Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ1: Design and Characterization of Responsive Gel Systems
Monday AM, November 30, 2009
Room 208 (Hynes)
9:30 AM - **QQ1.1
Covalent Aadaptable Networks for Smart Materials Applications.
Christopher Bowman 1 , Brian Adzima 1 , Heeyoung Park 1 , Christopher Kloxin 1 , Timothy Scott 2 Show Abstract
1 Chemical and Biological Engineering , University of Colorado, Boulder, Colorado, United States, 2 Center for Bioengineering, Mechanical Engineering, University of Colorado, Boulder, Colorado, United States
Typical covalently-bound polymer hydrogels, such as PEG diacrylate-based materials, have permanent network connectivity; however, the permanent nature of these materials renders them intractable to post-fabrication manipulation. Photolytic or thermally degradable linkages can extend the utility of a gel, but ordinarily at the expense of permanent destruction of the material. Here, we describe the incorporation of photo- and thermo-reversible crosslinks into a polymer network, allowing for readily and repeated post-polymerization manipulation. Unlike conventional gels, these covalent adaptable networks (CANs) respond to external stimuli without being destroyed. This novel class of material represents a paradigm shift in chemical design, possessing an entirely new set of mechanical properties and behavior such as mechanical actuation, stress relief, and a gel-to-sol transition.
10:00 AM - **QQ1.2
Confined Smart Hydrogels for Applications in Chemomechanical Sensors for Physiological Monitoring.
Jules Magda 1 , Genyao Lin 1 , Prashant Tathireddy 1 , Florian Solzbacher 1 , Volker Schulz 2 , Margarita Guenther 2 , Gerald Gerlach 2 Show Abstract
1 , University of Utah, Salt Lake City, Utah, United States, 2 , Technische Universität Dresden, Dresden Germany
Many sensing approaches have been proposed that employ smart hydrogels, but perhaps the simplest sensing scheme can be obtained by confining a microscopically-thin smart hydrogel between a porous membrane and the diaphragm of a miniature pressure transducer. In such a scheme, a change in the environmental analyte concentration, as sensed through the pores of the membrane, changes the hydrogel osmotic swelling pressure, thereby changing the mechanical pressure measured by the pressure transducer. Sensor selectivity can be enhanced by attaching moieties to the hydrogel that selectively bind the analyte of interest. In order to illustrate this versatile sensing approach, we discuss its use in the development of a promising real-time blood glucose sensor for diabetic patients. For this application, we employ recently-developed enzyme-free hydrogels that are reversibly crosslinked by glucose but not by fructose. These novel glucose-respsonive gels allow us to construct a sensitive and selective implantable glucose sensor that avoids the “oxygen deficit” problem that currently plagues commercial electrochemical glucose sensors.
11:00 AM - **QQ1.3
High Resolution Monitoring of Swelling of Biospecific Hydrogel Materials.
Sven Tierney 1 , Bjorn Stokke 1 Show Abstract
1 Dep. of Physics, The Norwegian University of Science and Technology, Trondheim Norway
Various hydrogel material designed to adopt an equilibrium swelling state selectively depending on a biological relevant molecule can be utilized for label-free biosensing. This requires sufficiently sensitive readout-technology for the monitoring changes in the hydrogel swelling. In this paper, we describe determination of hydrogel swelling employing an interferometric technique. Thus, 50-60 μm radius, hemispherical hydrogels manufactured at the end of an optical fiber constituting the biospecific sensing element makes up a Fabry-Perot cavity. The interference wave of the reflected light at the fiber-gel and gel-solution interfaces enables detection of the optical pathlength within the gel and thus the gel swelling. This interference technique supports detection of changes in optical length of the hydrogels with 2 nanometer resolution. The biological specific sensing group in the hydrogel can alter the conditions within the hydrogel materials by changes in the charge density (e.g., glucose oxidase catalyzing glucose oxidation) or crosslinking density that display sensitivity to a biological relevant molecule. A glucose sensitive hydrogel material was realized on this platform utilizing a boronic-acid moities with additionally cationic, tertiary amines incorporated to tune the carbohydrate selectivity. The resulting hydrogel was found to be applicable for continuous monitoring of glucose levels in the relevant physiological range, and at physiological temperatures. Glucose induced, reversible crosslinking of the incorporated boronic acid groups attached to topologically separated network chains is the possible molecular mechanism for this material. A hybid hydrogel material with hybridized dioligonucleotides grafted to the polymer network as network junctions in addition to the covalent crosslinks supports detection of complementary oligonucleotides or other biological molecules based on specific aptamers. The signal transduction principle for altered swelling state of the hydrogel is based on destabilizing the junction point by displacement hybridization thereby yielding altered cross-link density. Concentration sensitivity applied as specific label-free detection of oligonucleotide is estimated to be in the nanomolar region. The current design support detection in excess of 1x1012 sequences. The results show that a wide array of bioresponsive hydogels can be manufactured at the end of the optical fiber for high resolution readout of the specific signal, thus supporting development of biosensors.
