Thursday, November 14, 2019

Lateral Subunit Coupling Determines Intermediate Filament Mechanics

Charlotta Lorenz, Johanna Forsting, Anna V. Schepers, Julia Kraxner, Susanne Bauch, Hannes Witt, Stefan Klumpp, and Sarah Köster
The cytoskeleton is a composite network of three types of protein filaments, among which intermediate filaments (IFs) are the most extensible ones. Two very important IFs are keratin and vimentin, which have similar molecular architectures but different mechanical behaviors. Here we compare the mechanical response of single keratin and vimentin filaments using optical tweezers. We show that the mechanics of vimentin strongly depends on the ionic strength of the buffer and that its force-strain curve suggests a high degree of cooperativity between subunits. Indeed, a computational model indicates that in contrast to keratin, vimentin is characterized by strong lateral subunit coupling of its charged monomers during unfolding of α helices. We conclude that cells can tune their mechanics by differential use of keratin versus vimentin.


Wednesday, November 13, 2019

Laser Controlled 65 Micrometer Long Microrobot Made of Ni‐Ti Shape Memory Alloy

Min‐Soo Kim Hyun‐Taek Lee Sung‐Hoon Ahn
Microrobotics has many potential applications, such as environmental remediation, in the biomedical arena. However, existing microrobots exhibit practical limitations including inadequate biocompatibility and imprecise control. Here, a microrobot made of shape memory alloy (SMA) actuator which can be driven by laser scanning to perform microscale motions is introduced. The 65 µm long microrobot having crawling‐like motion can demonstrate the movement with 10.0 µm s−1 of the maximum speed. The microrobot is controlled by a laser affording wireless, spatiotemporally selective capabilities. During actuation, the robot exhibits crawling‐like motions including trigger via the SMA as removal of adhesion to surface, propulsion induced by optothermal and optical trapping effects. Both theoretical predictions and experimental results confirm that the SMA microrobot can be actuated and controlled via laser scanning. The principle of SMA microrobots, and the optical actuation method, can be broadened to other applications that require deformable microscale structures suitable for mass production.


Single molecule mechanics resolves the earliest events in force generation by cardiac myosin

Michael S Woody, Donald A Winkelmann, Marco Capitanio, E Michael Ostap, Yale E Goldman

Key steps of cardiac mechanochemistry, including the force-generating working stroke and the release of phosphate (Pi), occur rapidly after myosin-actin attachment. An ultra-high-speed optical trap enabled direct observation of the timing and amplitude of the working stroke, which can occur within <200 μs of actin binding by β-cardiac myosin. The initial actomyosin state can sustain loads of at least 4.5 pN and proceeds directly to the stroke or detaches before releasing ATP hydrolysis products. The rates of these processes depend on the force. The time between binding and stroke is unaffected by 10 mM Pi which, along with other findings, indicates the stroke precedes phosphate release. After Pi release, Pi can rebind enabling reversal of the working stroke. Detecting these rapid events under physiological loads provides definitive indication of the dynamics by which actomyosin converts biochemical energy into mechanical work.


Three-Dimensional Pose Estimation of Optically Transparent Microrobots

Maria Grammatikopoulou, Guang-Zhong Yang

The use of microrobots for cell manipulation has a range of applications in biomedical research. Direct sensing of microrobot three-dimensional (3D) position and orientation, however, is practically challenging due to the small scale involved. This letter proposes a vision-based method for estimating the 3D pose of optically transparent microrobots based on Optical Tweezers (OT) manipulation by using Convolutional Neural Networks (CNNs). A model-based approach is used to generate the large training set required for CNNs. Detailed validation is performed to demonstrate the experimental use of the technique.


Photonic crystal nanobeam/micro-ring hybrid-cavities for optical trapping

Shoubao Han, Yaocheng Shi

We propose a new type of hybrid-cavity structure for trapping nanoscale particles. The hybrid cavity is based on a ring waveguide with the incorporation of a photonic crystal nanobeam cavity (PCNC). We design the cavity and investigate the influence of geometric parameters on its performance and optical trapping ability. The numerical results show that high quality factor, low mode volume and strong optical trapping ability can be achieved. The radii of holes are quadratically tapered from the center to two sides with a smaller period in the mirrors of the PCNC and the optical trapping force is proportional to (1−T)Q/V of the cavity. Furthermore, a hybrid cavity with optimal trapping ability is realized with quality factor up to 1.98106, mode volume as low as 1.90()3 and its optical trapping force can be as high as −855 pN/mW, which is 15 times enhanced compared to that of a microring resonator with the same radius.


Tuesday, November 12, 2019

Structural characterization of the saxitoxin-targeting APTSTX1 aptamer using optical tweezers and molecular dynamics simulations

Nathalie Casanova-Morales, Nataniel L. Figueroa, Karol Alfaro, Felipe Montenegro, Nelson P. Barrera, J. R. Maze, Christian A. M. Wilson, Pablo Conejeros

Optical tweezers have enabled the exploration of picoNewton forces and dynamics in single–molecule systems such as DNA and molecular motors. In this work, we used optical tweezers to study the folding/unfolding dynamics of the APTSTX1–aptamer, a single-stranded DNA molecule with high affinity for saxitoxin (STX), a lethal neurotoxin. By measuring the transition force during (un)folding processes, we were able to characterize and distinguish the conformational changes of this aptamer in the presence of magnesium ions and toxin. This work was supported by molecular dynamics (MD) simulations to propose an unfolding mechanism of the aptamer–Mg+2 complex. Our results are a step towards the development of new aptamer-based STX sensors that are potentially cheaper and more sensitive than current alternatives.


