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Wednesday, August 23, 2017

Levitation and propulsion of a Mie-resonance particle by a surface plasmon

A. V. Maslov

It is predicted that the optical force induced by a surface plasmon can form a stable equilibrium position for a resonant particle at a finite distance from the surface. The levitated particle can be efficiently propelled along the surface without touching it. The levitation originates from the strong interaction of the particle with the surface.

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The effect of saliva on the fate of nanoparticles

Birgit J. Teubl, Biljana Stojkovic, Dominic Docter, Elisabeth Pritz, Gerd Leitinger, Igor Poberaj, Ruth Prassl, Roland H. Stauber, Eleonore Fröhlich, Johannes G. Khinast, Eva Roblegg

The design of nanocarriers for local drug administration to the lining mucosa requires a sound knowledge of how nanoparticles (NPs) interact with saliva. This contact determines whether NPs agglomerate and become immobile due to size- and interaction-filtering effects or adsorb on the cell surface and are internalized by epithelial cells. The aim of this study was to examine the behavior of NPs in saliva considering physicochemical NP properties. The salivary pore–size distribution was determined, and the viscosity of the fluid inside of the pores was studied with optical tweezers. Distinct functionalized NPs (20 and 200 nm) were dispersed in saliva and salivary buffers and characterized, and surface-bound MUC5B and MUC7 were analyzed by 1D electrophoresis and immunoblotting. NP mobility was recorded, and cellular uptake studies were performed with TR146 cells. The mode diameter of the salivary mesh pores is 0.7 μm with a peak width of 1.9 μm, and pores are filled with a low-viscosity fluid. The physicochemical properties of the NPs affected the colloidal stability and mobility: compared with non-functionalized particles, which did not agglomerate and showed a cellular uptake rate of 2.8%, functionalized particles were immobilized, which was correlated with agglomeration and increased binding to mucins. The present study showed that the salivary microstructure facilitates NP adsorption. However, NP size and surface functionalization determine the colloidal stability and cellular interactions.

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Equilibrium and out-of-equilibrium mechanics of living mammalian cytoplasm

Gupta, Satish Kumar; Guo, Ming

Living cells are intrinsically non-equilibrium systems. They are driven out of equilibrium by the activity of the molecular motors and other enzymatic processes. This activity along with the ever present thermal agitation results in intracellular fluctuations inside the cytoplasm. In analogy to Brownian motion, the material property of the cytoplasm also influences the characteristics of these fluctuations. In this paper, through a combination of experimentation and theoretical analysis, we show that intracellular fluctuations are indeed due to non-thermal forces at relatively long time-scales, however, are dominated solely by thermal forces at relatively short time-scales. Thus, the cytoplasm of living mammalian cells behaves as an equilibrium material at short time-scales. The mean square displacement of these intracellular fluctuations scales inversely with the cytoplasmic shear modulus in this short time-scale equilibrium regime, and is inversely proportional to the square of the cytoplasmic shear modulus in the long time-scale out-of-equilibrium regime. Furthermore, we deploy passive microrheology based on these fluctuations to extract the mechanical property of the cytoplasm at the high-frequency regime. We show that the cytoplasm of living mammalian cells is a weak elastic gel in this regime; this is in an excellent agreement with an independent micromechanical measurement using optical tweezers.

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Laser-Printing and 3D Optical-Control of Untethered Microrobots

Ebubekir Avci, Maria Grammatikopoulou, Guang-Zhong Yang

The two-photon photo-polymerization (2PP) method is used to manufacture an articulated micro-robot for indirect manipulation of cellular structures under laser light. To tackle the stickiness issue between the components of the proposed mechanism, optimizing the contact surface areas is carried out. A step-by-step procedure to manufacture and control an untethered articulated micro-robot under laser light is demonstrated. The manufacture and optical control of a floating multi-component micro-mechanism has not been achieved before. This is significant because it is anticipated that the articulated microrobots could be used in complex biomedical applications where control in 3D space is required such as single-cell analysis, embryo injection, polar-body biopsy, nuclear transplantation, and multi-dimensional imaging for microsurgery.

