Using Optical Tweezers to Study the Friction of the Red Blood Cells

In the last two decades the study of red blood cell elasticity using optical tweezers has known a rise appearing in the scientific research with regard to the various works carried out. Despite the various work done, no study has been done so far to study the influence of friction on the red blood cell indentation response using optical tweezers. In this study, we have developed a new approach to determine the coefficient of friction as well as the frictional forces of the red blood cell. This approach therefore allowed us to simultaneously carry out the indentation and traction test, which allowed us to extract the interfacial properties of the microbead red blood cell couple, among other things, the friction coefficient. This property would be extremely important to investigate the survival and mechanical features of cells, which will be of great physiological and pathological significance. But taking into account the hypothesis of friction as defined by the isotropic Coulomb law. The experiment performed for this purpose is the Brinell Hardness Test (DB).

Human Eosinophil Cell Manipulation by Optical Tweezers

In this work, lateral deformation of human eosinophil cell during the lateral indentation by an optically trapped microbead of diameter 4.5 µm is studied. The images were captured using a CCD camera and the Boltzmann statistics method was used for force calibration. Using the Hertz model, we calculated and compared the elastic moduli resulting from the lateral force, showing that the differences are important and the force should be considered. Besides the lateral component, the setup also allows us to examine the lateral cell-bead interaction. The mean values of the properties obtained, in particular the elastic stiffness and the shear stiffness, were Eh = (37.76 ± 2.85) µN/m and Gh = (12.57 ± 0.32) µN/m. These results show that the lateral indentation can therefore be used as a routine method for cell study, because it enabled us to manipulate the cell without contact with the laser.

Triboelectric‘electrostatic tweezers'for manipulating droplets on lubricated slippery surfaces prepared by femtosecond laser processing

The use of‘Electrostatic tweezers'is a promising tool for droplet manipulation,but it faces many limitations in manipulating droplets on superhydrophobic surfaces.Here,we achieve noncontact and multifunctional droplet manipulation on Nepenthes-inspired lubricated slippery surfaces via triboelectric electrostatic tweezers(TETs).The TET manipulation of droplets on a slippery surface has many advantages over electrostatic droplet manipulation on a superhydrophobic surface.The electrostatic field induces the redistribution of the charges inside the neutral droplet,which causes the triboelectric charged rod to drive the droplet to move forward under the electrostatic force.Positively or negatively charged droplets can also be driven by TET based on electrostatic attraction and repulsion.TET enables us to manipulate droplets under diverse conditions,including anti-gravity climb,suspended droplets,corrosive liquids,low-surface-tension liquids(e.g.ethanol with a surface tension of 22.3 mN·m^(-1)),different droplet volumes(from 100 nl to 0.5 ml),passing through narrow slits,sliding over damaged areas,on various solid substrates,and even droplets in an enclosed system.Various droplet-related applications,such as motion guidance,motion switching,droplet-based microreactions,surface cleaning,surface defogging,liquid sorting,and cell labeling,can be easily achieved with TETs.

K_2CO_3 catalyzed solvent-free synthesis of molecular tweezers containing chiral unsymmetrical urea unit under microwave irradiation

A facile,rapid and efficient method for the synthesis of molecular tweezers containing chiral unsymmetrical urea unit in solvent- free conditions using microwave was reported.

Theoretical study of the trapping efficiency of an optical tweezers array system

Optical tweezers have been a valuable research tool since their invention in the 1980s. One of the most important developments in optical tweezers in recent years is the creation of two-dimensional arrays of optical traps. In this paper, a method based on interference is discussed to form gradient laser fields, which may cause the spatial modulation of particle concentration. The parameters related to the optical tweezers array are discussed in detail and simulated by the Matlab software to show the influence of important parameters on the distribution of particle concentration. The spatial redistribution of particles in a laser interference field can also be predicted according to the theoretical analysis.

