Holographic Optical Tweezers Research Articles

Positional stability of holographic optical traps

The potential of digital holography for complex manipulation of micron-sized particles with optical tweezers has been clearly demonstrated. By contrast, its use in quantitative experiments has been rather limited, partly due to fluctuations introduced by the spatial light modulator (SLM) that displays the kinoforms. This is an important issue when high temporal or spatial stability is a concern. We have investigated the performance of both an analog-addressed and a digitally-addressed SLM, measuring the phase fluctuations of the modulated beam and evaluating the resulting positional stability of a holographic trap. We show that, despite imparting a more unstable modulation to the wavefront, our digitally-addressed SLM generates optical traps in the sample plane stable enough for most applications. We further show that traps produced by the analog-addressed SLM exhibit a superior pointing stability, better than 1 nm, which is comparable to that of non-holographic tweezers. These results suggest a means to implement precision force measurement experiments with holographic optical tweezers (HOTs).

Controlling ghost traps in holographic optical tweezers

Computer-generated holograms displayed by phase-modulating spatial light modulators have become a well-established tool for beam shaping purposes in holographic optical tweezers. Still, the generation of light intensity patterns with high spatial symmetry and simultaneously without interfering ghost traps is a challenge. We have implemented an iterative Fourier transform algorithm that is capable of controlling these ghost traps and demonstrate the benefit of this approach in the experiment.

Towards total photonic control of complex-shaped colloids by vortex beams

We demonstrate optical trapping and orientational control over colloidal particles having complex shapes in an anisotropic host fluid using a dynamic holographic optical tweezers system. Interactions between a colloidal particle and the toroidal intensity distributions of focused Laguerre-Gaussian beams allow for stable optical tweezing and provide a tunable tilt of the particle out of the focal plane. Use of an aligned nematic liquid crystal as the host fluid suppresses rotations about the optical axis arising from angular momentum transfer from the beam and effectively defines a rotational axis for the colloid within the trap.

Translation and manipulation of silicon nanomembranes using holographic optical tweezers

We demonstrate the use of holographic optical tweezers for trapping and manipulating silicon nanomembranes. These macroscopic free-standing sheets of single-crystalline silicon are attractive for use in next-generation flexible electronics. We achieve three-dimensional control by attaching a functionalized silica bead to the silicon surface, enabling non-contact trapping and manipulation of planar structures with high aspect ratios (high lateral size to thickness). Using as few as one trap and trapping powers as low as several hundred milliwatts, silicon nanomembranes can be rotated and translated in a solution over large distances.

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 EC50 of 5μM 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 25pN. 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.

Measuring Forces between Two Single Actin Filaments during Bundle Formation

Bundles of filamentous actin are dominant cytoskeletal structures, which play a crucial role in various cellular processes. As yet quantifying the fundamental interaction between two individual actin filaments forming the smallest possible bundle has not been realized. Applying holographic optical tweezers integrated with a microfluidic platform, we were able to measure the forces between two actin filaments during bundle formation. Quantitative analysis yields forces up to 0.2 pN depending on the concentration of bundling agents.

Integration of Optical Manipulation and Electrophysiological Tools to Modulate and Record Activity in Neural Networks

We present an optical system that combines IR (1064 nm) holographic optical tweezers with a sub-nanosecond-pulsed UV (355 nm) laser microdissector for the optical manipulation of single neurons and entire networks both on transparent and non-transparent substrates in vitro. The phase-modulated laser beam can illuminate the sample concurrently or independently from above or below assuring compatibility with different types of microelectrode array and patch-clamp electrophysiology. By combining electrophysiological and optical tools, neural activity in response to localized stimuli or injury can be studied and quantified at sub-cellular, cellular, and network level.

Surface imaging using holographic optical tweezers

We present an imaging technique using an optically trapped cigar-shaped probe controlledusing holographic optical tweezers. The probe is raster scanned over a surface, allowing animage to be taken in a manner analogous to scanning probe microscopy (SPM), withautomatic closed loop feedback control provided by analysis of the probe position recordedusing a high speed CMOS camera. The probe is held using two optical traps centred at least 10 µm from the ends, minimizing laser illumination of the tip, so reducing the chance of opticaldamage to delicate samples. The technique imparts less force on samples than contactSPM techniques, and allows highly curved and strongly scattering samples to beimaged, which present difficulties for imaging using photonic force microscopy. Tocalibrate our technique, we first image a known sample—the interface between two 8 µm polystyrene beads. We then demonstrate the advantages of this technique by imaging thesurface of the soft alga Pseudopediastrum. The scattering force of our laser applied directlyonto this sample is enough to remove it from the surface, but we can use our technique toimage the algal surface with minimal disruption while it is alive, not adhered andin physiological conditions. The resolution is currently equivalent to confocalmicroscopy, but as our technique is not diffraction limited, there is scope for significantimprovement by reducing the tip diameter and limiting the thermal motion of the probe.

