Ferenc Horkay National Institutes of Health
Noshir A. Langrana Rutgers University
Anthony J. Ryan The University of Sheffield
J. David Londono DuPont de Nemours
LL1: Thermodynamics and Modeling
Noshir A. Langrana
Monday PM, November 26, 2007
Room 207 (Hynes)
9:30 AM - **LL1.1
Phase Behavior of Polyelectrolyte Gels.
Murugappan Muthukumar 1 Show Abstract
1 , University of Massachusetts, Amherst, Massachusetts, United States
A general theory of thermodynamics of polyelectrolyte gels will be presented, by considering (a) hydrophobic interactions, (b) electrostatic interactions, (c) counterion distribution, (d) correlations of ions, (e) fluctuations of monomer concentrations, and (f) crosslink density. Predictions on the osmotic compressibility of a polyelectrolyte gel and on phase diagram for the volume transitions of gels will be presented. The role of valency of counterions on the various thermodynamic properties of gels will be addressed. Comparisons will be made between the theoretical predictions and experimental data.
10:00 AM - **LL1.2
Waves of Self-Assembly.
Jack Douglas 1 , Fredrick Phelan 1 , Fisher Daniel 2 , Kirill Efimenko 3 , Jan Genzer 3 Show Abstract
1 Polymers Division, NIST, Gaithersburg, Maryland, United States, 2 Ceramics division, NIST, Gaithersburg, Maryland, United States, 3 Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, United States
Wavefronts associated with reaction–diffusion and self-assembly processes are ubiquitous in the natural world. For example, propagating fronts arise in crystallization and diverse other thermodynamic ordering processes, in polymerization fronts involved in cell movement and division, as well as in the competitive social interactions and population dynamics of animals at much larger scales. Although it is often claimed that self-sustaining or autocatalytic front propagation is well described by mean-field 'reaction–diffusion' or 'phase field' ordering models, it has recently become appreciated from simulations and theoretical arguments that fluctuation effects in lower spatial dimensions can lead to appreciable deviations from the classical mean-field theory of this type of front propagation. The present work explores these fluctuation effects in a real physical system. In particular, we consider a high-resolution near-edge X-ray absorption fine structure spectroscopy (NEXAFS) study of the spontaneous frontal self-assembly of organosilane (OS) molecules into self-assembled monolayer (SAM) surface-energy gradients on oxidized silicon wafers. We find that these layers organize from the wafer edge as propagating wavefronts having well defined velocities. In accordance with two-dimensional simulations of this type of front propagation that take fluctuation effects into account, we find that the interfacial widths w(t) of these SAM self-assembly fronts exhibit a power-law broadening ('roughening') in time rather than the constant width predicted by mean field theory. Moreover, the observed exponent values accord rather well with previous simulation and theoretical estimates. These observations have significant implications for diverse types of ordering fronts that occur under confinement conditions in biological or materials-processing contexts.
11:00 AM - **LL1.3
Structure and Dynamics of Beta-peptide Networks and Gels.
Juan dePablo 1 Show Abstract
1 Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
We study the aggregation of beta-peptides into a variety of structures, including networks and gels, over a wide range of length and time scales. Monte Carlo, molecular dynamics, and Brownian dynamics simulations are used to examine several aspects of synthetic beta-peptides. We begin by exploring the role of sequence on the folded structure of individual peptides in both implicit and explicit solvents. The molecules considered in our experimental and theoretical studies are generally found to form stable 14-member helices. We then examine the behavior of multiple peptides in explicit and implicit solvents, and arrive at potentials of mean force that exhibit a distinct preference for particular molecular orientations and conformations as molecules approach each other. These potentials of mean force are used to explain a number of experimentally observed beta-peptide phases that include gels and liquid crystalline states. The potentials of mean force are also used to develop a coarse grain model of the peptides capable of describing the aggregation of beta-peptides over long time scales. The model is coupled to a formalism that takes into account fluctuating hydrodynamic interactions, thereby allowing us to provide a relatively complete picture of the structure and dynamics of semi-dilute and concentrated solutions of these molecules as they self-assemble into intriguing ordered liquid crystalline phases and amorphous gels.
11:30 AM - **LL1.4
Self-Assembled DNA Networks and Gels.
Francis Starr 1 Show Abstract
1 Physics, Wesleyan University, Middletown, Connecticut, United States
The use of DNA to control bond formation between otherwisenon-interacting particles offers a novel route to designinginterconnected and highly organized materials at the molecular scale.An additional valuable feature of using DNA strands is the possibilityof controlling the maximum number of bonded neighbors, or valency, a keyingredient to control the size of the region in which phase separationoccurs. We have developed coarse-grained models that capture theessential elements of "lock-and-key" binding of complementary DNAstrands and facilitate simulations of complex materials involving manystrands of DNA. We simulate DNA-dendrimers that consist ofsingle-strands of DNA tethered to a core. The number of tetherscontrols the number of neighbors that a dendrimer may bond with. Weshow how a system of these dendrimers self-assemble to form a stablelow-density fluid that can reversibly form a gel. We furtherdemonstrate a direct connection between the formation of the network andthe dynamics. The dendrimers also exhibit multiple fluid phases,offering the richness of polymorphic solids, but with amorphousordering. We discuss how the model can be adapted to mimic the behaviorof DNA-coated colloids that form networks due to "linker" DNA thatselectively binds the colloids. In this case, the concentration oflinker DNA controls the mean number of bonds formed, thereby offeringthe possibility to form stable low-density fluids and gels.
12:00 PM - LL1.5
Tuning the Collapse Temperature of poly (N-isopropylacrylamide) Gels.
Nily Dan 1 , David Weitz 2 , Jin Woong Kim 2 Show Abstract
1 Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Division of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts, United States
Temperature-sensitive polymeric gels, namely, gels that undergo a swelling transition upon a small change in the solution temperature, have been widely studied for a variety of in vitro and in vivo applications. Poly (N-isopropylacrylamide) (PNIPAM) is of special interest since it has a lower critical solution temperature in the range of 30 ± 5 0C. Previous studies have shown that the transition temperature of PNIPAM may be controlled by the degree of polymer charging. However, changing the gel electrostatic properties can reduce the sensitivity of the gel to temperature changes, as well as affect interactions between the gel chains and encapsulated components.In this paper we demonstrate that the transition temperature and sensitivity of PNIPAM gels and microgels may be accurately controlled through surface modification, thereby unaffecting the gel bulk characteristics.
12:15 PM - LL1.6
A Study of the Structure-Property Relationships in Self-Assembled beta-Hairpin Peptide Hydrogels by SANS and cryo-TEM.
Rohan Hule 1 , Radhika Nagarkar 2 , Boualem Hammouda 3 , Joel Schneider 2 , Darrin Pochan 1 Show Abstract
1 Materials Science and Engineering and Delaware Biotechnology Institute, University of Delaware, Newark, Delaware, United States, 2 Chemistry and Biochemistry, University of Delaware, Newark, Delaware, United States, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Hydrogels have been established as promising biomaterials for applications such as tissue engineering, controlled release of drugs and cell encapsulation. Knowledge of the precise nano-through microstructure can help draw specific structure-bioproperty relationships when studying in vitro and in vivo behavior of these new peptide scaffolds. De novo designed beta hairpin peptides, capable of undergoing intramolecular folding and consequent intermolecular self assembly and hydrogel formation, were investigated containing asymmetric beta strand arms surrounding a turn sequence. The stimuli responsive self assembly of the hydrogels occurs via a strand interdigitation mechanism, resulting in a fibrillar nanostructure. Fibril dimensions as measured by TEM and AFM corroborate the interdigitated assembly. SANS provides quantitative evidence of different fibril morphologies in terms of higher scattering intensities and persistence lengths for laminating fibrils vs. twisting or non-twisting structures. Inter and intramolecular associations during the self assembly process can also be related to exponents indicative of mass fractals at high Q values. The formation of fibrils as a result of the self-assembly can be precisely tracked by changes in the scattering exponents and correlation lengths. The laminating fibrils show a change from a surface (4) to a mass (3) fractal exponent at high Q with concentration, indicative of the faster assembly kinetics that disrupts regular lamination. Similar changes in concentration lead to a denser network of non-twisting fibrils, indicated by a change in the mass fractal dimension from 2 to 3. These changes in the hydrogel behavior, both at the network as well as the individual fibril length scales, at the same concentrations as SANS, can be directly visualized in situ by cryogenic TEM micrographs. Bulk material properties studied using oscillatory rheology can be accurately correlated to the difference in morphologies. Hydrogels consisting of laminated fibrils exhibit enhanced moduli over non-twisting or twisting fibrils and yield at lower strain values. Differences in the peptide registry that drive distinct nanostructures and hydrogel properties can be potentially targeted towards specific biomaterial applications.
12:30 PM - LL1.7
Synchronization in Heterogeneous Self-Oscillating Polymer Gels.
Victor Yashin 1 , Anna Balazs 1 Show Abstract
1 Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
The ability of polymer gels to significantly swell and deswell in the response to an external stimulation makes them optimal candidates for use as soft active materials. Chemoresponsive polymer gels that participate in the Belousov-Zhabotinsky reaction (BZ gels) are unique because they exhibit self-sustained, autonomous chemo-mechanical oscillations without external stimuli, in a stand-alone system. In the BZ gel, chemical oscillations due to the BZ reaction cause variations in the local degree of swelling because of the hydrating effect of the oxidized metal-ion catalyst linked to the polymer. The dynamic patterns in homogeneous BZ gels are determined mainly by the gel size, shape, and boundary constraints. Physical and chemical patterning of self-oscillating gels could provide more control over the gel dynamics, and could facilitate creating active materials that exhibit desirable spatiotemporal behavior. We explore these possibilities by computational modeling of a polymer gel, in which the spatial pattern is introduced through a heterogeneous distribution of the BZ catalyst. To exploit the specific polymeric degrees of freedom of the responsive polymer network, we vary the local gel stiffness within the catalyst patches by changing the local crosslink density of the polymer network. The catalyst patches within the gel matrix act as the local chemomechanical oscillators, which might exhibit highly correlated (synchronized) behavior due to the interaction through the concentration fields and the network distortions. We demonstrate and discuss how synchronization depends upon the distance between the patches, chemo-mechanical coupling, and local gel stiffness.
12:45 PM - LL1.8
Mechanically Induced Chemical Oscillations and Motion in Responsive Gels.
Olga Kuksenok 1 , Victor Yashin 1 , Anna Balazs 1 Show Abstract
1 , University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Mechanochemical transduction plays a vital role in biological processes. There have, however, been few studies on exploiting mechanical stimuli to trigger chemical signals in non-biological systems. Using computational modeling, we investigate how an applied mechanical pressure can be harnessed to initiate traveling chemical waves in polymer gels undergoing the Belousov-Zhabotinsky (BZ) reaction. We uncover a rich dynamic behavior, isolating systems where the applied pressure induces chemical oscillations in an initially non-oscillatory system. We also pinpoint a scenario where the compression induces both oscillations and the autonomous rotation of the entire sample. Determining factors that control mechanochemical transduction in BZ gels is necessary for establishing guidelines to create self-adjusting or adaptive materials that not only “sense” a localized impact, but also transmit a global chemical signal in response to the local mechanical perturbation. Such materials can potentially be used to fabricate touch-sensitive sensors and membranes, as well as self-reinforcing materials.
LL2: Synthesis and Characterization of Hydrogels
Monday PM, November 26, 2007
Room 207 (Hynes)
2:30 PM - **LL2.1
Surface Modification, Network Structure, Degradation, and Cellular Interactions of Thiol-Ene Networks.
Amber Rydholm 1 , Vaibhav Khire 1 , Danielle Benoit 1 , Kristi Anseth 1 , Christopher Bowman 1 Show Abstract
1 Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, United States
Thiol-ene photopolymers have several distinct advantages over crosslinked polymer networks formed via other reactions. Their specificity, step growth mechanism, the ability to perform surface initiated or bulk reactions, and the ability to form the networks without initiators or other catalysts enable these reactions to be used to form molecularly designed materials with intimate control of degradation, surface interactions, selective incorporation of biomolecular signals, and bulk material properties. Here, we will discuss the mechanism of these reactions as well as selective examples of the formation of polymer networks and polymer surfaces which utilize thiol-ene reactions. Examples of the ability to tune degradation will be presented that demonstrate control over both the mechanism and timescale of degradation. Further, the ability to fabricate surfaces with orthogonal gradients in surface chemistry, including biomolecular signals, and either graft polymer chain length or grafting density will be presented. These surfaces are subsequently used to study, in a high-throughput manner, the biomaterial-cell interaction.
3:00 PM - **LL2.2
Synthesis and Characterization of Stimulus-Responsive Biocompatible Triblock Copolymer Gelators.
Steven Armes 1 , Peter Madsen 1 , Andrew Lewis 2 Show Abstract
1 Chemistry, The University of Sheffield, Sheffield, Yorkshire, United Kingdom, 2 Biocompatibles UK Limited, Biocompatibles International plc, Farnham, Surrey, United Kingdom
4:00 PM - **LL2.3
Model Polymethacrylate Networks Containing Cleavable Linkages for Biological Applications.
Costas Patrickios 1 , Maria Rikkou 1 , Efrosyni Themistou 1 Show Abstract
1 Chemistry, University of Cyprus, Nicosia Cyprus
Various degradable polymethacrylate networks based on cleavable initiators and cleavable crosslinkers were prepared using group tansfer polymerization (GTP). These materials were characterized in terms of their swelling properties. Subsequently, they were subjected to degradation whose kinetics was investigated. Moreover, the kinetics of linear and star polymer analogs, also prepared by GTP, was studied too.
4:30 PM - **LL2.4
Nanoporous Materials from High Internal Phase Emulsions: Biodegradable Polymers and Hydrogels.
Michael Silverstein 1 Show Abstract
1 Materials Engineering, Technion, Haifa Israel
PolyHIPE are nanoporous polymers whose unique and advantageous structures and properties are of potential interest for tissue engineering and drug delivery applications. A high internal phase emulsion (HIPE) is defined as an emulsion in which the dispersed phase occupies more than 74% of the volume. PolyHIPE are typically based on water-in-oil (W/O) HIPE. The continuous organic phase (~10 vol%) contains monomers, crosslinking comonomers, and emulsifiers. The dispersed aqueous phase (~90 vol%), containing a water-soluble initiator and stabilizers, is removed following polymerization. Typical polyHIPE have bulk densities of less than 0.15 g/cc and surface areas of more than 5 sq.m/g. A typical polyHIPE open-pore structure consists of spherical voids (~10 µm) whose walls are perforated by numerous holes (~0.5 µm). Nanoporosity was introduced into the polyHIPE by adding a porogen to the HIPE’s continuous phase. Several different bio-inspired polyHIPE systems are presently being developed. PolyHIPE incorporating biodegradable polymers were synthesized through the formation of semi interpenetrating polymer networks and through macromonomer copolymerization. PolyHIPE in which the 90 vol% open porosity is replaced by a continuous hydrogel were synthesized through the simultaneous polymerization of a water-soluble monomer and crosslinking comonomer in the aqueous phase. PolyHIPE with 10 vol% hydrogel and 90 vol% open porosity were synthesized by inverting the phases and forming oil-in-water (O/W) HIPE. The continuous aqueous phase (~10 vol%) contains water-soluble monomers, crosslinking comonomers, emulsifiers, and initiator. The dispersed organic phase (~90 vol%) is removed following polymerization. The syntheses, molecular structures, porous structures, and properties will be discussed.
5:00 PM - LL2.5
Characterization of Kinetic Mechanisms of Three-component Photoinitiator Systems for Visible-light Free Radical Polymerizations.
