Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W18: Focus Session: Mechanical Force Spectroscopy and Device Applications of Polymeric and Biological Materials |
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Sponsoring Units: DPOLY Chair: Denis Pristinski, National Institute of Standards and Technology Room: 319 |
Thursday, March 19, 2009 11:15AM - 11:51AM |
W18.00001: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W18.00002: Transitions of a tethered chain under tension Jutta Luettmer-Strathmann, Wolfgang Paul, Kurt Binder When the end of a polymer chain tethered to an attractive surface is pulled away from the surface, the force required to extend the chain depends on interactions with the surface as well as intrachain interactions. Similarly, when the chain is held fixed and the temperature is reduced, both adsorption and collapse transitions have a signature in the force-extension curve. In this work, we performed Monte Carlo simulations of the bond-fluctuation model with a Wang-Landau algorithm to determine the density of states in the state space of monomer-monomer contacts, monomer-surface contacts, and chain extension. We study the effect of tension on the collapse and adsorption transitions and calculate force-extension curves that may be compared with experimental data. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W18.00003: Velocity Dependency of Dragging Force and Wetting Properties of High Viscous Liquids Using Constant Diameter Nanoneedle-Tipped AFM Probes. Mehdi Yazdanpanah, Mahdi Hosseini, Santosh Pabba, Charles Walter, Jayan Hewaparakrama, Robert Cohn A high aspect ratio and constant diameter Ag$_{2}$Ga nanoneedle grown on an AFM cantilever was used to perform F-D experiments on four different molecular weights of PDMS surfaces. The needle is partially inserted into and retracted from the liquid surface in various scan speeds. The viscous drag force causes the cantilever to deflect and recorded as a function of vertical displacement of the needle for each scan speed. The viscosity of the liquid is calculated by fitting a model into experimental data. The results show that the viscosity has strong correlation with the scan speed. Due to simple geometry of the needles, F-D curves are also interpreted to study the wetting properties (i.e. dynamic contact angle, meniscus height) of the PDMS at different scan speeds. Also, F-D curves are interpreted for polymer fiber formation during the capillary thinning and meniscus stretching that shown a strong correlation between the fiber length and the stretching velocity. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W18.00004: Mechanical Properties of Individual Microgel Particles Sara Hashmi, Eric Dufresne Microgels are important materials for both basic science and engineering and have wide applications from the study of phase transitions to the delivery of drugs. These micron and sub-micron particles, made of hydrogel materials, respond to solvent conditions. The most common microgels are environmentally sensitive, responding to temperature and pH. Our material of interest, poly(N-isopropylacrylamide) or NIPAM, undergoes a deswelling transition above a critical temperature. The deswelling behavior of this polymeric material has been thoroughly studied in ensemble microgel systems as well as in bulk hydrogel samples. We present measurements of the elastic properties of single microgel particles using atomic force microscopy. We observe a stiffening of the Young's modulus by an order of magnitude at temperatures well above the transition, where the cross-linked polymer network has fully collapsed. Interestingly, near the transition we observe a comparable softening of the material. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W18.00005: Intrinsically Disordered Titin PEVK as a Molecular Velcro: Salt-Bridge Dynamics and Elasticity Jeffrey Forbes, Wanxia Tsai, Richard Wittebort, Kuan Wang Titin is a giant modular protein (3-4 MDa) found in the muscle sarcomere, where the intrinsically disordered and elastic PEVK segment plays a major role in the passive tension of skeletal and heart tissues. We have proposed that salt-bridges play a central role in the elasticity of PEVK. The 50 kDa engineered PEVK polyprotein shows well-resolved NMR spectra at all concentrations. From long-range NOE's, we observed stable K to E salt-bridges. Simulated annealing with NMR restraints yielded a manifold of structures for an exon 172 trimer. Steered molecular dynamics simulations were done to study how the manifold of salt-bridges evolves during the stretching experiment. Repeated SMD simulations at slow velocity (0.0005 nm/ps) showed force spectra consistent with experimental AFM force spectra of the polyprotein. SMD shows that salt-bridges occur even at high degrees of stretch and that these short range interactions are in integral part of the mechanical properties of PEVK. We propose that the long-range, non-stereospecific nature of electrostatic interactions provide a facile mechanism to tether and untether the flexible chains, which in turn affect elasticity as well as control the accessibility of protein-protein interaction to these nanogel-like proteins. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W18.00006: Mechanical Signal Filtering by Viscoelastic Properties of Cuticle in a Wandering Spider Michael E. McConney, Clemens Schaber, Michael Julian, Joseph A.C. Humphrey, Friedrich Barth, Vladimir V. Tsukruk As recently found, in mechano-sensors of wandering spiders (Cupiennius salei) viscoelastic materials are important in signal filtering. We used atomic force microscopy to probe the time dependent mechanical behavior of these materials in live animals. We measured Young's modulus of a rubbery material located between a vibration receptor and the stimulus source. Earlier electrophysiological studies had demonstrated that the strain needed to elicit a sensory response (action potential) increased drastically as stimulus frequencies went below 10 Hz. Our surface force spectroscopy data similarly indicated a significant decrease in stiffness of the cuticular material and therefore less efficient energy transmission due to viscoelastic effects, as the frequency dropped to around 10 Hz. The stimulus transmitting cuticular material is acting as a high-pass filter for the mechanical stimulus on its way to the strain receptors. Again our results indicate that viscoelastic mechanical signal filtering is an important tool for arthropods to specifically respond to biologically relevant stimulus patterns. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W18.00007: Bacterial Cell Wall Peptidoglycan at Single Molecule Resolution Ahmed Touhami, Manfred Jericho, Valerio Matias, Anthony Clarke, Terry Beveridge, John Dutcher The major structural component of bacterial cell walls is the peptidoglycan sacculus, which is one of nature's strongest and largest macromolecules that maintains the large internal pressure within the cell while allowing the transport of molecules into and out of the cell and cell growth. The three-dimensional structure of this unique biopolymer is controversial, and two models have been proposed: the planar model, in which the glycan strands lie in the plane of the cell surface, and the scaffold model, in which the glycan strands lie perpendicular to the cell surface. We have used atomic force microscopy to investigate the high resolution structure of isolated, intact sacculi of Escherichia coli K12 bacteria. Atomic force microscopy-single molecule force spectroscopy was performed on single sacculi exposed to the tAmiB enzyme which cleaves the peptide-glycan bonds. Surprisingly, the measurements revealed individual strands of up to 250 nm in length. This finding combined with high resolution AFM images recorded on hydrated sacculi provide evidence for the validity of the planar model for the peptidoglycan structure in Gram-negative bacteria. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W18.00008: Unfolding polyelectrolytes in a strong DC electric field Pai-Yi Hsiao, Kun-Mao Wu The behavior of single polyelectrolytes in multivalent salt solutions under the action of an electric field is investigated by computer simulations. The variation of chain size against the strength of electric field displays a sigmoidal transition, which defines a critical field $E^\ast $ to unfold a chain. Above $E^\ast $, the chain is unfolded into a rodlike structure, aligned parallel to the field direction. We show that $E^\ast $ is a function of salt concentration and depends on the chain length via the scaling law $V^{-1/2}$ where $V$is the ellipsoidal volume occupied by the chain. Moreover, the magnitude of the electrophoretic mobility of chain drastically increases during the unfolding. These findings provide a solid foundation to a newly proposed mechanism to separate long charged homopolymers by their chain length in free-solution electrophoresis via the unfolding transition of globule polyelectrolytes condensed by multivalent salt. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W18.00009: Liquid Drop Pinning on Micro-patterned Surfaces Ahmed Soliman, Yevgeniy Kalinin, Robin Baur, Robert Thorne Pinning of liquid drops on surfaces is important in many areas of biotechnology. Micro-patterned surfaces provide a way to control drop pinning, and to investigate the mechanisms of pinning on real (rough) surfaces. Continuous circular rings on silicon wafers produced by etching the interior and surrounding silicon are shown to dramatically increase contact line pinning. The critical apparent contact angles and liquid drop volumes are measured and correlated with parameters that describe the ring geometry, such as ring-wall height and width, as well as with ring surface energy (hydrophilicity / hydrophobicity). Micro-patterning of surfaces in this way can be used to improve drop pinning, shape reproducibility and imaging in high-throughput protein crystallization. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W18.00010: An Optical Biosensing Platform using Reprecipitated Polyaniline Microparticles Louis Nemzer, Arthur Epstein A great deal of effort remains focused on the goal of developing a continuous \textit{in vivo} glucose monitoring system for patients with \textit{diabetes mellitus.