasonably accurate theory and computational models exist for predicting optical forces on a single particle in the close vicinity of a Gaussian laser beam. However, in practice the workspace includes multiple particles that are manipulated using individual optical traps. It has been experimentally shown that the presence of a particle can cast a shadow on a nearby particle and hence affect the optical forces acting on it. Computing optical forces in the presence of shadows in real-time is not feasible on CPUs. In this paper, we introduce a ray-tracing-based application optimized for GPUs to calculate forces exerted by the laser beams on microparticle ensembles in an optical tweezers system. When evaluating the force exerted by a laser beam on 32 interacting particles, our GPU-based approach is able to get a 66-fold speed up compared to a single core CPU implementation of traditional Ashkin's approach and a 10-fold speedup over the single core CPU-based implementation of our approach.

Nonequilibrium fluctuations of mechanically stretched single red blood cells detected by optical tweezers

Michal Wojdyla, Saurabh Raj, Dmitri Petrov
We study the thermal and out-of-equilibrium mechanical dynamics of single, living human red blood cells (RBCs) by combining two-probe passive and active microrheology techniques. Both experiments were performed quasisimultaneously on the same cell using two identical polystyrene probes, biochemically attached to the cell membrane. We obtained compelling evidence of nonequilibrium fluctuations in the RBCs under physiological condition and without the influence of any external chemicals. The spectral distributions of metabolically driven forces and viscoelastic response were evaluated in the relaxed and stretched states, intended to simulate the varying natural environment of the cells during blood circulation. We found that the internally generated forces are more pronounced in the stretched state, suggesting a stress-dependent RBC activity.

Friday, May 3, 2013

Construction and actuation of a microscopic gear assembly formed using optical tweezers

Jung-Dae Kim and Yong-Gu Lee
The assembly of micrometer-sized parts is an important manufacturing process; any development in it could potentially change the current manufacturing practices for micrometer-scale devices. Due to the lack of reliable microassembly techniques, these devices are often manufactured using silicon, which includes etching and depositions with little use of assembly processes. The result is the requirement of specialized manufacturing conditions with hazardous byproducts and limited applications where only simple mechanisms are allowed. Optical tweezers are non-contact type manipulators that are very suitable for assembling microparts and solve one of the most difficult problems for microassembly, which is the sticking of the physical manipulator to the micropart. Although contact type manipulators can be surface modified to be non-sticky, this involves extra preprocessing—optical tweezers do not require such additional efforts. The weakness of using optical tweezers is that the permanent assembly of parts is not possible as only very small forces can be applied. We introduce an advanced microassembly environment with the combined use of optical tweezers and a motorized microtip, where the former is used to position two parts and the latter is used to introduce deformation in the parts so that they form a strongly fitted assembly.

Construction of a High Resolution Microscope with Conventional and Holographic Optical Trapping Capabilities

Jacqualine Butterfield, Weili Hong, Leslie Mershon, Michael Vershinin
High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular organelles 2,3, as well as for high spatial and temporal resolution readout of their position relative to the center of the trap. The system described herein has one such "traditional" trap operating at 980 nm. It additionally provides a second optical trapping system that uses a commercially available holographic package to simultaneously create and manipulate complex trapping patterns in the field of view of the microscope 4,5 at a wavelength of 1,064 nm. The combination of the two systems allows for the manipulation of multiple refractive objects at the same time while simultaneously conducting high speed and high resolution measurements of motion and force production at nanometer and piconewton scale.

Thursday, May 2, 2013

Laser-driven microflow-induced bistable orientation of a nematic liquid crystal in perfluoropolymer-treated unrubbed cells

V. S. R. Jampani, M. Sǩarabot, H. Takezoe, I. Muševič, and S. Dhara

We demonstrate laser-driven microflow-induced orientational change (homeotropic to planar) in a dye-doped nematic liquid crystal. The homeotropic to planar director alignment is achieved in unrubbed cells in the thermal hysteresis range of a discontinuous anchoring reorientation transition due to the local heating by light absorption in dye-doped sample. Various bistable patterns were recorded in the cell by a programmable laser tweezers. The width of the patterns depend on the scanning speed of the tightly focussed laser beam and the minimum width obtained is ≃0.57μm which is about 35 times smaller than the earlier report in the rubbed cells. We show that the motion of the microbeam spot causes local flow as a result the liquid crystal director is aligned along that direction.