Dongkwan Kim 1 , Jeffrey Stansbury 1 Show Abstract
1 Craniofacial Biology, University of Colorado at Denver and Health Sciences Center, Aurora, Colorado, United States
Three-component photoinitiator systems generally include a light-absorbing photosensitizer (PS), an electron donor and an electron acceptor, which is usually an onium salt. To characterize the key factors involved with visible-light free radical polymerizations of three-component photoinitiators, we used thermodynamic feasibility and kinetic considerations with three-component photoinitiator systems containing rose bengal (RB) as the photosensitizer. The Rhem-Weller equation was used to verify the thermodynamic feasibility for the photo-induced electron transfer reaction. Using the thermodynamic feasibility, we suggested three different kinetic mechanisms, which are photo-reducible series mechanism, photo-oxidizable series mechanism and parallel-series mechanism. In the photo-reducible series mechanism, the primary photochemical reaction involves electron transfer and proton transfer from the electron donor to the photo-excited, PS and a series of electron transfer reactions between PS radical ion and an electron acceptor that occur in the secondary reaction step. In contrast, in the photo-oxidizable series mechanism, the primary photochemical reaction involves electron transfer from the electron acceptor to the photo-excited, PS with the unimolecular fragmentation reaction of electron acceptor and a series of electron transfer reactions between PS radical ion and an electron donor in the secondary reaction step. In the parallel-series mechanism, photo-excited, PS simultaneously involves photo-reducible and photo-oxidizable mechanism.To characterize the three mechanistic kinetics, the rates of polymerization were measured by photo-differential scanning calorimeter (Photo-DSC). The three-component initiator system containing RB/triethylamine (TEA)/iodonium salt (DPI), which involves photo-oxidizable series mechanism, produced the fastest photopolymerization reaction rate (maximum rate 0.0605 mol/L-sec observed at 4.20 min). The three-component initiator system containing RB/N-methyldiethanolamine (MDEA)/DPI, which undergoes parallel-series mechanism, showed the second fast reaction rate (maximum rate 0.0539 mol/L-sec observed at 4.78 min). The RB/MDEA/triphenylsulfonium salt (TPS) initiator system, which involves photo-reducible series mechanism, exhibited the slowest reaction rate (maximum rate 0.0258 mol/L-sec observed at 5.90 min). By the comparison of the kinetic mechanistic results, we conclude that the key factor to control the kinetics of visible-light polymerizations is to retard the back electron transfer step. Because the back electron transfer step is thermodynamically feasible, an approach that minimizes this step is required to achieve efficient formation of free radical active centers.
5:15 PM - LL2.6
Rheology of Thermoreversible Hydrogels from Multiblock Associating Polymers.
Jun Jiang 1 , Chunhua Li 1 , Daniel Cohn 2 , Min Lin 4 , Ralph Colby 3 , Miriam Rafailovich 1 , Jonathan Sokolov 1 Show Abstract
1 Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, New York, United States, 2 Casali Institute of Applied Chemistry, Hebrew University of Jersualem, Jerusalem Israel, 4 , Exxon Research and Engineering Corporation, Annandale, New Jersey, United States, 3 Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Multiblock copolymers of poly(ethylene oxide)99-poly(propylene oxide)67-poly(ethylene oxide)99 (F127) were synthesized by chain extending with hexamethylene diisocyanate (HDI). The resulting multiblock copolymer poly-F127 maintained the thermoreversible properties of the original F127 triblock unit. The rheological and structural properties of the gels were characterized as a function of temperature, composition and degree of polymerization. Neutron scattering reveals a large degree of alignment could be induced in the F127 gel, but no long-range order could be found in the multiblocks or the mixtures of F127 with multiblocks. The shear strain at yield in polymers having an average of 3.2 or more F127 repeats was nearly an order of magnitude higher than the F127 gel. For F127 solutions just below their gel point, substitution of F127 with as little as 1% multiblock succeeded in forming a physical gel. Percolation theory was used to understand the modulus growth when multiblock was added to F127 solutions just below their gel point, assuming the multiblocks form bridges between adjacent micelles.
5:30 PM - LL2.7
Hydrogel Nanoparticles as Injectable Dermal Fillers and Breast Implant Filler Materials with Engineered Physical Properties.
John St. John 1 , Daniel Moro 1 , Bill Ponder 1 , Kevin Shannon 1 , Daniel Hatef 2 , Spencer Brown 2 Show Abstract
1 Research, ULURU Inc., Addison, Texas, United States, 2 Plastic Surgery, University of Texas Southwestern Medical School, Dallas, Texas, United States
Soft filler materials are an important class of biological-inspired tissue construct that may fill a critical need for clinical reconstruction and augmentation procedures. We report on the development of two new classes of hydrogel materials composed of hydrogel nanoparticles that behave either as an injectable liquid or as a shape conforming gel. Upon exposure to physiological conditions, these hydrogel nanoparticles irreversibly form a permanent non-erodible, shape retentive material termed an aggregate.Nanoparticles were formed from poly-2-hydroxyethyl methacrylate and related monomers, subjected to rigorous purification and lyophilization and were dispersed either as a liquid suspension or as a gel suspension. Nanoparticles were stabilized in suspension with a surface charge that was quantified through zeta potential measurements. Upon exposure to high ionic strength, the surface charge was equalized through ion pairing and zeta potential measurements showed a drop in potential to zero. Irreversible aggregations forming a bulk hydrogel solids with physical properties determined by the nanoparticle composition were observed for eitherthe injectable suspension or the gelThe injectable suspension was stable and was combined with hyaluronic acid as a gel in water to form suspensions that ranged in composition from 100% hydrogel nanoparticles to 95 % hyaluronic acid. The suspensions were injected into dermal tissues forming tissue filler materials with different properties. The resulting permanent material can be formed with control over physical properties including: percentage of permanent structure remaining, porosity, strength and elasticity. Injections in murine and porcine studies demonstrated toleration and little or no local tissue responses to the mid and lower dermal injections. Preliminary murine studies showed no local irritation and no evidence of nanoparticle migration from the injectable aggregate The shape conforming gel was a concentrated form of hydrogel nanoparticle suspension with 10 to 15 % dry nanoparticle mass per unit volume of gel. The properties of the gel such as elasticity, and resistance to deformation were varied to provide materials that closely mimicked different types of adipose tissue. Silicone elastomeric shells can be filled with the hydrogel. Upon possible rupture or leakage of the shell, the hydrogel nanoparticle gel forms a shape retentive material that can be removed surgically Data were presented for the variations in physical properties and for in vivo studies in rabbit and porcine rupture studies.The injectable nanoparticle suspension is currently under development as a partially permanent dermal filler and the shape-confomring nanoparticle gel as a safe breast implant filler material.
5:45 PM - LL2.8
Flow-induced Conformational Changes in Gelatin Structure.
Mustafa Akbulut 1 , Robert Prud'homme 1 Show Abstract
1 Department of Chemical Engineering, Princeton university, Princeton, California, United States
We studied how the stability of gelatin adsorbed polystyrene nanoparticles changed as a function of their mixing velocities using circular dichroism spectroscopy and confinement impinging jet mixing method coupled with ex situ dynamics lights scattering. We found that the stability of the gelatin adsorbed-nanoparticles increased with increasing mixing velocities: when the mixing velocities were changed from 0.9 m/s to 550 m/s, the radius of the nano-clusters (aggregates) formed 12 hour after the mixing decreased from 2620 nm to 600 nm. Increasing temperature of the mixing chamber also gave rise to similar trends in the stability behavior with increasing mixing velocities. These results suggest that the dissipation energy produced by mixing can provide an activation energy to gelatin to change its conformation such a way that gelatin will unwrap its randomly coiled structure and have more binding sites to adsorb on the surfaces of the nanoparticles. Understanding such conformational changes and how these changes are related to the adsorption behavior of gelatin are very important because of both industrial and scientific reasons.
LL3: Poster Session
Tuesday AM, November 27, 2007
Exhibition Hall D (Hynes)
9:00 PM - LL3.10
PEG-POSS Hybrid Polyurethanes: Mechanically Robust Nanostructured Hydrogels.
Jian Wu 2 , Patrick Mather 1 Show Abstract
2 Chemical Engineering Department and Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 1 Department of Macromolecular Science and Engineering , Case Western Reserve University, Cleveland, Ohio, United States
A series of unique hybrid thermoplastic polyurethanes (TPUs) was synthesized using PEG as soft segment and incorporating an isobutyl-functionalized POSS diol (TMP POSS diol) in the hard segment. The molecular weight of PEG was systematically varied to include 10, 20, and 35 kg/mol while the mole ratio of PEG to POSS diol (as chain extender) was in range from 1:3 to 1:8 with samples featuring a PEG molecular weight of 10 kg/mol. The diisocyanate employed for TPU polymerization was 4,4’-methylenebis(phenyl-isocyanate) (MDI). We found that the hydrophobic hard segments (POSS) can form crystalline structures driven by micro-phase separation, itself due to significant thermodynamic incompatibility between POSS and ethylene oxide units. Consequently, the POSS nano-crystals serve as physical crosslinking points within an inorganic-organic hybrid network, in turn affording novel hybrid organic-inorganic hydrogels in the water-swollen state. The equilibrium swelling ratio has a positive correlation with PEG loading and is range from ~70% to ~600%. The shear modulus, G, of the hybrid hydrogels is in rubber range: 0.3 MPa < G < 4.0MPa. Indeed, the hydrogel stiffness can be finely tuned through the PEG:POSS molar ratio, as this predictably controls swelling in water.
9:00 PM - LL3.11
pH Oscillations of Ethyl Viologen/Ionic Liquid Driven by Electrochemical Reaction.
Su Ryon Shin 1 , Geoffrey Spinks 2 , Gordon Wallace 2 , Sun I. Kim 3 , Sun Hee Lee 1 , Seon Jeong Kim 1 Show Abstract
1 Center for Bio-Artificial Muscle (Dept. of Biomedical Engineering), Hanyang University, Seoul Korea (the Republic of), 2 Intelligent Polymer Research Institute, University of Wollongong, Wollongong Korea (the Republic of), 3 Dept. of Biomedical Engineering, Hanyang University, Seoul Korea (the Republic of)
Recently, several researchers have demonstrated synthetic chemical oscillators as a method of providing a free-running chemical motor in artificial biological systems, such as DNA-based nanomachines and pH-sensitive hydrogels. These systems are composed of an oxidant and either one or two reductants, yielding a two- or a three-component system in both quiescent and continuous-flow stirred tank reactors (CSTRs), since they were first reported in 1985 by Orbán et al. However, while these systems are interesting as biomimetic models, their ability to function in biological environments is limited, since they must be continuously supplied with toxic redox reactants, as these are irreversible reactions and they generate accumulated waste products from the oscillating reaction. For application in biological systems, among the many oscillating materials that can be developed for improved synthetic pH chemical oscillators, viologen is a good candidate due to its reversible redox behavior under a defined electrical potential. Viologen can be used to control the pH over a wide range in solution in a batch reactor, without the need for continuously supplied toxic redox reactants and the production of waste products. To improve a viologen pH oscillatory system substantially, an ionic liquid (IL) is required in a water-binary system. In this paper, reversible and robust electrochemical pH oscillator has been achieved using an ethyl viologen / IL aqueous solution under an applied redox potential in a batch reactor, where the IL incorporated into the pH oscillator increased the stability of the pH oscillation by acting as electron buffer solution.
9:00 PM - LL3.2
Effects of Poly(vinyl alcohol)(PVA) and Poly(ethylene glycol) Formulations on PVA Theta-gel Characteristics.
Jeeyoung Choi 1 2 , Hatice Bodugoz-Senturk 1 2 , Hsiang Kung 1 , Celia Macias 1 , Orhun Muratoglu 1 2 Show Abstract
1 Department of Orthopaedic Surgery, Harris Orthopaedic Biomechanics and Biomaterials Laboratory, Massachusetts General Hospital, Boston, Massachusetts, United States, 2 , Harvard Medical School, Boston, Massachusetts, United States
Poly(vinyl alcohol) (PVA) hydrogel is a candidate biomaterial for use as an artificial replacement of cartilage and meniscus of the knee joint or the nucleus pulposus of the intervertebral disc. Physically-crosslinked PVA hydrogels can be prepared by the well-known cryogel method  or the lesser-known theta-gel method . Whereas PVA cryogel is based on freezing and thawing process during which formation of ice crystals in PVA aqueous solutions brings PVA molecules into close vicinity to hydrogen-bond, PVA theta-gel is formed by thermally-induced phase separation in PVA solutions where PVA solvency of water is reduced in presence of low molecular weight poly(ethylene glycol) (PEG). PVA molecules confined in PVA rich phases are ordered into hydrogen-bonded PVA crystals, constructing a porous hydrogel structure where the pores contain water/PEG mixture. We hypothesized that the formulation parameters, namely, concentration and molecular weight (MW) of PVA and PEG would affect such gelation dynamics and ultimately, the resulting hydrogel properties. The effects of PVA and PEG formulations on the theta-gel characteristics were quantified by determining the equilibrium water content (EWC), creep resistance, and porosity. Increasing PVA (MW=200,000 g/mol) concentration from 15% to 30% with constant PEG (MW=400 g/mol) concentration decreased EWC from 90% to 76%, significantly lowering total creep strain (measured after 10hours of creep under 0.4MPa stress) from 86% to 56%, and reducing pore size (measured with a confocal microscope) from 10~20µm to <1µm. Similar trends were found with higher MW PVAs. Increasing PVA MW within the range of 137,000 ~ 400,000 g/mol with constant PVA and PEG concentrations did not change EWC or creep resistance, although the pore size was larger with higher MW PVA. Increasing PEG concentration from 25% to 28% decreased the pore size, EWC and total creep strain, while higher PEG concentrations up to 33% did not further alter the gel properties. Microstructure of PVA-theta gels was more sensitive to changes in PVA and PEG formulations than macroscopic gel properties such as creep resistance. Creep resistance of the PVA-theta gel showed strong correlation with EWC.  Hassan et al., Adv Polymer Sci 2000;  Ruberti et al., US Pat. Pub. No. 20040092653, 2004.
9:00 PM - LL3.3
Effects of Solvent Dehydration on Creep Resistance of Poly(vinyl alcohol) Hydrogels.
Jeeyoung Choi 1 2 , Hatice Bodugoz-Senturk 1 2 , Hsiang Kung 1 , Arnaz Malhi 1 , Orhun Muratoglu 1 2 Show Abstract
1 Department of Orthopaedic Surgery, Harris Orthopaedic Biomechanics and Biomaterials Laboratory, Massachusetts General Hospital, Boston, Massachusetts, United States, 2 , Harvard Medical School, Boston, Massachusetts, United States
As a synthetic replacement material for osteochondral defect repair, poly(vinyl alcohol) (PVA) hydrogels offer a great potential due to their high water content and strong mechanical integrity. To survive the high stress environment in the joint space where the axial loads can be as high as five times the body weight, high creep resistance becomes one of the key requirements for hydrogel implants. We hypothesized that reducing the equilibrium water content (EWC) of hydrogels would improve their creep resistance gauged by total creep strain (TCS, measured after 10 hours of creep under 0.4MPa stress). One method of reducing EWC of the hydrogel is through dehydration by soaking in a solvent followed by rehydration. In this study, we applied such methods on physically-crosslinked PVA theta-gels  made by thermally-induced phase separation in PVA solutions where PVA solvency of water is reduced in presence of low molecular weight poly(ethylene glycol) (PEG). Four solvents were investigated as dehydrating media, namely, 100% PEG400 (Molecular weight=400 g/mol), isopropyl alcohol (IPA), saturated aqueous sodium chloride (NaCl) solution, and 90% PEG600 (MW=600 g/mol) aqueous solution. PVA theta-gels were immersed in the respective solvents “as-gelled”, (immediately upon gelling) or “dePEGed” (first immersed in a 0.9% saline solution for removal of PEG400 from the hydrogel prior to immersion in the respective solvents). Subsequent to the equilibrium dehydration in respective solvents, hydrogels were immersed in a 0.9% saline solution for solvent removal and rehydration. This completed one cycle of solvent-dehydration/rehydration, which was sequentially repeated as many as four times in total on the same hydrogel samples. The results showed that the first dehydration cycle of the as-gelled hydrogel group in any of the solvents did not show significant changes in EWC or TCS, whereas large decreases in EWC and TCS were observed after the first dehydration cycle of the dePEGed hydrogel group for all solvents except for NaCl solution. The repeated dehydration-rehydration cycles could further reduce EWC and TCS in the dePEGed PVA gels, while as-gelled PVA gels remained much less responsive to repeated cycles than dePEGed ones. Overall, solvent dehydration was more effective with the dePEGed gels than the as-gelled ones and a working molecular model is presented to explain this phenomenon. The most effective medium was isopropyl alcohol for reducing the EWC and increasing the creep resistance of PVA theta-gels. There was a strong positive correlation between total creep strain and the EWC of the hydrogels in both the as-gelled and dePEGed groups, which suggests that the solvent dehydration is a useful tool in the fabrication of hydrogel medical implants requiring improved creep resistance and improved mechanical properties.  Ruberti et al., US Pat. Pub. No. 20040092653, 2004.
