Physics of Optical Tweezers

Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review

The careful arrangement of nanomaterials (NMs) holds promise for revolutionizing various fields, from electronics and biosensing to medicine and optics. This review delves into the intricacies of nano-assembly (NA) techniques, focusing on oriented-assembly methodologies and stimuli-dependent approaches. The introduction provides a comprehensive overview of the significance and potential applications of NA, setting the stage for review. The oriented-assembly section elucidates methodologies for the precise alignment and organization of NMs, crucial for achieving desired functionalities. The subsequent section delves into stimuli-dependent techniques, categorizing them into chemical and physical stimuli-based approaches. Chemical stimuli-based self-assembly methods, including solvent, acid-base, biomolecule, metal ion, and gas-induced assembly, are discussed in detail by presenting examples. Additionally, physical stimuli such as light, magnetic fields, electric fields, and temperature are examined for their role in driving self-assembly processes. Looking ahead, the review outlines futuristic scopes and perspectives in NA, highlighting emerging trends and potential breakthroughs. Finally, concluding remarks summarize key findings and underscore the significance of NA in shaping future technologies. This comprehensive review serves as a valuable resource for researchers and practitioners, offering insights into the diverse methodologies and potential applications of NA in interdisciplinary research fields.

Critical review of single-cell mechanotyping approaches for biomedical applications

Accurate mechanical measurements of cells has the potential to improve diagnostics, therapeutics and advance understanding of disease mechanisms, where high-resolution mechanical information can be measured by deforming individual cells. Here we evaluate recently developed techniques for measuring cell-scale stiffness properties; while many such techniques have been developed, much of the work examining single-cell stiffness is impacted by difficulties in standardization and comparability, giving rise to large variations in reported mechanical moduli. We highlight the role of underlying mechanical theories driving this variability, and note opportunities to develop novel mechanotyping devices and theoretical models that facilitate convenient and accurate mechanical characterisation. Moreover, many high-throughput approaches are confounded by factors including cell size, surface friction, natural population heterogeneity and convolution of elastic and viscous contributions to cell deformability. We nevertheless identify key approaches based on deformability cytometry as a promising direction for further development, where both high-throughput and accurate single-cell resolutions can be realized.

Laser-Induced Transfer of Functional Materials

Patterning is crucial for the large-scale application of functional materials. Laser-induced transfer is an emerging patterning method for additively depositing functional materials to the target acceptor. With the rapid development of laser technologies, this laser printing method emerges as a versatile method to deposit functional materials in either liquid or solid format. The emerging applications such as solar interfacial evaporation, solar cells, light-emitting diodes, sensors, high-output synthesis, and other fields are rising fields benefiting from laser-induced transfer. Following a brief introduction to the principles of laser-induced transfer, this review will comprehensively deliberate this novel additive manufacturing method, including preparing the donor layer and the applications, advantages, and limitations of this technique. Finally, perspectives for handling current and future functional materials using laser-induced transfer will also be discussed. Non-experts in laser technologies can also gain insights into this prevailing laser-induced transfer process, which may inspire their future research.

Optical trapping and light scattering in atmospheric aerosol science

Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

Optical trapping with holographically structured light for single-cell studies

A groundbreaking work in 1970 by Arthur Ashkin paved the way for developing various optical trapping techniques. Optical tweezers have become an established method for the manipulation of biological objects, due to their noninvasiveness and precise controllability. Recent innovations are accelerating and now enable single-cell manipulation through holographic light structuring. In this review, we provide an overview of recent advances in optical tweezer techniques for studies at the individual cell level. Our review focuses on holographic optical tweezers that utilize active spatial light modulators to noninvasively manipulate live cells. The versatility of the technology has led to valuable integrations with microscopy, microfluidics, and biotechnological techniques for various single-cell studies. We aim to recapitulate the basic principles of holographic optical tweezers, highlight trends in their biophysical applications, and discuss challenges and future prospects.