11:30 AM - QQ1.4
Dissociation of Thermally Reversible Polypeptide Hydrogels via Near Infrared Light.
Manoj Charati 1 , Ian Lee 1 , Kolin Hribar 1 , Jason Burdick 1 Show Abstract
1 Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Stimuli-responsive hydrogels are highly valued in fields ranging from controlled drug release, tissue repair and in microdevices. These hydrogels exhibit structural changes or dissociate based on changes in temperature, pH, and even with light exposure. With this in mind, polypeptide-based hydrogels, formed from a genetically engineered multi-block polypeptide have been previously developed by Tirrell and coworkers that exhibit a temperature dependent transition from a solid to liquid state (~40-45 C). The polypeptide consists of two associative leucine zipper end-blocks and a random coil midblock; self-assembly of the leucine zipper domains of the polypeptide leads to network formation, in which oligomer bundles serve as crosslinking points. For additional and triggered functionality, we formulated a composite of the thermo-reversible polypeptide gel with gold nanorods, where exposure to infrared light induces heating of the nanorods and consequently, melting of the gel. The gold nanorods were synthesized through a seed mediated growth process, imaged on a transmission electron microscope, and exhibited a characteristic morphology with an average length, width, and aspect ratio of 31.08 ± 4.52 nm, 9.24 ± 2.75 nm, and 3.64 ± 0.82, respectively, with the majority having elongated shapes and a small fraction that were rounded. The nanorods exhibited typical absorbances for both the longitudinal and transverse plasmon peaks and have a peak absorbance of ~800 nm. Gelation occurs within ~5 minutes (indicated by rheology plateau of ~1kPa) and is not hindered with the incorporation of the nanorods, encapsulated at 2.7×10-13 mol/ml. When exposed to near infrared light (~1W, 808 nm), the samples increase in temperature and dissociation occurs almost instantaneously, evident both macroscopically and through rheological studies. The dissociation of the thermally reversible network can be controlled effectively by the concentration of nanorods in the gel and the intensity and time of exposure to the near infrared light. The infrared light controlled dissociation of these hydrogels offers unique opportunities such as for delivery of drugs and growth factors at a precise dosage and a specific time or for incorporation of hydrogel actuators in microdevices. One additional benefit to this system is that near infrared light penetrates biological tissues quite well, as compared to other wavelengths, and this approach allows for triggered dissociation with transdermal light.
11:45 AM - QQ1.5
Thermal Responsive Hydrogels Formed by Triblock Copolymers with PEG and Oligomeric End-Blocks Bearing Cholesterol Moieties.
Yuxiang Zhou 1 , Nitin Sharma 2 , Rajeswari Kasi 1 2 Show Abstract
1 Chemistry Department, University of Connecticut, Storrs, Connecticut, United States, 2 The Institute of Materials Science , University of Connecticut, Storrs, Connecticut, United States
Physical hydrogels self-assembled through no-covalent intermolecular interactions have become a subject of great interest because they can be developed to smart materials which are responsive to external stimuli such as temperature variation, light, electric or magnetic field, pH, etc. These materials have found applications including controlled drug release, tissue engineering scaffolds, biosensors, actuators and so on. In the present study, three types of oligomeric triblock copolymers have been prepared, namely, oligo(cholesteryl methacrylate)n-PEG-oligo(cholesteryl methacrylate)n (OCMA-PEO-OCMA), oligo(5-cholesteryloxypencyl methacrylate)n-PEG-oligo(5-cholesteryloxypencyl methacrylate)n (OC5MA-PEO-OC5MA)and oligo(10-cholesteryloxydecyl methacrylate)n-PEG-oligo(10-cholesteryloxydecyl methacrylate)n (OC10MA-PEO-OC10MA). These three copolymers have the same PEG mid-block and different end-blocks containing side-on cholesterols via different lengths of methylene spacers (0, 5, 10 CH2). It is been found that these polymers can form hydrogels when dissolved in water with enough concentrations. And these hydrogels exhibit gel-sol transitions upon heating. Rheological studies show that by changing the spacer lengths and polymer concentrations, the modulus and the gel-sol transition temperatures (Tgel-sol) of the formed hydrogels can be tailored. Thus the hydrogel with desired Tgel-sol (~37 oC) and elastic modulus have been achieved by selecting the right polymer with the proper concentration. Scanning electron microscope images of these hydrogels show porous structures. The driving force of the gelation is presumed to be the stacking of cholesterol moieties that form the junctions of the polymer network. More work including X-ray diffraction and dynamic light scattering is under going to confirm this speculation. Since these triblock copolymers also feature the biocompatibility of PEO and the good cell affinity of cholesterol moieties, the prepared hydrogels can be good candidates for tissue engineering scaffolds or drug delivery systems.