A novel approach to study the interactions between polymeric stabilized micron-sized oil droplets by optical tweezers

The well understanding of interaction forces between single dispersed droplets is crucial to the understanding of emulsion stabilization mechanism.Recently,many studies have reported the direct quantitative measurements of interaction forces between 20-200μm single droplet coated polymers by atomic force microscope(AFM).These studies have revealed many important results about the relationship of the interaction forces and the droplet deformation.However,these studies of the quantitative relationship between the measured interaction forces and the separation distance of the front end of the droplet have rarely been reported.Optical tweezer instrument can make it possible to establish the quantitative relationship between the measured force and the separation distance of the front end of the droplet,which will make better understanding of the interaction mechanisms between droplets.Due to the differences of the measuring mechanism between atomic force microscopy(AFM)and optical tweezers,the theory model of AFM measurements cannot be fitted with the force measurement by optical tweezers.We have made an exhaustive comparison of the measuring differences between AFM and optical tweezer instrument in this work.Moreover,we built a numerical model to derive the repulsive pressure through the measured force curve in order to quantify the measured force of two micron-sized oil droplet coated polymers by optical tweezers.Furthermore,the novel method can be extended to other micron-sized emulsion systems,and these findings will be a vital progress on quantitative force measurements between micron-sized droplets.

Systematical study of the trapping forces of optical tweezers formed by different types of optical ring beams

The technique of optical tweezers has been improved a lot since its invention, which extends the application fields of optical tweezers. Besides the conventionally used Gaussian beams, different types of ring beams have also been used to form optical tweezers for different purposes. The two typical kinds of ring beams used in optical tweezers are the hollow Gaussian beam and Laguerre-Gaussian （LG） beam. Both theoretical computation and experiments have shown that the axial trapping force is improved for the ring beams compared with the Gaussian beam, and hence the trapping stability is improved, although the transverse trapping forces of ring beams are smaller than that of Gaussian beam. However, no systematic study on the trapping forces of ring beam has ever been discussed. In this article, we will investigate the axial and transverse trapping forces of different types of ring beams with different parameters systematically, by numerical computation in which the ray optics model is adopted. The spherical aberration caused by the refractive index mismatch between oil and water is also considered in the article. The trapping forces for different objectives that obey the sine condition and tangent condition are also compared with each other. The result of systematical calculation will be useful for the applications of optical tweezers formed by different types of ring beams.

3D dynamic motion of a dielectric micro-sphere within optical tweezers

Known as laser trapping,optical tweezers,with nanometer accuracy and pico-newton precision,plays a pivotal role in single bio-molecule measurements and controllable motions of micro-machines.In order to advance the flourishing applications for those achievements,it is necessary to make clear the three-dimensional dynamic process of micro-particles stepping into an optical field.In this paper,we utilize the ray optics method to calculate the optical force and optical torque of a micro-sphere in optical tweezers.With the influence of viscosity force and torque taken into account,we numerically solve and analyze the dynamic process of a dielectric micro-sphere in optical tweezers on the basis of Newton mechanical equations under various conditions of initial positions and velocity vectors of the particle.The particle trajectory over time can demonstrate whether the particle can be successfully trapped into the optical tweezers center and reveal the subtle details of this trapping process.Even in a simple pair of optical tweezers,the dielectric micro-sphere exhibits abundant phases of mechanical motions including acceleration,deceleration,and turning.These studies will be of great help to understand the particle-laser trap interaction in various situations and promote exciting possibilities for exploring novel ways to control the mechanical dynamics of microscale particles.

Robust and high-speed rotation control in optical tweezers by using polarization synthesis based on heterodyne interference

The rotation control of particles in optical tweezers is often subject to the spin or orbit angular momentum induced optical torque,which is susceptible to the mechanical and morphological properties of individual particle.Here we report on a robust and high-speed rotation control in optical tweezers by using a novel linear polarization synthesis based on optical heterodyne interference between two circularly polarized lights with opposite handedness.The synthesized linear polarization can be rotated in a hopping-free scheme at arbitrary speed determined electronically by the heterodyne frequency between two laser fields.The experimental demonstration of a trapped vaterite particle in water shows that the precisely controlled rotation frequency of 300 Hz can be achieved.The proposed method will find promising applications in optically driven micro-gears,fluidic pumps and rotational micro-rheology.

