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Results for “Optical Tweezers”

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At least 19 recordsLinked to original sources

PubMed2005

Axial optical trapping forces on two particles trapped simultaneously by optical tweezers.

Optical tweezers, which utilize radiation pressure to control and manipulate microscopic particles, are used for a large number of applications in biology and colloid science. In most applications a single optical tweezers is used to control one single particle. However, two or more particles can be trapped simultaneously. Although this characteristic has been used in applications, no theoretical analysis of the trapping force or the status of the trapped particles is available to our knowledge. We present our calculation, using a ray optics model, of the axial trapping forces on two rigid particles trapped in optical tweezers. The spherical aberration that results from a mismatch of the refractive indices of oil and water is also considered. The results show that the forces exerted by the optical tweezers on the two particles will cause the two particles to touch each other, and the two particles can be stably trapped at a joint equilibrium point. We also discuss the stability of axial trapping. The calculation will be useful in applications of optical tweezers to trap multiple particles.

Journal Article↗

PubMed2006

DNA as a metrology standard for length and force measurements with optical tweezers.

Optical tweezers have broad applications in studies of structures and processes in molecular and cellular biophysics. Use of optical tweezers for quantitative molecular-scale measurement requires careful calibration in physical units. Here we show that DNA molecules may be used as metrology standards for force and length measurements. Analysis of DNA molecules of two specific lengths allows simultaneous determination of all essential measurement parameters. We validate this biological-calibration method experimentally and with simulated data, and show that precisions in determining length scale factor ( approximately 0.2%), length offset ( approximately 0.03%), force scale factor ( approximately 2%), and compliance of the traps ( approximately 3%) are limited only by current measurement variation, much of which arises from polydispersity of the microspheres ( approximately 2%). We find this procedure to be simpler and more convenient than previous methods, and suggest that it provides an easily replicated standard that can insure uniformity of measurements made in different laboratories.

Biophysics↗

PubMed2006

Single molecule studies of DNA binding proteins using optical tweezers.

Optical tweezers have become a versatile tool in the biological sciences. Combined with various types of optical microscopy, they are being successfully used to discover the fundamental mechanism of biological processes. Recently, the study of proteins acting on DNA was aggressively undertaken at the single-molecule level. Here, we review the most recent studies which have revealed the dynamic behavior of individual protein molecules at work on DNA, providing detailed mechanistic insight that could not be revealed, at least not easily, using bulk-phase or ensemble approaches.

Animals↗

PubMed2001

Stretching short biopolymers using optical tweezers.

Optical tweezers is a useful technique to study single molecules. It has been applied to stretch biopolymers in several approaches. However, these approaches may not be appropriate to measure short biopolymers. In the present study, the optical signals of beads in different situations were studied. A new method was developed for stretching short biopolymers, as short as 300 nm in length. This new method was successfully applied to determine the stiffness of procollagen I molecules.

Biopolymers↗

PubMed2006

Binding of TmHU to single dsDNA as observed by optical tweezers.

Optical tweezers are employed to study the action of the histone-like protein from Thermotoga maritima (TmHU) on DNA at a single molecule level. Binding and disruption of TmHU to and from DNA are found to take place in discrete steps of 4-5 nm length and a net binding enthalpy of about 16kBT. This is in reasonable agreement with a microscopic model that estimates the extension of the binding sites of the protein and evaluates the energetics mainly for bending of the DNA in the course of interaction.

Bacterial Proteins↗

PubMed2002

Membrane tether formation from outer hair cells with optical tweezers.

Optical tweezers were used to characterize the mechanical properties of the outer hair cell (OHC) plasma membrane by pulling tethers with 4.5-microm polystyrene beads. Tether formation force and tether force were measured in static and dynamic conditions. A greater force was required for tether formations from OHC lateral wall (499 +/- 152 pN) than from OHC basal end (142 +/- 49 pN). The difference in the force required to pull tethers is consistent with an extensive cytoskeletal framework associated with the lateral wall known as the cortical lattice. The apparent plasma membrane stiffness, estimated under the static conditions by measuring tether force at different tether length, was 3.71 pN/microm for OHC lateral wall and 4.57 pN/microm for OHC basal end. The effective membrane viscosity was measured by pulling tethers at different rates while continuously recording the tether force, and estimated in the range of 2.39 to 5.25 pN x s/microm. The viscous force most likely results from the viscous interactions between plasma membrane lipids and the OHC cortical lattice and/or integral membrane proteins. The information these studies provide on the mechanical properties of the OHC lateral wall is important for understanding the mechanism of OHC electromotility.