12:00 PM - QQ1.6
Swelling and Diffusion Characteristics of Modified Poly (N-isopropylacrylamide) Hydrogels.
Guoguang Fu 1 , W. Soboyejo 1 Show Abstract
1 Mechanical Engineering, Princeton Univ., Princeton, New Jersey, United States
Thermo-responsive hydrogels are capable of swelling changes to external temperature. A series of modified poly (N-isopropylacrylamide) (PNIPA) hydrogels were synthesized by free radical polymerization in aqueous solution. Acrylamide (AAm) was used to increase the lower critical solution temperature (LCST), while sodium alginate (SA) was used to improve the swelling performance of the hydrogels. Experiments show that 5.5% mass ratio of AAm increased the LCST by about 9oC above that of conventional PNIPA. Also, SA significantly improved the equilibrium swelling ratio associate with temperature change. Trypan blue diffusion revealed significant differences in the fluid release obtained from hydrogels with modified LCST and swelling properties. The implications of the modified fluid release and swelling characteristics are also discussed for the device design of thermo-sensitive hydrogels for localized drug delivery.
12:15 PM - QQ1.7
Thermal and Mechanical Properties of Double-Gelling Thermosensitive Polymers.
Hanin Bearat 1 , Jorge Valdez 1 , Brent Vernon 1 Show Abstract
1 Harrington Department of Bioengineering, Arizona State University, Tempe, Arizona, United States
Stimuli-responsive materials have been widely used in various fields of biomedical research, with thermosensitive materials being of interest as hydrogels. Poly(N-isopropyl acrylamide) (poly(NIPAAm)) is an interesting polymer since it has temperature sensitivity around physiological conditions, with its lower critical solution temperature (LCST) being around 32°C. Its LCST can be altered with addition of different monomers; an addition of a hydrophobic monomer lowers its LCST while a hydrophilic monomer increases it. We have investigated two different Michael-type addition systems by conjugation of poly(NIPAAm) with hydroxyethylmethacrylate-acrylate (HEMA-acrylate) and cysteamine for system 1, as well as cysteamine-vinylsulfone and cysteamine for system 2, synthesized through free radical polymerization. Upon combination, the thiol group on the cysteamine attacks the olefin groups on either the HEMA-acrylate or the cysteamine-vinylsulfone, resulting in a chemical crosslink. Our system not only undergoes chemical crosslinking, but physical crosslinking due to the presence of NIPAAm. A comparison study was conducted on the properties of both systems. The polymers were characterized through HNMR, FTIR and HPLC for chemical composition verification and molecular weight distribution, as well as DSC and rheology for the thermosensitive and mechanical properties of the gels. Samples for DSC and rheology were prepared in PBS at pH 7.4, at 5wt% and 30wt%, respectively. All polymers demonstrated LCST points lower than 37°C, thus inducing physical gelation at body temperature. Various mixing times were tested for mechanical properties of both hydrogel systems using rheology. The polymers were mechanically mixed for 5 seconds, 2 or 4 minutes. A time sweep was conducted for each mixing time at 37°C for 75 minutes at a frequency of 1Hz and an oscillation stress of 10Pa. Results show that after mixing the two components for 5 seconds, system 1 did not form a gel whereas system 2 underwent gel formation within 30 seconds. However, after 2 minutes of mixing, system 1 formed a gel after 54 minutes and system 2 had formed a gel prior to its deposition on the rheometer plate. As the mixing time increased to 4 minutes, the time to gelation decreased to 48.5 minutes for system 1, indicating that longer duration of mechanical mixing allows for more chemical crosslinks to form prior to physical gelation at 37°C. Mechanical and thermal properties of the two systems demonstrate qualifications for potential use as in situ gels for endovascular embolization.