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Dielectrophoretic force measurement of red blood cells exposed to oxidative stress using optical tweezers and a microfluidic chip

Hee-Jae Jeon, Hyungbeen Lee, Dae Sung Yoon, Beop-Min Kim

Red blood cell (RBC) dysfunction is often associated with a pathological intervention, and it has been proposed as a critical risk factor for certain lethal diseases. Examining the cell viability of RBCs under various physiological conditions is essential and of importance for precise diagnosis and drug discovery in the field of medicine and pharmacy. In this paper, we report a new analytical method that employs dielectrophoretic (DEP) force measurements in absolute units to assess the viability, and potentially the functionality of RBCs. We precisely quantify the frequency-dependent DEP forces of the RBCs by using a micro-electrode embedded chip combined with optical tweezers. DEP characteristics are known to be well-correlated with the viability of biological cells, and DEP forces are measured in both fresh and long-term stored RBCs to investigate the effect that the storage period has on the cell viability. Moreover, we investigate the DEP behavior of RBCs when exposed to oxidative stress and verify whether EDTA protects the RBCs from an oxidant. From the experiments, it is found that the fresh RBCs without oxidative stress display very high DEP forces over the entire frequency range, exhibiting two cutoff frequencies. However, both the RBCs stored for the long-term period and exposed to oxidative stress reveals that there exist no significant DEP forces over the frequency range. The results indicate that the DEP forces can serve as a useful parameter to verify whether the RBCs in certain blood are fresh and not exposed to oxidative stress. Therefore, it is believed that our system can be applied to a diagnostic system to monitor the cell viability of the RBCs or other types of cells.

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Tuesday, August 22, 2017

Single-molecule imaging reveals the translocation and DNA looping dynamics of hepatitis C virus NS3 helicase

Chang-Ting Lin, Felix Tritschler, Kyung Suk Lee, Meigang Gu, Charles M. Rice, Taekjip Ha

Non-structural protein 3 (NS3) is an essential enzyme and a therapeutic target of hepatitis C virus (HCV). Compared to NS3-catalyzed nucleic acids unwinding, its translation on single stranded nucleic acids have received relatively little attention. To investigate the NS3h translocation with single-stranded nucleic acids substrates directly, we have applied a hybrid platform of single-molecule fluorescence detection combined with optical trapping. With the aid of mechanical manipulation and fluorescence localization, we probed the translocase activity of NS3h on laterally stretched, kilobase-size single-stranded DNA and RNA. We observed that the translocation rate of NS3h on ssDNA at a rate of 24.4 nucleotides per second, and NS3h translocates about three time faster on ssRNA, 74 nucleotides per second. The translocation speed was minimally affected by the applied force. A subpopulation of NS3h underwent a novel translocation mode on ssDNA where the stretched DNA shortened gradually and then recovers its original length abruptly before repeating the cycle repetitively. The speed of this mode of translocation was reduced with increasing force. With corroborating data from single-molecule fluorescence resonance energy transfer (smFRET) experiments, we proposed that NS3h can cause repetitive looping of DNA. The smFRET dwell time analysis showed similar translocation time between sole translocation mode versus repetitive looping mode, suggesting that the motor domain exhibits indistinguishable enzymatic activities between the two translocation modes. We propose a potential secondary nucleic acids binding site at NS3h which might function as an anchor point for translocation-coupled looping.

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Optical spin torque induced by vector Bessel (vortex) beams with selective polarizations on a light-absorptive sphere of arbitrary size