Forces of Change: Optical Tweezers in Membrane Remodeling Studies

Optical tweezers allow precise measurement of forces and distances with piconewton and nanometer precision, and have thus been instrumental in elucidating the mechanistic details of various biological processes. Some examples include the characterization of motor protein activity, studies of protein-DNA interactions, and characterizing protein folding trajectories. The use of optical tweezers (OT) to study membranes is, however, much less abundant. Here, we review biophysical studies of membranes that utilize optical tweezers, with emphasis on various assays that have been developed and their benefits and limitations. First, we discuss assays that employ membrane-coated beads, and overview protein-membrane interactions studies based on manipulation of such beads. We further overview a body of studies that make use of a very powerful experimental tool, the combination of OT, micropipette aspiration, and fluorescence microscopy, that allow detailed studies of membrane curvature generation and sensitivity. Finally, we describe studies focused on membrane fusion and fission. We then summarize the overall progress in the field and outline future directions.

Determination of protein-protein interactions at the single-molecule level using optical tweezers

Biomolecular interactions are at the base of all physical processes within living organisms; the study of these interactions has led to the development of a plethora of different methods. Among these, single-molecule (in singulo) experiments have become relevant in recent years because these studies can give insight into mechanisms and interactions that are hidden for ensemble-based (in multiplo) methods. The focus of this review is on optical tweezer (OT) experiments, which can be used to apply and measure mechanical forces in molecular systems. OTs are based on optical trapping, where a laser is used to exert a force on a dielectric bead; and optically trap the bead at a controllable position in all three dimensions. Different experimental approaches have been developed to study protein–protein interactions using OTs, such as: (1) refolding and unfolding in trans interaction where one protein is tethered between the beads and the other protein is in the solution; (2) constant force in cis interaction where each protein is bound to a bead, and the tension is suddenly increased. The interaction may break after some time, giving information about the lifetime of the binding at that tension. And (3) force ramp in cis interaction where each protein is attached to a bead and a ramp force is applied until the interaction breaks. With these experiments, parameters such as kinetic constants (k_off, k_on), affinity values (K_D), energy to the transition state ΔG^≠, distance to the transition state Δx^≠ can be obtained. These parameters characterize the energy landscape of the interaction. Some parameters such as distance to the transition state can only be obtained from force spectroscopy experiments such as those described here.

Wave-Driven Assembly of Quasiperiodic Patterns of Particles

We theoretically show that a superposition of plane waves causes small (compared to the wavelength) particles dispersed in a fluid to assemble in quasiperiodic two or three-dimensional patterns. We experimentally demonstrate this theory by using ultrasound waves to assemble quasiperiodic patterns of carbon nanoparticles in water using an octagonal arrangement of ultrasound transducers, and we document good agreement between theory and experiments. The theory also applies to obtaining quasiperiodic patterns in other situations where particles move with linear waves, such as optical lattices.

Contact and macroscopic ageing in colloidal suspensions

The ageing behaviour of dense suspensions or pastes at rest is almost exclusively attributed to structural dynamics. Here, we identify another ageing process, contact-controlled ageing, consisting of the progressive stiffening of solid-solid contacts of an arrested colloidal suspension. By combining rheometry, confocal microscopy and particle-scale mechanical tests using laser tweezers, we demonstrate that this process governs the shear-modulus ageing of dense aqueous silica and polymer latex suspensions at moderate ionic strengths. We further show that contact-controlled ageing becomes relevant as soon as Coulombic interactions are sufficiently screened out that the formation of solid-solid contacts is not limited by activation barriers. Given that this condition only requires moderate ion concentrations, contact-controlled ageing should be generic in a wide class of materials, such as cements, soils or three-dimensional inks, thus questioning our understanding of ageing dynamics in these systems.

Optical Tweezers in Studies of Red Blood Cells

Optical tweezers (OTs) are innovative instruments utilized for the manipulation of microscopic biological objects of interest. Rapid improvements in precision and degree of freedom of multichannel and multifunctional OTs have ushered in a new era of studies in basic physical and chemical properties of living tissues and unknown biomechanics in biological processes. Nowadays, OTs are used extensively for studying living cells and have initiated far-reaching influence in various fundamental studies in life sciences. There is also a high potential for using OTs in haemorheology, investigations of blood microcirculation and the mutual interplay of blood cells. In fact, in spite of their great promise in the application of OTs-based approaches for the study of blood, cell formation and maturation in erythropoiesis have not been fully explored. In this review, the background of OTs, their state-of-the-art applications in exploring single-cell level characteristics and bio-rheological properties of mature red blood cells (RBCs) as well as the OTs-assisted studies on erythropoiesis are summarized and presented. The advance developments and future perspectives of the OTs' application in haemorheology both for fundamental and practical in-depth studies of RBCs formation, functional diagnostics and therapeutic needs are highlighted.