12:30 PM - QQ1.8
Responsive Polymer Scrolls Made by Strain Engineering.
Brian Simpson 1 , Kyriaki Kalaitzidou 1 Show Abstract
1 G. W. W. School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Responsive polymer scrolls have the ability to change geometry, flow characteristics, and adsorption properties upon the stimulation of an environmental change, such as temperature. These scrolls are fabricated by utilizing residual stress that is developed at the interface of a bi-layer structure. The focus of this work is to demonstrate the reversible response of polymer scrolls through the use of rheology as a means of capturing the transition from 3-D cylindrical to 2-D flat structures upon application of a trigger. The material system under investigation is poly dimethylsiloxane (PDMS)-gold (Au) bilayers. Temperature is used as the trigger to tune the residual stress locked at the interface of the bilayer. A theoretical equation that relates the material properties with the processing conditions and the residual strain is provided and used to estimate the residual strain. The experimental analysis involves determining the material modulus, thickness of the two layers, and the radius of curvature of the scrolls , This analysis can provide the scheme for optimizing the size, shape, and behavior of responsive polymer scrolls so that they can be utilized for numerous applications within the electronics, biomedical, and material science fields.
12:45 PM - QQ1.9
Development of a Flexible Conductive Polymer Membrane on Electroactive Hydrogel Microfibers.
Maria Bassil 1 2 , Mario El Tahchi 1 , Michael Ibrahim 1 4 , Eddy Souaid 1 , Georges El Haj Moussa 1 , Gisele Boiteux 2 , Joel Davenas 2 , Senentxu Lanceros-Mendez 3 , Joseph Farah 1 Show Abstract
1 LPA-GBMI, Department of Physics, Lebanese University, Beirut, Jdeidet, Lebanon, 2 IMP/LMPB Laboratoire des Matériaux Polymères et Biomatériaux, CNRS, UMR5223, Ingénierie des Matériaux Polymères, Claude Bernard University -Lyon I, , Lyon, Villeurbanne, France, 4 IMP-LMI, Claude Bernard University -Lyon I, , Lyon, Villeurbanne, France, 3 Department of Physics, , University of Minho, Braga Portugal
Polyacrylamide (PAAM) hydrogel is wet and soft material like the tissue of living organism and presents a remarkable stimuli response [1-2]. In addition, it is biocompatible and not biodegradable. Recently, we have presented a new artificial muscle design [3-5] based on this hydrogel. This electro-bio-active device consists on a fiber like elements of hydrolyzed PAAM, working in parallel, embedded in a thin conducting gel membrane that plays the role of electrodes .The first step in realizing the design is achieved by the production of PAAM microfibers. The influence of surface properties on the rate of shrinking of hydrogel microfibers was studied. It is shown that the geometrical distribution of microfibers influences their response to electrical and chemical stimuli. While the number of microfibers placed in parallel increases the rate of shrinkage decreases but it stays much higher than that of bulky gel .The second step is the development of the gel membrane which holdsthe electroactive hydrogel microfibers together. In fact, the main task in artificial muscle development is the control of the volumetric changes. Since the volume variation of the gel structure is under the kinetic control of the driving electrical power; developing a thin, flexible and conductive membrane which plays the role of electrodes can:- ensure a large surface area to stimulate the gel with an electrical input while improving the gel’s time of response.- hold the hydrogel microfibers together and enhance the mechanical properties of the microfibrous structure while moving smoothly with it.In this study, a thin gel membrane presenting such properties has been integrated in the PAAM structure. The geometrical properties of the layers and their composition are systematically modified and the effect of these modifications on the actuation rate of the structure has been investigated.Polymers presenting electrically controlled properties could lead to a revolution in a number of applications especially in the field of artificial muscle fabrication. Y.Bar-Cohen (2001) Electroactive Polymer Actuators as Artificial Muscles: Reality, Potential, and Challenges Second Edition, Bellingham, SPIE Press Monograph Vol. PM136. P. Calvert, Hydrogels for Soft Machines, Adv. Mater. 21 (2009) 743–756 M.Bassil, J.Davenas and M.El Tahchi, Sensors and Actuators B: Chemical 134 (2008) 496–501. M.Bassil, M.El Tahchi and J.Davenas, Advances in Science and Technology 61 (2008) 85-90. M.Bassil, M.Ibrahim, M.El Tahchi, J.Farah and J.Davenas, Mater. Res. Soc. Symp. Proc. 1134 (2009) BB01-10. M.Bassil, M.Ibrahim, M.El Tahchi, S.Lanceros-Mendez, G.Boiteux, J.Davenas and J.Farah, Proc. of the 2009 E-MRS Spring meeting, June 8–12, Strasbourg, France.