Renxian Li, Chunying Ding, F.G. Mitri

The optical spin torque (OST) induced by vector Bessel (vortex) beams can cause a particle to rotate around its center of mass. Previous works have considered the OST on a Rayleigh absorptive dielectric sphere by a vector Bessel (vortex) beam, however, it is of some importance to analyze the OST components for a sphere of arbitrary size. In this work, the generalized Lorenz-Mie theory (GLMT) is used to compute the OST induced by vector Bessel (vortex) beams on an absorptive dielectric sphere of arbitrary size, with particular emphasis on the beam order, the polarization of the plane wave component forming the beam, and the half-cone angle. The OST is expressed as the integration of the moment of the time-averaged Maxwell stress tensor, and the beam shape coefficients (BSCs) are calculated using the angular spectrum decomposition method (ASDM). Using this theory, the OST exerted on the light-absorptive dielectric sphere in the Rayleigh, Mie or the geometrical optics regimes can be considered. The axial and transverse OSTs are numerically calculated with particular emphasis on the sign reversal of the axial OST and the vortex-like character of the transverse OST, and the effects of polarization, beam order, and half-cone angle are discussed in detail. Numerical results show that by choosing an appropriate polarization, order and half-cone angle, the sign of the axial OST can be reversed, meaning that the sphere would spin in opposite handedness of the angular momentum carried by the incident beam. The vortex-like structure of the total transverse OSTs can be observed for all cases. When the sphere moves radially away from the beam axis, it may rotate around its center of mass in either the counter-clockwise or the clockwise direction. Conditions are also predicted where the absorptive sphere experiences no spinning. Potential applications in particle manipulation and rotation in optical tweezers and tractor beams would benefit from the results.

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Guiding cellular activity with polarized light

Colin Constant, Andrea Bergano, Kiminobu Sugaya, Aristide Dogariu

Actin, cytoskeleton protein forming microfilaments, play a crucial role in cellular motility. Here we show that exposure to very low levels of polarized light guide their orientation in-vivo within the live cell. Using a simple model to describe the role of actin-filament orientation in directional cellular motion, we demonstrate that the actin polymerization/depolymerization mechanism develops primarily along this direction and, under certain conditions, can lead to guidance of the cell movement. Our results also show a dose dependent increase in actin activity in direct correspondence to the level of laser irradiance. We found that total expression of Tau protein, which stabilize microtubules, was decreased by the irradiance, indicating that exposure to the light may change the activity of kinase, leading to increased cell activity.

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Investigating the effect of cell substrate on cancer cell stiffness by optical tweezers

Muhammad Sulaiman Yousafzai, Giovanna Coceano, Serena Bonin, Joseph Niemela, Giacinto Scoles, Dan Cojoc

The mechanical properties of cells are influenced by their microenvironment. Here we report cell stiffness alteration by changing the cell substrate stiffness for isolated cells and cells in contact with other cells. Polydimethylsiloxane (PDMS) is used to prepare soft substrates with three different stiffness values (173, 88 and 17 kPa respectively). Breast cancer cells lines, namely HBL-100, MCF-7 and MDA-MB-231 with different level of aggressiveness are cultured on these substrates and their local elasticity is investigated by vertical indentation of the cell membrane. Our preliminary results show an unforeseen behavior of the MDA-MB-231 cells. When cultured on glass substrate as isolated cells, they are less stiff than the other two types of cells, in agreement with the general statement that more aggressive and metastatic cells are softer. However, when connected to other cells the stiffness of MDA-MB-231 cells becomes similar to the other two cell lines. Moreover, the stiffness of MDA-MB-231 cells cultured on soft PDMS substrates is significantly higher than the stiffness of the other cell types, demonstrating thus the strong influence of the environmental conditions on the mechanical properties of the cells.

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Scanning holographic optical tweezers

L. A. Shaw, Robert M. Panas, C. M. Spadaccini, and J. B. Hopkins

The aim of this Letter is to introduce a new optical tweezers approach, called scanning holographic optical tweezers (SHOT), which drastically increases the working area (WA) of the holographic-optical tweezers (HOT) approach, while maintaining tightly focused laser traps. A 12-fold increase in the WA is demonstrated. The SHOT approach achieves its utility by combining the large WA of the scanning optical tweezers (SOT) approach with the flexibility of the HOT approach for simultaneously moving differently structured optical traps in and out of the focal plane. This Letter also demonstrates a new heuristic control algorithm for combining the functionality of the SOT and HOT approaches to efficiently allocate the available laser power among a large number of traps. The proposed approach shows promise for substantially increasing the number of particles that can be handled simultaneously, which would enable optical tweezers additive fabrication technologies to rapidly assemble microgranular materials and structures in reasonable build times.

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