Wednesday, May 1, 2013

Resonant Excitation Effect on Optical Trapping of Myoglobin: The Important Role of a Heme Cofactor

Tatsuya Shoji , Noboru Kitamura , and Yasuyuki Tsuboi
We demonstrate the efficient trapping of myoglobin (Mb), which is a small protein, in aqueous solution. Conventional optical tweezers exert barely sufficient radiation force (RF) to manipulate nanometer-sized dielectric objects such as small proteins and organic molecules. One possible candidate to enhance the RF is to use laser light with a wavelength that is electronically resonant with the electronic transition of the object to be trapped; resonant optical trapping (ROT). In this study, ROT of Mb was investigated in aqueous solution using a 1064 nm laser beam. We found that trapping of Mb was accompanied by an increase in the near-infrared absorbance of a heme cofactor in Mb. By contrast, trapping of Mb without the heme cofactor (apomyoglobin) was not detected at all. This clearly indicates that the heme cofactor plays a crucial role in the ROT of Mb. Furthermore, confocal Raman microspectroscopy indicated structural conformational changes to the trapped Mb. Such a ROT technique would open up new channels in the development of nano-bioscience, such as transportation/ crystallization techniques and a selectively molecular sorting technique for biomolecules.

Electrically tunable optoelastic interaction range of nematic colloids

Luigino Criante, Francesco Bracalente, Liana Lucchetti, Francesco Simoni and Etienne Brasselet
We report on the electrical tuning of the interaction range between colloidal particles in nematic liquid crystals. The experimental demonstration relies on the study of the dynamics of a real colloidal particle made of a glass sphere in the presence of an artificial laser-induced colloid that results from the local distortion of the nematic by a Gaussian light beam. The optoelastic interaction range between such a pair of colloids is shown to be fully controlled electrically. Moreover, all the observations are well described by an analytical model that accounts for the field-induced reorientation of the liquid crystal and the overlap between the long-range director distortion around both colloids.


DNA Interactions in Crowded Nanopores

Nadanai Laohakunakorn, Sandip Ghosal, Oliver Otto, Karolis Misiunas, and Ulrich F Keyser
The motion of DNA in crowded environments is a common theme in physics and biology. Examples include gel electrophoresis and the self interaction of DNA within cells and viral capsids. Here we study the interaction of multiple DNA molecules within a nanopore by tethering the DNA to a bead held in a laser optical trap to produce a `molecular tug-of-war'. We measure this tether force as a function of the number of DNA molecules in the pore and show that the force per molecule decreases with the number of molecules. A simple scaling argument based on a mean field theory of the hydrodynamic interactions between multiple DNA strands explains our observations. At high salt concentrations, when the Debye length approaches the size of the counterions, the force per molecule becomes essentially independent of the number of molecules. We attribute this to the sharp decrease in electroosmotic flow which makes the hydrodynamic interactions ineffective.

Cell Signaling Experiments Driven by Optical Manipulation

Francesco Difato, Giulietta Pinato, and Dan Cojoc
Cell signaling involves complex transduction mechanisms in which information released by nearby cells or extracellular cues are transmitted to the cell, regulating fundamental cellular activities. Understanding such mechanisms requires cell stimulation with precise control of low numbers of active molecules at high spatial and temporal resolution under physiological conditions. Optical manipulation techniques, such as optical tweezing, mechanical stress probing or nano-ablation, allow handling of probes and sub-cellular elements with nanometric and millisecond resolution. PicoNewton forces, such as those involved in cell motility or intracellular activity, can be measured with femtoNewton sensitivity while controlling the biochemical environment. Recent technical achievements in optical manipulation have new potentials, such as exploring the actions of individual molecules within living cells. Here, we review the progress in optical manipulation techniques for single-cell experiments, with a focus on force probing, cell mechanical stimulation and the local delivery of active molecules using optically manipulated micro-vectors and laser dissection.