Periodic particle arrangements using standing acoustic waves

We determine crystal-like materials that can be fabricated by using a standing acoustic wave to arrange small particles in a non-viscous liquid resin, which is cured afterwards to keep the particles in the desired locations. For identical spherical particles with the same physical properties and small compared to the wavelength, the locations where the particles are trapped correspond to the minima of an acoustic radiation potential which describes the net forces that a particle is subject to. We show that the global minima of spatially periodic acoustic radiation potentials can be predicted by the eigenspace of a small real symmetric matrix corresponding to its smallest eigenvalue. We relate symmetries of this eigenspace to particle arrangements composed of points, lines or planes. Since waves are used to generate the particle arrangements, the arrangement's periodicity is limited to certain Bravais lattice classes that we enumerate in two and three dimensions.

Influence of Pulsed He-Ne Laser Irradiation on the Red Blood Cell Interaction Studied by Optical Tweezers

Optical Tweezers (OT), as a revolutionary innovation in laser physics, has been extremely useful in studying cell interaction dynamics at a single-cell level. The reversible aggregation process of red blood cells (RBCs) has an important influence on blood rheological properties, but the underlying mechanism has not been fully understood. The regulating effects of low-level laser irradiation on blood rheological properties have been reported. However, the influence of pulsed laser irradiation, and the origin of laser irradiation effects on the interaction between RBCs remain unclear. In this study, RBC interaction was assessed in detail with OT. The effects of both continuous and pulsed low-level He-Ne laser irradiation on RBC aggregation was investigated within a short irradiation period (up to 300 s). The results indicate stronger intercellular interaction between RBCs in the enforced disaggregation process, and both the cell contact time and the initial contact area between two RBCs showed an impact on the measured disaggregation force. Meanwhile, the RBC aggregation force that was independent to measurement conditions decreased after a short time of pulsed He-Ne laser irradiation. These results provide new insights into the understanding of the RBC interaction mechanism and laser irradiation effects on blood properties.

Drag controlled formation of polymeric colloids with optical traps

Optical trapping is a powerful optical manipulation technique for controlling various mesoscopic systems that allows formation of tailor-made polymeric micro-sized colloids by directed coalescence of nucleation sites. However, control over the size of a single colloid requires constant monitoring of the growth process and deactivation of the optical trap once it reaches the required dimensions. Moreover, producing more than one colloid requires moving the sample to a pristine location where the process must be repeated. Here, we present a novel method for continuous control over formation of polydimethylsiloxane colloids based on directed coalescence induced by optical traps under flow inside microfluidic channels. Once the drag force on a growing colloid exceeds the trapping force, it leaves the optical trap, and a new colloid starts to form at the same location. We demonstrate repeatability of the process and selectively produce colloids with radii of ∼1-14 μm by controlling the laser intensity and flow rate. In addition, holographic optical tweezers are used to show how multiple optical traps in 3D could be used to influence a significant cross section of the micro-channel, thus forming a light-controlled assembly line for colloidal formation.

Digital Assembly of Colloidal Particles for Nanoscale Manufacturing

From unravelling the most fundamental phenomena to enabling applications that impact our everyday lives, the nanoscale world holds great promise for science, technology and medicine. However, the extent of its practical realization would rely on manufacturing at the nanoscale. Among the various nanomanufacturing approaches being investigated, the bottom-up approach involving assembly of colloidal nanoparticles as building blocks is promising. Compared to a top-down lithographic approach, particle assembly exhibits advantages such as smaller feature size, finer control of chemical composition, less defects, lower material wastage, and higher scalability. The capability to assemble colloidal particles one by one or "digitally" has been heavily sought as it mimics the natural way of making matter and enables construction of nanomaterials with sophisticated architectures. This progress report provides an insight into the tools and techniques for digital assembly of particles, including their working mechanisms and demonstrated particle assemblies. Examples of nanomaterials and nanodevices are presented to demonstrate the strength of digital assembly in nanomanufacturing.