Equilibrium folding and unfolding dynamics to reveal detailed free energy landscape of src SH3 protein by magnetic tweezers

Src SH3 protein domain is a typical two-state protein which has been confirmed by research of denaturant-induced unfolding dynamics.Force spectroscopy experiments by optical tweezers and atomic force microscopy have measured the force-dependent unfolding rates with different kinds of pulling geometry.However,the equilibrium folding and unfolding dynamics at constant forces has not been reported.Here,using stable magnetic tweezers,we performed equilibrium folding and unfolding dynamic measurement and force-jump measurement of src SH3 domain with tethering points at its N-and C-termini.From the obtained force-dependent transition rates,a detailed two-state free energy landscape of src SH3 protein is constructed with quantitative information of folding free energy,transition state barrier height and position,which exemplifies the capability of magnetic tweezers to study protein folding and unfolding dynamics.

Manipulations of micro/nanoparticles using gigahertz acoustic streaming tweezers

Contactless acoustic manipulation of micro/nanoscale particles has attracted considerable attention owing to its near independence of the physical and chemical properties of the targets,making it universally applicable to almost all biological systems.Thin-film bulk acoustic wave(BAW)resonators operating at gigahertz(GHz)frequencies have been demonstrated to generate localized high-speed microvortices through acoustic streaming effects.Benefitting from the strong drag forces of the high-speed vortices,BAW-enabled GHz acoustic streaming tweezers(AST)have been applied to the trapping and enrichment of particles ranging in size from micrometers to less than 100 nm.However,the behavior of particles in such 3D microvortex systems is still largely unknown.In this work,the particle behavior(trapping,enrichment,and separation)in GHz AST is studied by theoretical analyses,3D simulations,and microparticle tracking experiments.It is found that the particle motion in the vortices is determined mainly by the balance between the acoustic streaming drag force and the acoustic radiation force.This work can provide basic design principles for AST-based lab-on-a-chip systems for a variety of applications.

Forces of RBC interaction with single endothelial cells in stationary conditions:Measurements with laser tweezers

Red blood cells(RBCs)are able to interact and communicate with endothelial cells(ECs).Under some pathological or even normal conditions,the adhesion of RBCs to the endothelium can be observed.Presently,the mechanisms and many aspects of the interaction between RBCs and ECs are not fully understood.In this work,we considered the interaction of single RBCs with single ECs in vitro aiming to quantitatively determine the force of this interaction using laser tweezers.Measurements were performed under different concentrations of proaggregant macromolecules and in the presence or absence of tumor necrosis factor(TNF-α)activating the ECs.We have shown that the strength of interaction depends on the concentration of fibrinogen or dextran proaggregant macromolecules in the environment.A nonlinear increase in the force of cells interaction(from 0.4 pN to 21 pN)was observed along with an increase in the fibrinogen con-centration(from 3 mg/mL to 9 mg/mL)in blood plasma,as well as with the addition of dextran macromolecules(from 10 mg/mL to 60 mg/mL).Dextran with a higher molecular mass(500 kDa)enhances the adhesion of RBCs to ECs greater compared to the dextran with a lower molecular mass(70 kDa).With the preliminary activation of ECs with TNF-α,the force of interaction increases.Also,the adhesion of echinocytes to EC compared to discocytes is significantly higher.These results may help to better understand the process of interaction between RBCs and ECs.

Simultaneous, hybrid single-molecule method by optical tweezers and fluorescence

As studies on life sciences progress toward the single-molecule level,new experiments have put forward more requirements for simultaneously displaying the mechanical properties and conformational changes of biomolecules.Optical tweezers and fluorescence microscopy have been combined to solve this problem.The combination of instruments forms a new generation of hybrid single-molecule technology that breaks through the limitations of traditional biochemical analysis.Powerfulmanipulation and fluorescence visualization have beenwidely used,and these techniques provide new possibilities for studying complex biochemical reactions at the singlemolecule level.This paper explains the features of this combined technique,including the application characteristics of single-trap and dual-traps,the anti-bleaching method,and optical tweezers combined with epifluorescence,confocal fluorescence,total internal reflection fluorescence,and other fluorescence methods.Using typical experiments,we analyze technical solutions and explain the factors and principles that instrument designers should consider.This review aims to give an introduction to this novel fusion technology process and describe important biological results.