QQ2: Modeling and Simulation
Monday PM, November 30, 2009
Room 208 (Hynes)
2:30 PM - **QQ2.1
Molecular Dynamics Simulations of Polyleucine and Polyleucine-Modified HA.
Grant Smith 1 , Dmitry Bedrov 1 , Ben Hanson 1 Show Abstract
1 Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States
There is growing interest in exploiting the self-assembly of biocompatible amphiphilic polymers in aqueous solution to create novel structures and materials for a wide variety of biomedical applications. We are exploring the utilization of polyleucine as a hydrophobic modifier in order to control the structure and rheological properties of HA networks. Here we hope to exploit the fact that hydrophobic polyleucine side chains covalently attached to HA at the appropriate frequency and molecular weight will self-assembly to form physical crosslinks. We have utilized atomistic molecular dynamics simulations to study the conformations and interactions of polyleucine in aqueous solution, and, utilizing knowledge of polyleucine self-assembly gleaned from these studies, have conducted molecular dynamics simulations of polyleucine-modified HA in order to understand the role of side chain frequency and molecular weight on network properties.
3:00 PM - **QQ2.2
Gelation in Semi-Flexible Polymer Solutions.
Sanat Kumar 1 Show Abstract
1 , Columbia U, New York, New York, United States
Computer simulations have been employed to study the formation of a physical gel by semi-flexible polymer chains. The formation of a geometrically connected network of these chains is investigated as a function of temperature. As the temperature is lowered, a percolated homogeneous solution phase separates to form a non-percolated nematic fluid and upon further decrease in the temperature, it goes back to a percolated gel state. The gelation, at lower temperatures, is due to the dynamic arrest of chains, preventing them from completing the phase separation process. The effect of cooling rate is also studied. Quenching the system to the final temperature at a faster rate yields gelation while slower quenches result in phase separation. Depending on the rate of cooling, we either get a percolated gel or a single nematic bundle, both with the same local density.
4:00 PM - QQ2.3
The Effect of Nucleic Acid Length and Sequence on Effectiveness of the DNA Film Growth: A Molecular Dynamics Study.
Yaroslava Yingling 1 , Stacy Snyder 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Nucleic acids, such as RNA and DNA molecules, are especially appealing for nanobiotechnological applications due to their versatility in function and structure and molecular recognition properties of base pairing. We investigate the effectiveness of self-assembly of nucleic acids into thin, multilayered films, which have been prepared using single-stranded DNA deposited via a layer-by-layer technique. These thin films can form novel hollow multilayer capsules that posses unique engineered features such as size, shape, composition, porosity, stability, surface functionality, and biodegradability. Using extensive molecular dynamics simulations we explored the effects of nucleic acid strand length and nucleotide sequence on the efficiency of film growth. We monitor the dynamics and conformation of successive DNA oligonucleotides as film is grown in order to explain experimental results, including anomalous changes in growth efficiency with strand length and sequence. Our simulations predict the effective structural modifications that the film undergoes as a function of length and sequence and explains the experimental observation of a required minimum length of 20 nucleotides for film growth and a saturation limit at higher length or change in a sequence. Clearer understanding of the self-assembly process is expected to make possible the algorithmic self-assembly of nucleic acid thin films for applications in drug delivery and biological sensing.
4:15 PM - QQ2.4
Atomistic Modeling of the Structural and Mechanical Properties of Silk Nanocrystals and Nanocomposites.