} We report a proof-of-concept study on a reagentless optical biosensing platform that circumvents the problems usually associated with direct glucose detection by utilizing the UV-VIS absorption properties of polyaniline, a biocompatible polymer. When the enzyme glucose oxidase is entrapped within reprecipitated polyaniline microparticles, a glucose molecule readily donates two protons and two electrons to the polyaniline, reversibly altering the polymer's oxidation state. The resultant change can be monitored by measuring the absorption at wavelengths that fall within the ``optical window'' for skin. The micro-structured morphology also insures a high surface-area to volume ratio. Data from \textit{in vitro} prototype devices indicate that in the low enzyme-loading regime, the response can be fit to the Michaelis-Menten model for enzyme kinetics, but at higher enzyme loading, diffusion effects dominate. As a biosensing platform, the system also has the potential to be adapted to detect other biologically relevant analytes, including cholesterol and ethanol. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W18.00011: Patterning of Ferritin Nanoparticles on Gold Posts of Silicon Substrate Yunxia Hu, Dian Chen, Soojin Park, Todd Emrick, Thomas Russell Patterning and immobilizing protein nanoparticles with nanometer-scale control has been proven integral to a range of applications in the development of biochip arrays, biosensor and electronic devices. Protein nanoparticles, such as ferritin nanoparticles, have a uniform size distribution and shape that can be used to construct well-defined patterns with nanoscale features. Here, the gold posts on silicon were produced using block copolymer PS (47.6 kg/mol)-b-P4VP (20.9 kg/mol) (PDI: 1.14) as a template and then gold chloride solution~was loaded into P4VP domain. After reducing gold salt into gold and removing the block copolymer using anoxygen plasma, producing a pattern of gold posts. Thiol modified horse spleen ferritin are anchored to gold posts of silicon substrate by the binding of thiol and gold. Scanning electron microscopy (SEM) shows that the feature size of gold posts decreased from 30 nm to 13 nm after attached with modified ferritin nanoparticles, which is consistent with size of modified ferritin. Also XPS result shows nitrogen and ion elements on ferritin-attached gold posts, and the signal of gold was attenuated after ferritin attached. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W18.00012: Rapid Hydrogel Microactuator Using Elastic Instability Howon Lee, Chunguang Xia, Nicholas Fang Rapid Hydrogel Microactuator Using Elastic Instability Inspired by rapid movement of sensitive plants such as Venus flytrap [1], we present an innovative way to enhance actuation speed of hydrogel micro devices by exploiting elastic instability. In this work, hydrogel micro devices in doubly curved shape are designed and fabricated using projection micro-stereolithography[2], with embedded microfluidic channels on the surface. Local swelling of hydrogel around channels causes bending which subsequently induces stretching of the soft structure. Such coupling gives rise to elastic instability, the onset of which triggers rapid conversion of stored elastic energy into kinetic energy in fast motion. We further designed a set of devices with different dimensions, which leads to different coupling of elastic energy in bending and stretching [1]. Our experimental results verified the critical coupling parameter that triggers snap-buckling motion. Ongoing experiments are investigating the actuation speed as a function of coupling parameter. This novel approach promises new potential applications for hydrogel based devices in various fields of study including microfluidics, soft robotics, artificial muscle, and drug delivery. Reference [1] Forterre, Y., et al, Nature, 433, 421-425 (2005) [2] Sun, C., et al, Sensors and Actuators A, 121:1, 113-120 (2005) [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W18.00013: Effect of Single Bacterium Cell and DNA Attachment on the Electrical Properties of Chemically Modified Graphene Sheets Nihar Mohanty, Vikas Berry Chemically modified graphene (CMG) sheets are expected to have a considerably different electrical sensitivity to molecular attachment than the pristine graphene sheets. Here we present the electrical-interfacing properties of (a) CMG's hybrids with single bacterial cells, (b) CMG with DNA (single and double stranded) tethered on graphene-surface and (c) CMG with polyelectrolyte-layer assembled on surface. These hybrids function as: (a) single bacterium devices, (b) DNA hybridization sensor and (c) charge polarity sensitive chemical-detector, respectively. A single bacterium attachment leads to generation of $\sim $1400 holes on a CMG while hybridization of $\sim $4 DNA molecules on graphene-DNA-carpets lead to generation of one hole. Further explanation of the attachment-potential, system-reversibility and sensitivity will also be presented. [Preview Abstract] |
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