Animals↗

PubMed1999

A new determination of the shear modulus of the human erythrocyte membrane using optical tweezers.

Optical tweezers are used to apply calibrated forces to human erythrocytes, via small silica beads bound to their membrane. The shear modulus mu of the membrane is inferred from measurements of the cell deformation in the small strain linear regime. We find the same result mu = 2.5 +/- 0.4 microN/m for both discotic and nearly spherical swollen cells. This value is smaller than the one deduced from micropipettes experiments. However the two methods do not operate in the same deformation regime and are not expected to lead to the same result.

Cell Membrane↗

PubMed1993

Micromanipulation of chromosomes in PTK2 cells using laser microsurgery (optical scalpel) in combination with laser-induced optical force (optical tweezers).

An optical scalpel and optical tweezer have been combined to perform intracellular microsurgery and micromanipulation in vivo. When only laser microsurgery was performed on metaphase chromosomes, the dissected sister chromatid fragments drifted off to either the side of the spindle or completely off the spindle. At anaphase the fragments separated and the two arms generally moved to their respective daughter cells. When the chromosome arm was cut during anaphase A and B, the distal chromosome fragment separated from the rest of the chromosome and moved toward the pole, following the proximal chromosome fragment. Distal chromosome fragments laser-dissected during metaphase were held together throughout anaphase using the optical trap. Optical trapping of dissected chromosome fragments during anaphase A and B inhibited movement of the chromosome fragment to its pole. As a result, the trapped chromosome fragments were (1) incorporated into the opposite daughter cell, (2) lost in the cleavage furrow during cytokinesis, or (3) eventually incorporated into the correct daughter cell. These results indicate that optical traps are effective in holding laser-dissected chromosome fragments throughout mitosis. This new tool should be useful for studies on chromosome movement and cell genetics.

Animals↗

PubMed1998

Micromanipulation of retinal neurons by optical tweezers.

Micromanipulation by optical tweezers has been tested in cultures of mature isolated retinal cells to determine its potential for use in creating synaptic circuits in vitro. Rod and cone photoreceptors as well as other retinal nerve cell types could be optically trapped with a 980 nm diode laser mounted on an inverted light microscope using a 40x oil immersion objective numerical aperture of 1.3. Manipulation was done under sterile conditions using transparent culture dishes. To form cell groups, one half of a culture dish was made less adhesive by application of a thin layer of silicone elastomer. Unattached cells were trapped and relocated next to cells lying on an adhesive culture substrate. Optical trapping did not affect the ability of neurons to subsequently attach to the culture substrate. Up to 60% of trapped cells survived for 2 or more days. The pattern and rate of process outgrowth for manipulated cells was comparable to unmanipulated cells and by 2 days, cell-cell contacts were observed. Cultures were fixed at 2 and 5 days for electron microscopy. Organelle, nuclear and cytoplasmic structure of manipulated cells was completely normal and in photoreceptors, synaptic vesicles and ribbons were intact. Optical tweezers, therefore, provide a benign technique with which to micromanipulate whole neurons. The procedures also bestow increased precision to the study of cell-cell interactions by allowing the selection of potentially interacting cell types at a single cell level.

Ambystoma↗

PubMed1999

Mechanical manipulation of bone and cartilage cells with 'optical tweezers'.

The single beam optical gradient trap (optical tweezers) uses a single beam of laser light to non-invasively manipulate microscopic particles. Optical tweezers exerting a force of approximately 7 pN were applied to single bone and cartilage derived cells in culture and changes in intracellular calcium levels were observed using Fluo-3 labelling. Human derived osteoblasts responded to optical tweezers with an immediate increase in [Ca2+]i that was inhibited by the addition of a calcium channel blocker nifedipine. Force applied to different regions of cells resulted in a variable response. [Ca2+]i elevation in response to load was lower in rat femur derived osteoblasts, and not apparent in primary chondrocytes and the osteocytic cell line (MLO Y4).

Calcium↗

PubMed2006

Optical tweezer micromanipulation of filamentous fungi.

