Red blood cell membrane damage by light-induced thermal gradient under optical trap

Tired and stressed: direct holographic quasi-static stretching of aging echinocytes and discocytes in plasma using optical tweezers [Invited]

Red blood cells (RBCs) undergo a progressive morphological transformation from smooth biconcave discocytes into rounder echinocytes with spicules on their surface during cold storage. The echinocytic morphology impacts RBCs' ability to flow through narrow sections of the circulation and therefore transfusion of RBC units with a high echinocytic content are thought to have a reduced efficiency. We use an optical tweezers-based technique where we directly trap and measure linear stiffness of RBCs under stress without the use of attached spherical probe particles or microfluidic flow to induce shear. We study RBC deformability with over 50 days of storage performing multiple stretches in blood plasma (serum with cold agglutinins removed to eliminate clotting). In particular, we find that discocytes and echinocytes do not show significant changes in linear stiffness in the small strain limit (∼ 20 % change in length) up to day 30 of the storage period, but do find differences between repeated stretches. By day 50 the linear stiffness of discocytes had increased to approximately that measured for echinocytes throughout the entire period of measurements. These changes in stiffness corresponded to recorded morphological changes in the discocytes as they underwent storage lesion. We believe our holographic trapping and direct measurement technique has applications to directly control and quantify forces that stretch other types of cells without the use of attached probes.

Comparison of the human's and camel's red blood cell deformability by optical tweezers and Raman spectroscopy

Red blood cells of vertebrates have undergone evolutionary changes over time, leading to the diversification of morphological and mechanical properties of red blood cells (RBCs). Among the vertebrates, camelids have the most different RBC characteristics. As a result of adaptation to the desert environment, camelid RBCs can expand twice as much of their total volume in the case of rapid hydration yet are almost undeformable under mechanical stress. In this work, the mechanical and chemical differences in the RBC properties of the human and camelid species were examined using optical tweezers and Raman spectroscopy. We measured the deformability of camel and human RBCs at the single-cell level using optical tweezers. We found that the deformability index (DI) of the camel and the human RBCs were 0.024 ± 0.019 and 0.215 ± 0.061, respectively. To investigate the chemical properties of these cells, we measured the Raman spectra of the whole blood samples. The result of our study indicated that some of the Raman peaks observed on the camel's blood spectrum were absent in the human blood's spectrum, which further points to the difference in chemical contents of these two species' RBCs.

Advancing optothermal manipulation: decoupling temperature and flow fields in quasi-isothermal microscale streaming

By decoupling temperature and flow fields through symmetry-correlated laser scan sequences, ISO-FLUCS enables quasi-isothermal optofluidic microscale streaming. This technique offers precise control over fluid manipulation while minimizing thermal damage.

Modelling red blood cell optical trapping by machine learning improved geometrical optics calculations

Optically trapping red blood cells allows for the exploration of their biophysical properties, which are affected in many diseases. However, because of their nonspherical shape, the numerical calculation of the optical forces is slow, limiting the range of situations that can be explored. Here we train a neural network that improves both the accuracy and the speed of the calculation and we employ it to simulate the motion of a red blood cell under different beam configurations. We found that by fixing two beams and controlling the position of a third, it is possible to control the tilting of the cell. We anticipate this work to be a promising approach to study the trapping of complex shaped and inhomogeneous biological materials, where the possible photodamage imposes restrictions in the beam power.

Cell mechanical responses to subcellular perturbations generated by ultrasound and targeted microbubbles

The inherently dynamic and anisotropic microenvironment of cells imposes not only global and slow physical stimulations on cells but also acute and local perturbations. However, cell mechanical responses to transient subcellular physical signals remain unclear. In this study, acoustically activated targeted microbubbles were used to exert mechanical perturbations to single cells. The cellular contractile force was sensed by elastic micropillar arrays, while the pillar deformations were imaged using brightfield high-speed video microscopy at a frame rate of 1k frames per second for the first 10s and then confocal fluorescence microscopy. Cell mechanical responses are accompanied by cell membrane integrity changes. Both processes are determined by the perturbation strength generated by microbubble volumetric oscillations. The instantaneous cellular traction force relaxation exhibits two distinct patterns, correlated with two cell fates (survival or permanent damage). The mathematical modeling unveils that force-induced actomyosin disassembly leads to gradual traction force relaxation in the first few seconds. The perturbation may also influence the far end subcellular regions from the microbubbles and may propagate into connected cells with attenuations and delays. This study carefully characterizes the cell mechanical responses to local perturbations induced by ultrasound and microbubbles, advancing our understanding of the fundamentals of cell mechano-sensing, -responsiveness, and -transduction. STATEMENT OF SIGNIFICANCE: Subcellular physical perturbations commonly exist but haven't been fully explored yet. The subcellular perturbation generated by ultrasound and targeted microbubbles covers a wide range of strength, from mild, intermediate to intense, providing a broad biomedical relevance. With µm² spatial sensing ability and up to 1ms temporal resolution, we present spatiotemporal details of the instantaneous cellular contractile force changes followed by attenuated and delayed global responses. The correlation between the cell mechanical responses and cell fates highlights the important role of the instantaneous mechanical responses in the entire cellular reactive processes. Supported by mathematical modeling, our work provides new insights into the dynamics and mechanisms of cell mechanics.

Optical tweezers across scales in cell biology

Optical tweezers (OT) provide a noninvasive approach for delivering minute physical forces to targeted objects. Controlling such forces in living cells or in vitro preparations allows for the measurement and manipulation of numerous processes relevant to the form and function of cells. As such, OT have made important contributions to our understanding of the structures of proteins and nucleic acids, the interactions that occur between microscopic structures within cells, the choreography of complex processes such as mitosis, and the ways in which cells interact with each other. In this review, we highlight recent contributions made to the field of cell biology using OT and provide basic descriptions of the physics, the methods, and the equipment that made these studies possible.