9:00 PM - LL3.5
Mechanical Properties and Drug Release Behavior of Biopolymer-based Semi-IPN Hydrogels.
Soumitra Choudhary 1 , Surita Bhatia 1 Show Abstract
1 Chemical Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts, United States
9:00 PM - LL3.6
Novel Applications of the QCM Technique in Biomaterials Science.
Candida Silva 1 , David Lin 1 , Iren Horkayne-Szakaly 1 , Peter Basser 1 , Ferenc Horkay 1 Show Abstract
1 NICHD, NIH, Bethesda, Maryland, United States
Biosensors based on acoustic waves propagating in piezoelectric quartz crystals, have been extensively studied in the past decade. The quartz crystal microbalance (QCM) proved to be a sensitive device for investigating biopolymer systems at the solution-surface interface. It detects the mass change as a shift in the resonant frequency with sensitivity in the ng/cm2 range. The QCM can be used to determine monolayer surface coverage by either small molecules or polymers. The technique is also capable of detecting much larger masses (e.g., biological tissues and cells) attached to the surface of the quartz crystal. Moreover, QCM provides information about the energy dissipating properties of the bound material. However, interpretation of the change in the resonance frequency can be ambiguous when the resonator is loaded with inhomogeneous biological materials. In order to interpret the QCM results in terms of the mechanical properties of the attached sample it is necessary to describe the experimental situation using an appropriate physical model. The development of such a model requires a detailed understanding of the dependence of the QCM response on the physical properties of the specimen (e.g., rigidity, heterogeneity, degree of swelling). While the QCM technique has been used extensively to investigate adsorption from solution, only few studies have utilized its ability to detect controlled changes in the vapor pressure of the surrounding environment. Here we report three applications of the QCM in the latter area. (i)The QCM with dissipation monitoring capability has been used to investigate the osmotic properties of biological tissues. This method allows us to determine the thermodynamic response of small specimens (< 100 ng). We present osmotic swelling pressure data obtained for a variety of tissue samples (engineered cartilage, mouse cartilage, etc.).(ii)We utilize the surface sensitivity of the QCM to study the effect of film rigidity on the QCM signal. Experimental results measured on thin poly(vinyl acetate) films over the glass transition range are reported. The film thickness is varied from 10 to 1000 nm. The data are analyzed by fitting the frequency shift, and dissipation change to a viscoelastic model. (iii)The formation of thin polymer films on surfaces is attractive for many practical applications such as surface modification of certain materials (e.g., encapsulation of active components in controlled release formulations). We demonstrate that QCM is a powerful tool to study the kinetics of water uptake by layered polymeric structures composed of immiscible polymers. Our preliminary measurements indicate that ultrathin polymer films behave as a barrier for the hydration of polymer molecules previously under its layer. These experiments provide valuable hints about the contribution of aqueous compartments that are known to exist in inhomogeneous biological tissues.
9:00 PM - LL3.7
Modified Microparticles of Calcium Alginate Gel for Controlled Release of Anesthetics.
Erkesh Batyrbekov 1 , Rinat Iskakov 1 , Turar Akylbekova 1 Show Abstract
1 Polymer Department, Institute of Chemical Sciences, Almaty Kazakhstan
9:00 PM - LL3.8
Sol-Gel Transition by X-ray Irradiation.
Byung Mook Weon 1 , Keun Ho Lee 1 , Jung Ho Je 1 , Yeukuang Hwu 2 , Giorgio Margaritondo 3 Show Abstract
1 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, GyungBuk, Korea (the Republic of), 2 Institute of Physics, Academia Sinica, Taipei Taiwan, 3 , Ecole Polytechnique Fádárale de Lausanne, Lausanne Switzerland
Stimuli-responsive transitions become a key requirement for intelligent materials, and recently a central issue in diverse fields including materials science. Hydrogels, which are three-dimensional networks of polymer chains, are known to exhibit transitions in response to various stimuli such as electric field, temperature, pH, concentration, enzyme, electron beam, sound, and light. Hydrogels are actively studied with the aim of creating future technologies for the control of fluidity, viscoelasticity, solvent volatility, optical transmission, ion transport, and so on. Light irradiation among various stimuli, is, in particular, an attractive candidate for a “remote stimulus”, yet versatile remote control remains a challenge, when applied to a transition into a thick opaque object, mostly due to weak penetration power by long wavelengths of lights such as ultraviolet and visible lights.To make a breakthrough, we use X-ray irradiation to trigger a gel-to-sol transition in a polyethylene glycol (PEG)-based hydrogel. The PEG-based hydrogels have been widely studied for biotechnological applications because of their high biocompatibility, hydrophilicity, and versatility. X-ray irradiation has great advantage of high penetration by short wavelength of X-rays. This work is the first demonstration of X-ray-responsive sol-gel transition in hydrogel. Our idea is based on a hypothesis that hydrogen-based products will proliferate in hydrogels through X-ray radiolysis of water, during a brief irradiation with high energy X-rays (10–60 keV). Accurate imaging and analysis provide evidence for an X-ray-responsive sol-gel transition, taken by using synchrotron X-ray microradiography (from the PLS 7B2 beamline in Pohang, Korea). The X-ray-responsive gel-to-sol transition is attributed to an increasing solubility of the hydrogel in water owing to the strong hydrogen bonding induced by X-ray radiolysis.This work opens a remote, versatile control way of sol-gel transitions using X-ray irradiation, which will be applicable even into a thick object like a human body.
9:00 PM - LL3.9
A New Class of Biodegradable Polymer Nanocomposites.
Seongchan Park 1 , Ezra Bobo 2 , Jonathan Sokolov 1 , Miriam Rafailovich 1 Show Abstract
1 Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States, 2 , University of Pennsylvania, Philadelphia, Pennsylvania, United States
Due to increasing environmental concerns there has been much effort with developing new polymer blends which are biocompatible. Since starch is a traditionally primary component, many of these blends are very brittle and hence can not be used for structural products where high impact is desired. Here we show that a new set of compounds can be formed using the high amylase corn-based starch compounded with glycerol at room temperature and then the compounds was blended with the aliphatic-aromatic copolyester, another biodegradable polymer which is flexible and impact resistant. Even though these two polymers are immiscible, the blends can be produced using natural clays which are absorbed with RDP, a phosphorus-based flame retardant agent. Here we correlated dynamic mechanical analysis data, tensile testing, with biodegradation properties.
Ferenc Horkay National Institutes of Health
Noshir A. Langrana Rutgers University
Anthony J. Ryan The University of Sheffield
J. David Londono DuPont de Nemours
LL4: Molecular Conformation and Self-Assembly
Tuesday AM, November 27, 2007
Room 207 (Hynes)
9:30 AM - **LL4.1
Proteins Behaving Badly - Aggregation and Gelation.
Athene Donald 1 Show Abstract
1 Cavendish Laboratory, Cambridge University, Cambridge United Kingdom
The ability of proteins to form amyloid fibrils is well documented. These are usually found to form away from the protein’s isoelectric point under conditions of partial denaturation (often heat). As well as individual amyloid fibrils themselves, we have recently found a suprafibrillar aggregate, structurally similar to synthetic polymer spherulites, which appears to consist of amyloid fibrils growing outwards from some amorphous core. These form under essentially the same pH conditions as the fibrils. Many proteins appear to form these structures, but the propensity of different proteins to form the spherulites does vary, as do the details of the perfection of the internal packing. Around the isoelectric point itself we find that many proteins form rather monodisperse particulates. This structure, although previously only documented in detail for beta-lactoglobulin, can also therefore be considered as generic. Under some conditions these particles are also found to contain some beta sheet elements, the key element of the amyloid fibril.Gelation is a term often used to describe the sample both when amyloid fibrils or particulates form, although the nature of the gel is not usually discussed, and rheological measurements are rarely attempted. We are exploring the nature of the gels that form, to see to what extent they are more than simply complex fluids, using techniques such as particle tracking during the gelation process itself to determine the nature of the viscoelastic parameters. Only by understanding to what extent these materials really are gels, will we fully be able to utilize them.
10:00 AM - **LL4.2
Folding of RNA, SAXS Studies of a Model System.
Robert Briber 1 , Joon-Ho Roh 1 2 , Gokhan Caliksan 2 , Seema Chauhan 2 , Sarah Woodson 2 , D. Thirumalai 4 , Liang Guo 3 , Sarvin Moghaddam 1 4 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 T. C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland, United States, 4 Institute for Physical Science and Technology, University of Maryland, College Park, Maryland, United States, 3 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States
Small Angle X-Ray Scattering (SAXS) from RNA aqueous solutions has been used to observe the size of the Azoarcus ribozyme as a function of Mg2+ concentration. The equilibrium pathway for Azoarcus folding in the presence of Mg2+ involves collapse to an intermediate state IC at about 0.5mM Mg2+ and folding to the final native tertiary structure, N, which occurs at about 2mM Mg2+. Our results have shown there is a collapse to a compact state for Azoarcus that corresponds to the IC state and we see no large changes in Rg for the IC -> N transition. We have used singular value decomposition analysis to establish that a two-state transition can be used to model the folding process for Azoarcus. We have also studied the effect of tertiary interactions on the collapse transition by disrupting of the interactions through mutations. Our measurements indicate that the tertiary interactions play an important role in the stabilization of the collapsed state with the midpoint of the collapse transition increasing significantly and the transition becoming less cooperative. More recent work on the kinetics of folding as measured by stopped-flow SAXS will be discussed as time permits. The SAXS measurements were carried out at Argonne National Lab Advanced Photon Source BIOCAT beamline ID18. X-rays with 1.05 Å wavelength were used (11.8 keV). A momentum transfer range from ~0.007 to ~0.266 Å-1 covered. RNA stocks were diluted to 0.4 mg/ml in 20 mM Tris (pH 7.5) plus 0-10 mM MgCl2, then incubated 5 min at 50°C before use. For Mg2+ titrations, RNA solutions in 0 and 10 mM Mg2+ were mixed using a Hamilton syringe pump, with a 5 min incubation at 32°C after each addition. To minimize damage from hydroxyl radicals, the sample was passed through a quartz capillary flow cell with 1.5 mm inner diameter at a flow rate of 7.7μl/sec. Measurements at various flow rates showed that the radiation damage to the samples was negligible under these conditions. Each measurement was the average of four 2s exposures. Stopped-flow measurements were done using a Biologics SFM-400 and a CCD camera in streak mode with approximately 4ms time resolution. The SAXS data were radially averaged and corrected for the background signal due to the buffer.
11:00 AM - **LL4.3
Gels, Networks, and Measurement Tools.
Eric Amis 1 Show Abstract
1 Materials Science and Engineering Laboratory, NIST, Gaithersburg, Maryland, United States
11:30 AM - **LL4.4
Bacterial Exopolymer Nucleate the Formation of Self Assembled Marine Microgels.
Pedro Verdugo 1 2 , Chin Wei-Chun 3 , Ding Yong-Xue 2 , Hung Chin-Chang 4 , Peter Santschi 4 Show Abstract
1 Bioengineering, University of Washington, Friday Harbor, WA, Washington, United States, 2 Bioengineering, University of Washington Friday Harbor Labs, Friday Harbor, Washington, United States, 3 School of Engineering, University of California, Merced, Merced, California, United States, 4 Department of Marine Sciences and Oceanography, Texas A&M University, Galverston, Texas, United States
The oceans hold the second most important photosynthetic agents in our planet and contain in one of the biggest stocks of reduced carbon in our planet. Carbon is largely found as free moieties in the dissolved organic matter (DOM) pool. DOM is composed of a polydisperse pool of biopolymers that although present at micromolar concentrations reach a total mass of 7×10^17 g of carbon. The discovery that DOM polymers can reversibly self-assemble forming microscopic gels with e yield at equilibrium that reach about 70 Gt of carbon (Chin et al, Nature 1998) has broad implications for the understanding of how carbon is cycled in the ocean (Wells, Nature 1998, Verdugo et al, Mar Chem 2005). Marine self-assembled microgels are physical gels containing a polymer network stabilized not by covalent bonds but by tangles and Ca cross-links. Although microgel formation can increase the microbial susceptibility of DOM (Wells, Nature 1998), the interaction of exopolymers released by bacteria (BE) and formation of microgel still remains largely unexplored. Using exopolymers released by the marine bacteria Sagitulla stellata as a model, our results show that nanomolar concentrations of BE can drastically promote the self-assembly of DOC polymers. Sagitulla expolymer can switch self-assembly of DOM polymer from a second to a first order kinetics, and turn microgel formation independent of Ca cross-linking. An alternative source of low energy polymer cross-linking of is via the formation of hydrophobic bonds. Sagitulla expolymers can self-assemble in a Ca-independent, temperature-dependent mode, strongly suggesting that SE cross-linking might result not from electrostatic but from hydrophobic bonding. Our studies using fluorescence Energy Transfer (FET), ESCA, and To-SIMS show that Sagitulla expolymer chains do indeed contain hydrophobic domains that could potentially drive their own assembly as well as binding to solid surfaces, and nucleate the assembly of DOC. These observations suggest that BE can form self-assembling networks that could serve to bind bacteria to specific substrates. In seawater however, bacteria need not to release a large amount of BE to create polymer networks since trace amounts of BE can efficiently recruit polymers found in the DOC pool to nucleate the formation of microgels. These findings open a new frontier of exploration to understand the microgel/bacteria interaction in the ocean and the role of bacteria in marine cycling of carbon. Bacteria could colonize preexisting gels. However, by releasing exopolymers they might as well nucleate the assembly of DOM polymers and the formation of microgels, securing a rich source of substrate. Our results further provide intriguing new paradigm to investigate both the mechanisms of biofilm formation and a model for the development of nucleating agents for water treatment.
12:00 PM - LL4.5
Dynamics and Transport of Molecules in Polymer and Colloidal-Rod Networks.
Kyongok Kang 1 Show Abstract
1 IFF-Weiche Materie, Forschungszentrum Juelich, Juelich, NRW, Germany
Re-orientational dynamics of liquid crystal molecules in a polymer network subjected to an electric field is studied by means of light diffraction . When the optical pitch of the electric-field induced cholesteric phase is small compared as the optical wavelength of light, i.e. 0.5-1.5 um, dynamic light scattering (DLS) can be performed to extract the relaxation dynamics of the chiral nematic molecules in the presence of the polymer network. Intriguingly, the reactive mesogenic type of polymer network underlies the confinement effect, which is within the limited range of scattering angles that comply with the structural correlation length in the system, as compared to the isotropic polymer network in the same host liquid crystals . Diffusive transport of molecules through a rod network can be studied via fluorescence correlation spectroscopy (FCS) and DLS. Long time self-diffusion of tracer spheres (silica and proteins) in isotropic and nematic colloidal-rod networks (fd-viruses) is systematically studied for various tracer-sphere sizes as compared to the mesh size of the network . In addition, by varying the salt concentration, the relative contribution of electrostatic interactions can be tuned, from which the effect of screened hydrodynamic interactions between the tracer sphere and the rods is quantified [4-5]. We are also interested in an in-situ electric field DLS setup combined with SALS (small angle static light scattering) and electric birefringence to investigate the dynamics (both translational and rotational motion) and structures of macromolecules and colloidal rods (fd viruses and its mixtures with isotropic and mesogenic monomers) as induced by the electric field. K. Kang, L. C. Chien, and S. Sprunt, Liquid Crystals, Vol. 29, No. 1, 9-18, 2002. K. Kang, S. Sprunt, Phys. Rev. E. 72, 031702, 2005. K. Kang et al, J. Chem. Phys. 122, 044905, 2005. K. Kang et al, J. Chem. Phys., 124, 044907, 2006. K. Kang, A. Wilk, A. Patkowski, Jan K. G. Dhont, J. Chem. Phys. 126, 214501, 2007.