On-the-fly particle metrology in hollow-core photonic crystal fibre

Abhinav Sharma, Shangran Xie, Richard Zeltner, and Philip St.J. Russell

Efficient monitoring of airborne particulate matter (PM), especially particles with aerodynamic diameter less than 2.5 µm (PM2.5), is crucial for improving public health. Reliable information on the concentration, size distribution and chemical characteristics of PMs is key to evaluating air pollution and identifying its sources. Standard methods for PM2.5 characterization require sample collection from the atmosphere and post-analysis using sophisticated equipment in a laboratory environment, and are normally very time-consuming. Although optical methods based on analysis of scattering of free-space laser beams or evanescent fields are in principle suitable for real-time particle counting and sizing, lack of knowledge of the refractive index in these methods not only leads to inevitable sizing ambiguity but also prevents identification of the particle material. In the case of evanescent wave detection, the system lifetime is strongly limited by adhesion of particles to the surfaces. Here we report a novel technique for airborne particle metrology based on hollow-core photonic crystal fibre. It offers in situ particle counting, sizing and refractive index measurement with effectively unlimited device lifetime, and relies on optical forces that automatically capture airborne particles in front of the hollow core and propel them into the fibre. The resulting transmission drop, together with the time-of-flight of the particles passing through the fibre, provide unambiguous mapping of particle size and refractive index with high accuracy. The technique offers unique advantages over currently available real-time particle metrology systems, and can be directly applied to monitoring air pollution in the open atmosphere as well as precise particle characterization in a local environment such as a closed room or a reaction vessel.


Orientation of swimming cells with annular beam optical tweezers

Isaac C.D. Lenton, Declan J. Armstrong, Alexander B. Stilgoe, Timo A. Nieminen, Halina Rubinsztein-Dunlop

Optical tweezers are a versatile tool that can be used to manipulate small particles including both motile and non-motile bacteria and cells. The orientation of a non-spherical particle within a beam depends on the shape of the particle and the shape of the light field. By using multiple beams, sculpted light fields or dynamically changing beams, it is possible to control the orientation of certain particles. In this paper we discuss the orientation of the rod-shaped bacteria Escherichia coli (E. coli) using dynamically shifting annular beam optical tweezers. We begin with examples of different beams used for the orientation of rod-shaped particles. We discuss the differences between orientation of motile and non-motile particles, and explore annular beams and the circumstances when they may be beneficial for manipulation of non-spherical particles or cells. Using simulations we map out the trajectory the E. coli takes. Estimating the trap stiffness along the trajectory gives us an insight into how stable an intermediate rotation is with respect to the desired orientation. Using this method, we predict and experimentally verify the change in the orientation of motile E. coli from vertical to near-horizontal with only one intermediate step. The method is not specific to exploring the orientation of particles and could be easily extended to quantify the stability of an arbitrary particle trajectory.


X-typed curvilinear transport of strongly absorbing particle in a dual-beam fiber optical trap

Zhihai Liu, Lu Wang, Yu Zhang, Siyu Lin, Yaxun Zhang, Xinghua Yang, Jianzhong Zhang, Jun Yang, and Libo Yuan

We propose and demonstrate a novel approach to transport a strongly absorbing particle in an X-typed trajectory reciprocally in pure liquid glycerol based on a dual-beam optical fiber trap. We perform the X-typed light field by integrating a glass microsphere on the tip of a two-core fiber. The motion of the absorbing particle in pure liquid glycerol is dominated by the Δα-type photophoretic forces (FΔα). The incident laser power determines the direction of FΔα. Therefore, we may perform the reciprocating transport of the absorbing particle by changing and controlling the laser power. It is simple to manufacture the fiber probe and convenient to operate the transport of the microparticle. Our research expands the applications of absorbing particles in targeted drug delivery, biological sampling, and optically mediated particle clearing.


Characterization of non-linearities through mechanical squeezing in levitated optomechanics

Ashley Setter, Jamie Vovrosh, and Hendrik Ulbricht

We demonstrate a technique to estimate the strength of nonlinearities present in the trapping potential of an optically levitated nanoparticle. By applying a brief pulsed reduction in the trapping laser power of the system such as to squeeze the phase space distribution and then matching the time evolution of the shape of the phase space distribution to that of numerical simulations, one can estimate the strength of the nonlinearity present in the system. We apply this technique to estimate the strength of the Duffing nonlinearity present in the optical trapping potential.We would like to thank C. Timberlake for comments on this manuscript as well as M. Toroš, T. Georgescu, and M. Rashid for discussions. We also wish to thank the Leverhulme Trust and the Foundational Questions Institute (FQXi) for funding. A. Setter is supported by the Engineering and Physical Sciences Research Council (EPSRC) under the Center for Doctoral Training Grant No. EP/L015382/1. We also acknowledge support from the EU FET project TEQ (Grant Agreement No. 766900). In addition, the authors acknowledge the use of the IRIDIS High Performance Computing Facility and associated support services at the University of Southampton. All data supporting this study are openly available from the University of Southampton repository at https://doi.org/10.5258/SOTON/D0967. The code used to analyze the data is openly available at https://doi.org/10.5281/zenodo.1042526.