Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y20: Biological-Synthetic Hybrid Materials |
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Sponsoring Units: DPOLY Chair: Ali Dhinojwala, University of Akron Room: 321 |
Friday, March 20, 2009 8:00AM - 8:36AM |
Y20.00001: Peptide Folding and Consequent Self-assembly for Shear Thinning Hydrogels with Immediate Recovery Invited Speaker: The local nano- and overall network structure, and resultant viscoelastic properties, of hydrogels that are formed via beta-hairpin self-assembly will be presented. The 20 amino acid peptides have been shown to intramolecularly fold and intermolecularly self-assemble into a rigid hydrogel based on environmental cues such as pH, salt, and temperature including physiological conditions. The hydrogel is composed of a network of fibrils that are 3 nm wide that physically crosslink (i.e. entangle and branch) with no covalent crosslinking required. Slight design variations of the peptide sequence allow for tunability of the self-assembly/hydrogelation kinetics. In turn, by controlling hydrogel self-assembly kinetics, one dictates the ultimate stiffness of the resultant network. This physical assembly process allows the encapsulation of desired payloads into the gel network such as large macromolecules or living cells. Importantly, once formed into a solid, the self-supporting gel network can be disrupted by the introduction of a shear stress. The system can shear thin but immediately reheal to a stiff solid on the cessation of the shear stress. This shear thinning and recovery behavior makes them interesting candidates for injectable delivery in vivo. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y20.00002: Designing `smart' hydrogels using peptide-responsive conjugates Alberto Saiani, Jean-Baptiste Guibaud, Aline Miller Polymers displaying phase transitions in aqueous solution from a hydrophilic to hydrophobic state are of widespread interest as `smart' biomedical materials. Hydrogels formed from self-assembling peptides have also attracted considerable attention in the past decade. These hydrogels are based on the self-assembly of short peptides into supra-molecular fibres that entangle or associate to form three dimensional networks and, ultimately, self-supporting hydrogels. In order to create a new generation of hydrogel based on self-assembling peptides but possessing an internal transition that can be used as a trigger to release a drug molecule or a specific biological signal we have synthesized a new family of peptide-polymer conjugates using free radical polymerization. We were able to create a new generation of `smart' hydrogel in which gelation is driven by the peptide while the polymer phase transition can be used to release in a controlled fashion a specific signal or drug molecules. These materials are though to be of prime interest for tissue culture applications where they can be used to deliver specific signals stimulating a specific cell response. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y20.00003: ``Backpack'' Functionalized Living Immune Cells Albert Swiston, Soong Ho Um, Darrell Irvine, Robert Cohen, Michael Rubner We demonstrate that functional polymeric ``backpacks'' built from polyelectrolyte multilayers (PEMs) can be attached to a fraction of the surface area of living, individual lymphocytes. Backpacks containing fluorescent polymers, superparamagnetic nanoparticles, and commercially available quantum dots have been attached to B and T-cells, which may be spatially manipulated using a magnetic field. Since the backpack does not occlude the entire cellular surface from the environment, this technique allows functional synthetic payloads to be attached to a cell that is free to perform its native functions, thereby synergistically utilizing both biological and synthetic functionalities. For instance, we have shown that backpack-modified T-cells are able to migrate on surfaces for several hours following backpack attachment. Possible payloads within the PEM backpack include drugs, vaccine antigens, thermally responsive polymers, nanoparticles, and imaging agents. We will discuss how this approach has broad potential for applications in bioimaging, single-cell functionalization, immune system and tissue engineering, and cell-based therapeutics where cell-environment interactions are critical. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y20.00004: Reversible Structural Transition of a DNA Lipid Film. Matthew Tirrell, Surekha Gajria, Thorsten Neumann, Luc Jaeger Polyanions such as nucleic acids (RNA and DNA) can self-assemble with cationic lipids via electrostatic complexation, driven thermodynamically by the release of counterions. The structures of these complexes in water have been studied extensively and are recognized as potentially useful in the field of gene delivery. The structure of films in water is dominated by the nature of the lipid. Within these lamellar complexes in aqueous solution the lipid assumes a bilayer formation and the DNA is a double helix. It is possible to obtain dry nucleic acid-lipid films when the dissolved cationic lipid complex of DDAB and nucleic acid is cast on a solid. These self-standing films have been characterized by tensile properties and nucleic acid intercalation experiments. The tensile properties of these films are adjustable by mixing different molecular weights. It was expected that these films would have the same characteristic structure as these complexes in water. However, our work shows that the film undergoes a transition from double stranded helical DNA complexed with a bilayer of DDAB in the wet state, while in the dry state we observed a repeat unit of single stranded DNA complexed with a monolayer of DDAB. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y20.00005: Gecko-Inspired Carbon Nanotube-Based Adhesives Liehui Ge, Sunny Sethi, Anubha Goyal, Lijie Ci, Pulickel Ajayan, Ali Dhinojwala Nature has developed hierarchical hairy structure on the wall-climbing gecko's foot, consisting of microscopic hairs called setae, which further split into hundreds of smaller structures called spatulas. In the last five years, numerous attempts to mimic gecko foot-hair using polymer soft molding and photolithography methods have been reported. However, most of these polymer-based synthetic gecko hairs fall short of the clinging ability of geckos. Vertically aligned carbon nanotubes (CNT) have shown strong adhesion at nanometer scale. Here, we present our work on developing CNT-based macroscopic flexible tape mimicking the hierarchical structure found on gecko's foot. The synthetic gecko tape is made by transferring aligned CNT array onto flexible polymer tape. The unpatterned CNT-gecko tape can support a shear force stress similar to gecko foot (10 N/cm$^{2})$. The supported shear stress increase by a factor of four, when we use micro-patterned CNT patches (50 to 500 $\mu $m). We find that both setae (replicated by CNT bundles) and spatulas (individual CNT) are necessary to achieve large macroscopic shear adhesion. The carbon nanotube-based tape offers an excellent synthetic option as a dry conductive reversible adhesive in microelectronics, robotics, and space applications. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y20.00006: Sensing Structures Inspired by Blind Cave Fish Michael E. McConney, Nannan Chen, David Lu, Kyle D. Anderson, Huan Hu, Chang Liu, Vladimir V. Tsukruk Blind cave fish, with degenerated non-functioning eyes, have evolved to ``see'' their hydrodynamic environment by using the flow receptors of the lateral line system. The hair-cell receptors are encapsulated in a hydrogel-like material, called a cupula, which increases the sensitivity of the hair-cell receptors by coupling their motion to the surrounding flowing media. We characterized the viscoelastic properties and of blind cave fish cupulae by using colloidal-probe spectroscopy in fluid. A photo-patternable hydrogel with similar properties was developed to mimic the fish receptor coupling structure. Flow-based measurements indicated that the hydrogels enhance drag through increased surface area, but also inherent material properties. These bio-inspired structures endowed micro-fabricated flow sensors with sensitivities rivaling that of fish. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y20.00007: Giant super-helix formation from aqueous bioinspired block copolymers. H.K. Murnen, A.M. Rosales, R.N. Zuckermann, R.A. Segalman Polypeptoids are a class of bioinspired polymers based on N-substituted glycines with the side group bonded to the backbone nitrogen rather than the alpha carbon as in natural polypeptides. Due to the lack of backbone hydrogen bonding and the sequence specific synthesis of these materials, side chain interactions can be designed to induce the formation of macromolecular structures in aqueous solution. An amphiphilic block copolypeptoid consisting of a hydrophobic block, poly[N-(2-phenylethyl)glycine] and a hydrophilic block, poly[N-(2-carboxyethyl)glycine] is found to form giant superhelices in aqueous solution by scanning and transmission electron microscopy and atomic force microscopy. With a diameter greater than 600nm, these helices are much larger than the fully extended length of the molecules (7 nm). Furthermore, while the molecules are completely achiral, the helices are all left handed and remarkably regular (pitch 670nm, length greater than 2 microns). We will discuss possible preferential chain conformations that may provide the driving force for the superstructure. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y20.00008: The effects of supramolecular network topology on hapten-receptor avidity Jason Benkoski, Andrew Mason, Jill La Favors, Joshua Wolfe Antibodies produced in the early stages of the immune response have much lower affinities for a given antigen than those produced later on. Nature compensates for the initial weakness of these associative bonds by synthesizing multivalent antibodies. The total binding strength, represented by the avidity constant, is equal to the product of the affinity constants for the individual hapten/receptor sites. However, under realistic conditions the individual binding sites do not act independently. Factors such as steric hindrance, intramolecular stresses, and competitive binding can significantly alter the relationship between affinity and avidity. We investigate the influence of these factors on a model system consisting of synthetic multifunctional nanoparticles and polymers. Each polymer or nanoparticle is decorated with either multiple antigens (thromboxane B2) or multiple antibodies. We then measure the association and dissociation in real time using Surface Plasmon Resonance Spectrometry (SPR). By using synthetic polymers and nanoparticles, we are able to systematically control the degree of functionality, flexibility, and distance between receptor and hapten sites. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y20.00009: Dynamic DNA Interactions with Functionalized Colloids Lu Zhang, Yingxi Elaine Zhu Many biomedical processes, such as protein adsorption, DNA hybridization and enzyme reactivity, are intimately related to their interactions with surfaces and complex ionic environments, yet the details of biomacromolecular interaction remain insufficiently understood. In this work, we use confocal laser scanning microscopy to examine the interaction between DNA molecules and functionalized colloidal particles in aqueous suspension. We observe an intriguing attractive interaction between DNAs and carboxyl-functionalized silica particles of varied sizes from 50 nm to 3 um, resulting in complex DNA-colloid aggregation with a strong dependence on DNA/colloid size ration and ionic strength. As colloidal size becomes larger than DNA dimensions, colloidal doublets and triplets with adsorbed DNAs are observed at high DNA concentration and ionic strength. The intriguing DNA-colloid complex structures are further confirmed by SEM and appear stable for at least 2 weeks. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y20.00010: Growth of well-defined metal and oxide nanoparticles on biological surfaces Vladimir Tsukruk We present a brief overview of our recent studies in the field of bio-enabled surface-mediated growth of inorganic nanoparticles at room temperature and ambient conditions. We demonstrate that all titania, gold, and silver nanoparticles can be grown with relatively monodisperse diameter within 4-6 nm surrounded by biological shells of 1-2 nm thick. As biological templates we utilized ultrathin, molecular uniform and micropatterned surface layers of two different proteins: silk fibroin (for growth of gold and silver nanoparticles) and silaffin (for growth of titania nanoparticles). To identify the grown nanophases and chemical composition/secondary structure of biological templates we applied combined AFM, SEM, TEM, XPS, SERS, UV-vis, and ATR-FTIR techniques. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y20.00011: Fluorescent DNA-bound Ag nanoclusters Patrick O'Neill, Lourdes Velazquez, Kim Weirich, Deborah Fygenson Few-atom fluorescent Ag nanoclusters self-assemble on short, synthetic DNA strands, and exhibit sequence and structure dependent fluorescence ranging from the blue to the near infrared. Here we report UV excitation as a ubiquitous feature of these emitters. Each emitter thus has two excitation peaks: a visible peak which is cluster-dependent, and a UV peak which has the same wavelength for all DNA-bound Ag clusters. This UV peak corresponds to resonant absorbance by the DNA bases, and produces the same emission spectra as visible excitation, suggesting energy transfer from the DNA bases to the Ag cluster. We make use of this UV excitation to image the emitters in unstained polyacrylamide gels, and show that electrophoresis can be used to create a pure solution of green DNA:Ag11 clusters from an inhomogeneous red solution of DNA:Ag$>$12 clusters. [Preview Abstract] |
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