Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P59: Driving, Actuating, and Triggering Activity in Biopolymer NetworksFocus
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Sponsoring Units: GSOFT DBIO Chair: Jennifer Ross, University of Massachusetts Amherst Room: BCEC 257B |
Wednesday, March 6, 2019 2:30PM - 2:42PM |
P59.00001: Closed Loop Mechanical Actuation of Cardiac Microtissue Josh Javor, Subramanian Sundaram, Anant Chopra, Chris Chen, David John Bishop Mechanical forces play a significant role in the maturation and function of stem cell derived cardiac tissue. We present a micromechanical test bed with closed loop actuation to control tissue strain with sub-micron spatial and 10 ms temporal resolution. A hydrogel is self-assembled between two custom elastomer posts, where one post is functionalized with a rare-earth magnet. Opposing it is a spherical pillar top facilitating hydrogel attachment. An anti-Helmholtz coil imposes gradient magnetic fields on the magnet translating to nanoNewton forces and tissue strain. The cardiomyocytes beat spontaneously, imposing another force on the magnet. The magnet position is detected without contact by imbedding a hall sensor. The detected signal is processed by analog filters and phase-sensitive detection. The loop is closed with an Arduino MEGA, which digitizes the signal, performs an algorithm, and drives the coils with an arbitrary analog signal. |
Wednesday, March 6, 2019 2:42PM - 2:54PM |
P59.00002: Microscale Mechanics of triggered bundling and unbundling of actin networks Bekele Gurmessa, Leila Farhadi, Michael Rust, Jennifer Ross, Moumita Das, Rae Robertson-Anderson Networks of semiflexible actin filaments play key roles in many mechanical processes. The functionality of actin networks arises from the ability for actin filaments to dynamically entangle, crosslink, and bundle with one another. For example, simply increasing divalent salt concentration can trigger varying degrees and types of bundling in entangled actin networks. However, how the mechanical properties vary with varying degrees of salt-induced bundling remains unknown. More importantly, how the mechanical properties vary in time as actin networks transition between bundled and unbundled states has yet to be explored. Here, we couple optical tweezers microrheology with microfluidics to measure the viscoelastic response of entangled actin networks during triggered bundling and unbundling. We measure the frequency-dependent viscoelastic moduli in set time intervals as we cyclically vary the salt concentration via microfluidic perfusion chambers. We also use fluorescence confocal microscopy to image labeled filaments and characterize the corresponding time-varying network mobility and structure. Our measurements shed new light into how bundling and unbundling can dynamically tune the mechanical properties of actin networks. |
Wednesday, March 6, 2019 2:54PM - 3:06PM |
P59.00003: Modeling dividing actomyosin droplets as colloids in liquid crystal Kinjal Dasbiswas, Eli Alster, Kimberly Weirich, Thomas A Witten, Margaret Gardel, Suriyanarayanan Vaikuntanathan Biopolymer networks are often organized into oriented bundles both in reconstituted in vitro situations and in the cell. Motivated by dividing fluid droplet-like bundles of actomyosin observed in experiments, we propose a theoretical mechanism for division of a droplet of liquid crystal material induced by a colloidal particle. The preferred alignment of actin filaments at the droplet surface and that at the motor cluster, modeled as a colloidal inclusion wet by the droplet, together result in a deformed droplet having minimal free energy. The dynamics of the model are illustrated by continuum simulations which show droplet deformation and pinching off into two equal daughter droplets. While colloidal inclusions are widely known to induce defects in a surrounding bulk liquid crystal, we predict here that the colloid can in principle deform and divide droplets of liquid crystal. Our description of these liquid crystal droplet dynamics, where the active forces of molecular motors is accounted for effectively through aligning interactions, explores the physical aspects of complex fluid phase separation in biology and suggests that self-assembly of motors may occur through interactions mediated by ordered biopolymers. |
Wednesday, March 6, 2019 3:06PM - 3:42PM |
P59.00004: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 3:42PM - 3:54PM |
P59.00005: Elucidating the consequences of heterogeneous activity in an actin based liquid crystal Steven Redford, Rui Zhang, Nitin Kumar, Paul Ruijgrok, Ali Mozaffari, Aaron Dinner, Vincenzo Vitelli, Zev Bryant, Juan De Pablo, Margaret Gardel Active matter is generally studied in cases in which activity is spatially uniform. However, many of the biological systems that inspire this line of research, such as forces in a cell, feature activity that is highly spatially inhomogeneous. While generating spatially inhomogeneous stress has historically been difficult, here we present an experimental system in which spatial control of myosin activity in an actin liquid crystal allows for patterning of activity within the sample. Using this system, and comparing with hydrodynamic simulations, we show that patterned activity has the potential to direct the motion of +1/2 defects within a liquid crystal and constrain fluid flow. |