Design and synthesis of chiral single-armed molecular tweezers derived fromα-hyodeoxycholic acid

A novel type of molecular tweezers with different chiral center and cleft has been designed and prepared by using α- hyodeoxycholic acid as spacer and D/L-amino acid methyl ester as chiral arm attached at 3-position. Their structures were elucidated by ^1H NMR, FTIR and elemental analysis. Their recognition properties for various D/L-amino acid methyl esters were also investigated. The preliminary results indicated that these chiral single-armed molecular tweezers exhibited good recognition ability for D/L-amino acid methyl esters with high association constant （up to 5.24 × 10^3 L mol^-1）.

Ultrasound Tweezers Trigger Single-Cell Mechanical Dynamics for Cell Mechanophenotyping

Cells actively modulate mechanobiological circuitry against external perturbations to stabilize whole cell/tissue physiology.The dynamic adaption of cells to mechanical force is critical for cells to perform vital biological functions,from single cell migration to embryonic development.Dysregulation of such dynamics has been associated with pathophysiological conditions in cardiovascular diseases,cancer,aging,and developmental disorders[1].Therefore,a direct understanding of cell’s biomechanical adaptive/maladaptive behaviors and the trigger factors causing the transformation of healthy adaption to maladaptation can help reveal the regulatory role of single cell mechanosensitive dynamics in the progression of various degenerative diseases and aging.However,current efforts for uncovering fundamental associations between disease and cell architecture have been focusing on'static'measurements of biophysical properties,which is limited by the requirement of large sample sizes to obtain statistically significant data.We therefore developed a single and highly integrated platform with mechanical stimulation and fine spatiotemporal sensing functions to probe the single cell mechanical dynamics at subcellular level to determine cell’s mechanophenotypes in healthy and disease conditions.We developed an integrated micromechanical system composed of an’ultrasound tweezer’stimulator[2]and a PDMS micropillar array [3] cellular force sensor to in situ noninvasively probe and monitor single cell mechanical dynamics.Vascular smooth muscle cells(VSMCs)from healthy mouse and mouse with induced abdominal aorta aneurysm(AAA)were used for cell mechanobiological study.An ultrasound transducer(V312-SM,Olympus)was used to generate ultrasound pulses to excite lipid-encapsulated microbubbles(Targeson)binding to cell membrane through an RGD-integrin linkage to apply a transient nanonewton force to VSMCs seeded on the PDMS micropillar array.PDMS micropillar array was fabricated and functionalized as previously described [3] and acts as the mechanical force sensor in our platform.Upon a 1 HZ and 10-second ultrasound stimulation,calcium influx was clearly detected in both healthy and AAA-VSMCs by using the fluo-4 calcium sensor,suggesting the microbubble-integrin-actin cytoskeleton(CSK)linkage can serve as a mechanosensory to sense the ultrasound stimulation.We then examined how healthy and AAA VSMCs would exhibit adaptions to mechanical stimulation at a global cellular scale.After the onset of a 10-second ultrasound stimulation,control and AAA-VSMCs displayed distinct dynamics of CSK tension within 30 mins,in which the CSK tension of healthy VSMCs increased within the reinforcement period(0-5 min)and restored to their ground state with the relaxation period(5-10 min);yet AAA-VSMCs displayed compromised dynamics of such CSK tension upon calcium influx.Quantitative analysis and theoretical modelling revealed the critical roles of myosin motor contraction,F-actin filament polymerization in regulating cell mechanosensitive dynamics in response to a transient mechanical perturbation.The distinct force and CSK dynamics in healthy and AAA conditions indicates that the force-dependent CSK molecular kinetics is a critical factor governing the distinct mechanosensitive dynamics of cells under pathologically dysfunctional conditions.Our results reveal that the mechanical adaptive process of cells to mechanical stimulus can measure the cellular mechanobiological phenotypes featured in both pathologically healthy and diseased context.We demonstrated that an altered mechanobiological phenotype,i.e.AAA-VSMCs with distinct actomyosin-CSK properties potentiates a mechanical maladaptation that reflects progressive accumulation of cellular damage and dysfunction.This may further reveal the pathogenic contexts and their physical mediators featuring biophysical dysregulation in cardiovascular diseases.

