Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W49: Focus Session: Migration of Cells, Droplets, and Particles on Substrates |
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Sponsoring Units: GSOFT Chair: Kim Weirich, University of Chicago Room: 217D |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W49.00001: Kinetic Description for Formation and Dissolution of Living Colonies Christoph Weber, Yen Ting Lin, Nicolas Biais, Vasily Zaburdaev Pathogen bacteria, such as N. gonorrhoeae or N. meningitidis form colonies due to encounters of nearby individuals while the effect of cell division is in general negligible. They use long and thin filaments, called pili, which attach to a substrate, retract and thereby pull the cell forward. Even though it is known that these bacteria interact by pili and adhesion, the question of how single cell motility and cell-cell interactions affect the process of colony formation is poorly understood. To bridge this gap we propose a kinetic description that keeps track of the length scales related to the underlying interactions between the cells and with the substrate. We derive the corresponding hydrodynamic equation and find an ordering instability leading to the formation of colonies. However, colonies can also dissolve which is a key survival mechanism in rapidly deteriorating environmental conditions. Recent experimental studies indicate that colonies can dissolve by switching off either the adhesive or pili-mediated interaction. Remarkably, within the same framework we can show that dissolution is possible, however, there is a region in parameter space where it is precluded. Both scenarios can be explained in terms of the underlying microscopic interactions. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W49.00002: Coarse-grained model for a motor protein walker on a bead-spring substrate Jutta Luettmer-Strathmann, Nabina Paudyal, Maral Adeli Koudehi Motor proteins play an important role in many biological processes. For example, kinesin molecules are responsible for the transport of vesicles in nerve cells and their malfunction has been linked to neurodegenerative diseases. Unfortunately, the complexity of motor proteins and their environment makes it difficult to model the detailed dynamics of molecular motors over long time scales. In this work, we develop a simple coarse-grained model for a motor protein on a bead-spring substrate under tension. In our model, different pair potentials describe interactions between substrate and motor, motor components and substrate components. The movement of motor proteins entails ATP hydrolysis, which is modeled in terms of mechano-chemical states that couple positional and chemical degrees of freedom. We apply the model to the problem of cargo transport and the effect of motor-protein activity on the mechanical response of a single chain molecule. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W49.00003: Effect of silane molecular length on initial attachment of bacteria to silanized glass surfaces Andrea Jaimes-Lizcano, Sumedha Sharma, Jacinta Conrad Bacteria adhered to surfaces can form biofilms, which foul biomedical implants, industrial equipment and marine vessels, leading to deleterious costs. Designing surfaces to control biofilm formation first requires understanding the role of surface chemistry on initial attachment of bacteria, the first step in biofilm formation. We characterize the initial attachment of \textit{Escherichia coli} to glass surfaces that are coated with silane molecules with the same functional end group but two different carbon chain lengths. Bacteria are deposited from flow in a microfluidic channel at shear rates ranging from 3.1 s$^{\mathrm{-1}}$ to 25 s$^{\mathrm{-1}}$ and imaged and tracked using confocal microscopy and high-throughput image processing algorithms. The initial rate at which bacteria deposit on the surface is independent of shear rate for the shorter three-carbon chains but depends on shear rate for the longer nine-carbon chains. We found longer bacterial residence times on the shorter silane molecules at the highest shear rate. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W49.00004: Dissecting Subcellular Actomyosin Mechanics with Magnetically Actuated Micropost Arrays Yu Shi, Steven Henry, John Crocker, Daniel Reich The cellular actomyosin cytoskeleton is widely regarded as an archetypal example of an active matter system. However, the extent to which the wide range of observed cellular motility behaviors arise from active-matter physics is not well understood. Characterizing an active matter system requires simultaneous measurement of the fluctuation spectrum of the internal force generators and also the local viscoelasticity to separate the distinct effects of the material's internal stresses from its viscoelastic response to those stresses. By placing cells on top of PDMS micropost arrays with magnetic nanowires embedded in selected posts, we can actuate local regions of the cells by applying AC magnetic fields to dynamically probe the local viscoelasticity, while simultaneously using the posts as ``probe particles'' for passive microrheology measurements of the cytoskeletal force fluctuations. The range of active and passive responses observed for different subcellular regions of fibroblast cells will be presented, and the results compared to simple active material models based on known or predicted behavior of molecular motors in viscoelastic networks. Effects of coupling between local cellular regions as measured by correlations in the microposts' motion will also be described. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W49.00005: Confocal Microscopy Indentation for Hydrogel Donghee Lee, Md. Mahmudur Rahman, You Zhou, Sangjin Ryu It is well known that the stiffness of extracellular matrix affects cellular behaviors, and such effects were observed by culturing cells on hydrogel substrates. Thus it is required to measure the elasticity of the hydrogel substrate rigorously and efficiently. Here we propose a confocal microscopy indentation method for hydrogels. We indented fluorescently stained polyacrylamide gel with a sub-mm-sized ball indenter, and imaged the indented gel using confocal microscopy. Having formed a three-dimensional image stack of the gel, we measured the indentation depth based on automated image processing, and then evaluated the elasticity of the gel. We also validated our method using other well established indentation methods. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W49.00006: Schwann Cells and the Importance of Finite 3D Deformations in Soft Gels Christian Franck, Eyal Bar-Kochba, Cristina Lopez-Fagundo, Liane Livi, Diane Hoffman-Kim Schwann cells (SCs) are specialized glial cells that are critical for the development, regeneration, and maintenance of nerves in the peripheral nervous system (PNS). Recent studies have shown that the mechanical properties of the extracellular matrix can significantly affect cell structure and function. Studying the mechanical interactions between SCs and their microenvironment can aid in understanding their physical and morphological changes as well as their native function. Using a recently developed 3-D large deformation traction force microscopy (3D-LDTFM) technique, we investigate the mechanosensitivity of SCs across a physiologically relevant substrate stiffness range (0.24 kPa to 4.80 kPa) in vivo. As oppose to other cell types, we find that the SC spreading area and prominent stress fiber formation was relatively insensitive to substrate stiffness. Consistent with these structural findings, the SCs generated large surface tractions on stiff substrates and large material deformations on soft substrates. Across all moduli, we observed a significant contribution from the out-of-plane traction component, locally giving rise to rotational moments similar to those reported for mesenchymal embryonic fibroblasts. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 4:18PM |
W49.00007: "Please choose a title, something about migration on subtrates" Invited Speaker: Erin Rericha |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W49.00008: Complex multi-cellular manifolds Tapomoy Bhattacharjee, Kyle G. Rowe, Suhani Jain, Steven M. Zehnder, Ryan M. Nixon, W. Gregory Sawyer, Thomas E. Angelini Investigation of collective cell behavior is critical for developing an understanding of tissue regeneration, embryonic morphogenesis, wound healing, and cancer invasion. Collective behavior has been widely studied in 2D cell monolayers, providing great fundamental understanding of multi-cellular motion and mechanics. Living tissues, by contrast, are densely permeated with complex 3D structures including curved manifolds and tubular networks. Exploration of collective cell behavior within such complex 3D structures is essential to connect our knowledge of cells in 2D monolayers to their motion and mechanics in tissues. In this study, complex structures have been generated by 3D printing living cells into a viscoelastic cell growth medium, creating cellular manifolds with a wide range of mean and Gaussian curvature, such as linear cylinders and branched tubular networks. Preliminary data describing collective cell behavior within these complex manifolds will be presented. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W49.00009: Long range self-assembly of microcapsules regulated via the repressilator signaling network Henry Shum, Victor Yashin, Anna Balazs Communication to produce collective motion is a biological characteristic realized by few synthetic systems. Inspired by biological regulatory networks, we design a collection of microcapsules that move in response to self-generated chemical signals. Three microcapsules act as localized sources of distinct chemicals that diffuse through surrounding fluid. Production rates are modulated by the ``repressilator'': each chemical species represses the production of the next in a cycle. Depending on the maximum production rates and capsule separation distances, we show that immobile capsules either exhibit steady or oscillatory chemical production. We then consider movement of the microcapsules over the substrate, induced by gradients in surface energy due to adsorbed chemicals. We numerically simulate this advection-diffusion-reaction system with solid-fluid interactions by combining lattice Boltzmann, immersed boundary and finite difference methods, and thereby, construct systems where the three capsules spontaneously assemble, forming a close-packed triad. Chemical oscillations are shown to be critical to this assembly. By adjusting parameters, the triad can either remain stationary or translate as a cohesive group. Stationary triads can also be made to ``turn off'' after assembly. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W49.00010: Asymmetric oscillation and dynamic clustering of water-in-oil droplets by hydrodynamic interactions Takuya Ohmura, Ken-Ichiro Kamei, Masatoshi Ichikawa, Yusuke Maeda Ordered motion or patterns are widely observed in many body systems far from equilibrium. Microfluidic droplet crystal that is the ensemble of water-in-oil droplet moving in immiscible fluid in a microchannel is known to exhibit normal vibrational mode due to inter-droplet hydrodynamic interactions. In this study we study dynamic ordering in the ensemble of different-sized droplets in order to investigate the effect of heterogeneity in the droplet crystal. We find asymmetric back-and-forth motion of single small droplet of 20 $\mu$m, which is placed between two droplets of 140 $\mu$m, is emerged. As a group of small droplets comes in the middle of larger droplets, their motion turns dynamic clustering from oscillation. Numerical analysis indicates that hydrodynamic interactions with boundary wall and large droplets break symmetry of flow field and results in closed streamlines sustaining asymmetric oscillation. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W49.00011: Tensional Homeostasis in Single Fibroblasts Probed with Traction Force Microscopy Rostislav Boltyanskiy, Henry Foote, Aaron Mertz, Kathryn Rosowski, Holly Lauridsen, Valerie Horsley, Jay Humphrey, Martin Schwartz, Eric Dufresne Many tissue types, including skin and blood vessels, respond to mechanical perturbations by remodeling to maintain a constant level of stress. This is called tensional homeostasis. Does similar remodeling and adaptation occur in single cells? To address this question, we have developed a technique to measure cell traction forces as the extra-cellular matrix is stretched. The time- and strain-dependent cellular response sheds light on active adaptive processes, like tensional homeostasis, and passive mechanical properties, such as stiffness. [Preview Abstract] |
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