Calibration of force detection for arbitrarily shaped particles in optical tweezers

Shining Light in Mechanobiology: Optical Tweezers, Scissors, and Beyond

Mechanobiology helps us to decipher cell and tissue functions by looking at changes in their mechanical properties that contribute to development, cell differentiation, physiology, and disease. Mechanobiology sits at the interface of biology, physics and engineering. One of the key technologies that enables characterization of properties of cells and tissue is microscopy. Combining microscopy with other quantitative measurement techniques such as optical tweezers and scissors, gives a very powerful tool for unraveling the intricacies of mechanobiology enabling measurement of forces, torques and displacements at play. We review the field of some light based studies of mechanobiology and optical detection of signal transduction ranging from optical micromanipulation-optical tweezers and scissors, advanced fluorescence techniques and optogenentics. In the current perspective paper, we concentrate our efforts on elucidating interesting measurements of forces, torques, positions, viscoelastic properties, and optogenetics inside and outside a cell attained when using structured light in combination with optical tweezers and scissors. We give perspective on the field concentrating on the use of structured light in imaging in combination with tweezers and scissors pointing out how novel developments in quantum imaging in combination with tweezers and scissors can bring to this fast growing field.

Opto-hydrodynamic tweezers

Optical fiber tweezers offer a simple, low-cost and portable solution for non-invasive trapping and manipulation of particles. However, single-fiber tweezers require fiber tip modification (tapering, lensing, etc.) and the dual-fiber approach demands strict alignment and positioning of fibers for robust trapping of particles. In addition, both tweezing techniques offer a limited range of particle manipulation and operate in low flow velocity regimes (a few 100 μm s-1) when integrated with microfluidic devices. In this paper, we report a novel opto-hydrodynamic fiber tweezers (OHT) platform that exploits the balance between the hydrodynamic drag force and optical scattering forces to trap and manipulate single or multiple particles of various shapes, sizes, and material compositions in a microfluidic channel. 3D hydrodynamic flow focusing offers an easy and dynamic alignment of the particle trajectories with the optical axis of the fiber, which enables robust trapping of particles with high efficiency of >70% and throughput of 14 particles per minute (operating flow velocity: 1000 μm s-1) without the need for precision stages or complex fabrication. By regulating the optical power and flow rates, we were able to trap single particles at desired positions in the channel with a precision of ±10 μm as well as manipulate them over a long range upstream or downstream with a maximum distance of 500 μm. Our opto-hydrodynamic tweezers offer an alternative to conventional optical fiber tweezers for several applications in physics, biology, medicine, etc.

A disturbance suppression micro-Newton force sensor based on shadow method

The precision of micro-force measurement depends on the force sensor sensitivity and the environmental disturbance magnitude. However, micro-force sensors generally have the poor anti-disturbance ability. Inspired by the shadow formed by water striders walking on water surface under sunlight, a viscoelastic-polymer micro-force (VPMF) sensor based on the shadow method was proposed, which could suppress disturbances effectively due to the high damping ratio of 0.22. The shadow diameter change and the applied force were proportional. The experimental results indicated that the sensitivity could reach 2.15 μN/pixel with a good linear performance. Furthermore, compared with the cantilever, it was capable of the reduction of the disturbance influence by approximately 96.35%. Therefore, the VPMF sensor can be applied to reliable micro-force measurement in complex environments such as industrial sites.

Particle Localization Using Local Gradients and Its Application to Nanometer Stabilization of a Microscope

Particle localization plays a fundamental role in advanced biological techniques such as single-molecule tracking, superresolution microscopy, and manipulation by optical and magnetic tweezers. Such techniques require fast and accurate particle localization algorithms as well as nanometer-scale stability of the microscope. Here, we present a universal method for three-dimensional localization of single labeled and unlabeled particles based on local gradient calculation of particle images. The method outperforms state-of-the-art localization techniques in high-noise conditions, and it is capable of 3D nanometer accuracy localization of nano- and microparticles with sub-millisecond calculation time. By localizing a fixed particle as fiducial mark and running a feedback loop, we demonstrate its applicability for active drift correction in sensitive nanomechanical measurements such as optical trapping and superresolution imaging. A multiplatform open software package comprising a set of tools for local gradient calculation in brightfield, darkfield, and fluorescence microscopy is shared for ready use by the scientific community.

Fabrication and Manipulation of Non-Spherical Particles in Microfluidic Channels: A Review

Non-spherical shape is a general appearance feature for bioparticles. Therefore, a mechanical mechanism study of non-spherical particle migration in a microfluidic chip is essential for more precise isolation of target particles. With the manipulation of non-spherical particles, refined disease detection or medical intervention for human beings will be achievable in the future. In this review, fabrication and manipulation of non-spherical particles are discussed. Firstly, various fabrication methods for non-spherical microparticle are introduced. Then, the active and passive manipulation techniques for non-spherical particles are briefly reviewed, including straight inertial microchannels, secondary flow inertial microchannels and deterministic lateral displacement microchannels with extremely high resolution. Finally, applications of viscoelastic flow are presented which obviously increase the precision of non-spherical particle separation. Although various techniques have been employed to improve the performance of non-spherical particle manipulation, the universal mechanism behind this has not been fully discussed. The aim of this review is to provide a reference for non-spherical particle manipulation study researchers in every detail and inspire thoughts for non-spherical particle focused device design.