Opto-thermophoretic fiber tweezers

Recent advances in opto-thermophoretic tweezers open new avenues for low-power trapping and manipulation of nanoparticles with potential applications in colloidal assembly, nanomanufacturing, life sciences, and nanomedicine. However, to fully exploit the opto-thermophoretic tweezers for widespread applications, the enhancement of their versatility in nanoparticle manipulations is pivotal. For this purpose, we translate our newly developed opto-thermophoretic tweezers onto an optical fiber platform known as opto-thermophoretic fiber tweezers (OTFT). We have demonstrated the applications of OTFT as a nanoparticle concentrator, as a nanopipette for single particle delivery, and as a nanoprobe. The simple setup and functional versatility of OTFT would encourage its use in various fields such as additive manufacturing, single nanoparticle-cell interactions, and biosensing.

Analysis of Egg White Protein Composition with Double Nanohole Optical Tweezers

We use a double nanohole optical tweezer to analyze the protein composition of egg white through analysis of many individual protein trapping events. The proteins are grouped by mass based on two metrics: standard deviation of the trapping laser intensity fluctuations from the protein diffusion and the time constant of these fluctuations coming from the autocorrelation. Quantitative analysis is demonstrated for artificial samples, and then, the approach is applied to real egg white. The composition found from real egg white corresponds well to past reports using gel electrophoresis. This approach differs from past works by allowing for individual protein analysis in heterogeneous solutions without the need for denaturing, labeling, or tethering.

Out-of-Plane Rotation Control of Biological Cells With a Robot-Tweezers Manipulation System for Orientation-Based Cell Surgery

In many cell surgery applications, cell must be oriented properly such that the microsurgery tool can access the target components with minimum damage to the cell. In this paper, a scheme for out of image plane orientation control of suspended biological cells using robotic controlled optical tweezers is presented for orientation-based cell surgery. Based on our previous work on planar cell rotation using optical tweezers, the dynamic model of cell out-of-plane orientation control is formulated by using the T-matrix approach. Vision-based algorithms are developed to extract the cell out of image plane orientation angles, based on 2-D image slices obtained under an optical microscope. A robust feedback controller is then proposed to achieve cell out-of-plane rotation. Experiments of automated out of image plane rotational control for cell nucleus extraction surgery are performed to demonstrate the effectiveness of the proposed approach. This approach advances robot-aided single cell manipulation and produces impactful benefits to cell surgery applications such as nucleus transplantation and organelle biopsy in precision medicine.

An improved optical tweezers assay for measuring the force generation of single kinesin molecules

Numerous microtubule-associated molecular motors, including several kinesins and cytoplasmic dynein, produce opposing forces that regulate spindle and chromosome positioning during mitosis. The motility and force generation of these motors are therefore critical to normal cell division, and dysfunction of these processes may contribute to human disease. Optical tweezers provide a powerful method for studying the nanometer motility and piconewton force generation of single motor proteins in vitro. Using kinesin-1 as a prototype, we present a set of step-by-step, optimized protocols for expressing a kinesin construct (K560-GFP) in Escherichia coli, purifying it, and studying its force generation in an optical tweezers microscope. We also provide detailed instructions on proper alignment and calibration of an optical trapping microscope. These methods provide a foundation for a variety of similar experiments.

Quantitative analysis of shape and volume changes in activated thrombocytes in real time by single-shot spatial light modulator-based differential interference contrast imaging

We suggest to use a combination of optical tweezers and single-image quantitative differential interference contrast (DIC) emulated by a spatial light modulator (SLM) to study physiological shape changes in thrombocytes after activation and demonstrate the effectiveness of this system for the given task. A specially designed phase mask displayed at the SLM enables quantitative phase calculation from only a single recording. The optical tweezers stabilize trapped thrombocytes for long-time monitoring of changes in the optical thickness profile of thrombocytes during activation by adenosine diphosphate (ADP).

Genetically encoded force sensors for measuring mechanical forces in proteins

THERE ARE THREE SOURCES OF FREE ENERGY FOR CELLS: chemical potential, electrical potential and mechanical potential. There is little known about the last one since there have not been simple ways to measure stress in proteins in cells. we have now developed genetically encoded force sensors to assess the stress in fibrous proteins in living cells. These FReT based fluorescence sensors can be read out at video rates and provide real time maps of the stress distribution in cells, tissues and animals. The sensors can be inserted into specific proteins and in general do not disturb the normal function or anatomy. The original sensors used mutant GFPs linked by elastic linkers. These sensors provide a linear output with applied stress but the response is linear in strain. To improve contrast and dynamic range we have now developed a new class of sensors that are smaller making them less invasive, and have much higher intrinsic sensitivity since force modulates the angle between the donor and acceptor much more than the distance between them. Known as cpstFRET, the probe shows improved biocompatibility, wider dynamic range and higher sensitivity.