12:15 PM - LL4.6
Polymer Gel Transducers as an Interface for Optical Detection of Biological Molecules.
Sven Tierney 1 , Bjorn Stokke 1 , Dag Hjelme 2 Show Abstract
1 Physics Department, Norwegian University of Science and Technology,NTNU, Trondheim Norway, 2 , Invivo Sense, Trondheim Norway
Hydrogels are crosslinked, three-dimensional hydrophilic polymer networks, which swell but do not dissolve when brought into contact with water. These materials have attracted considerable attention in biochemical and biomedical fields, due to volume variations as a response to environmental changes such as ionic strength, pH, temperature and surfactants. In recent years, hydrogels have been synthesized incorporating biological active molecules, making theses gels sensitive also to specific molecules. Here we present a novel technique for detection of hydrogel swelling which is intended to be used as a chemical/biological sensor. Briefly, the sensor consists of a hydrogel bound to the end of an optical fiber, which in turn is connected to a detector. Light sent continuously from the detector reflected both at the fiber-gel and gel-solution surfaces to an interference wave. Swelling changes in the bound gel alter the reflected interference wave thus enabling optical detection. The gel is approximately 60µm in length, and due to the miniature size of the sensor, it is aimed to be used for in vivo measurements. Preliminary experiments have proven the instrument to be highly sensitive, where length changes of ~4nm are detectable. Furthermore the detection rate is 0.98 seconds, making it is possible to measure the swelling kinetics of the gel.The synthetic hydrogels utilised as transducers are acrylamide based, and so far we have managed to fabricate a glucose sensitive hydrogel. This has been achieved by incorporating a phenylboronic monomer into the gel. Under physiological conditions, however, phenylboronic acid does not react very well with glucose. We have overcome this problem by incorporating another co-monomer into the gel which has a large impact on the reactivity towards glucose. The blood sugar level for healthy humans is in the region 4-8mM, and with this improved sensor, we are able to monitor glucose concentrations in the micromolar region.
12:30 PM - LL4.7
Electrochemical Biosensors Using Covalently Coimmobilized Glucose Oxidase and Ferrocene Mediators Within Hydrogel Films on Microdic Electrode Arrays.
Walter Torres 1 2 , Anthony Guiseppi-Elie 1 2 Show Abstract
1 Center for Bioelectronics, Biosensors, and Biochips, Clemson University, Anderson, South Carolina, United States, 2 Chemical and Biomolecular Engineering, Clemson University, Clemson, South Carolina, United States
12:45 PM - LL4.8
Mechanical Properties of Central Force Networks.
Haiyi Liang 1 , Alexandre Kabla 1 , Matthieu Wyart 1 , L. Mahadevan 1 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Central force networks arise naturally in the study of granular and fibrous materials with implications for the mechanical behavior of hierarchical materials such as nano-composites, cytoskeleton mechanics and tissue engineering. Using simulations based on a damped molecular dynamics approach, we study the soft and stiff modes of deformation in both Gaussian and Poissonian networks of random linear springs with the goal of understanding the onset of elasticity, and the eventual strain-stiffening in these networks. Although the individual springs are linear, we see that collectively the systems are nonlinear due to the effects of rotation and alignment. For small to intermediate deformations, we observe that the systems have negative Poisson's ratio. We observe a strong dependence of the above quantities on the coordination number in systems, which effect may be rationalized using some classical arguments going back to the work of Maxwell. Floppy (almost zero energy) modes play a central role in the deformation process, and are responsible for both the softening and stiffening behavior of these networks; in fact strain stiffening is due to the annihilation of these floppy modes. Some implications of these results for the design and behavior of hierarchical materials will be outlined.
LL5: Biological and Biomimetic Networks
Ronald A. Siegel
Tuesday PM, November 27, 2007
Room 207 (Hynes)
2:30 PM - **LL5.1
Cartilage Biopolymeric Networks and Macromolecules.
Alan Grodzinsky 1 2 3 , H. Lee 1 , C. Ortiz 4 , J. Kisiday 4 , D. Chai 3 , P. Kopesky 3 Show Abstract
1 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 3 Biological Engineering, MIT, Cambridge, Massachusetts, United States, 4 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Cells in articular cartilage synthesize an extracellular matrix (ECM) consisting of many members of the proteoglycan and collagen superfamilies, as well as other non-collagenous proteins, to create a dense biopolymeric tissue that can withstand joint loading in vivo. Tissue engineered constructs for cartilage repair often utilize cell-seeded hydrogel scaffolds in an attempt to synthesize a cartilage-like matrix that can integrate with adjacent normal tissue. Since there are not enough primary chondrocytes within native cartilage to enable autologous repair, investigators have increasingly used progenitor cells, e.g., bone marrow-derived progenitors (MPCs), in combination with growth factor and mechanical stimulation to achieve chondrogenesis. Mechanical forces acting on chondrocytes in vivo, and in tissue explants and tissue engineering constructs in vitro, can differentially regulate the gene expression and biosynthesis of ECM macromolecules, as well as their precise nanomolecular structure. This study compares the biomechanical properties and biochemical composition of native cartilage to that of tissue engineered constructs made using a self-assembling peptide hydrogel with equine marrow-derived cells. MPCs were seeded in 0.35% KLD12 peptide hydrogel at a density of 107 cells/mL, and the hydrogel cultures were maintained in medium supplemented with 10 ng/mL TGF-b1. Total GAG retained in the constructs and lost to the media were quantified, and proteoglycans extracted from the hydrogel scaffold were examined via AFM imaging to study the structure of aggrecan-like morphologies as a measure of chondrogenesis of the MPCs. The equilibrium moduli of the macroscopic constructs were compared to that of native tissue. In parallel studies, the effects of mechanical loading on the gene expression of aggrecan and other ECM molecules, proteolytic enzymes, growth factors and cytokines, were studied in cartilage organ culture explants as well as in agarose hydrogel disk scaffolds seeded with primary chondrocytes. We observed that changes in expression were very sensitive to the type of applied mechanical stimulus; in particular, static versus dynamic mechanical compression showed distinctly different trends in expression. These results are consistent with the known mechano-biological feedback pathways that are critical for maintenance of healthy cartilage, and for the success of cartilage tissue engineered constructs being developed for the repair of cartilage defects and diseases such as osteoarthritis.
3:00 PM - **LL5.2
A Polymer Physics/Materials Science Approach to Explain the Material Properties of Cartilage.
Peter Basser 1 , David Lin 1 , Candida Silva 1 , Iren Horkayne-Szakaly 1 , Ferenc Horkay 1 Show Abstract
1 , NIH, Bethesda, Maryland, United States
Several paradigms have contributed to our understanding of structure/function relationships in extracellular matrix (ECM) in general, and in cartilage in particular. One is the "biomechanical/geophysical" approach, in which cartilage is modeled as a poroelastic medium, like an elastic saturated rock, having interpenetrating fluid and solid phases. This macrocontinuum modeling approach has been extended to including charged ions in the bathing medium and fixed charges decorating the elastic network, much like a charged clay. The biomechanical/geophysical approach has been successful in describing certain phenomena, such as the time constant for a tissue to drain under different loading conditions, the origin of electrokinetic flows and forces, and a greater understanding of cartilage lubrication.Another paradigm is the "chemical engineering/physical chemistry" approach, in which the material properties of the tissue are ascribed primarily to osmotic forces arising within and among different macromolecular compartments within the tissue. Here, ECM is viewed as a composite medium having macromolecular-sized collagen network that traps highly charged polysaccharides within them. It is useful in explaining changes in cartilage functional properties following different biochemical interventions and in aging and degeneration,A complementary paradigm we have been promulgating is the "polymer physics/material science" approach, in which we attempt to explain ECM's material properties using a multi-scale experimental and theoretical framework focused on studying properties of the constituents of ECM and cartilage and their interactions at a different length scales. This approach is being used to address several interesting biological questions, such as why cartilage remains mechanically stable unlike other anionic polyelectrolyte gels that collapse in the presence of high calcium concentrations present in joints, (particularly near the bone-cartilage interface); whether there are cross-links between the collagen and polysaccharides they entrap and how would such cross-linking affect the tissues mechanical properties; whether one can derive a macroscopic constitutive law of cartilage that is consistent with molecular and macromolecular interactions among collagen, proteoglycans, and water, and which provides a framework for calculating stress and strain distributions within the tissue based upon the deformation of all constituents. In general, a goal is to understand the underlying physics of load bearing and joint lubrication in development, aging and degeneration. This talk will review some work being performed in our laboratory that incorporates molecular, macromolecular, mesoscopic and macroscropic measurements on systems ranging from individual cartilage constituents to the tissue itself, leading to a self consistent physical understanding of tissue structure/function relationships.
4:00 PM - **LL5.3
Bernard Yurke 1 Show Abstract
1 , Bell Laboratories, Murray Hill, New Jersey, United States
Polyacrylamide hydrogels can be given novel functionality through the incorporation of DNA-based nanostructures as crosslinks. This has allowed the construction of DNA-responsive gels which reversibly assemble and disassemble or which reversibly change stiffness in response to externally applied DNA. Such gels may be viewed as biomimics of cytoplasm which can undergo sol-gel transformations or stiffness change in response to external stimuli. Such gels may find application in drug delivery, cell culture, and tissue engineering.The incorporation of DNA in hydrogels is of particular interest, since it allows the exploitation of DNA nanotechnology to create gels with enhanced functionality. DNA has proven to be a versatile material for the construction of nanostructures, nanodevices, and molecular motors. A property of DNA that has facilitated the construction of such DNA-based devices is the fact that two DNA strands bind with each other most strongly if their base sequences are complementary. This provides a kind of recognition property that allows the design of sets of DNA strands which spontaneously self-assemble into a desired nanostructure. The incorporation of such nanostructures within a polyacrylamide gel is facilitated by the availability of Acrydite-functionalized oligomers, which are readily incorporated as side chains in polyacrylamide during polymerization. Two properties of DNA greatly facilitate the design of DNA-responsive gels. The first is the fact that duplex DNA is stiffer than single-stranded DNA. This allows the construction of structural units that straighten and stiffen when a region of single-stranded DNA is converted to duplex through hybridization with another strand of DNA. The second is that one member of duplex DNA can be displaced from the second member by a DNA strand that can form more base pairs with the second member. This process is called strand displacement. These two properties allow the construction of nanodevices that can be cycled between two or more distinct states. They allow the construction of gels that can be induced to undergo sol-gel transformations through the application of particular DNA strands. They also allow the construction of gels that undergo stiffness changes in response to particular DNA strands.DNA-crosslinked gels that can be reversibly assembled and disassembled at any temperature between the freezing point of water and the melting point of DNA may be useful in drug packaging and delivery. DNA-crosslinked gels that can undergo reversible stiffness changes may find application in cell culture and tissue engineering.
4:30 PM - **LL5.4
Multiscale Approaches to Cartilage Tissue Engineering Using Alginate Hydrogels.
Lawrence Bonassar 1 Show Abstract
1 Biomedical Engineering, Cornell University, Ithaca, New York, United States
5:00 PM - LL5.5
The Effect of Network Architecture on the Mechanics of Three-dimensional Cross-linked Actin Networks.
Patrick Onck 1 , Liesbeth Huisman 1 , Teun Van Dillen 1 , Erik Van der Giessen 1 Show Abstract
1 Micromechanics of Materials, Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
The mechanics of biological tissues and gels play an important role in biological functions such as cell motility and mechanotransduction. These materials respond to deformation by exhibiting an increasing stiffness. This has been demonstrated by micropipette experiments on individual cells and through rheological experiments on in-vitro cytoskeletal gels and extracellular matrices such as fibrin and collagen. One of the most prevalent protein filaments in eukaryotic cells is F-actin which, when cross-linked, defines the actin cortex. In this work we study the mechanical response of cross-linked F-actin when subjected to large shear deformations. Theoretical studies either consider networks that distort in an affine manner (e.g. ), or are based on a two-dimensional representation of the network structure (e.g. ). In contrast to the existing literature, our numerical model is based on an explicit representation of the discrete, three-dimensional network architecture .The generation starts by placing filaments at random positions and orientations inside a fully periodic unit cell. Each filament is discretized using beam elements, accounting for bending, twisting and stretching. To cross-link the filaments, an attractive force field is applied between the nodes of the filaments. When two nodes approach each other within a certain cut-off distance, a rigid cross-link of zero length is formed between these nodes. This procedure enables us to generate networks of realistic architectures, with control of the actin concentration, cross-link concentration and the network topology. The generated periodic network serves as a representative volume element (RVE) and is subjected to a macroscopic shear strain. Simulation results show that the shear stress increases with increasing shear strain. The associated stiffening is caused by nonaffine network rearrangements that govern a transition from a bending-dominated response at small strains to a stretching-dominated response at large strains. By comparing the response for straight and undulated filaments it is found that filament undulations only play a role when the persistence length is on the order of the distance between cross-links. Furthermore, the discrete representation of the network allows to independently investigate the effect of the filament length, at constant actin concentration and cross-link distance. The results show a pronounced increase in stiffness with increasing filament length, which originates from a change in cross-link connectivity, being larger for longer filaments. References C. Storm, J.J. Pastore, F.C. MacKintosh, T.C. Lubensky, P.A. Janmey, Nature 435, 191 (2005). P.R. Onck, T. Koeman, T. Van Dillen, E. Van der Giessen, Phys. Rev. Lett. 95, 178102 (2005). E.M. Huisman, T. van Dillen, P.R. Onck, and E. Van der Giessen, Three-dimensional cross-linked F-actin networks: relation between network architecture and mechanical behavior (under review).
5:15 PM - LL5.6
Constitutive Modeling of the Stress-Strain Behavior of F-actin Filament Networks.
Jeffrey Palmer 1 2 , Mary Boyce 1 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , MIT Lincoln Laboratory, Lexington, Massachusetts, United States
The central role of the cytoskeleton in both healthy and diseased cellular functions makes it a compelling subject for detailed three-dimensional (3D) micromechanical modeling. Microstructural features of the cytoskeleton govern the cell’s mechanical behavior in many of the regulating cellular functions including cell division, adhesion, spreading, migration, contraction, and other mechanotransductive effects which influence many biochemical processes such as gene expression. Actin microfilaments (AF) combine to form one of the predominant cytoskeletal filament networks during these biological processes. Here, the AF cytoskeletal microstructure and stress-strain behavior is modeled via a microstructurally-informed continuum mechanics approach using a variant of the worm-like chain (WLC) constitutive relationship for the force-extension behavior of individual chains in conjunction with the Arruda-Boyce eight-chain network model to capture the 3D multiaxial stress-strain behavior. The 3D cytoskeletal network constitutive model presented provides the ability to track and adjust microstructural stretch and orientation states under macroscopic stretching conditions, which allows the model to effectively simulate load sharing among actin filaments and the accommodation of the shear strain through filament rotation and a small amount of filament stretch. These characteristics enable the WLC network model, using physically realistic material properties, to capture the AF network’s initial stiffness as well as the complex nonlinear strain stiffening response at large stresses. The WLC network model predictions, when enhanced to include bundling effects, compare favorably with published microrheological data of in vitro AF networks cross-linked and bundled with scruin cross-linking proteins at varying concentrations. The overall constitutive framework enables predictions of large-strain multi-axial deformation states of 3D isotropic F-actin filament networks, and can be extended to model in vivo F-actin networks or in vitro networks of other filaments once updated with the proper material properties.
5:30 PM - LL5.7
Elasticity Models for the Spherical Indentation of Gels and Soft Biological Tissues.