Emerging Diamond Quantum Sensing in Bio-Membranes

Bio-membranes exhibit complex but unique mechanical properties as communicative regulators in various physiological and pathological processes. Exposed to a dynamic micro-environment, bio-membranes can be seen as an intricate and delicate system. The systematical modeling and detection of their local physical properties are often difficult to achieve, both quantitatively and precisely. The recent emerging diamonds hosting quantum defects (i.e., nitrogen-vacancy (NV) center) demonstrate intriguing optical and spin properties, together with their outstanding photostability and biocompatibility, rendering them ideal candidates for biological applications. Notably, the extraordinary spin-based sensing enable the measurements of localized nanoscale physical quantities such as magnetic fields, electrical fields, temperature, and strain. These nanoscale signals can be optically read out precisely by simple optical microscopy systems. Given these exclusive properties, NV-center-based quantum sensors can be widely applied in exploring bio-membrane-related features and the communicative chemical reaction processes. This review mainly focuses on NV-based quantum sensing in bio-membrane fields. The attempts of applying NV-based quantum sensors in bio-membranes to investigate diverse physical and chemical events such as membrane elasticity, phase change, nanoscale bio-physical signals, and free radical formation are fully overviewed. We also discuss the challenges and future directions of this novel technology to be utilized in bio-membranes.

Raman tweezers as an alternative diagnostic tool for paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare disease characterized by hemolysis of red blood cells (RBC) and venous thrombosis. The gold standard method for the diagnosis of this disease is flow cytometry. Here, we propose a combined optical tweezers and Raman spectral (Raman tweezers) approach to analyze blood samples from volunteers with or without PNH conditions. Raman spectroscopy is a well-known method for investigating a material's chemical structure and is also used in molecular analysis of biological compounds. In this study, we trap individual RBCs found in whole blood samples drawn from PNH patients and the control group. Evaluation of the Raman spectra of these cells by band component analysis and machine learning shows a significant difference between the two groups. The specificity and the sensitivity of the training performed by support vector machine (SVM) analysis were found to be 81.8% and 78.3%, respectively. This study shows that an immediate and high accuracy test result is possible for PNH disease by employing Raman tweezers and machine learning.

Optical microlever assisted DNA stretching

Optical microrobotics is an emerging field that has the potential to improve upon current optical tweezer studies through avenues such as limiting the exposure of biological molecules of interest to laser radiation and overcoming the current limitations of low forces and unwanted interactions between nearby optical traps. However, optical microrobotics has been historically limited to rigid, single-body end-effectors rather than even simple machines, limiting the tasks that can be performed. Additionally, while multi-body machines such as microlevers exist in the literature, they have not yet been successfully demonstrated as tools for biological studies, such as molecule stretching. In this work we have taken a step towards moving the field forward by developing two types of microlever, produced using two-photon absorption polymerisation, to perform the first lever-assisted stretches of double-stranded DNA. The aim of the work is to provide a proof of concept for using optical micromachines for single molecule studies. Both styles of microlevers were successfully used to stretch single duplexes of DNA, and the results were analysed with the worm-like chain model to show that they were in good agreement.

Quantifying the influences of radiation therapy on deformability of human red blood cells by dual-beam optical tweezers

Radiation therapy is widely used as a treatment tool for malignancies. However, radiation-related complications are still unavoidable risks for off-target cells. Little is known about radiation therapy's possible effects on mechanical features of the off-target cells such as human red blood cells (RBCs). RBCs are nucleus-free circulating cells that can deform without losing functionality in healthy conditions. Thus, to evaluate in vitro effects of radiation therapy on the healthy plasma membrane of cells, RBCs were selected as a primary test model. RBCs were exposed to clinically prescribed radiotherapy doses of 2 Gy, 12 Gy and, 25 Gy, and each radiotherapy dose group was compared to a non-irradiated group. Cells were characterized by stretching using dual-beam optical tweezers and compared using the resulting deformability index. The group receiving the highest radiation dose was found statistically distinguishable from the control group (DI_0Gy = 0.33 ± 0.08), and revealed the highest deformability index (DI_25Gy = 0.38 ± 0.11, p = 0.0068), while no significant differences were found for 2 Gy (DI_2Gy = 0.33 ± 0.08, p = 0.9) and 12 Gy (DI_12Gy = 0.31 ± 0.09, p = 0.2) dose groups. Based on these findings, we conclude that radiotherapy exposure may alter the deformability of red blood cells depending on the dose amount, and measurement of deformability index by dual-beam optical tweezers can serve as a sensitive biomarker to probe responses of cells to the radiotherapy.

Little is known about radiation therapy's possible effects on mechanical features of off-target cells such as human red blood cells. Here, irradiated human red blood cells were stretched using dual-beam optical tweezers and compared using the resulting deformability index.

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.

Optical Tweezers: Phototoxicity and Thermal Stress in Cells and Biomolecules

For several decades optical tweezers have proven to be an invaluable tool in the study and analysis of myriad biological responses and applications. However, as with every tool, they can have undesirable or damaging effects upon the very sample they are helping to study. In this review the main negative effects of optical tweezers upon biostructures and living systems will be presented. There are three main areas on which the review will focus: linear optical excitation within the tweezers, non-linear photonic effects, and thermal load upon the sampled volume. Additional information is provided on negative mechanical effects of optical traps on biological structures. Strategies to avoid or, at least, minimize these negative effects will be introduced. Finally, all these effects, undesirable for the most, can have positive applications under the right conditions. Some hints in this direction will also be discussed.