In situ calibrating optical tweezers with sinusoidal-wave drag force method

We introduce a corrected sinusoidal-wave drag force method （SDFM） into optical tweezers to calibrate the trapping stiffness of the optical trap and conversion factor （CF） of photodetectors. First, the theoretical analysis and experimental result demonstrate that the correction of SDFM is necessary, especially the error of no correction is up to 11.25% for a bead of 5μm in diameter. Second, the simulation results demonstrate that the SDFM has a better performance in the calibration of optical tweezers than the triangular-wave drag force method （TDFM） and power spectrum density method （PSDM） at the same signal-to-noise ratio or trapping stiffness. Third, in experiments, the experimental standard deviations of calibration of trapping stiffness and CF with the SDFM are about less than 50% of TDFM and PSDM especially at low laser power. Finally, the experiments of stretching DNA verify that the in situ calibration with the SDFM improves the measurement stability and accuracy.

Design and Synthesis of Novel Molecular Tweezers Derived from Chenodeoxycholic Acid

A novel type of chiral molecular tweezers has been designed and synthesized by using chenodeoxy cholic acid as spacer and the aromatic compounds as arm. Their structures were characterized by 1HNMR, IR, MS spectra and elemental analysis. These chiral molecular tweezers showed good enantioselectivity for D-amino acid methyl esters.

Design and Synthesis of Chiral Molecular Tweezers Based on Deoxycholic Acid

A series of new chiral molecular tweezers have been designed and synthesized by using deoxycholic acid as spacer and aromatic amines as arms. Instead of using toxic phosgene, the triphosgene was employed in synthesis of the molecular tweezers receptors. These chiral molecular tweezers showed good enantioselectivity for D-amino acid methyl esters.

Influence of viscous force on the dynamic process of micro-sphere in optical tweezers

With the advantages of noncontact,high accuracy,and high flexibility,optical tweezers hold huge potential for micro-manipulation and force measurement.However,the majority of previous research focused on the state of the motion of particles in the optical trap,but paid little attention to the early dynamic process between the initial state of the particles and the optical trap.Note that the viscous forces can greatly affect the motion of micro-spheres.In this paper,based on the equations of Newtonian mechanics,we investigate the dynamics of laser-trapped micro-spheres in the surrounding environment with different viscosity coefficients.Through the calculations,over time the particle trajectory clearly reveals the subtle details of the optical capture process,including acceleration,deceleration,turning,and reciprocating oscillation.The time to equilibrium mainly depends on the corresponding damping coefficient of the surrounding environment and the oscillation frequency of the optical tweezers.These studies are essential for understanding various mechanisms to engineer the mechanical motion behavior of molecules or microparticles in liquid or air.

Mechanobiology of the cell surface:Probing its remodeling dynamics using membrane tether pulling assays with optical tweezers

Mammalian cell surfaces consist of the plasma membrane supported by an underneath cortical cytoskeleton.Together,these structures can control not only the shape of cells but also a series of cellular functions ranging from migration and division to exocytosis,endocytosis and differentiation.Furthermore,the cell surface is capable of exerting and reacting to mechanical forces.Its viscoelastic properties,especially membrane tension and bending modulus,are fundamental parameters involved in these responses.This viewpoint summarizes our current knowledge on how to measure the viscoelastic properties of cell surfaces employing optical tweezers-based tether assays,paving the way for a better understanding of how cells react to external mechanical forces,with a glance on their remodeling dynamics and possible consequences on downstream cellular processes.