David Lin 1 , Emilios Dimitriadis 1 , Ferenc Horkay 1 Show Abstract
1 , National Institutes of Health, Bethesda, Maryland, United States
In numerical simulations or uniaxial and biaxial mechanical tests, polymer gels and biological tissues are often modeled successfully using linear elasticity theory at small strains and rubber elasticity theory at large strains. For measurement of elasticity at micron and submicron length scales, the prevalence of atomic force microscopy in materials research has allowed nanoindentation to become one of the leading techniques. However, despite advancements in instrumentation and analysis methods, its accuracy when applied to soft matter remains equivocal. One source of this uncertainty is the lack of practicable nonlinear contact mechanics models; many investigators rely on models based on the Hertz theory to analyze force curves. Consequently, errors are frequently incurred by applying these linear elastic representations beyond their functional range or at the small-strain range where the indentation process is most prone to noise. In this work, we developed approximate relationships for the spherical indentation of rubber-like materials. Starting with the Hertz equation and the three-term Ogden constitutive model for uniaxial loading of rubber elastic materials, a force-indentation equation was formulated. Mooney-Rivlin, Neo-Hookean, and single-term Ogden forms of the equation were then derived as special cases of the Ogden model. We tested each equation by fitting it to data obtained from the large-strain indentation of swollen poly(vinyl alcohol) gels and of native and tissue-engineered cartilage samples. The average residual error of each fit was calculated and extracted values of the initial Young’s modulus were compared with those obtained by utilizing the Hertz equation at small strains. In the case of the synthetic gels, results from macroscopic compression testing served as standards for appraising accuracy. The three-term Ogden model provided the best fit of the indentation data. However, the computational expense was concomitant with its considerable complexity relative to the other models. The Mooney-Rivlin and single-term Ogden equations were found to be suitable for fitting most datasets at much lower expense. Because it is a special case of each of the other models, the Neo-Hookean model provided poorer fits by comparison; the results suggest that it is still capable of representing the nonlinear stress-strain behavior of the tested materials. Finally, the Hertz model proved to be acceptable at small strains (<15% for the samples tested), with the stipulation that the retained portions of the data exhibit low levels of noise. Although this finding supports the generally accepted view that many soft elastic materials can be assumed to be linear elastic at small strains, low signal-to-noise ratios common in the vicinity of initial contact preclude the accurate analysis of data in that regime. For modeling the large-strain spherical indentation of rubber-like gels and tissues, we propose the use of the nonlinear models.
5:45 PM - LL5.8
Elastic Fiber Alignments in Collagen Networks.
Alexandre Kabla 1 , David Vader 2 , David Weitz 2 , L. Mahadevan 2 Show Abstract
1 Engineering Department, University of Cambridge, Cambridge United Kingdom, 2 SEAS, Harvard University, Cambridge, Massachusetts, United States
Collagen is the most abundant network forming protein: it is the main component of the extracellular matrix and plays a key role in tissue engineering techniques. Cells in such a material typically generate local tensile forces which might affect the overall network morphology. We explore experimentally the mechanical response of fibrous networks by mimicking in collagen gels the typical mode of deformation that live cells impose on the network. We observe upon local stretching a strong alignment of the fibers and a large densification of the network. For pure type I collagen gels, this alignment is irreversibly imprinted in the network; this offers a simple mechanism for self-organization at the microscale. However, the same behavior is maintained and becomes reversible when the binding strength is increased by addition of a crosslinker. This demonstrates that fiber alignment is primarily an elastic effect and part of the fundamental non-linear properties of fibrous biological networks. This provides a baseline on top of which hydrodynamics effects can be conveniently studied.
LL6: Poster Session
Anthony J. Ryan
Wednesday AM, November 28, 2007
Exhibition Hall D (Hynes)
9:00 PM - LL6.1
Novel Stimuli Sensitive Structures Formed by the Directed Assembly of Microgels in Droplets.
Rhutesh Shah 1 , Jinwoong Kim 1 , David Weitz 2 1 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Physics, Harvard University, Cambridge, Massachusetts, United States
9:00 PM - LL6.10
Nanostructured Polymers as Better Bladder Tissue Substitutes.
Young Wook Chun 1 , Karen Haberstroh 1 , Martin Kaefer 2 , Thomas Webster 1 Show Abstract
1 , Brown University, Providence, Rhode Island, United States, 2 , Indiana University, Indianapolis, Indiana, United States
Nano-structured polymers are excellent candidates as biomaterials to regenerate native bladder tissue. In previous work, nano PU (poly ether urethane) and nano PLGA (poly-lactic-co-glycolic acid) films with nanometer surface features (created via chemical etching procedures) enhanced bladder smooth muscle cell functions. Here, we investigated reasons why and also determined urothelial cell function (adhesion, proliferation, and synthesis of extracellular matrix proteins) on the same nano-structured compared to conventional polymers (PU and PLGA). Results showed that nano PU adsorbed more calcium than other polymers (such as conventional PU). This may be a reason for the previously observed enhanced bladder cell functions on nano compared to conventional polymers. In addition, in vivo studies supported these in vitro results by showing a quicker regeneration of the bladder wall (with no calcium stones) on nano compared to conventional PU when implanted into rat bladders. Such results suggest that nano-structured polymers should be further studied for improved bladder regeneration properties.
9:00 PM - LL6.2
Open Chain Sugar Gelators by Biocatalytic Synthesis Pathway: A Possible Drug Delivery Vehicle.
Swapnil Jhadav 1 , Praveen Kumar Vemula 1 , George John 1 Show Abstract
1 Department of Chemistry, City College of the City University of New York, New York, New York, United States
9:00 PM - LL6.3
Computer Simulation of Compressive Failure in Silica Aerogels.
Brian Good 1 Show Abstract
1 , NASA GRC, Cleveland, Ohio, United States
Historically, the low thermal conductivities of silica aerogels have made them of interest to the aerospace community for applications such as cryotank insulation. However, recent advances in the application of conformal polymer coatings to these gels have made them significantly stronger, and potentially useful as lightweight structural materials as well. In this work, we perform atomistic computer simulations to investigate the compressive strength and failure behavior of silica and polymer-coated silica aerogels.The gels' nanostructure is simulated via the Diffusion Limited Cluster Aggregation (DLCA) process, which produces fractal aggregates that are structurally very similar to experimentally observed gels. The largest distinct feature of the clusters is the so-called secondary particle, typically tens of nm in diameter, which are composed of primary particles of amorphous silica an order of magnitude smaller. The secondary particles are connected by amorphous silica bridges that are typically smaller in diameter than the particles they connect.We investigate compressive failure via the application of a uniaxial compressive strain to the DLCA clusters. In computing the energetics of the compression, the detailed structure of the secondary particles is ignored, and the interaction among secondary particles is described by a Morse pair potential parameterized such that the potential range is much smaller than the secondary particle size; an angular potential contribution is included in some of the simulations as well. The Morse parameters are obtained by atomistic simulation of models of the interparticle bridges, with the compressive behavior of these bridges modeled via molecular statics.We consider the energetics of compression and compressive failure, and compare qualitative features of low-and high-density gel failure.
9:00 PM - LL6.4
Assembly and Dissociation Behaviours of Poly(ethylene glycol) and α-cyclodextrin Induced by Small Drug Competitors.
Zhao Sanping 1 , Lee Jonghwi 1 Show Abstract
1 , Department of Chemical Engineering and Materials Science, Chung-Ang University, Seoul Korea (the Republic of)
Cyclodextrins(CDs),mainly α-, β-, and γ-CDs, and their derivatives have been extensively studied as host molecules in supramolecular chemistry due to their hydrophobic cavities capable of selectively accommodating various small compounds and polymers. Since the first report in 1990 referred to as a “molecular necklace”, CD-based polymer inclusion complexes (PICs) have been widely investigated because of their non-covalent binding behavior based on macromolecular recognition and their potential applications. Recently, supramolecular hydrogels based on PICs formation have attracted much attention due to their potential applications in biotechnology and bioengineering. Li et al.developed a new class of supramolecular hydrogels for injectable drug-delivery system using high molecular weight poly(ethylene glycol) (PEG) or Pluronic to partially penetrate the inner cavity of α-CD. Huh et al. prepared thermoreversible hydrogels based on graft copolymers consisting of dextran as a hydrophilic backbone and PEG or PPG as a linear side chain for inclusion complexation with CD. In these systems, the hydrogels showed a thermally reversible sol-gel transition based on non-covalent assembly and dissociation between host and guest moieties. However, an unexpected disruption of the supramolecular hydrogel formed from PEG and α-CD was observed, when a small drug molecule, i.e., α-lipoic acid (ALA) or naproxen, was introduced into this system (see Table 1). These competitive guests were able to fill the α-CD cavities, expulsed PEG moieties, leading to disruption of such association and inhibition of gel formation. Depending on the association capability between small drug molecules and α-CD, the introduction of ALA caused the collapse of the hydrogel(Figure 1(a))and naproxen only led to the drastic reduction of viscosity of the hydrogel system(Figure 1(b)). The effect of stability of complexes with α-CD at different pH values on the gel formation was also studied. It was observed that a gelation occurred when mixing PEG solution and α-CD solution containing naproxen (the molar ratio of naproxen to CD is 1:2) at high pH (pH=9), however a viscosity solution was obtained at the same condition except different pH value (pH=2). This difference is possibly due to higher stability of complexes with α-CD at low pH value than at high pH value. So assembly and dissociation of PEG and α-CD could be controlled by choosing an appropriate drug competitor. Detailed studied on mechanism and kinetic of a ternary system of PEG, α-CD and small drug molecule are in progress.
9:00 PM - LL6.5
Stimuli-Sensitive Polymers Based on PNIPAM, LIPOIC ACID, and GOLD COLLOID.
Hye Ri Yun 1 , Chul Ho Park 1 , Jonghwi Lee 1 Show Abstract
1 School of Chemical Engineering Materials Science , Chung Ang University, Seoul Korea (the Republic of)
Alpha-lipoic acid (ALA) can be a base material for the drug delivery system because of its chemical reactivity, pH-sensitivity, and functional groups. A pH-sensitive polymer (PL) is prepared by the ring-opening polymerization of ALA, which is degradable in a base condition. Many stimuli-sensitive hydrogels have been reported to show changes in their structures and physical properties in response to external stimuli. Poly (N-isopropylacrylamide) (PNIPAM) has played an important role in designing temperature-sensitive carriers. The polymer was reported on its phase transition phenomena (swelling/shrinking) at a single temperature (near 32°C), despite various modification of hydrogels into Interpenetrating Polymer Networks (IPNs). IPNs of PNIPAM/PL/Au are successfully synthesized, and they showed the characteristics of interpenetrating structures. Polymer IPNs were prepared by a radical polymerization. PNIPAM as an initial network was reacted with ALA and Au colloid(156 ug/ml) in ethyl alcohol. Hydrogels were kept in an oven at 80 °C for a day. After polymerization, the hydrogels were immersed in a 99.5% methyl alcohol to remove unreacted chemicals and were cleaned in distilled water. The hydrogels were dried in a vacuum oven. Equilibrium swelling properties of IPNs were studied at a 5-50 °C range in distilled water. On measuring the swelling ratio of PNIPAM and IPNs, equilibration time was continued for an hour at each temperature. A characteristic of ALA disulfide polymer is pH-sensitive, and disulfide polymer is degradable and converted into monomeric ALA at the base condition. The degradation of PL, PNIPAM/PL, and PNIPAM/PL/Au were measured at pH 9 for a week. The disulfide bond of ALA could be detected by the UV measurement at 340 nm. PL showed the degradation at pH 9, but PNIPAM/PL and PNIPAM/PL/Au didn’t show degradation, possibly because the latter were formed into IPNs. Swelling of PNIPAM, which was known to be a thermosensitive polymer, showed an inflection point at 32 °C. The IPNs (PNIPAM/PL/Au) also showed its lower critical solution temperature (LCST) at 32 °C. It has been suggested that LCST behavior is caused by a critical hydrophilic/ hydrophobic balance of polymer side groups. For PNIPAM, the CONH groups are hydrophilic and for ALA, the aliphatic groups are hydrophobic. The ability of two polymers to form a complex depends upon the nature of polymer. It shows the consequence of Differential Scanning Calorimeter (DSC) results showing the glass transition temperature (Tg) of IPNs as a function of the content of ALA, suggesting the possible formation of copolymer. The IPNs of PNIPAM/PL/Au were successfully synthesized, and they showed the formation IPNs. PL is degradable at base condition but the IPNs didn’t undergo the degradation at pH 9, and IPNs were not detected the absorption peak of disulfide at 340 nm. The IPNs showed their lower critical solution temperatures (LCST) at 32 °C.
9:00 PM - LL6.6
Perfluorocarbon-based Microemulsion Gels With Triblock Copolymers.
Xiaoming Pan 1 , Surita Bhatia 1 Show Abstract
1 Chemical Engineering , University of Massachusetts Amherst, Amherst, Massachusetts, United States
9:00 PM - LL6.7
Micellar Solutions and Gels of Diblock Copolymers of Styrene and N-tert-Butylacrylamide Studied by Rheology and Light Scattering.
Nitin Sharma 1 , Rajeswari Kasi 2 Show Abstract
1 Polymer Program,Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Polymer Program, Institute of Materials Science and Chemistry Department, University of Connecticut, Storrs, Connecticut, United States
Soft polymeric materials have gained importance in recent years, namely in food, adhesives, pharmaceuticals, photographic media, actuators, in microfluidic devices. The ability of these materials to self-organize to form supramolecules of long range order imparts liquid-like behavior on molecular length scales combined with solid-like macroscopic properties, makes them unique systems and hence important in many industrial processes. In analogy with hydrogels, which are the soft materials of interest for last few decades, organogels attract lively research in recent years and their expanding use in different potential applications requires fundamental understanding of the material behavior subjected to a variety of external factors and agencies. We have studied the mechanical, relaxation and structural properties of thermoreversible gels made with polystyrene-b-poly(N-tert-butylacrylamide)[PS-b-PNtBAM] diblock copolymers in octanol-1, a good solvent for poly (N-tert-butylacrylamide) using light scattering and rheology. The diblock copolymer was synthesized by employing controlled free radical polymerization techniques. Effects of poly(N-tert-butylacrylamide)[PNtBAM] block length, temperature and gel concentration on the gel properties were investigated.
9:00 PM - LL6.8
Refractive Index Change in Nanoscale Thermosensitive Hydrogel for Optoelectronic and Biophotonic Applications.
Arup Neogi 1 , Brett Garner 1 , Zhibing Hu 1 , Floyd McDaniel 1 , Miguel Rojas 2 Show Abstract
1 Physics, University of North Texas, Denton, Texas, United States, 2 Physics, Universidad Autonoma del Edo de Mex, Toluca Mexico
9:00 PM - LL6.9
Hyaluronic Acid-Based Hydrogel Particles and Particle-Crosslinked Networks for Soft Tissue Regeneration.
Amit Jha 1 , Nurettin Sahiner 1 , Xinqiao Jia 1 Show Abstract
1 Material Sc & Eng, University of Delaware, Newark, Delaware, United States
We have previously demonstrated the synthesis and characterization of hyaluronic acid (HA)-based soft hydrogel particles (HGPs, average diameter 15-20 μm) and particle crosslinked networks (PXNs) as injectable materials for use in vocal fold restoration. In this presentation, we report a versatile technique for the preparation of HA HGPs with diameters varying from 200 nm to several micron. The hydrogel particles were synthesized via its chemical crosslinking with divinyl sulfone using an AOT (sodium bis(2-ethylhexyl)sulfosuccinate)/isooctane reverse micelle system in the presence of a co-surfactant (1-heptanol). Sodium periodate oxidation was employed to introduce aldehyde groups to the particles. The presence of aldehyde groups was confirmed by multi-photon confocal microscope upon fluorescence staining using cascade blue hydrazide. The aldehyde groups were subsequently used as reactive handles for subsequent crosslinking with HA that has been previously modified with adipic acid dihydrazide. The resulting macroscopic hydrogels are highly elastic and do not break until 400% strain. In-vitro cytotoxicity study was carried out by direct exposure of HGPs to the cultured NIH 3T3 fibroblasts. The results suggest that the HA HGPs are well tolerated by the cells. These novel hydrogels are promising implant materials for soft tissue regeneration.
Ferenc Horkay National Institutes of Health
Noshir A. Langrana Rutgers University
Anthony J. Ryan The University of Sheffield
J. David Londono DuPont de Nemours
LL7: Drug and Gene Delivery
Wednesday AM, November 28, 2007
Room 207 (Hynes)
9:30 AM - **LL7.1
Identification of Factors Affecting Performance of an Enzyme/Hydrogel Oscillator.