Mechanical induction of gene expression in connective tissue cells

The extracellular matrices of mammals undergo coordinated synthesis and degradation, dynamic remodeling processes that enable tissue adaptations to a broad range of environmental factors, including applied mechanical forces. The soft and mineralized connective tissues of mammals also exhibit a wide repertoire of mechanical properties, which enable their tissue-specific functions and modulate cellular responses to forces. The expression of genes in response to applied forces are important for maintaining the support, attachment, and function of various organs including kidney, heart, liver, lung, joint, and periodontium. Several high-prevalence diseases of extracellular matrices including arthritis, heart failure, and periodontal diseases involve pathological levels of mechanical forces that impact the gene expression repertoires and function of bone, cartilage, and soft connective tissues. Recent work on the application of mechanical forces to cultured connective tissue cells and various in vivo force models have enabled study of the regulatory networks that control mechanically induced gene expression in connective tissue cells. In addition to the influence of mechanical forces on the expression of type 1 collagen, which is the most abundant protein of mammals, new work has shown that the expression of a wide range of matrix, signaling, and cytoskeletal proteins are regulated by exogenous mechanical forces and by the forces generated by cells themselves. In this chapter, we first discuss the fundamental nature of the extracellular matrix in health and the impact of mechanical forces. Next we consider the utilization of several, widely employed model systems for mechanical stimulation of cells. Finally, we consider in detail how application of tensile forces to cultured cardiac fibroblasts can be used for the characterization of the signaling systems by which mechanical forces regulate myofibroblast differentiation that is seen in cardiac pressure overload.

Bacterial translocation motors investigated by single molecule techniques

Translocation of DNA and protein fibers through narrow constrictions is a ubiquitous and crucial activity of bacterial cells. Bacteria use specialized machines to support macromolecular movement. A very important step toward a mechanistic understanding of these translocation machines is the characterization of their physical properties at the single molecule level. Recently, four bacterial transport processes have been characterized by nanomanipulation at the single molecule level, DNA translocation by FtsK and SpoIIIE, DNA import during transformation, and the related process of a type IV pilus retraction. With all four processes, the translocation rates, processivity, and stalling forces were remarkably high as compared with single molecule experiments with other molecular motors. Although substrates of all four processes proceed along a preferential direction of translocation, directionality has been shown to be controlled by distinct mechanisms.

Light at work: the use of optical forces for particle manipulation, sorting, and analysis

We review the combinations of optical micro-manipulation with other techniques and their classical and emerging applications to non-contact optical separation and sorting of micro- and nanoparticle suspensions, compositional and structural analysis of specimens, and quantification of force interactions at the microscopic scale. The review aims at inspiring researchers, especially those working outside the optical micro-manipulation field, to find new and interesting applications of these methods.

Cell mechanics at multiple scales

The response of cells to mechanical stresses is a field of growing inquiry. It is well known that both the morphologic and molecular expression of cells depend, in part, on the local mechanical environment, especially for cells such as endothelial cells that experience shear stress, stretch, and pressures. To systematically study the large variety of responses of cells to physical forces (e.g., signaling, adhesion, or stiffness changes), a number of techniques have been developed and used. Here we present methods for three types of cell mechanical studies, from the multicellular to the subcellular scales, and describe the basic principle and main use of each technique along with some design and setup considerations.

Negative axial radiation forces on solid spheres and shells in a Bessel beam

Prior computations predict that fluid spheres illuminated by an acoustic Bessel beam can be subjected to a radiation force directed opposite the direction of beam propagation. The prediction of negative acoustic radiation force is extended to the cases of a solid poly(methylmethacrylate) PMMA sphere in water and an empty aluminum spherical shell in water. Compared with the angular scattering patterns for plane wave illumination, the scattering into the back hemisphere is suppressed when the radiation force is negative. This investigation may be helpful in the development of acoustic tweezers and in the development of methods for manipulating objects during space flight.