Holographic optical tweezers and their relevance to lab on chip devices

Miles Padgett and Roberto Di Leonardo

During the last decade, optical tweezers have been transformed by the combined availability of spatial light modulators and the speed of low-cost computing to drive them. Holographic optical tweezers can trap and move many objects simultaneously and their compatibility with other optical techniques, particularly microscopy, means that they are highly appropriate to lab-on-chip systems to enable optical manipulation, actuation and sensing.

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Simultaneous Observation of Tail and Head Movements of Myosin V during Processive Motion

Hailong Lu, Guy G. Kennedy, David M. Warshaw, and Kathleen M. Trybus

Processive stepping of myosin Va (myoV) has been tracked by monitoring either the tail position (center of mass) or the position of one or both heads. Here, we combine these two approaches by attaching a quantum dot to one of the motor domains and a bead to the tail. Using laser trapping and total internal reflection microscopy, the position of one head and the tail are observed simultaneously as myoV moves processively on an actin filament bundle against the resistive load of the laser trap. The head moves one step (73 ± 10 nm) for every two steps of the tail (35 ± 9 nm). One tail step occurs concurrently with quantum dot-labeled head movement, whereas the other occurs with movement of the unlabeled head, consistent with a hand-over-hand model. Load increases the probability of the motor taking a back step. The back step is triggered by the motor taking a shorter forward step (head step, 68 ± 11 nm; tail step, 32 ± 10 nm), likely one actin monomer short of its preferred binding site. During a back step, the motor reverses its hand-over-hand motion, with the leading head detaching and reattaching to one of multiple actin sites behind the trailing head. After a back step, the motor can correct its mistake and step processively forward at resistive loads <0.7 piconewton or stall or detach at higher loads. Back stepping may provide a mechanism to ensure efficient cargo delivery even when myoV encounters obstacles within the actin cytoskeletal meshwork or when other motors are attached to the same cargo.

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Enhancement by optical force of separation in pinched flow fractionation

Kyung Heon Lee, Sang Bok Kim, Kang Soo Lee and Hyung Jin Sung

A method for improving the size-based particle separation technique known as pinched flow fractionation (PFF) has been demonstrated experimentally and analyzed by performing numerical calculations. Since the particles in the pinched region are pushed by an optical scattering force, the original particle position with respect to the wall is modulated. This position modulation in the pinched region is amplified in the broadened region along the streamline. This enhancement of separation is achieved by imposing an optical force on the original PFF design. Three different polystyrene latexes (PSLs) with diameters of 2, 5, and 10 μm were separated with PFF and optically enhanced PFF (OEPFF) devices. The separations achieved with the two devices were compared and enhancements in the separation distance by factors of up to approximately 15 were achieved. Theoretical calculations were also performed to interpret these results.

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Rotation of single bacterial cells relative to the optical axis using optical tweezers

G. Carmon and M. Feingold

Using a single-beam, oscillating optical tweezers, we demonstrate trapping and rotation of rod-shaped bacterial cells with respect to the optical axis. The angle of rotation, θ, is determined by the amplitude of the oscillation. It is shown that θ can be measured from the longitudinal cell intensity profiles in the corresponding phase-contrast images. The technique allows viewing the cell from different perspectives and can provide a useful tool in fluorescence microscopy for the analysis of three-dimensional subcellular structures.

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An interdisciplinary systems approach to study sperm physiology and evolution

Linda Z. Shi, Jaclyn Nascimento, Elliot Botvinick, Barbara Durrant, Michael W. Berns

Optical trapping is a noninvasive biophotonic tool that has been developed to study the physiological and biomechanical properties of cells. The custom-designed optical system is built to direct near-infrared laser light into an inverted microscope to create a single-point three-dimensional gradient laser trap at the microscope focal point. A real-time automated tracking and trapping system (RATTS) is described that provides a remote user-friendly robotic interface. The combination of laser tweezers, fluorescent imaging, and RATTS can measure sperm swimming speed and swimming force simultaneously with mitochondrial membrane potential (MMP). The roles of two sources of adenosine triphosphate in sperm motility/energetics are studied: oxidative phosphorylation, which occurs in the mitochondria located in the sperm midpiece, and glycolysis, which occurs along the length of the sperm tail (flagellum). The effects of glucose, oxidative phosphorylation inhibitors, and glycolytic inhibitors on human sperm motility are studied. This combination of photonic physical and engineering tools has been used to examine the evolutionary effect of sperm competition in primates. The results demonstrate a correlation between mating type and sperm motility: sperm from polygamous (multi-partner) primate species swim faster and with greater force than sperm from polygynous (single partner) primate species. In summary, engineering and biological systems are combined to provide a powerful interdisciplinary approach to study the complex biological systems that drive the sperm toward the egg.

