Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N25: Focus Session: Biopolymers I: Mechanical Properties |
Hide Abstracts |
Sponsoring Units: DPOLY DBP Chair: Ting Xu, University of California, Berkeley Room: Colorado Convention Center 203 |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N25.00001: Probing Polarization Dynamics and Energy Dissipation in Ferroelectric Polymers on the Nanoscale Invited Speaker: Ferroelectric polymers are emerging as prominent materials for ultrasonic actuators, gate materials for non-volatile ferroelectric memories, and energy storage. The nature of ferroelectricity in polymers is significantly different from that in inorganic perovskites, resulting in significant interest to elementary mechanism of switching and the role of local microstructure. In this talk, I briefly delineate Piezoresponse Force Microscopy and Spectroscopy as applied for characterization of Langmuir-Blodgett ferroelectric PVDF polymer films. The slow polarization switching in PVDF can be attributed to the grain-by grain switching mechanism. Recent advances in PFM probing of polarization dynamics and electromechanical energy dissipation are discussed. In particular, switching spectroscopy PFM is used to probe the spatial variability of switching behavior and role of grain boundaries on switching. Local energy dissipation imaging through the changes of the Q-factor of electrically driven cantilever in contact with the surface is developed to study energy losses in the ferroelectric switching processes. In collaboration with Brian J. Rodriguez and Stephen Jesse, Materials Sciences and Technology Division and The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory; Jihee Kim and Steven Ducharme, Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience University of Nebraska, Lincoln. \newline \newline Research was supported by the U.S. Department of Energy Office of Basic Energy Sciences Division of Materials Sciences and Engineering (SVK, BJR, and SJ) and user proposal of The Center for Nanophase Materials Sciences (JK and SD) and was performed at Oak Ridge National Laboratory which is operated by UT-Battelle, LLC. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N25.00002: Synchrotron X-ray Diffraction Study on the Effect of the Tau protein on the Mechanical Properties of Microtubules Myung Chul Choi, Uri Raviv, Herbert Miller, Michelle Massie, Youli Li, Leslie Wilson, Stuart Feinstein, Mahn Won Kim, Cyrus Safinya Microtubules (MTs) are 25 nm protein nanotubes used as tracks for intracellular trafficking of biomolecules, for example, those involved in transmitting signals between neurons. In neurons, MTs are long-lived both in axons and dendrites. A distinct member of microtubule-associated-proteins (MAPs) regulates microtubule assembly, although the mechanisms of regulation resulting from different tau isoforms remains to be fully elucidated. Incorrectly phosphorylated MAP tau is implicated in a large number of neurodegenerative diseases where altered tau-MT interactions and MT depolymerization and tangles of taus lead to detrimental consequences for neuronal survival. We will describe our recent finding on the effect of tau isoforms on the mechanical properties of MTs, probed by synchrotron X-ray diffraction. Supported by NSF DMR-0503347, DOE DE-FG02-06ER46314, and NIH GM59288. M.C.Choi received partial support from the Korean Foundation Grant KRF-2005-2214-C00202. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N25.00003: Time-resolved studies of actin organization by multivalent ions and actin-binding proteins Ghee Hwee Lai, Kirstin Purdy, James R. Bartles, Gerard Chee Lai Wong Actin is one of the principal components in the eukaryotic cytoskeleton, the architecture of which is highly regulated for a wide range of biological functions. In the presence of multivalent salts or actin-binding proteins, it is known that F-actin can organize into bundles or networks. In this work, we use time-resolved confocal microscopy to study the dynamics of actin bundle growth induced by multivalent ions and by espin, a prototypical actin binding protein that is known to induce bundles. For divalent ion induced bundles, we observe a rapid lateral saturation followed by longitudinal growth of bundles, in sharp contrast to the bundling mechanism of espin, which favors finite length bundles. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N25.00004: Elastic Behavior of Composite Actin and Microtubule Networks Yi-Chia Lin, Gijsje Koenderink, Frederick Mackintosh, David Weitz We explore the non-linear shearing behavior of composite actin and microtubule networks. Large bending rigid microtubules are used as a probe of the deformation mode of cross-linked actin networks. For a sparsely cross-linked actin network that deforms non-affinely, adding microtubules can drive the system back to affine by suppressing local rearrangements of actin filaments. It applies to both permanently rigid cross-linker, such as scruin, and flexible cross-linker, such as filamin. This experiment also shows that filamin cross-linked actin networks are deforming in an affine manner. