Introduction to Optical Tweezers

Application of biomechanics in tumor epigenetic research

The field of cancer research is increasingly recognizing the complex interplay between biomechanics and tumor epigenetics. Biomechanics plays a significant role in the occurrence, development, and metastasis of cancer and may exert influence by impacting the epigenetic modifications of tumors. In this review, we investigate a spectrum of biomechanical tools, including computational models, measurement instruments, and in vitro simulations. These tools not only assist in deciphering the mechanisms behind these epigenetic changes but also provide novel methods for characterizing tumors, which are significant for diagnosis and treatment. Finally, we discuss the potential of new therapies that target the biomechanical properties of the tumor microenvironment. There is hope that by altering factors such as the stiffness of the extracellular matrix or interfering with mechano-sensing pathways, we can halt tumor progression through epigenetic mechanisms. We emphasize the necessity for multidisciplinary efforts to integrate biomechanics with tumor epigenetics more comprehensively. Such collaboration is anticipated to advance therapeutic strategies and enhance our understanding of cancer biology, signaling the dawn of a new era in cancer treatment and research.

Advancing Transfusion Medicine through Raman Tweezers Spectroscopy: A Review of Recent Progress and Future Perspectives

Background

Raman tweezers spectroscopy (RTS) is a powerful tool that combines optical tweezers and Raman spectroscopy to study single living cells. RTS has become increasingly popular in biomedical and clinical research due to its high molecular specificity and sensitivity, which enable the study of cell viability, cell deformation, cell-protein, cell-nanoparticle, cell-cell interaction, etc. In transfusion medicine, RTS can give valuable insights into the storage lesions and effects of various preservatives and intravenous fluids on blood cells.

Summary

By analyzing the Raman spectra of individual blood cells, RTS can detect changes in the cellular blood components which can be used to monitor the quality of blood products during storage and transfusion. The present review article highlights the principle and clinical applications of RTS in transfusion medicine.

Key Messages

Raman spectroscopy is a versatile analytical method for biomedical research. Combining the Raman spectroscopy method with the optical tweezers technique will allow us to explore the dynamics of live single cells in their physiological medium.

Microscale Diffractive Lenses Integrated into Microfluidic Devices for Size-Selective Optical Trapping of Particles

Integration of optical components into microfluidic devices can enhance particle manipulations, separations, and analyses. We present a method to fabricate microscale diffractive lenses composed of aperiodically spaced concentric rings milled into a thin metal film to precisely position optical tweezers within microfluidic channels. Integrated thin-film microlenses perform the laser focusing required to generate sufficient optical forces to trap particles without significant off-device beam manipulation. Moreover, the ability to trap particles with unfocused laser light allows multiple optical traps to be powered simultaneously by a single input laser. We have optically trapped polystyrene particles with diameters of 0.5, 1, 2, and 4 μm over microlenses fabricated in chromium and gold films. Optical forces generated by these microlenses captured particles traveling at fluid velocities up to 64 μm/s. Quantitative trapping experiments with particles in microfluidic flow demonstrate size-based differential trapping of neutrally buoyant particles where larger particles required a stronger trapping force. The optical forces on these particles are identical to traditional optical traps, but the addition of a continuous viscous drag force from the microfluidic flow introduces tunable size selectivity across a range of laser powers and fluid velocities.

Biophysical characterization of synthetic adhesins for predicting and tuning engineered living material properties

Bacterial synthetic multicellular systems are promising platforms for engineered living materials (ELMs) for medical, biosynthesis, environmental, and smart materials applications. Recent advancements in genetically encoded adhesion toolkits have enabled precise manipulation of cell-cell adhesion and the design and patterning of self-assembled multicellular materials. However, in contrast to gene regulation in synthetic biology, the characterization and control of synthetic adhesins remains limited. Here, we demonstrate the quantitative characterization of a bacterial synthetic adhesion toolbox through various biophysical methods. We determine key parameters, including number of adhesins per cell, in-membrane diffusion constant, production and decay rates, and bond-breaking force between adhesins. With these parameters, we demonstrate the bottom-up prediction and quantitative tuning of macroscopic ELM properties (tensile strength) and, furthermore, that cells inside ELMs are connected only by a small fraction of available adhesins. These results enable the rational engineering, characterization, and modeling of other synthetic and natural adhesins and multicellular consortia.

Investigation of metal ion binding biomolecules one molecule at a time

Metal ions can perform multiple roles ranging from regulatory to structural and are crucial for cell function. While some metal ions like Na+ are ubiquitously present at high concentrations, other ions, especially Ca2+ and transition metals, such as Zn2+ or Cu+/2+ are regulated. The concentrations above or below the physiological range cause severe changes in the behavior of biomolecules that bind them and subsequently affect the cell wellbeing. This has led to the development of specialized protocols to study metal ion binding biomolecules in bulk conditions that mimic the cell environment. Recently, there is growing evidence of influence of post-transcriptional and post-translational modifications on the affinity of the metal ion binding sites. However, such targets are difficult to obtain in amounts required for classical biophysical experiments. Single molecule techniques have revolutionized the field of biophysics, molecular and structural biology. Their biggest advantage is the ability to observe each molecule's interaction independently, without the need for synchronization. An additional benefit is its extremely low sample consumption. This feature allows characterization of designer biomolecules or targets obtained coming from natural sources. All types of biomolecules, including proteins, DNA and RNA were characterized using single molecule methods. However, one group is underrepresented in those studies. These are the metal ion binding biomolecules. Single molecule experiments often require separate optimization, due to extremely different concentrations used during the experiments. In this review we focus on single molecule methods, such as single molecule FRET, nanopores and optical tweezers that are used to study metal ion binding biomolecules. We summarize various examples of recently characterized targets and reported experimental conditions. Finally, we discuss the potential promises and pitfalls of single molecule characterization on metal ion binding biomolecules.