Ronald Siegel 1 , Amardeep Bhalla 1 , Siddharthya Mujumdar 1 Show Abstract
1 Pharmaceutics, University of Minnesota, Minneapolis, Minnesota, United States
Oscillating hydrogels have been received considerable attnetion over the past decade. Typically, swelling of a hydrogel is coupled to a chemical oscillator such as the Belousov-Zhabotinsky reaction or an inorganic pH oscillator. Such systems are interesting because they support numerous spatiotemporal phenomena such as wave-propagation, but their use in biomedical applications is limited since they require toxic redox reactants. Pursuing an interest in rhythmic hormone delivery, we are investigating a biochemomechanical oscillator that functions by nonlinear negative feedback between a reaction catalyzed by the enzyme glucose oxidase, and permeability of a hydrogel that gates access of the substrate, glucose, to that enzyme. The hydrogel is a copolymer of N-isopropyl acrylamide and methacrylic acid. Swelling state, and hence glucose permeability depend on concentration of hydrogen ion (i.e. pH) inside the reactor, with a bistable characteristic. Under proper conditions, reactor pH and hydrogel swelling state display rhythmic behaviors, with constant feed of glucose into the system. However, oscillation period increases with time, and eventually oscillations cease. Also, oscillations inside the reactor occur near pH 5, far below physiologic pH. The present studies were performed to determine conditions that permit oscillations, to identify the source of the increase in periodicity and ultimate cessation of oscillations with time, and to shift the operating point of the oscillator closer to physiologic pH. In the first set of studies, it was shown that the range of glucose concentrations supporting rhythmic behavior depends on other controllable system parameters. Typically, this range is bounded from above and below, i.e. too little or too much glucose leads to stationary, nonoscillatory behavior. In the second set of studies, it was shown that slowing of oscillations is most likely caused by accumulation of gluconate, a reaction product and pH buffer, inside the reactor. In the third set of studies, it was shown that slow pH transients can cause the hydrogel to “find” an intermediate swelling state leading to stationary behavior of the system and quenching of oscillations. Optical methods showed that this stationary state is probably associated with formation of stress patterns in the hydrogel and appearance of domains that are relatively high in their permeability to glucose. These patterns, which occur during slow oscillations, appear to be reversible. In the final study, methyl methacrylate was replaced by a series of α-alkyl acrylic acids (ethyl-, propyl-, and butylacrylic acid). It was shown that the swelling transition pH increases by about 0.5 units per methylene group added to the α-alkyl sidechain, and that the range of pH oscillations shifts accordingly. Thus, it is possible to move the range of pH oscillations closer to the physiologic pH range, rendering the system more realistic for biomedical applications.
10:00 AM - **LL7.2
Rational Design of Oligonucleotide Delivery Vectors.
Charles Roth 1 2 Show Abstract
1 Biomedical Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Chemical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey, United States
The ability to modulate cellular behavior through genetic modification has great potential in molecular therapeutics, functional genomics and tissue engineering. For instance, antisense oligonucleotides (AS ONs), which are most commonly single-stranded DNA molecules 15-25 nucleotides in length, modulate gene expression by binding to a complementary segment on a target mRNA, thus repressing translation of the encoded protein. While antisense technology is becoming a viable therapeutic entity and platform for functional genomics, intracellular delivery of the polynucleic acid remains a major barrier to more widespread development. For antisense technology and alternative approaches involving oligonucleotides, such as RNA interference (RNAi), delivery systems are borrowed primarily from those used for non-viral gene delivery.A significant part of our development effort builds upon principles that are inspired by the mechanisms of viral infection. Viruses self-assemble, protect nucleic acids from nucleases, enable their own delivery into the cytoplasm of cells, disassemble, and mediate nuclear entry and integration. In particular, we have adapted viral mechanisms of endosomal escape via pH-dependent peptides and applied them to polymer-mediated endosomal escape. For example, we have shown that the synthetic, pH-sensitive, membrane-disrupting polyanion, poly(propylacrylic acid) (PPAA) improves the in vitro efficiency of the cationic lipid DOTAP with regards to oligonucleotide delivery and activity for either antisense or RNAi. We have also noted the importance of disassembly in the effectiveness of a delivery vector and shown that optimizing intracellular release of oligonucleotides corresponds to enhanced antisense activity. It is also important to understand that delivery of oligonucleotides is different from delivery of plasmid DNA and to modify design principles accordingly. Oligonucleotides are much smaller than genes, and they must be maintained at sufficient intracellular concentration to drive hybridization with target mRNA. A manifestation of these differences is that the linear, 25 kDa form of the cationic polymer, polyethyleneimine (PEI), which is a gold standard for non-viral delivery of plasmid DNA, is ineffective for oligonucleotide delivery. We have developed structure-activity relationships for the delivery of oligonucleotides using PEI of varying molecular weight and architecture (i.e., linear vs. branching). We show that the extent and dynamics of antisense activity are affected by subtle changes in molecular properties, and we show that we can use this understanding to design mixed-backbone oligonucleotides exhibiting improved activity via a balance of nuclease stability, polyelectrolyte complexation strength, and avoidance of non-specific binding.
11:00 AM - **LL7.3
Diffusion of Nanoparticles in Concentrated Polymer Solutions and Gels.
Hacene Boukari 1 , Ariel Michelman-Ribeiro 1 , Ralph Nossal 1 , Ferenc Horkay 1 Show Abstract
1 , National Institutes of Health, Bethesda, Maryland, United States
The transport of particles in concentrated solutions and gels is relevant to many technological and biological processes. Recently, we have exploited the advantages of fluorescence correlation spectroscopy, a relatively non-intrusive optical technique, to probe interactions of various nanoparticles (~1-100 nm) in synthetic and natural polymeric solutions and gels. In this talk we focus on two model polymer systems: Poly(vinyl-alcohol) (PVA, MW=85 kDa) and Ficoll 70 (MW=70 kDa). PVA is a neutral, water-soluble, linear polymer commonly used as a component of tissue engineering matrices. Ficoll70 is a water-soluble, highly-branched sucrose-polymer used in perfusion experiments and studies of the effects of crowding on, for example, protein stability. We have measured characteristic diffusion times of various probes [simple dyes(~1.8 nm); dextran(~3 nm); BSA (~7.2 nm); phycoerythrin (~10.3 nm); polystyrene beads (28,44-nm)…] in non-fluorescent -hence ‘invisible’-PVA or Ficoll70 solutions as a function of the polymer concentration, exploring various length scales of the polymer solutions as defined by the polymer-polymer correlation length. For small probes, the decrease of the diffusion coefficient (D) with increasing polymer concentration (c), in both systems, cannot be accounted for by the Stokes-Einstein equation. Instead, D(c) varies as a stretched exponential exp(-Bcn), where n is related to the solvent quality. For PVA solutions we find 0.73 ≤ n ≤ 0.84 (good solvent) and a roughly linear relation between B and the size of the small probes. In contrast, n= 1 (single exponential) for the Ficoll70 suggesting a theta-like behavior of the Ficoll-water solutions. Cross-linking of PVA chains to form gels can further slow down the diffusion of some probe nanoparticles (dyes: TAMRA and R6G). The more the polymer chains are cross-linked, the slower the nanoparticles diffuse. Here, we find a simple linear relation between the elastic modulus and the diffusion time. As the gels are subjected to drying, the elastic modulus of the gels increases monotonically and the probe diffusion coefficient appears to decrease. We discuss these results within available theoretical models.
11:30 AM - **LL7.4
Bioinspired Stimulisensitive Microgels and Hydrogels.
Walter Richtering 1 , Martina Keerl 1 , Bastian Brugger 1 , Judith Musch 1 Show Abstract
1 Physical Chemistry, RWTH Aachen University, Aachen Germany
Hydrogels and microgels of thermo-responsive polymers have promising potential as materials for technical applications especially in the biomedical field e.g. for drug delivery or sensors. A prominent example is poly(N-isopropylacrylamide (PNIPAM). Aqueous solutions of PNIPAM undergo a reversible phase transition at ca. 32 °C. The phase transition of the thermo-sensitive polymers is attributed to the balance between the hydrophobic interaction and hydrogen bonding. One important aspect is to find means for tuning the volume phase transition temperature (VPTT) of such polymer systems as this allows tailoring them for specific needs in technical applications. In this contribution we discuss different routes: (i) copolymerisation of monomers to exploit specific hydrogen bonding; (ii) preparation of core-shell particles,(iii) modification of microgel particles via adsorption of polyelectrolytes and (iv) incorporation of magnetic nanoparticels.Structure and properties of these materials are probed by means of light and neutron scattering, FTIR, NMR as well as fluorescence spectroscopy and microscopy.
12:00 PM - LL7.5
Smart Membranes: Hydroxypropyl Cellulose for Flavor Delivery.
Kevin Heitfeld 1 , Dale Schaefer 1 Show Abstract
1 Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio, United States
Flavorings often contain volatile compounds, so in the absence of encapsulation, they are vulnerable to premature release. The ideal flavor would survive the temperature excursion during cooking, remain in the product for days to months and then be released instantaneously in the mouth. This work has focused on the use of temperature responsive gels (TRGs) featuring a lower critical solution temperature (LCST) for flavor retention at cooking temperatures. In the collapsed state, the polymer acts as a transport barrier, keeping the volatile flavors inside. We have successfully encapsulated a flavor inside a TRG controlling the release rate at high temperatures. Diffusion results indicate hydroxypropyl cellulose (HPC) performs better or as well as current flavor encapsulation technologies at room temperature. At high temperature, HPC performs ten times better than current technology requiring ten times less flavor resulting in a substantial decrease in costAn encapsulation system was designed utilizing the solution behavior (phase separation) of a TRG to control flavor diffusion. The gel morphology was determined through small angle scattering, and diffusive properties were tailored through morphology manipulation. Heterogeneous and homogeneous gels were processed by understanding the effect of temperature on gel structure. A model was developed linking bulk diffusive properties to molecular morphology.Flavor was encapsulated within the gel and the emulsifying capability was determined. The capsules responded to temperature similarly to the pure polymer. The release kinetcs were compared to commercial gelatin capsules and the temperature responsive polymer took longer to release.Chemistry was developed following guidelines for the Food and Drug Administration for food use. The food grade crosslinking was coupled with commercial scale-up equipment to develop large scale commercial production procedures.This work resulted in a new commercial encapsulation system. The system is able to tailor release kinetics through processing conditions. New crosslinking methods were developed with the possibility of opening new markets in food, flavor, and fragrance.
12:15 PM - LL7.6
Thermosensitive Copolymer, poly(N-isopropylacrylamide-co-dimethyl-gamma-butyrolactone acrylate-co-acrylic acid), as Injectable, in-situ Gelling Material for Controlled Drug Release with Hydrolysis-dependent LCST.
Zhanwu Cui 1 , Baehoon Lee 1 , Brent Vernon 1 Show Abstract
1 the Harrington Department of Bioengineering, Arizona State University, Tempe, Arizona, United States
Significant research has been directed toward developing temperature-sensitive poly(N-isopropylacrylamide) (poly(NIPAAm)) and its copolymers for biomedical applications, such as cell immobilization, drug delivery, etc. The lower critical solution temperature (LCST) of poly(NIPAAm) copolymers can be controlled by varying comonomer type and content. Incorporation of hydrophobic monomers leads to a lower LCST and hydrophilic monomers to a higher LCST. Our previous work has shown the synthesis of thermosensitive polymer, poly(N-isopropylacrylamide-co- dimethyl-gamma-butyrolactone acrylate) with hydrolysis-dependent lower critical solution temperature (LCST) for controlled drug release. These initial materials had an LCST below room temperature, had slow degradation, and high molecular weights, making application difficult. To solve these problems, a third monomer, acrylic acid was copolymerized with N-isopropylacrylamide and dimethyl-gamma-butyrolactone acrylate in tetrahydro- furan (THF) with a feed ratio of 94:5:1. Properties of this new copolymer were characterized by DSC, HPLC, NMR, and acidic titration. By using THF as the polymerization solvent, the molecular weight of the copolymer was lowered to 11kDa, allowing potential for kidney clearance after in vivo dissolution. Incorporation of acrylic acid resulted in increased initial LCST of 28.63°C. A time-dependent hydrolysis study was conducted in 0.1N PBS solution of pH 7.4 at 37°C and showed that the LCST increased above body temperature after 60 days. Also, a cloud point test was conducted at 1 wt.% and 500nm. The onset temperature and peak temperature of DSC showed the same trend as in cloud point test. This new material shows faster increase in LCST with time with an LCST closer to room temperature compared to previous polymers without acrylic acid.
12:30 PM - LL7.7
Intracellular Delivery of Membrane-impermeable Macromolecules Using Biodegradable pH-sensitive Core-shell Nanoparticles.
Yuhua Hu 1 , Arpun Nagaraja 2 , James Lu 4 6 , Tamara Litwin 5 , Darrell Irvine 2 3 Show Abstract
1 ChemEng, MIT, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 4 Biology , MIT, Cambridge, Massachusetts, United States, 6 Cancer Research Center, MIT, Cambridge, Massachusetts, United States, 5 Chemistry, MIT, Cambridge, Massachusetts, United States, 3 Bioengineering, MIT, Cambridge, Massachusetts, United States
Many therapeutic strategies require the delivery of drugs into the cytosolic or nuclear compartments of cells. Examples include gene therapy mediated by plasmid DNA, gene silencing or RNA interference via oligonucleotides, anti-tumor toxin delivery, and therapeutic protein delivery. However, most of the drugs are membrane-impermeable macromolecules and internalized by cells via endocytosis or phagocytosis, through which drug molecules are confined in endosomes or phagosomes. These intracellular compartments, with reduced pH at 5-6, fuse with lysosomes in which drug molecules may be denatured at a low pH of 4-5 in the presence of many degradative enzymes. Thus escape of drug molecules to the cytosol before destruction in endolysosomes is a major challenge for drug delivery. We synthesized monodisperse pH-sensitive hydrogel nanoparticles designed to chaperone macromolecules into the cytosol of target cells following endocytic uptake. A core-shell structure was utilized to physically and compositionally segregate the functions of the particle into an endosome-disrupting pH-responsive core that would absorb protons at endolysosomal pH, and a shell whose composition could be separately tuned to facilitate particle targeting, cell binding, and/or drug binding. Two-stage surfactant-free emulsion polymerization of diethylamino ethyl methacrylate (DEAEMA, core) and amino ethyl methacrylate (shell) in the presence of poly(ethylene glycol) dimethacrylate as a crosslinker was used for the synthesis of nanoparticles 200nm in diameter. The protonation of tertiary amine groups on the polyDEAEMA core on moving from extracellular/cytosolic to endolysosomal pH resulted in reversible swelling of the nanoparticles with a 2.8-fold diameter change. We postulated that the pH sensitivity of the particles could facilitate molecular delivery to the cytosol via the proton sponge effect. This hypothesis was proven by the cytosolic delivery of calcein (with ~95% efficiency), ovalbumin (with ~30% efficiency) in dendritic cells, and siRNA (with ~30% knockout rate) in epithelial cells. By sequestering the hydrophobic pH-responsive component of the particles under the more hydrophilic shell layer, negligible cytotoxicity was achieved as determined by MTT or colony forming assays. To make the particles dissolve on reaching the cytosol, cystaminebisacrylamide (BAC) was used as a crosslinker for particle synthesis; the disulfide bonds of BAC can be digested by the high glutathione levels in the cytosol, allowing the particles to break down into nontoxic micelles. These biodegradable nanoparticles retained their pH-sensitivity and were able to deliver calcein and other membrane-impermeable molecules to the cytosol similarly to nondegradable particles. These particles may provide a useful means to deliver immunomodulatory oligonucleotides or therapeutic molecules to the cytosol of target cells in vivo due to their biodegradability and highly efficient endosome-escape properties.
12:45 PM - LL7.8
Radiation Induced Intra- and Inter-Crosslinked Poly(vinyl pyrrolidone) Nanohydrogels for Drug Delivery.