Wednesday, March 6, 2019 3:54PM - 4:30PM |
P59.00006: Driving, Actuating, and Triggering Activity in Biopolymer Networks Invited Speaker: Weihong Tan Active and driven biopolymer networks, such as networks of cytoskeleton proteins, have been intensely investigated over the past decade due to their promise for designing smart materials and understanding cell mechanics. These materials continuously alter their mechanical properties by varying the structural properties and interactions of the comprising biopolymers. Non-equilibrium activity can be driven by external triggers such as light, salt, temperature, or magnetic or electric fields. Activity can also be internally driven via molecular motors. This session will bring together studies on a wide-range of non-equilibrium biopolymer networks to elucidate the functional design principles of driven soft matter, as well as the time-dependent structural and rheological properties of these non-equilibrium networks. Advances in modulating and characterizing driven networks will also be discussed. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P59.00007: Myosin II filament size and activity influences localization in nematic actin droplets Kimberly Weirich, Kinjal Dasbiswas, Thomas A Witten, Suriyanarayanan Vaikuntanathan, Margaret Gardel Soft, active materials self-organize from macromolecules in cells to form precisely structured assemblies, such as the mitotic spindle, that orchestrate specific biological functions. A central question is how these self-organized assemblies arise from their macromolecular components. We investigate mechanisms of self-organization in structured biopolymer assemblies, using a minimal model system of biopolymer droplets constructed from cross-linked actin filaments. These droplets have nematic structure, which arises from the actin filaments. Myosin II motor proteins, which form filaments that bind to and translocate actin filaments, spatially self-organize in these actin droplets. We find that motors large compared to the characteristic structure of the nematic liquid self-organize to the center of the droplet, evocative of mitotic spindle configurations. In contrast, motors small compared to the liquid are dispersed throughout the droplet. We investigate the influence of motor size and activity on the dynamics and localization within the droplet and capture the spatial localization with a continuum model based on liquid crystal theory. Our results reveal potential physical mechanisms of self-organization in biological assemblies and bio-inspired soft materials design. |
Wednesday, March 6, 2019 4:42PM - 5:18PM |
P59.00008: The paradoxical material properties of living matter Invited Speaker: Gijsje Koenderink This talk will focus on the mechanics of the polymeric load-bearing structures that support living matter: cells have a fibrous cytoskeleton, whilst tissues are supported by the extracellular matrix. Living matter uses dynamic materials in order to combine mechanical resistance with the ability to adapt and self-heal. Whereas synthetic transient networks readily fracture due to the inherent force sensitivity of dynamic bonds, biological networks are surprisingly strong. How does biological matter achieve the ability to flow without risking mechanical failure? I will discuss our recent findings on the nonlinear time-dependent mechanical properties and rupture of passive and active biopolymer networks reconstituted from purified components, which begin to shed some light on this question. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P59.00009: Nonequilibrium dynamics of semiflexible filaments in an active fluid Junang Li, Shreyas Gokhale, Sami Kaya, Alexandre Solon, Jeffrey Gore, Nikta Fakhri Active fluids exhibit a variety of complex dynamical phenomena that are not observed in their passive counterparts, largely due to the breaking of detailed balance at the particle level. This breaking of detailed balance can also be manifested in the dynamics of passive objects immersed in an active fluid. For instance, Nikola et al. have demonstrated numerically that a semiflexible filament exhibits spontaneous buckling and directed motion when immersed in an active bath. Despite considerable progress on the theoretical and numerical front, experimental investigations of the dynamics of passive objects in active media were so far restricted to the simplest cases. Here, we have developed an experimental system to systematically analyze the nonequilibrium dynamics of semiflexible filaments in an active fluid. We perform optical video microscopy on DNA-linked magnetic colloidal chains immersed in a quasi-2D bacterial bath, and quantify the diffusivity and the amplitudes of the bending mode fluctuations of the chains as a function of chain length, stiffness and bacterial density. Using fluorescent bacteria, we compute the active flow fields to uncover the underlying mechanism of the nonequilibrium dynamical phenomena. |
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