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Optical trapping force combining an optical fiber probe and an AFM metallic probe

Binghui Liu, Lijun Yang, and Yang Wang

A high-resolution optical trapping and manipulating scheme combining an optical fiber probe and an AFM metallic probe is proposed. This scheme is based on the combination of evanescent illumination and light scattering at the metallic probe apex, which shapes the optical field into a localized, three-dimensional optical trap. Detailed simulations of the electromagnetic fields in composite area and the resulting forces are described the methods of Maxwell stress tensor and three-dimensional FDTD. Calculations show that the scheme is able to overcome the disturbance of other forces to trap a polystyrene particle of up to 10nm in radius with lower laser intensity (~1040W/mm2) than that required by conventional optical tweezers (~105W/mm2). Based on the discussion of high manipulating efficiency dependent on system parameters and the implementing procedure, the scheme allowing for effective manipulation of nano-particles opens a way for research on single nano-particle area.

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Three-dimensional parallel particle manipulation and tracking by integrating holographic optical tweezers and engineered point spread functions

Donald B. Conkey, Rahul P. Trivedi, Sri Rama Prasanna Pavani,Ivan I. Smalyukh, and Rafael Piestun

We demonstrate an integrated holographic optical tweezers system with double-helix point spread function (DH-PSF) imaging for high precision three-dimensional multi-particle tracking. The tweezers system allows for the creation and control of multiple optical traps in three-dimensions, while the DH-PSF allows for high precision, 3D, multiple-particle tracking in a wide field. The integrated system is suitable for particles emitting/scattering either coherent or incoherent light and is easily adaptable to existing holographic tweezers systems. We demonstrate simultaneous tracking of multiple micro-manipulated particles and perform quantitative estimation of the lateral and axial forces in an optical trap by measuring the fluid drag force exerted on the particles. The system is thus capable of unveiling complex 3D force landscapes that make it suitable for quantitative studies of interactions in colloidal systems, biological materials, and a variety of soft matter systems.

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In vivo lipidomics using single-cell Raman spectroscopy

Huawen Wu, Joanne V. Volponi, Ann E. Oliver, Atul N. Parikh, Blake A. Simmons, and Seema Singh

We describe a method for direct, quantitative, in vivo lipid profiling of oil-producing microalgae using single-cell laser-trapping Raman spectroscopy. This approach is demonstrated in the quantitative determination of the degree of unsaturation and transition temperatures of constituent lipids within microalgae. These properties are important markers for determining engine compatibility and performance metrics of algal biodiesel. We show that these factors can be directly measured from a single living microalgal cell held in place with an optical trap while simultaneously collecting Raman data. Cellular response to different growth conditions is monitored in real time. Our approach circumvents the need for lipid extraction and analysis that is both slow and invasive. Furthermore, this technique yields real-time chemical information in a label-free manner, thus eliminating the limitations of impermeability, toxicity, and specificity of the fluorescent probes common in currently used protocols. Although the single-cell Raman spectroscopy demonstrated here is focused on the study of the microalgal lipids with biofuel applications, the analytical capability and quantitation algorithms demonstrated are applicable to many different organisms and should prove useful for a diverse range of applications in lipidomics.

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All-optical nano modulator, sensor, wavelength converter, logic gate, and flip flop based on a manipulated gold nanoparticle

Asaf Shahmoon, Yoed Abraham, and Ofer Limon, Liora Bitton and Aviad Frydman, Ron Unger, Zeev Zalevsky
We developed a device in which one can shift and control the position of a gold nanoparticle by using special type of optical tweezers realized by guiding and confining light in a nanosize void structure in which the nanoparticle is placed. The nanosize void is positioned inside a multimode interference (MMI) region of a silicon waveguide. The coupling of light from two opposite sides of the optical device generates standing interference waves in the MMI region. The relative phase between the two coupled beams is controllable and therefore also the position of the fringes of the standing waves. Evanescent tails coming from the guided standing waves interfere in the void and allow control the position of the trapped nanoparticle. A nanoparticle with diameter of 30 nm was experimentally implanted in the void. The particle was trapped by one of the high intensity evanescent fringes. Changing the relative phase between the two inputs to the chip allowed us experimentally to modify the location of the fringes and the position of the particle (similarly to what happens in optical tweezers). This experimentally demonstrated capability may be useful for all-optical nano modulators, sensors, wavelength converters, logic gates and even a state machine (e.g. a flip flop).

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The observable pressure of light in dielectric fluids

Brandon A. Kemp and Tomasz M. Grzegorczyk

By considering a perfect reflector submerged in a dielectric fluid, we show that the Minkowski formulation describes the optical momentum transfer to submerged objects. This result is required by global energy conservation, regardless of the phase of the reflected wave. While the electromagnetic pressure on a submerged reflector can vary with phase of the mirror reflection coefficient between twice the Abraham momentum and twice the Minkowski momentum, the Minkowski momentum is always restored due to the additional pressure on the dielectric surface. This analysis also gives further evidence for use of the Minkowski stress tensor at the boundary of a dielectric interface, which has been the subject of a long-standing debate in physics and the source of uncertainty in the modeling of optical forces on submerged particles.