Exploring cell and tissue mechanics with optical tweezers

Cellular and tissue biosystems emerge from the assembly of their constituent molecules and obtain a set of specific material properties. To measure these properties and understand how they influence cellular function is a central goal of mechanobiology. From a bottoms-up, physics or engineering point-of-view, such systems are a composition of basic mechanical elements. However, the sheer number and dynamic complexity of them, including active molecular machines and their emergent properties, makes it currently intractable to calculate how biosystems respond to forces. Because many diseases result from an aberrant mechanotransduction, it is thus essential to measure this response. Recent advances in the technology of optical tweezers have broadened their scope from single-molecule applications to measurements inside complex cellular environments, even within tissues and animals. Here, we summarize the basic optical trapping principles, implementations and calibration procedures that enable force measurements using optical tweezers directly inside cells of living animals, in combination with complementary techniques. We review their versatility to manipulate subcellular organelles and measure cellular frequency-dependent mechanics in the piconewton force range from microseconds to hours. As an outlook, we address future challenges to fully unlock the potential of optical tweezers for mechanobiology.

Determining Trap Compliances, Microsphere Size Variations, and Response Linearities in Single DNA Molecule Elasticity Measurements with Optical Tweezers

We previously introduced the use of DNA molecules for calibration of biophysical force and displacement measurements with optical tweezers. Force and length scale factors can be determined from measurements of DNA stretching. Trap compliance can be determined by fitting the data to a nonlinear DNA elasticity model, however, noise/drift/offsets in the measurement can affect the reliability of this determination. Here we demonstrate a more robust method that uses a linear approximation for DNA elasticity applied to high force range (25-45 pN) data. We show that this method can be used to assess how small variations in microsphere sizes affect DNA length measurements and demonstrate methods for correcting for these errors. We further show that these measurements can be used to check assumed linearities of system responses. Finally, we demonstrate methods combining microsphere imaging and DNA stretching to check the compliance and positioning of individual traps.

Optical Tweezers Exploring Neuroscience

Over the past decade, optical tweezers (OT) have been increasingly used in neuroscience for studies of molecules and neuronal dynamics, as well as for the study of model organisms as a whole. Compared to other areas of biology, it has taken much longer for OT to become an established tool in neuroscience. This is, in part, due to the complexity of the brain and the inherent difficulties in trapping individual molecules or manipulating cells located deep within biological tissue. Recent advances in OT, as well as parallel developments in imaging and adaptive optics, have significantly extended the capabilities of OT. In this review, we describe how OT became an established tool in neuroscience and we elaborate on possible future directions for the field. Rather than covering all applications of OT to neurons or related proteins and molecules, we focus our discussions on studies that provide crucial information to neuroscience, such as neuron dynamics, growth, and communication, as these studies have revealed meaningful information and provide direction for the field into the future.

Optical Manipulation of Lanthanide-Doped Nanoparticles: How to Overcome Their Limitations

Since Ashkin's pioneering work, optical tweezers have become an essential tool to immobilize and manipulate microscale and nanoscale objects. The use of optical tweezers is key for a variety of applications, including single-molecule spectroscopy, colloidal dynamics, tailored particle assembly, protein isolation, high-resolution surface studies, controlled investigation of biological processes, and surface-enhanced spectroscopy. In recent years, optical trapping of individual sub-100-nm objects has got the attention of the scientific community. In particular, the three-dimensional manipulation of single lanthanide-doped luminescent nanoparticles is of great interest due to the sensitivity of their luminescent properties to environmental conditions. Nevertheless, it is really challenging to trap and manipulate single lanthanide-doped nanoparticles due to the weak optical forces achieved with conventional optical trapping strategies. This limitation is caused, firstly, by the diffraction limit in the focusing of the trapping light and, secondly, by the Brownian motion of the trapped object. In this work, we summarize recent experimental approaches to increase the optical forces in the manipulation of lanthanide-doped nanoparticles, focusing our attention on their surface modification and providing a critical review of the state of the art and future prospects.

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.

Optical Micromachines for Biological Studies

Optical tweezers have been used for biological studies since shortly after their inception. However, over the years research has suggested that the intense laser light used to create optical traps may damage the specimens being studied. This review aims to provide a brief overview of optical tweezers and the possible mechanisms for damage, and more importantly examines the role of optical micromachines as tools for biological studies. This review covers the achievements to date in the field of optical micromachines: improvements in the ability to produce micromachines, including multi-body microrobots; and design considerations for both optical microrobots and the optical trapping set-up used for controlling them are all discussed. The review focuses especially on the role of micromachines in biological research, and explores some of the potential that the technology has in this area.