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N25.00005: Mechanics of actin networks crosslinked with mutant human $\alpha$-actinin-4 Sabine Volkmer, Daniel Blair, Karen Kasza, David Weitz Globular actin can be polymerized {\em in vitro} to form F-actin in the presence of various binding proteins. These networks often exhibit dramatic nonlinear rheological response to imposed strains. We study the rheological properties of F-actin networks crosslinked with human $\alpha$-actinin-4. A single genetic mutation of the $\alpha$-actinin-4 protein is associated with focal and segmented glomerulosclerosis (FSGS), a genetic disorder which leads to renal failure. Mechanically, the mutant crosslinker has an increased binding strength compared to the wild type. We will show that human $\alpha$-actinin-4, displays a unique stiffening response. Moreover, we also demonstrate that a single point mutation dramatically effects the inherent relaxation time of the crosslinked network. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N25.00006: Viscoelastic properties of Ionomer Melt Monojoy Goswami, Sanat Kumar Viscoelastic prperties of a model telechelic ionomer, i.e., a melt of non-polar polymers with a charge at each chain end along with neutralizing counterions, have been examined using molecular dynamics simulation. Equlibrium calculation of the loss modulus $G^{\prime\prime}(\omega)$ and storage modulus $G^\prime(\omega)$ shows plateau at lower temperatures when the systems are not relaxed. In this situation the specific heat ($C_v$) peak corresponds to the self-assembly of the system, at lower temperatures the specific heat begins to plateau. Similarities of the dynamic features found for telechelic melts with those observed in glass-forming liquids and entangled polymers have been shown. Furthremore, using an athermal 'probe', the properties of these materials is being distinctly classified as 'strong' glass or physical gels. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N25.00007: Controlling the Properties of Thermoreversible Protein Hydrogels Hui Yan, Alberto Saiani, Aline Miller In this work we have explored the potential of using self-assembling protein molecules as the basic unit for novel biomaterials for biomedical applications. Here we will show how thermo-reversible fibrillar hydrogels can be formed from an aqueous solution of hen egg white lysozyme by adding the reductant dithiothreitol. The elastic modulus of the hydrogels formed has been examined and micro differential scanning calorimetry experiments confirmed that the hydrogels were thermally reversible and that gelation and melting occurs through a solid-liquid like first order transition. Infra-red and transmission electron microscopy studies of very dilute samples revealed the presence of beta-sheet rich fibrils that were 4--6 nm in diameter and 1micron in length. These fibrils self-assemble along their long axes to form larger fibers that become physically entangled to form the 3D network observed in both cryoSEM and small angle neutron scattering studies. We will also demonstrate that we can control and manipulate gel properties by varying the protein concentration, reductant concentration and ionic strength of the matrix. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N25.00008: Electrospinning of Hyaluronic acid (HA) and HA/Gelatin Blends Aihua He, Junxing Li, Charles Han, Dufei Fang, Benjamin Hsiao, Benjamin Chu It was found that the processability of HA solution with high viscosity had been improved greatly by using a DMF-water solvent mixture or/and by adding gelatin(GE) into the HA solution. Nano-fibrous membranes with different average fiber diameters and different HA/GE compositions could be obtained. Measurements on viscosity indicated that the HA solution in DMF-water mixed solvent still showed high viscosity. The decrease in surface tension contributed to the fiber formation of HA and HA/GE by electrospinning. Therefore, this study not only provided a novel and simpler way to electrospin the natural polyanion HA solution, but also provided the fundamental physical insight and solution to this spinning difficulty. The HA-GE nanofibrous membranes at different HA/GE compositions are expected to be useful in the biomedical field as novel scaffolds for many applications. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N25.00009: Rheology and lubricity of hyaluronic acid Jing Liang, Wendy E. Krause The polyelectrolyte hyaluronic acid (HA, hyaluronan) is an important component in synovial fluid ($i.e$., the fluid that lubricates our freely moving joints). Its presence results in highly viscoelastic solutions. In comparison to healthy synovial fluid, diseased fluid has a reduced viscosity and loss of lubricity. In osteoarthritis the reduction in viscosity results from a decline in both the molecular weight and concentration of HA. In our investigation, we attempt to correlate the rheological properties of HA solutions to changes in lubrication and wear. A nanoindenter will be used to evaluate the coefficient of friction and wear properties between the nanoindenter tip and ultrahigh molecular weight polyethylene in both the presence and absence of a thin film of HA solution. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N25.00010: Physical Control of Stem Cells via Matrix Elasticity Florian Rehfeldt, Dennis Discher Most of our cells reside in soft tissue, but it has only become clear over the last decade that substrate elasticity exerts a major influence on cell motility, contractility, and overall cell function. The mechanical properties of the matrix can even direct the differentiation of human adult stem cells as reported by our group recently (Engler et al. Cell 2006). Basically, the greater the resistance to matrix deformation, the larger the force with which the cell pulls on the matrix, driving the assembly of cytoskeleton and adhesions. For a deeper understanding of the molecular mechanisms of force generation and transduction, various biophysical and biochemical tools must be combined with well-defined extracellular matrix (ECM) models. Past studies have been conducted mostly with synthetic and uncharged polyacrylamide (PA) gel matrices, motivating more bio-relevant gel models. We have developed such a biocompatible hydrogel system of widely and finely tunable elasticity using hyaluronic acid (HA), which is ubiquitous in development and in particular adult tissues. The effective Young's modulus $E$ of these negatively charged hydrogels measured by AFM can be finely tuned by variation of cross-linker and HA concentration yielding a stiffness of 0.1 kPa to 150 kPa. $E$ scales with the concentration of HA to the power of $n$=2.6 and is a biphasic function of cross-linker concentration. We will describe the influence of these unique gels on stem cell differentiation. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N25.00011: Elasticity of Short DNA Molecules: Quantitative Agreement Between Theory and Experiment Yeonee Seol, Jinyu Li, Philip Nelson, Thomas Perkins, M. D. Betterton Single-molecule experiments have yielded new insight into the mechanical behavior of individual DNA molecules and protein-DNA interactions. Single-molecule force experiments require a model to deduce the polymer's intrinsic contour length ($L)$ from measurements of force and extension. To date, the worm-like chain model (WLC) provides the best description of DNA elasticity. This theory requires parameters, the contour length $L$ and the persistence length $p$. Using both theory and experiment, we studied the elasticity of dsDNA as function of $L$ using the classic WLC solution, for $L$ between 632 nm and 7.03 microns. When the elasticity data were analyzed using the classic WLC, the fit value of $p$ depended $L$. Therefore we developed the finite worm-like chain solution (FWLC) by including the finite length of the chain and bead rotation. After incorporating these two corrections, our FWLC solution was used to predict elasticity curves and to analyze experimental data. The FWLC provides a single theoretical framework in which to analyze single molecule experiments over a broad range of experimentally accessible DNA lengths, including both short and very long molecules. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N25.00012: Tube Radius in Entangled Networks of Semiflexible Polymers Hauke Hinsch, Jan Wilhelm, Erwin Frey The mechanical properties of the cytoskeleton play an important role in many cellular functions like locomotion or adhesion. One of the cytoskeleton's dominant constituents is a network structure composed of the semiflexible polymer F-Actin. To connect the single polymer properties to the macroscopic behavior of the network, a single polymer is considered to be constrained to a tube established by neighboring filaments. Here we focus on the tube's diameter in entangled networks. While scaling laws for the tube diameter are well established, the absolute value is still under debate and different theoretical concepts and experimental measurements exist. We present a new approach to the problem and have conducted extensive computer simulations to check the validity of our assumptions. A model of independent rods is used to describe the confinement of a single semi-flexible polymer in the network environment. A self-consistency approach allows us then to derive an absolute tube radius for the network as a function of several parameters and compare our results to experimental measurements. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N25.00013: Stretching and bending in cross-linked biopolymer networks Claus Heussinger, Erwin Frey The elastic response of cross-linked biopolymer networks is usually interpreted in terms of affine stretching models, adopted from the theory of rubber-elasticity valid for flexible polymer gels. Unlike flexible polymers, however, stiff polymers have a highly anisotropic elastic response, where the low-energy elastic excitations are actually of bending nature. As a consequence, similar to springs connected in series, one would expect the softer bending mode to dominate the elastic energy rather than the stiff stretching mode. We propose a theory that, unlike recent affine models, properly accounts for the soft bending response of stiff polymers. It allows calculating the macroscopic elastic moduli starting from a microscopic characterization of the (non-affine) deformation field. The calculated scaling properties for the shear modulus are in excellent agreement with the results of recent simulations obtained in simple two-dimensional model networks, and can also be applied to rationalize bulk rheological data in reconstituted actin networks. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700