Removal of Pseudomonas type IV pili by a small RNA virus

The retractile type IV pilus (T4P) is important for virulence of the opportunistic human pathogen Pseudomonas aeruginosa. The single-stranded RNA (ssRNA) phage PP7 binds to T4P and is brought to the cell surface through pilus retraction. Using fluorescence microscopy, we discovered that PP7 detaches T4P, which impairs cell motility and restricts the pathogen's virulence. Using cryo-electron microscopy, mutagenesis, optical trapping, and Langevin dynamics simulation, we resolved the structure of PP7, T4P, and the PP7/T4P complex and showed that T4P detachment is driven by the affinity between the phage maturation protein and its bound pilin, plus the pilus retraction force and speed, and pilus bending. Pilus detachment may be widespread among other ssRNA phages and their retractile pilus systems and offers new prospects for antibacterial prophylaxis and therapeutics.

Development and Automation of a Bacterial Biosensor to the Targeting of the Pollutants Toxic Effects by Portable Raman Spectrometer

Water quality monitoring requires a rapid and sensitive method that can detect multiple hazardous pollutants at trace levels. This study aims to develop a new generation of biosensors using a low-cost fiber-optic Raman device. An automatic measurement system was thus conceived, built and successfully tested with toxic substances of three different types: antibiotics, heavy metals and herbicides. Raman spectroscopy provides a multiparametric view of metabolic responses of biological organisms to these toxic agents through their spectral fingerprints. Spectral analysis identified the most susceptible macromolecules in an E. coli model strain, providing a way to determine specific toxic effects in microorganisms. The automation of Raman analysis reduces the number of spectra required per sample and the measurement time: for four samples, time was cut from 3 h to 35 min by using a multi-well sample holder without intervention from an operator. The correct classifications were, respectively, 99%, 82% and 93% for the different concentrations of norfloxacin, while the results were 85%, 93% and 81% for copper and 92%, 90% and 96% for 3,5-dichlorophenol at the three tested concentrations. The work initiated here advances the technology needed to use Raman spectroscopy coupled with bioassays so that together, they can advance field toxicological testing.

Art Ashkin and the Origins of Optical Trapping and Particle Manipulation

A brief history of optical forces, the invention of optical tweezers, and their application to biological problems.

Competitive binding of independent extension and retraction motors explains the quantitative dynamics of type IV pili

Type IV pili (TFP) function through cycles of extension and retraction. The coordination of these cycles remains mysterious due to a lack of quantitative measurements of multiple features of TFP dynamics. Here, we fluorescently label TFP in the pathogen Pseudomonas aeruginosa and track full extension and retraction cycles of individual filaments. Polymerization and depolymerization dynamics are stochastic; TFP are made at random times and extend, pause, and retract for random lengths of time. TFP can also pause for extended periods between two extension or two retraction events in both wild-type cells and a slowly retracting PilT mutant. We developed a biophysical model based on the stochastic binding of two dedicated extension and retraction motors to the same pilus machine that predicts the observed features of the data with no free parameters. We show that only a model in which both motors stochastically bind and unbind to the pilus machine independent of the piliation state of the machine quantitatively explains the experimentally observed pilus production rate. In experimental support of this model, we show that the abundance of the retraction motor dictates the pilus production rate and that PilT is bound to pilus machines even in their unpiliated state. Together, the strong quantitative agreement of our model with a variety of experiments suggests that the entire repetitive cycle of pilus extension and retraction is coordinated by the competition of stochastic motor binding to the pilus machine, and that the retraction motor is the major throttle for pilus production.

Structured Back Focal Plane Interferometry (SBFPI)

Back focal plane interferometry (BFPI) is one of the most straightforward and powerful methods for achieving sub-nanometer particle tracking precision at high speed (MHz). BFPI faces technical challenges that prohibit tunable expansion of linear detection range with minimal loss to sensitivity, while maintaining robustness against optical aberrations. In this paper, we devise a tunable BFPI combining a structured beam (conical wavefront) and structured detection (annular quadrant photodiode). This technique, which we termed Structured Back Focal Plane Interferometry (SBFPI), possesses three key novelties namely: extended tracking range, low loss in sensitivity, and resilience to spatial aberrations. Most importantly, the conical wavefront beam preserves the axial Gouy phase shift and lateral beam waist that can then be harnessed in a conventional BFPI system. Through a series of experimental results, we were able to tune detection sensitivity and detection range over the SBFPI parameter space. We also identified a figure of merit based on the experimental optimum that allows us to identify optimal SBPFI configurations that balance both range and sensitivity. In addition, we also studied the resilience of SBFPI against asymmetric spatial aberrations (astigmatism of up to 0.8 λ) along the lateral directions. The simplicity and elegance of SBFPI will accelerate its dissemination to many associated fields in optical detection, interferometry and force spectroscopy.