Sinan Keten 1 , Britni Ihle 1 , Markus Buehler 1 Show Abstract
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Spider silk is a unique material that can blend disparate properties such as extensibility and high strength, making it one of the toughest materials known. Despite significant developments in our understanding of secondary structures in silk and their link to mechanical behavior, the molecular mechanisms of silk assembly and fracture are have not yet been elucidated. Through full-atomistic molecular dynamics simulations in explicit and implicit solvent as well as simplistic continuum formulations, we present a study that rides the gap between macro and nano-scale descriptions of silk by providing a first-principles based model for the constitutive behavior of silk nanocrystals. Replica exchange simulations on short segments of spider silk proteins predict a multi-phase representative model structure for the material that contains orderly beta-sheet regions dispersed within less orderly domains. Simulations elucidate the influence of mechanical tensile and shear forces on structure formation and fracture. The model illustrates the size, geometry and sequence dependent mechanical properties of spider silk, shedding light on the validity of earlier hypotheses and lower resolution models on silk mechanics.
4:30 PM - QQ2.5
Multiscale Mechanics of Large Random Fiber Networks.
Catalin Picu 1 , Hamed Hatami-Marbini 1 Show Abstract
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Random fiber networks are ubiquitous in biological and non-biological systems. The cytoskeleton, on which the cell integrity and structure depend, is a typical example of such network. We investigate the mechanics of such ensembles on multiple scales and observe that the mechanical fields (stress, strain, stiffness) exhibit power law scaling over a range of scales bounded below by the mean fiber segment length and above by the fiber persistence length. This conclusion results from the analysis of spatial correlations of local elastic moduli evaluated over network sub-domains at multiple probing scales. Therefore, the network deforms similarly with a stochastic fractal heterogeneous elastic object. Based on this observation, a multiscale method is developed to solve boundary value problems defined on large fiber network domains, without accounting for every fiber in the system.
4:45 PM - QQ2.6
Local Measurements of Phase Transitions in Biological Polymers (Lambda DNA and Bacteriorhodopsin Membrane).
Maxim Nikiforov 1 , Roger Proksch 2 , Sophia Hohlbauch 2 , Stephen Jesse 1 , Sergei Kalinin 1 Show Abstract
1 CNMS, ORNL, Oak Ridge, Tennessee, United States, 2 , Asylum Research Co., Santa Barbara, California, United States
Phase transitions play an important role in biology. Specifically the thermodynamic stability of internal membrane proteins and bio-polymers (DNA) is an important issue of biophysics. Purple membranes from Halobacterium halobium contain bacteriorhodopsin, an integral protein 70-80% of whole mass is intramembraneous. There are heated debates in the field about the parameters of thermal denaturation of bacteriorhodopsin, such as the denaturation temperature, enthalpy etc. Recently, bacteriorhodopsin and DNA are materials proposed as a components of biomolecular electronics. Thus, reliable information about the phase transitions of supported smaples of bacteriorhodopsin membranes and DNA is necessary.Phase transitions in polymer/biopolymer materials are associated with the large changes in mechanical properties of the samples. We developed the technique for the measurements of the temperature dependence of the mechanical properties with high spatial resolution. This technique is based on the measurements of the contact stiffness of the atomic force microscopy tip – sample system as a function of temperature. The contact stiffness was monitored using dual frequency resonance tracking methodology developed by Asylum Research Co.We measured softening temperature of the bacteriorhodopsin membranes deposited on mica substrate using the developed technique. It was found that extracellular and intracellular membranes have similar softening temperatures. The tip – sample contact are had 20 nm radius meaning that the response from less than 20 molecules was measured. The temperature dependence of mechanical properties of single stranded DNA was also measured and used for the calculations of softening temperature of the single biological molecule.A portion of this research at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. A portion of this research was sponsored by Asylum Research Co.
5:00 PM - QQ2.7
Nanoscale Pattern Formation in Polyelectrolyte Gels.
Prateek Jha 1 , Francisco Solis 2 , Juan de Pablo 3 , Monica Olvera de la Cruz 1 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States, 2 , Arizona State University, Glendale, Arizona, United States, 3 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Polyelectrolyte (PE) gels with hydrophobic backbones exhibit complex phase behavior that includes the formation of nanophases, that is, the local segregation of the monomers. The formation of these inhomogeneous phases is possible due to the presence of different length scales for interactions; in this case with electrostatic effects providing long length-scales. We demonstrate the presence of these nanophases in a model for a PE gel that incorporates entropic, elastic, electrostatic, and solvent interactions. We analyze the model with both linear and non-linear methods and show that the linear approximation properly identifies the region of instability against nanophase formation. Nonlinear effects lead, in addition, to features not accessible through the linear approach, such as the formation of very sharp interfaces between the nano-segregated regions. We discuss the dependence of the periodicity and monomer and charge distributions for the gel as functions of the gel physical parameters. We find that the higher the charge fraction and the lower the crosslink density, the larger the range of solvent quality for nanophase segregation.