Stimulation of human red blood cells leads to Ca 2+-mediated intercellular adhesion

Red blood cells (RBCs) are a major component of blood clots, which form physiologically as a response to injury or pathologically in thrombosis. The active participation of RBCs in thrombus solidification has been previously proposed but not yet experimentally proven. Holographic optical tweezers and single-cell force spectroscopy were used to study potential cell–cell adhesion between RBCs. Irreversible intercellular adhesion of RBCs could be induced by stimulation with lysophosphatidic acid (LPA), a compound known to be released by activated platelets. We identified Ca 2+ as an essential player in the signaling cascade by directly inducing Ca 2+ influx using A23187. Elevation of the internal Ca 2+ concentration leads to an intercellular adhesion of RBCs similar to that induced by LPA stimulation. Using single-cell force spectroscopy, the adhesion of the RBCs was identified to be approximately 100 pN, a value large enough to be of significance inside a blood clot or in pathological situations like the vasco-occlusive crisis in sickle cell disease patients.

Combined optical tweezers and laser dissector for controlled ablation of functional connections in neural networks

Regeneration of functional connectivity within a neural network after different degrees of lesion is of utmost clinical importance. To test pharmacological approaches aimed at recovering from a total or partial damage of neuronal connections within a circuit, it is necessary to develop a precise method for controlled ablation of neuronal processes. We combined a UV laser microdissector to ablate neural processes in vitro at single neuron and neural network level with infrared holographic optical tweezers to carry out force spectroscopy measurements. Simultaneous force spectroscopy, down to the sub-pico-Newton range, was performed during laser dissection to quantify the tension release in a partially ablated neurite. Therefore, we could control and measure the damage inflicted to an individual neuronal process. To characterize the effect of the inflicted injury on network level, changes in activity of neural subpopulations were monitored with subcellular resolution and overall network activity with high temporal resolution by concurrent calcium imaging and microelectrode array recording. Neuronal connections have been sequentially ablated and the correlated changes in network activity traced and mapped. With this unique combination of electrophysiological and optical tools, neural activity can be studied and quantified in response to controlled injury at the subcellular, cellular, and network level.

Indirect optical gripping with triplet traps

We describe an indirect optical gripper for noninvasive micromanipulation of sensitive objects, such as cells. Our optical gripper, driven by dynamic holographic optical tweezers, consists of an arrangement of six silica beads, each held in place by an optical trap. The beads are moved so as to grip an object, which itself is not significantly exposed to laser light. Six beads are sufficient to grip an object reliably, consistent with robotics design rules. We find that improved gripping is achieved when beads are arranged into groups of triplets, since each triplet of trapped beads forms a weakly attractive pocket into which other objects are drawn.

Adding functionalities to precomputed holograms with random mask multiplexing in holographic optical tweezers

In this study, we present a method designed to generate dynamic holograms in holographic optical tweezers. The approach combines our random mask encoding method with iterative high-efficiency algorithms. This hybrid method can be used to dynamically modify precalculated holograms, giving them new functionalities-temporarily or permanently-with a low computational cost. This allows the easy addition or removal of a single trap or the independent control of groups of traps for manipulating a variety of rigid structures in real time.

Optical assembly of microparticles into highly ordered structures using Ince–Gaussian beams

Ince–Gaussian (IG) beams are a third complete family of solutions of the paraxial Helmholtz equation. While many applications of Hermite–Gaussian and Laguerre–Gaussian beams have been demonstrated for manipulation of microparticles, the potential of the more general class of IG beams has not yet been exploited at all. We describe the unique properties of IG beams with respect to optical trapping applications, demonstrate a flexible experimental realization of arbitrary IG beams and prove the concept by creating two- and three-dimensional, highly ordered assemblies of typical microparticles. The concept is universal and can easily be integrated into existing holographic optical tweezers setups.