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Multilayered polymeric nanotube bending elasticity from optical-tweezers micromechanics

Bin Huang, Jan A. van Heiningen, Reghan J. Hill and Theo G. M. van de Ven

We measure the bending deformation of SMA/PEI polymer multilayered nanotubes induced by a transverse hydrodynamic flow past optically trapped nanotube-micro-sphere assemblies in overhanging and end-supported beam configurations. Theoretical analysis of the deformation furnishes the bending stiffness and elastic modulus, the latter of which we compare to values from dry “nanopaper” sheets and other polymer multilayer systems reported in the literature. The results suggest that covalent cross-linking between the SMA and PEI layers produces stiff, water-stable laminates, whereas neither component forms a stable solid in water on its own. The proof of principle for optical-tweezers micromechanics demonstrated here furnishes a persistence length EI/(kBT) 10 m with elastic modulus E 1 GPa. Further quantitative refinements of the technique may lead to a robust material characterization for similarly dimensioned soft nano-particulates. The multilayered nanotubes in this study have a higher elastic modulus than generally achieved with polyelectrolyte multilayers, and the mechanical properties are qualitatively consistent with predictions of macro-scale continuum theory.

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Simulation of lazer light propagation and thermal processes in red blood cells exposed to infrared laser tweezers

I. Krasnikov, A. Seteikin and I. Bernhardt

Continuous-wave laser micro-beams are generally used as diagnostic tools in laser scanning microscopes or, in the case of near-infrared micro-beams, as optical traps for cell manipulation and force characterization. Because single beam traps are created with objectives of high numerical aperture, typical trapping intensities and photon flux densities are in the order of 106 W/cm2 and 103 cm−2 s−1, respectively. These extremely high fields may induce two-photon absorption processes and anomalous biological effects. We studied effects occurring in red blood cells (RBCs) radiated by near-infrared laser tweezers λ = 1064 nm). The main idea of our study was to investigate the thermal reaction of RBCs irradiated by laser micro-beam. It is supported by the fact that many experiments have been carried out on RBCs using laser near infrared tweezers. Usually they are relatively long lasting and the thermal aspects of such experiments are not examined. In the present work it has been identified that the laser affects a RBC with a density of absorbed energy at approximately 107 J/cm3, which causes a temperature rise in the cell of about 10–15°C.

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Stretching and relaxation of vesicles

Hernan Zhou, Beatriz Burrola Gabilondo, Wolfgang Losert, and Willem van de Water

We study the shape relaxation of spherical giant unilamellar vesicles which have been deformed far from equilibrium into ellipsoids using optical tweezers. The relaxation back to a sphere is determined by elastic constants of the vesicles, and their excess area, parameters that are obtained for each stretched vesicle from shape fluctuations in thermal equilibrium, as well as low Reynolds number fluid flow. The relaxation time could be compared favorably to a simple formula which encompasses the joint effect of membrane rigidity and fluid flow. The time constant of the stretched vesicle is slower than that of its thermal fluctuations, which agrees with a recent theory; however, it is one order of magnitude faster than predicted.

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All-optical controllable trapping and transport of subwavelength particles on a tapered photonic crystal waveguide

Pin-Tso Lin and Po-Tsung Lee

We propose that a tapered photonic crystal waveguide design can unify optical trapping and transport functionalities to advance the controllability of optical manipulation. Subwavelength particles can be trapped by a resonance-enhanced field and transported to a specified position along the waveguide on demand by varying the input wavelength. A simulated transport ability as high as 148 (transport distance/wavelength variation) is obtained by the waveguide with 0.1° tilted angle. Stable trapping of a 50nm polystyrene particle can be achieved with input power of 7mW. We anticipate that this design would be beneficial for future life science research and optomechanical applications.

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Volumetric Stress-Strain Analysis of Optohydrodynamically Suspended Biological Cells

Sean S. Kohles, Yu Liang and Asit K. Saha

Ongoing investigations are exploring the biomechanical properties of isolated and suspended biological cells in pursuit of understanding single-cell mechanobiology. An optical tweezer with minimal applied laser power has positioned biologic cells at the geometric center of a microfluidic cross-junction, creating a novel optohydrodynamic trap. The resulting fluid flow environment facilitates unique multiaxial loading of single cells with site-specific normal and shear stresses resulting in a physical albeit extensional state. A recent two-dimensional analysis has explored the cytoskeletal strain response due to these fluid-induced stresses [Wilson and Kohles, 2010, “Two-Dimensional Modeling of Nanomechanical Stresses-Strains in Healthy and Diseased Single-Cells During Microfluidic Manipulation,” J Nanotechnol Eng Med, 1(2), p. 021005]. Results described a microfluidic environment having controlled nanometer and piconewton resolution. In thispresent study, computational fluid dynamics combined with multiphysics modeling has further characterized the applied fluid stress environment and the solid cellular strain response in three dimensions to accompany experimental cell stimulation. A volumetric stress-strain analysis was applied to representative living cell biomechanical data. The presented normal and shear stress surface maps will guide future microfluidic experiments as well as provide a framework for characterizing cytoskeletal structure influencing the stress to strain response.

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