Optical tweezers have been little used in experimental studies on filamentous fungi. We have built a simple, compact, easy-to-use, safe and robust optical tweezer system that can be used with brightfield, phase contrast, differential interference contrast and fluorescence optics on a standard research grade light microscope. We have used this optical tweezer system in a range of cell biology applications to trap and micromanipulate whole fungal cells, organelles within cells, and beads. We have demonstrated how optical tweezers can be used to: unambiguously determine whether hyphae are actively homing towards each other; move the Spitzenkörper and change the pattern of hyphal morphogenesis; make piconewton force measurements; mechanically stimulate hyphal tips; and deliver chemicals to localized regions of hyphae. Significant novel experimental findings from our study were that germ tubes generated significantly smaller growth forces than leading hyphae, and that both hyphal types exhibited growth responses to mechanical stimulation with optically trapped polystyrene beads. Germinated spores that had been optically trapped for 25min exhibited no deleterious effects with regard to conidial anastomosis tube growth, homing or fusion.

Cytological Techniques↗

PubMed1998

Optical tweezers in pharmacology.

Current applications of optical tweezers in pharmacology are presented. The manufacture of cellular biosensor arrays employing optical tweezers is reviewed. Using this technique, a new approach to cellular drug screening, based on single cells patterned with the laser tweezers was introduced. Specific stimulation of different immobilized, viable cells could be shown simultaneously. Furthermore, the usefulness of optical tweezers for analyzing the interactions of ligands with cellular membrane receptors is demonstrated. The laser tweezers could successfully be used to compare neuron interactions with glycoproteins of the extracellular matrix by applying the optical tweezers as picotensometer. The forces of interactions between polystyrene beads coated with different proteins of the extracellular matrix and the cell membrane receptors of cerebellar neurons from postnatal day 6 (P6) mice were measured. When antibodies to the extracellular matrix proteins were added, forces were significantly reduced for the corresponding antigens but not for the other glycoproteins. This proved the specificity of the measured interactions. Information regarding the receptor anchorage of tenascin-C could be deduced.

Animals↗

PubMed2005

Controlled induction, enhancement, and guidance of neuronal growth cones by use of line optical tweezers.

We report an optical tweezers based approach for efficient and controlled manipulation of neuronal growth cones. The approach exploits asymmetric transverse gradient force created in a line optical tweezers to transport actin monomers in the desired growth direction. With this approach induction of artificial growth cones from the neuronal cell body and enhancement of the growth rate of the natural growth cones have been achieved. The use of this approach to bring two growth cones into close proximity for establishing a neuronal connection is also discussed.

Animals↗

PubMed1995

Single kinesin molecules stressed with optical tweezers.

Using the optical tweezers to pull on microtubules, we have stretched and twisted single kinesin molecules adsorbed to glass surfaces. Preliminary measurements suggest that the mechanical system is very compliant, with an apparent stretch of 120 nm with < 2 pN of force. Although measurements of the series compliance of the bead-microtubule structure are still in progress, the kinesin attachment site does not slip with stretch. However, under torsional stress, kinesin appears to slip. With torques < 2 pN-microns approximately 1 Hz in 2 mM AMP-PNP, there is no apparent limit to the number of revolutions that the microtubule can rotate around the kinesin attachment site (n = 44). Preliminary data from other nucleotide conditions are similar. Although there are rare instances of torsional elasticity where the attachment site unwinds, the restoring forces are not constant with angular position, also indicating slippage. Mechanisms of mechanochemical transduction must account for linear force generation in the presence of angular "slippage."

Adenylyl Imidodiphosphate↗

PubMed1995

Micromanipulation of statoliths in gravity-sensing Chara rhizoids by optical tweezers.