Jung-Chul An 1 , Dianne Poster 2 , Wyatt Vreeland 3 , Joseph Silverman 1 , Mohamad Al-Sheikhly 1 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 Chemistry Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Analytical Chemistry, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Nanohydrogels for medical applications such as drug delivery and imaging have been actively studied in recent years. We have used high energy electron beam to synthesize nanostructure of poly(vinyl pyrrolidone) (PVP) in aqueous solution. The nanogel structure was achieved through radiation induced inter- and intra-crosslinking of PVP. Upon the radiation, the radiolytically produced free radical of PVP undergoes inter- and intra-crosslinking by radical-radical reaction. Pulsed electron beam irradiation at high repetition rate generates more time-averaged number of carbon centered free radicals along the each single polymeric chain simultaneously. In dilute solution, those radicals react with each other bimolecularly forming preferred intra-molecular crosslinked PVP hydrogel structure. The diffusive motion of polymeric free radicals contributes to the buildup of molar mass through inter-molecular crosslinking reaction. Smaller nanohydrogel particles with hydrodynamic radius (Rh) value of 11.5 ± 0.2 nm were attained at higher pulse repetition rate (300 pulses per second). From the pulse radiolysis experiments, the second order reaction rate constants (2k) of PVP radical recombination at 28 °C are determined ca. 1.0 ± 0.1 x 109 L mol-1 s-1. The Rh of hydrogel nanoparticles was measured from the dynamic light scattering (DLS) experiments and the molecular weight distribution were analyzed with asymmetric flow field flow fractionation (AFFF) chromatography method. Atomic force microscopy (AFM) was used to investigate the morphology of the synthesized nanohydrogels.
LL8: Biological Gels and Tissue Engineering
Wednesday PM, November 28, 2007
Room 207 (Hynes)
2:30 PM - **LL8.1
Networking with Nature for Lessons that Gel: From Slugs in America to Hippos in Zambia.
Christopher Viney 1 Show Abstract
1 , University of California at Merced, Merced, California, United States
Gel-forming polymers can assemble into coherent films and fibers at significantly lower concentrations than non-gel-forming polymers. Nature capitalizes abundantly on this property, especially in the context of surface-coating films, where the combination of molecular mobility, network stabilization and overall cohesivity promotes the maintenance of a continuous layer.A common feature of many of the polymer gels encountered in Nature is the presence of liquid crystalline order. This observation serves as a reminder and/or lesson that material property optimization depends on the control of molecular order over a range of length scales. Although the tertiary structure of a biopolymer may exhibit no net anisotropy, this does not preclude significant local conformational order (secondary structure) from developing within molecules and synergizing with the formation of locally anisotropic correlations between segments of adjacent molecules. Even molecules that do have a random coil structure can still promote liquid crystalline phase formation, if there are intermolecular contacts that encourage the coils to associate into supramolecular anisotropic structures. Liquid crystalline phases can serve as useful precursors to the nucleation of even more highly ordered, crystalline phases when the concentration of polymer is subsequently increased.This presentation will look at how liquid crystalline gels contribute to the properties of four quite diverse biopolymer systems.(1) In spider silk secretions, supramolecular assembly of high axial ratio rods allows a given volume fraction of polymer gel to be optimally effective at instigating orientational order and thus promoting spinnability of the secretion.(2) Slugs and snails use liquid crystalline gels (mucus) in several contexts: (a) to produce a continuous, protective coating; (b) to facilitate the molecular order changes associated with the transition from lubricating mucus (that enables their progress over dangerous topography) to cement-like mucus (that allows them to adhere firmly to even the smoothest glass surfaces); (c) to facilitate the nucleation of anisotropic structures that convey directional information in their trails; and (d) to produce a “rope” on which they can abseil from vertical overhangs. (3) Sebum secreted by the Meibomian glands in mammals serves to coat and lubricate the eyes; it forms liquid crystalline films that are able to spread but also coat easily and reduce the evaporation rate of tears.(4) Hippopotamus “sweat” is a highly heterogeneous composition that forms a great variety of dilute liquid crystalline phases. Initial studies indicate that the physical organization of molecules in the liquid crystalline sweat gives rise to a sun-blocking ability that augments the intrinsic sun-screening propensity of some of the ingredients. In addition to affording protection from photodamage, this excellent barrier film has antiseptic and insect-repellant properties.
3:00 PM - **LL8.2
Injectable Materials that form Macroporous Scaffolds with Defined Surface Chemistry and Growth Factor Release Profiles.
Kevin Shakesheff 1 Show Abstract
1 School of Pharmacy, University of Nottingham, Nottingham United Kingdom
Regenerative medicine offers the promise of new clinical treatments that cure patients by restoring functional tissues. Materials play an important role in this field by defining the three-dimensional architecture within which tissue regeneration occurs. The materials, normally presented as highly porous scaffolds, act as a template for the biological actions of cells (including stem cells), growth factors and surface-tethered signalling molecules. Numerous clinical applications, including regeneration of bone, cornea, liver, skeletal muscle and cartilage are under development as targets for scaffolds.We have invented a new material that within the body has the desirable properties of a macroporous scaffold but outside of the body the material is a low visocosity paste or suspension that can be administered by injection. The ability to administer a scaffold via injection allows a space filling and highly porous material to be introduced my minimally invasive methods.This presentation will describe the mechanism of conversion of a paste into a solid scaffold using a 10○C temperature increase, from room to body temperature, as the trigger. Next the use of the injectable scaffold as a drug and cell delivery system will be explored using clinical applications in orthopaedics as the example.
4:00 PM - **LL8.3
Fibrin Based Scaffolds for Neural Wound Repair.
Lisa Flanagan 1 , Penelope Georges 2 , Ivo Laidmae 3 , Raivo Uibo 3 , Evelyn Sawyer 4 , Paul Janmey 2 Show Abstract
1 , University of California-Irvine College of Medicine, Irvine, California, United States, 2 , University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 , Tartu University, Tartu Estonia, 4 , Sea Run Holdings, Inc., Freeport, Maine, United States
Injury to the mammalian spinal cord induces astrocyte activation and glial scarring that impairs the regrowth of neuronal axons and hinders functional recovery. Approaches to stimulate repair include the use of three-dimensional matrices as bridging materials at the site of injury. Biocompatible filament networks provide a scaffold with cell adhesion sites and a means to concentrate growth factors that can be covalently or non-covalently bound to the filaments. Fibrin gels derived from purified clotting factors form matrices suitable for neuronal wound healing because in addition to their biocompatible structure, the mechanical properties of fibrin can be optimized for growth of nervous system cells. Soft matrices with Youngs modulus 50-500 Pa maximize neurite extension and branching of neurons and inhibit the hyper-activation of astrocytes. Fibrin prepared from salmon coagulation proteins provides better support for axonal extension than mammalian fibrin, collagen, or matrigel, and allows long term culture in vitro. In our current study, we show that salmon fibrin has no deleterious immunological effects or effects on coagulation profiles when administered in vivo. Salmon fibrin also shows significant potential in prevention of inflammation and restoration of neuronal function when used as a bridging material after spinal cord injury in a rodent model.
4:30 PM - **LL8.4
Deconstructing the Cell-Biomaterial Interface: Lessons and Challenges in Vascular Tissue Engineering.
Joyce Wong 1 Show Abstract
1 Biomedical Engineering, Boston University, Boston, Massachusetts, United States
A deeper understanding of the cell-biomaterial interface can promote the development of biomimetic materials for tissue engineering. Specifically, our laboratory is interested in understanding the integration of biomaterial compositional, structural and physical properties on controlling physiological function at the cellular and tissue levels. Our approach is to develop model systems of bioengineered materials coupled with biophysical techniques to quantify structure-function relationships of biological processes across multiple lengthscales. To achieve this, we have developed a suite of techniques based on microfluidics, microfabrication, and colloid and polymer science and engineering. Our long-term goals are to reveal fundamental principles for the rational design of new ‘biologically-inspired’ and ‘hierarchically-ordered’ materials for tissue engineering.
5:00 PM - LL8.5
Morphology of Microtubule Networks Grown in Agarose Gels.
Javier Castro 1 , Pierre Deymier 1 Show Abstract
1 Materials Science and Engineering, University of Arizona, Tucson, Arizona, United States
5:15 PM - LL8.6
A Cell’s Perspective of its Culture Surface.
Ruchirej Yongsunthon 1 , David Baker 1 , Wanda Walczak 1 , Theresa Chang 1 , Wageesha Senaratne 1 , Odessa Petzold 1 , Randall Youngman 1 Show Abstract
1 , Corning Incorporated, Corning, New York, United States
Widely-used commercial materials such as tissue-culture-treated (TCT) or protein-coated polystyrene, Corning® CellBIND® Surface, and BD Matrigel™ have a long-standing history of successful eukaryotic cell culture. However, current research demands often require specific functionality of cultured cells to more closely mimic in vivo cells. Not surprisingly, the biological surfaces tend to produce more desirable cell response (e.g. attachment and proliferation) but lack both the lot-to-lot reproducibility and well-defined composition of synthetic materials. Development of biomimetic synthetic materials necessitates increased understanding of the fundamental mechanisms of cell-substrate interactions. To that end, atomic force microscopy (AFM) was used to characterize a series of commercial substrates from a cell’s perspective, i.e. in terms of how the surface presents itself on cell-relevant size scales and how the surface evolves under cell culture conditions. A reference set of surface properties was measured using a series of well-defined materials (e.g. TCT) and protein coatings (e.g. Laminin). Basic mechanical materials properties were obtained with standard silicon nitride and etched silicon AFM probes. Additionally, these AFM tips were chemically or biologically functionalized, e.g. with octa-decyl-trichlorosilane for hydrophobic probe properties, to allow exploration of chemical surface properties with sub-micron accuracy. Force spectroscopy was also used to examine the conformation of exposed proteins, which are believed to mediate cell-substrate interactions. This reference set was then used to explore and characterize more complex biological substrates.
5:30 PM - LL8.7
Developing New Hydrogels for Spinal Cord Injury.
Erin Lavik 1 , Millicent Rauch 1 , Sara Hynes 1 , Andy Redmond 1 Show Abstract
1 , Yale, New Haven, Connecticut, United States
There have been a number of approaches to promote repair following spinal cord injury (SCI) but most have had limited success. Neural stem cells (NSCs) are capable of replacing all of the major cells of the central nervous system (CNS) and have shown potential for treating SCI. However, for such therapies to be successful and safe, the NSCs must survive and terminally differentiate appropriately following transplantation. Previously, we developed polymer systems which appeared to improve the survival of stem cells following transplantation but did not promote differentiation. To assess whether materials properties can influence the fate of stem cells, we developed a library of hydrogels which permitted the independent variation of chemical and mechanical properties. We designed a library of poly(ethylene glycol)/poly-L-lysine (PEG/PLL) hydrogels which could achieve a range of elastic moduli and free amine concentrations. We fabricated the library by carbonyldiimidazole activation of linear or 4-arm PEG. The activated PEG is subsequently reacted with PLL. Properties were modified by varying the molecular weights and ratios of the macromers resulting in a library of 52 unique gels. Hydrogel properties spanned a range of mechanical moduli (100 Pa to 20 kPa) as well as a range of free amine concentrations. The gels were characterized using a range of techniques including parallel plate rheometry to determine the mechanical properties, SEM analysis for the architecture, FITC labeling to determine the extent of the PLL network, chemical analysis of the free amine concentration, trypsin digestion to determine the relative degradation rates, and swelling analysis. By culturing NSCs on these gels, we identified a subset of the gels which promoted the neuronal differentiation of NSCs as indicated by neurofilament 200 immunostaining. This subset exhibited a range of free amine concentrations but a relatively limited range of elastic moduli between 3500 and 5500 Pa. Further experiments have evaluated the NSC-seeded hydrogels in a hemisection model of spinal cord injury. The NSCs survive and express markers for differentiation. Our data suggest that the materials properties can influence the fate of stem cells in vitro and vivo.
5:45 PM - LL8.8
Novel Ionic Block Copolymers as Templates for Biomineralization.
Mathumai Kanapathipillai 1 , Chieh-Tsung Lo 2 , Pappannan Thiyagarajan 3 , Surya Mallapragada 1 Show Abstract
1 Chemical engineering, Iowa state university, Ames laboratory, Ames, Iowa, United States, 2 Advanced photon source, Argonne national laboratory, Argonne, Illinois, United States, 3 Intense pulse neutron source, Argonne national laboratory, Argonne, Illinois, United States
Organic-inorganic ionic interactions play an important role in natural mineral formation. The interaction of carboxyl groups on collagen nano fiber templates with calcium ions in physiological fluids is known to facilitate the nucleation and growth of hydroxyapatite in bone. To mimic this, we have synthesized a family of novel ionic pentablock copolymers with excellent self-assembling properties that can form hierarchical gel structures at different pH and temperature conditions to serve as templates for biominerlization. The polymers are based on Pluronic® triblock copolymers with anionic poly(acrylic acid) and zwitterionic poly(sulfobetaine) blocks. We have further extended these families of polymers by conjugating them with hydroxyapatite nucleating peptides (DSKSDSSKSESDSS) and titanium binding peptides (RKLPDAPGMHTW) using N-hydroxysuccinimide chemistry. The polymers were characterized by NMR, gel permeation chromatography (GPC), differential scanning calorimetry, light scattering, X-ray, neutron scattering and rheology to understand the phase behavior and thermodynamics of these polymers in aqueous solutions. The lower critical solution temperature of the PEO blocks of Pluronic® micelles form micellar liquid crystal mesophases resulting in macroscopic gelation. The pentablock copolymers, in addition to retaining the thermoreversible gelation properties of the Pluronic® copolymer, also exhibit excellent pH responsiveness due to their cationic, anionic and zwitterionic blocks. Conjugation of peptides increases the specificity of mineralization, thereby enhancing the nucleation and growth of the mineral. X-ray and neutron scattering on these polymer gels showed nanometer length scale structures resembling FCC ordering. Due to their excellent self-assembling properties, these bioinspired polymers are excellent candidates for the synthesis of polymer-inorganic nanocomposites using a bottom-up approach.
Ferenc Horkay National Institutes of Health
Noshir A. Langrana Rutgers University
Anthony J. Ryan The University of Sheffield
J. David Londono DuPont de Nemours
LL9: Functional Biopolymer Assemblies
Thursday AM, November 29, 2007
Room 207 (Hynes)
9:30 AM - LL9.1
Two-photon Microfabrication of Structures Containing the Biopolymer Chitosan.
Cleber Mendonca 1 2 , Daniel Correa 1 2 , Prakriti Tayalia 1 , Gatien Cosendey 1 , David dos Santos Jr. 3 , Ricardo Aroca 3 , Eric Mazur 1 Show Abstract
1 Department of Physics and Division of Engineering and Applied , Harvard University, Cambridge, Massachusetts, United States, 2 Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, SP, Brazil, 3 , University of Windsor, Windsor, Ontario, Canada
Two-photon polymerization is a powerful tool in the fabrication of three-dimensional micro-structures for applications ranging from photonics to biology. The nonlinear nature of the multiphoton absorption confines the polymerization to the focal volume of the laser, allowing the fabrication of microstructures by moving the laser focus three-dimensionally through the resin. However, studies exploring the possibilities of doping microstructures fabricated by two-photon polymerization are still sparse. In this way, it is desirable to look for new resin formulations containing active components, which still can be polymerized using two-photon absorption. To explore the possibilities in this direction, we present here the two-photon microfabrication of structures containing the bio-polymer Chitosan.We prepared a variety of blends in a guest/host scheme. The host resin we used consists of tris(2-hydroxyethyl)isocyanurate triacrylate and ethoxylated(6) trimethyl-lolpropane triacrylate. As guest material we used the Chitosan [(1→4)-2 amino –2 –deoxy-β-D-glucan], which is a linear cationic polysaccharide obtained by deacetylation of chitin [(1→4)-2 acetamide –2 –deoxy-β-D-glucan], a structural polysaccharide normally encountered in crustaceans. There are several potential applications for chitosan, mainly due to its biodegradability and biocompatibility. Several application for chitosan films have already been proposed, such as in blood coagulation, sensing unit in a taste sensors, soft tissue and bone regeneration and antibacterial action.We induced the two-photon absorption polymerization with a Ti:sapphire laser oscillator that produces 130-fs pulses at 800 nm. To fabricate structures we used an average laser power of 20 mW, measured after the 0.65-NA objective that focuses the laser beam into the sample. The sample was positioned in the axial z-direction using a motorized stage, and the laser beam was scanned in the resin x-y-direction with a set of galvanic mirrors. After fabrication, the unpolymerized resin is washed away with ethanol and dried at room temperature. Here we report on microstructures with dimensions on the order of tenths of micrometers, although much smaller ones can be fabricated. Scanning electron micrographs of the fabricated microstructures reveal good definition and excellent integrity. The distribution of chitosan in the microstructure was evaluated by micro-Raman spectroscopy. We found that the chitosan distribution in the microstructure is not uniform, but still compatible with the ones usually observed in polymeric blends. We also performed mechanical and biocompatibility experiments in the proposed resin. The approach employed here is a promising alternative for the fabrication of scaffolds containing biopolymers. Tailor-made scaffolds for tissue engineering and bone reconstruction can be fabricated in short time using two-photon polymerization, opening new venues for advanced drug delivery and tissue regeneration.