Stereoscopic particle tracking for 3D touch, vision and closed-loop control in opticaltweezers

Force measurement in an interactive 3D micromanipulation system can allowthe user to make delicate adjustments, and to explore surfaces with touch aswell as vision. We present a system to achieve this on the micron scale usingstereoscopic particle tracking combined with holographic optical tweezers, whichcan track particles with nanometre accuracy. 2D tracking of particles in eachof the stereo images gives 3D positions for each particle. This takes less than200 µs per image pair, using a 1D ‘symmetry transform’ applied to each row andcolumn of a 2D image, which can maintain tracking of particles throughout the10 µm axial range. The only parameters required are the geometry of the imagingsystem, and therefore there is no need to recalibrate for different particle sizesor refractive indices. Consequently, we can calculate the force exerted by theoptical trap in real time at 1 kilohertz, allowing us to implement a force-feedbackinterface (with a loop rate of 400 Hz). In combination with our OpenGL hologramcalculation engine, the system has a closed-loop bandwidth of 20 Hz. This allowsus to stabilize trapped particles axially through active feedback, cancellingout some Brownian motion. For the weak traps we use here (spring constantk≈2 pN µm − 1), this results in a threefold increase in axial stiffness. We demonstrate the3D interface by probing an oil droplet, mapping out its surface in they–z plane.

ITweezers: optical micromanipulation controlled by an Apple iPad

The 3D interactive manipulation of multiple particles with holographic optical tweezers isoften hampered by the control system. We use a multi-touch interface implemented on anApple iPad to overcome many of the limitations of mouse-based control, and demonstratean elegant and intuitive interface to multi-particle manipulation. This interface connects tothe tweezers system hardware over a wireless network, allowing it to function as a remotemonitor and control device.

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

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.

Investigation of Intercellular Adhesion of Red Blood Cells by Means of Holographic Optical Tweezers and Single Cell Force Spectroscopy

Red blood cells are a major component of blood clots which form physiologically as a response to injury or pathologically in thrombosis. The active participation of RBCs in thrombus solidification was previously proposed, but not yet experimentally proven. Holographic optical tweezers and single cell force spectroscopy were used to study potential cell-cell adhesion in between RBCs. Irreversible intercellular adhesion of RBCs could be induced by stimulation with lysophosphatidic acid (LPA), a compound known to be released by activated platelets. We identified Ca2+ as an essential player in the signaling cascade by directly inducing Ca2+ influx using A23187. The elevation of internal Ca2+ concentration leads to intercellular adhesion of RBCs similar to that induced by LPA stimulation. Based on single cell force spectroscopy adhesion of the RBCs was identified to be approximately 100 pN, a value large enough to be of significance inside a blood clot or in pathological situations like the vasco-occlusive crisis in sickle cell disease patients.

Ballistic Motion of Spirochete Membrane Proteins

Borrelia burgdorferi spirochetes are pathogenic bacteria which can cause Lyme disease. B. burgdorferi use membrane proteins to anchor themselves inside ticks which act as intermediate hosts. Human immune cells that fight bacterial infections bind to bacteria to internalize them. This binding can be mediated by antibodies against bacterial membrane proteins. Therefore these proteins and their mechanical behavior in the membrane play a key role for the disease transmission and the immune response.The mechanical behavior of proteins in bacterial membranes is still poorly understood. We investigated membrane protein dynamics in B. burgdorferi with microparticles coated with monoclonal antibodies against the spirochete outer surface protein A (OspA). We used holographic optical tweezers to attach the microparticles to membranes of live B. burgdorferi and we tracked the subsequent particle motion.Surprisingly the particles were transported ballistically to one end of the bacteria. The average speed of this transport was 3.1 ± 0.2 um/s. To determine if the microparticle motion is driven by the proteins, we are using speckle microscopy with fluorescently labeled OspA. In addition, we measured the stall force of the transport and found that a few piconewtons were sufficient to inhibit the motion. Although B. burgdorferi have a symmetric morphology, the protein motion showed a preferred direction of transport towards one end in individual bacteria. The origin of this symmetry breaking still needs to be unraveled. Mutant B. burgdorferi which lack the flagella that are found in wild-type bacteria did not show directed transport of membrane proteins, but only diffusive motion. Therefore we hypothesize that the protein transport is enabled by the bacterial motility machinery. We will discuss the proposition that this transport of membrane proteins could be a novel type of bacterial defense mechanism against immune cells.[HK and RB contributed equally]

Holographic optical tweezers and their relevance to lab on chip devices

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.

3D trapping and manipulation of micro-particles using holographic optical tweezers with optimized computer-generated holograms

A multi-plane adaptive-additive algorithm is developed for optimizing computer-generated holograms for the reconstruction of traps in three-dimensional (3D) spaces. This algorithm overcomes the converging stagnation problem of the traditional multi-plane Gerchberg{Saxton algorithm and improves the diffraction efficiency of the holograms effectively. The optimized holograms are applied in a holographic optical tweezers (HOT) platform. Additionally, a computer program is developed and integrated into the HOT platform for the purpose of achieving the interactive control of traps. Experiments demonstrate that the manipulation of micro-particles into the 3D structure with optimized holograms can be carried out effectively on the HOT platform.