Infrared laser traps (optical tweezers) were used to micromanipulate statoliths in gravity-sensing rhizoids of the green alga Chara vulgaris Vail. We were able to hold and move statoliths with high accuracy and to observe directly the effects of statolith position on cell growth in horizontally positioned rhizoids. The first step in gravitropism, namely the physical action of gravity on statoliths, can be simulated by optical tweezers. The direct laser microirradiation of the rhizoid apex did not cause any visible damage to the cells. Through lateral positioning of statoliths a differential growth of the opposite flank of the cell wall could be induced, corresponding to bending growth in gravitropism. The acropetal displacement of the statolith complex into the extreme apex of the rhizoid caused a temporary decrease in cell growth rate. The rhizoids regained normal growth after remigration of the statoliths to their initial position 10-30 micrometers basal to the rhizoid apex. During basipetal displacement of statoliths, cell growth continued and the statoliths remigrated towards the rhizoid tip after release from the optical trap. The resistance to statolith displacement increased towards the nucleus. The basipetal displacement of the whole complex of statoliths for a long distance (>100 micrometers) caused an increase in cell diameter and a subsequent regaining of normal growth after the statoliths reappeared in the rhizoid apex. We conclude that the statolith displacement interferes with the mechanism of tip growth, i.e. with the transport of Golgi vesicles, either directly by mechanically blocking their flow and/or, indirectly, by disturbing the actomyosin system. In the presence of the actin inhibitor cytochalasin B the optical forces required for acropetal and basipetal displacement of statoliths were significantly reduced to a similar level. The lateral displacement of statoliths was not changed by cytochalasin B. The results indicate: (i) the viscous resistance to optical displacement of statoliths depend mainly on actin, (ii) the lateral displacement of statoliths is not impeded by actin filaments, (iii) the axially directed actin-mediated forces against optical displacement of statoliths (for a distance of 10 micrometers) are stronger in the basipetal than in the acropetal direction, (iv) the forces acting on single statoliths by axially oriented actin filaments are estimated to be in the range of 11-110 pN for acropetal and of 18-180 pN for basipetal statolith displacements.

Actins↗

PubMed2002

Stress response in Caenorhabditis elegans caused by optical tweezers: wavelength, power, and time dependence.

Optical tweezers have emerged as a powerful technique for micromanipulation of living cells. Although the technique often has been claimed to be nonintrusive, evidence has appeared that this is not always the case. This work presents evidence that near-infrared continuous-wave laser light from optical tweezers can produce stress in Caenorhabditis elegans. A transgenic strain of C. elegans, carrying an integrated heat-shock-responsive reporter gene, has been exposed to laser light under a variety of illumination conditions. It was found that gene expression was most often induced by light of 760 nm, and least by 810 nm. The stress response increased with laser power and irradiation time. At 810 nm, significant gene expression could be observed at 360 mW of illumination, which is more than one order of magnitude above that normally used in optical tweezers. In the 700-760-nm range, the results show that the stress response is caused by photochemical processes, whereas at 810 nm, it mainly has a photothermal origin. These results give further evidence that the 700-760-nm wavelength region is unsuitable for optical tweezers and suggest that work at 810 nm at normal laser powers does not cause stress at the cellular level.

Animals↗

PubMed2006

Optical trapping of a spherically symmetric sphere in the ray-optics regime: a model for optical tweezers upon cells.

Since their invention in 1986, optical tweezers have become a popular manipulation and force measurement tool in cellular and molecular biology. However, until recently there has not been a sophisticated model for optical tweezers on trapping cells in the ray-optics regime. We present a model for optical tweezers to calculate the optical force upon a spherically symmetric multilayer sphere representing a common biological cell. A numerical simulation of this model shows that not only is the magnitude of the optical force upon a Chinese hamster ovary cell significantly three times smaller than that upon a polystyrene bead of the same size, but the distribution of the optical force upon a cell is also much different from that upon a uniform particle, and there is a 30% difference in the optical trapping stiffness of these two cases. Furthermore, under a small variant condition for the refractive indices of any adjacent layers of the sphere, this model provides a simple approximation to calculate the optical force and the stiffness of an optical tweezers system.

Animals↗

PubMed1991

Application of laser optical tweezers in immunology and molecular genetics.

Optical tweezers, based on a compact diode pumped Nd:YAG laser providing 350 mW at 1,064 nm coupled into a Zeiss IM 35 microscope, were used to sort CD4+ T cells into a capillary for further mechanical handling and to establish contact between single human natural killer (NK) cells and human erythroleukemia cells (K562) as targets. After contact and a lag phase of a few tens of seconds, the target cell starts to change its morphology and membrane blebbing occurs. The kinetics of the attack of the NK cell on K562 cells is not straightforward but governed by temporal oscillations in the shape of the target cell (zeosis). In a second application, the optical tweezers are combined with a UV laser microbeam based on a pulsed UV laser and with flow cytometry and sorting. With the pulsed laser, segments of sorted chromosome 1 of the chinese hamster karyotype (CHV 79) can be easily micro-dissected and subsequently collected using the optical tweezers. This allows preparation of a few hundred chromosome segments per day without mechanical contact and in an absolutely sterile way and thus may provide an interesting basic technique in any type of genome sequencing project.

Animals↗