9:45 AM - LL9.2
Magnetically Controlled Thermoresposive Hydrogel for Microfluidics and Biomemetics.
Santaneel Ghosh 2 , Tong Cai 1 , Zhibing Hu 1 , Arup Neogi 1 Show Abstract
2 Department of Physics and Engineering Physics, Southeast Missouri State University, Cape Girardeau, Missouri, United States, 1 Department of Physics , University of North Texas, Denton, Texas, United States
10:00 AM - LL9.3
Analytical Approach to Quantifying the Non-Affine Behavior of Fiber Networks.
Catalin Picu 1 , Hamed Hatami-Marbini 1 Show Abstract
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Fiber networks deform nonaffinely due to their inhomogeneous structure. The degree of nonaffinity depends on many factors such as the density and properties of fibers, the intensity and nature of the applied load, the type of joints and the length scale of observation. The “homogenized” response on given scale depends on the non-affine mechanics on all sub-scales. Therefore, it is important to develop tools to quantify the nonaffinity observed in random networks and to study how it depends on the geometric parameters. Here, we develop a method to quantify the nonaffine strains and use it to study their scaling properties and dependence on the architecture of the network. Furthermore, a method is developed to map the non-affine mechanics of a regular network with a large density of defects into an equivalent continuum, which is then used to determine the homogenized elastic properties of the network. This provides a semi-analytic tool linking the structure of the network to its overall elastic properties.
10:15 AM - LL9.4
Protein-Based Hydrogels for Cell Transplantation under Constant Physiological Conditions.
Cheryl Wong Po Foo 1 , Sarah Heilshorn 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
A promising treatment for multiple neurological disorders including stroke, Huntington's, and Parkinson's is the transplantation of stem cells into the diseased site to promote regeneration of neural tissue. However, viability of transplanted cells is often low (15-35%) and unpredictable. Cell viability has been directly correlated with functional outcome of the treatment, motivating the development of more reliable cell transplantation procedures. To protect transplanted cells from shear stress during injection and from the hostile, inflammatory environment of the diseased brain tissue, many research groups are exploring physical hydrogels as a protective, growth-permissive matrix to enhance cell viability. However, physical hydrogels require an environmental trigger to induce gelation. These environmental triggers include sudden changes in temperature, pH, or salt concentration - all of which are detrimental to encapsulated cells and proteins and complicate their use in a clinical environment. To address this need, we have designed a two-component, protein-based hydrogel system that can self-assemble under constant physiological conditions.Both components of the hydrogel system are created using recombinant protein technology, which allows synthesis of exact monomer sequences within monodisperse polymers. The first hydrogel component is a block copolymer made of several repeats of a peptide sequence encoding the WW domain-fold, a short triple-stranded, anti-parallel-beta-sheet. The WW domains are interspersed with a random-coil, hydrophilic spacer to enhance polymer flexibility and solubility. The second hydrogel component is made of several repeats of a polyproline rich peptide sequence interspersed with a random-coil, hydrophilic spacer. Upon mixing the two hydrogel components together, the WW-domains in component 1 and the polyproline rich peptides in component 2 bind together with 1:1 stoichiometry. This binding has an apparent association constant of 2.2x105 M, as measured by isothermal titration calorimetry. This peptide-binding event serves as the physical crosslinks to form a polymeric network composed of the two components. Because gelation is initiated by simply mixing the two components together at physiological pH, temperature, and ionic strength, this system is highly biocompatible and easy to use. Furthermore, the precision of protein engineering allows both components to be easily modified. For example, increasing the length of the hydrophilic spacers will increase the resulting network pore size. Additionally, bioactive peptide sequences, such as the RGD cell-binding domain, have been introduced into the hydrophilic spacers to modify cell-scaffold interactions. Our long-term objective is to design a self-assembling hydrogel system for cell delivery that will both improve cell viability and mimic many of the essential cues in the developmental niche to encourage cell differentiation and outgrowth.
11:00 AM - **LL9.5
Asphaltene Aggregation: Thermo- and Molecular Dynamics and NMR Applications in Petroleum Exploration.
Natalia Lisitza 1 , Denise Freed 1 , Pabitra Sen 1 , Yi-Qiao Song 1 Show Abstract
1 , Schlumberger, Cambridge, Massachusetts, United States
Aggregation and self-aggregation phenomena are of central importance for characterization of many complex chemical and biological systems. Collective molecular dynamics can be critical for the system's properties in these cases. However, heterogeneity in natural materials often makes it difficult to study the molecular dynamics directly. An example can be found in asphaltene-rich oils that exhibit a series of aggregation phenomena; the molecular dynamics and mechanism of this aggregation are poorly understood. Here we use a suite of Nuclear Magnetic Resonance methods to study the molecular dynamics and thermodynamics of aggregation in asphaltene solutions. We observed a reduction in spin-echo signal due to restricted motion of the alkyl chains upon aggregation and this behavior was used to establish the critical concentration of aggregation. The temperature dependence of such behavior allowed the determination of the entropy and enthalpy of aggregation. The enthalpy is negative, most likely due to pi-stacking interactions. The entropy of aggregation is found to be positive, which is unexpected for a non-polar solution possibly due to the entropy gain of the solvent during aggregation.Furthermore, we observed an abrupt drop of the asphaltene diffusion constant and show that this drop is indicative of a molecular conformation change at the level of single molecules prior to aggregation. We have also developed a relaxation based NMR method to measure the time scale of formation of the aggregation.The continuous rise of global demand for energy and the difficulty of significantly increasing production capacity have made heavy oil and deep-sea reservoirs much more attractive. As a result, there is an urgent need of advanced technologies for better reservoir characterization, such as NMR technology. The importance of NMR in petroleum exploration has been enhanced by the recent progress in NMR well-logging techniques, such as 2D NMR, for the characterization of both rocks and the natural fluids. Such advanced techniques are increasing being accepted as a valuable logging service especially in the technically challenging areas, such as deep-sea exploration. We will briefly review the NMR applications of fluid properties in petroleum exploration, in particular, those in fluid composition.
11:30 AM - LL9.6
Human Cervical Tissue Anisotropy and Structure-Function Relationships.
Kristin Myers 1 , Anastassia Paskaleva 1 , Michael House 2 , Simona Socrate 1 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Division of Materal Fetal Medicine, New England Medical Center, Boston, Massachusetts, United States
The cervix plays a crucial role in maintaining a healthy pregnancy. The cervix acts as a mechanical barrier to hold the fetus inside the uterus during gestation. Altered biochemical and mechanical properties of the cervical tissue are suspected to play an important role in spontaneous preterm birth. Slight changes in the biochemical properties are known to cause softening of the cervical tissue thus weakening the structure. Premature softening of the cervical tissue is associated with spontaneous preterm birth and the clinical condition known as cervical insufficiency. “Cervical insufficiency” is clinically defined as a history of spontaneous preterm birth associated with a painless contraction-free delivery. Mechanical and biochemical properties are suspected to play a role in spontaneous preterm birth, however, the relationship between mechanical properties and biochemical content has not been established. Therefore, there is a need to uncover the etiology of cervical insufficiency to aid in the identification and management of patients. This study explores the relationship between the biochemical and mechanical properties of cervical tissue. Specifically, the anisotropic mechanical properties were measured by loading cervical stroma specimens in uniaxial compression and tension. The orientation and location of the tissue specimens was marked, and specimens were loaded uni-axially along three orthogonal anatomical directions (longitudinal, radial, circumferential) using a load-unload, ramp-relaxation deformation protocol. All lateral deformation were captured using a video extensometer and a right angle prism. To visualize collagen fibers, 2nd Harmonic Generation and birefringence images were obtained for tissue sections normal to the three anatomical directions. These images investigate a possible correlation between the preferential alignment of the collagen fibers in the cervical stroma and the anisotropy of its deformation behavior. Experimental data for the tissue mechanical response were fit to a micro-structurally based one-dimensional constitutive relationship. Values of the constitutive model parameters corresponding to the best fit to the material response in all modes of deformation were taken to represent the “mechanical properties” of each sample. Further, structure-function relationships were investigated by searching for statistically significant correlations between model parameters and measured biochemical properties. An improved knowledge of the structure-function relationship is an important first step in understanding how the microstructure of the extracellular matrix (ECM) relates to its macroscopic mechanical response.
11:45 AM - LL9.7
Chemomechanical Tuning of Polymeric Surfaces to Modulate Prokaryotic Cell Adhesion.
Todd Thompson 1 , Jenny Lichter 1 , Michael Rubner 1 , Krystyn Van Vliet 1 Show Abstract
1 Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
It is increasingly appreciated that both mechanical and chemical characteristics of the material interface to which mammalian (eukaryotic) cells adhere can affect cell morphology and function. For example, the attachment and proliferation of mammalian cells such as microvascular endothelial cells correlates directly with the elastic stiffness of polyelectrolyte multilayers, synthetic polymeric thin films to which these cells adhere . It is generally accepted that mammalian cells are chemomechanically responsive via complex molecular signaling pathways that include transduction of the extracellular stiffness from the cytoskeleton to the nucleus (inducing transcription and translation of new proteins). This can be demonstrated by patterning these polymeric surfaces with specific ligands to deconvolute the chemical and mechanical cues . Here, we show that the adhesion of prokaryotic cells, including gram positive bacterium staph. epi. and gram negative e. coli, can also be controlled via mechanical stiffness of the polymeric substrata, despite the fact that these cells are thought to lack the genetic machinery to transduce extracellular mechanical stimuli. We demonstrate that this mechanically tuned adhesion of bacteria via the polymeric biointerface presents the opportunity to decouple the effects of substrata stiffness, surface charge, and specific functional groups required to prevent the undesired adhesion and growth of bacterial colonies and plaques. MT Thompson, MC Berg, IS Tobias, MF Rubner and KJ Van Vliet. Tuning compliance of polyelectrolyte multilayers to modulate cell adhesion. Biomaterials 26: 6836(2005). MT Thompson, MC Berg, J Lichter, IS Tobias, MF Rubner and KJ Van Vliet. Biochemical functionalization of polyelectrolyte multilayers can alter mechanical compliance. Biomacromolecules 7: 1990 (2006).
12:00 PM - LL9.8
Hierarchies, Multiple Energy Barriers and Robustness Govern the Fracture Mechanics of Alpha-helical Proteins.
Theodor Ackbarow 1 , Markus Buehler 1 Show Abstract
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The fundamental fracture mechanisms of biological protein materials remain largely unknown, partly due to a lack of understanding of how individual protein building blocks respond to mechanical load. For instance, it remains controversial if the free energy landscape of the unfolding behavior of proteins consists of multiple, discrete transition states or if the location of the transition state changes continuously with the pulling velocity. This lack in understanding has thus far prevented from developing predictive strength models for protein materials. Here we report for the first time direct atomistic simulation over four orders of magnitude in time scales of the unfolding behavior of an alpha-helical (AH) protein, the key building block of hair, hoof, wool and the cell’s cytoskeleton. We find that there exist two discrete transition states corresponding to two fracture mechanisms. Whereas the unfolding mechanism at fast pulling rates is sequential rupture of hydrogen bonds (HBs), unfolding at slow pulling rates proceeds by simultaneous rupture of three parallel HBs. Based on Bell’s theory, we derive a theoretical model that explicitly considers the hierarchical architecture of HBs in the AH protein, accurately predicting the observed unfolding behavior, providing a rigorous structure-property relationship. Our theory reveals that 3-4 parallel HBs per convolution are most favorable in light of the protein’s mechanical and thermodynamical stability, in agreement with experimental findings that AHs feature 3.6 HBs per convolution. Our results provide strong evidence that the particular molecular structure of AHs follows Pareto’s principle in maximizing robustness at minimal use of building materials.
12:15 PM - LL9.9
On Demand Release of Nucleic Acid Constructs via Enzymatic and Physical Triggers.
Sid Venkatesh 1 , Jacek Wower 2 , Mark Byrne 1 Show Abstract
1 Biomimetic & Biohybrid Materials, Biomedical Devices, and Drug Delivery Laboratories, Department of Chemical Engineering, Auburn University, Auburn, Alabama, United States, 2 Department of Animal Sciences, Auburn University, Auburn, Alabama, United States
The incorporation of nucleic acid constructs into hydrogels can produce novel biomaterials with programmable on-demand switches or modulatory mechanisms, with unprecedented control and sensitivity. Our work is the first to harness the mechanisms of nucleic acid based molecular recognition to develop advanced biomaterials. Custom oligonucleotides, which were partially complementary to each other and had a programmed recognition site for the restriction endonuclease BamHI, were modified with a polymerizable acrylate functionality and 32P-labeled using T4 RNA ligase. In-vitro hybridization and restriction enzyme digests were performed and confirmed using polyacrylamide gel electrophoresis. Approximately 10-11% of the 32P-labeled oligonucleotide self-annealed. Capture layers in a polyacrylamide gel enabled the highly efficient separation of unincorporated double stranded DNA and unhybridized single strands from the double stranded DNA incorporated into the networks. Quantification via phosphoimaging indicated-70% capture, 25 % unincorporated acrylated DNA, and 5% 32P-labeled oligonucleotide. The release of DNA due to the penetration of restriction enzyme was shown to be highly specific, with no release in the absence of BamHI, and the presence of another endonuclease, EcoRI. As an alternative trigger, temperature was used to release the 32P-labeled oligonucleotide. Temperature responsive release characteristics corresponded to the theoretical melting temperature of the helix (58°C). The physiological significance of these biomaterials was demonstrated by delivering a deoxyribozyme using BamHI in order to cleave a HIV Tat/Rev mRNA. Scission digests due to restriction enzyme or deoxyribozyme treatment were analyzed using electrophoretic mobility shift assays on denaturing polyacrylamide gels. Such a model can be exploited in siRNA-treatment regimes.We have also produced modulatory devices by programming the swelling states of the hydrogels, based on the binding and release of ligands and aptamers selected for them. Ligand-binding RNA pseudoknots were identified using SELEX and affinity chromatography. They were synthesized in vitro using the T7 RNA polymerase and synthetic DNA templates. Transcription of these pseudoknots was controlled by promoters that are known for producing RNA with very high efficiency and yield. Templates and transcripts were examined by agarose and polyacrylamide gel electrophoresis, respectively. Modified nucleotides were co-transcriptionally incorporated into RNA pseudoknots to render them resistant against ribonucleases. A pseudoknot-hairpin-pseudoknot construct was shown to bind and release the ligand by metal ion-chelating agent switching. This new class of biomaterials, which involves aptamer selection for different therapeutics, is based on the guiding principles of molecular biology and is expected to profoundly influence the delivery of novel RNA based therapeutics.
12:30 PM - LL9.10
Biocompatible Nanoparticle-Reinforced Hydrogels for Tissue Engineering.
Sarvesh Agrawal 1 , Naomi Sanabria-Delong 2 , Kyuongsik Chin 1 , Susan Roberts 1 , Gregory Tew 2 , Surita Bhatia 1 Show Abstract
1 Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States, 2 Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States