5:15 PM - QQ2.8
Artificial Polymers Mimic Bacteriophage Capsid Proteins and Encapsulate Nucleic Acids.
David Robinson 1 , Michael Kent 1 , George Buffleben 1 , Ronald Zuckermann 2 Show Abstract
1 Energy Nanomaterials, Sandia National Laboratories, Livermore, California, United States, 2 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
The filamentous bacteriophage m13 and related viruses encapsulate DNA with protein, forming an organic wire about 1 micrometer long and less then 10 nanometers wide. The length of the wire is formed from many copies of a single protein, which is a single alpha helix formed from about 50 amino acids. It can be viewed as a very sophisticated surfactant, with hydrophilic regions that interact with the DNA and form the outer surface, and hydrophobic regions that pack against each other. We have implemented these design principles in peptoids (sequence-specific N-functional glycine oligomers) and have found that they form well-defined aggregates with DNA. This approach may prove applicable to the design of hydrogels with tailored filament length, stiffness, and functionality. It may complement phage display methods, providing new approaches to gene transfection and nanofabrication that do not involve bacterial intermediates. We have adjusted the peptoid-DNA interaction by tuning the sequence and length of the peptoids and observing by gel electrophoresis. We have observed evidence of filamentous assemblies by neutron and light scattering and electron microscopy. Molecular dynamics simulations, circular dichroism, and NMR provide confidence that the peptoids have a helical conformation.This work was performed under the Laboratory-Directed Research and Development Program at Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
5:30 PM - QQ2.9
The Effect of Friction on the Structure and Dynamics of Unbonded Random Fiber Networks.
Gopinath Subramanian 1 , Catalin Picu 2 1 Show Abstract
1 Scientific Computation Research Center, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Insitute, Troy, New York, United States
The role of interfiber friction in defining the structure and dynamics of entangled fiber networks is studied. The fibers are not bonded to each other and the excluded volume condition is imposed in all simulations. When subjected to isostatic compression, the fiber mass undergoes a transition from the sparse network structure to a dense state. The critical density at which this transition occurs decreases with increasing fiber aspect ratio. This transition, in the case of frictionless fibers is sharp and occurs over a small range of densities. In the presence of friction, the transition is more gradual. In the network state, frictionless fibers were observed to display an exponential distribution of elastic energy at fiber-fiber contacts. Fibers with friction displayed a power law distribution. Cyclic compression/relaxation shows hysterisis behaviour both in the presence and absence of friction. However, in the presence of friction, multiple cycles of compression/relaxation result in the fiber network forming one single entangled mass, held together solely by friction, and this phenomenon is not observed in frictionless fibers. The density of this resultant mass is seen to increase with the coefficient of friction.
5:45 PM - QQ2.10
A Large Deformation Theory for Thermo-chemo-mechanically Coupled Polymeric Gels.
Shawn Chester 1 , Lallit Anand 1 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Many stimulus responsive polymer gels operate in non-isothermal, chemically saturated environments. Such situations require not only thermo-mechanical coupling, but thermo-chemo-mechanical coupling. Thermo-chemo-mechanical coupling is a coupling between the temperature, ``chemical'', and deformation fields. All fields may be inhomogeneous and evolve with the system over time. The mostfamiliar type of material coupling is thermo-mechanical coupling, between the temperature and deformation fields. What is non-standard is the chemical coupling. In the specific case considered in this talk, the chemical coupling is the diffusion of fluid into the polymer network causing large volume changes. Many continuum level theories in the literature do not fully couple the temperature, chemical, and deformation fields. Instead, they take the limit of isothermal, or having a constant fluid concentration field. We have developed a general, thermodynamically consistent, continuum level thermo-chemo-mechanically coupled theory for large deformations of polymer gels. In discussing special constitutive equations, we limit our attention to isotropic materials, and a special model for the free energy based on standard Gaussian statistical mechanics considerations of changes in configurational entropy due the stretching of the polymer chains, along with a simple Flory-Huggins model for the free energy change due to mixing of the fluid with the polymer. Furthermore, current commercial finite element codes do not provide for a coupling between the large deformation, temperature, and the chemical fields. Therefore, the corresponding numerical procedure in the context of standard finite element methods is also developed. The numerical simulation capability is then implemented for some representative examples of swelling in polymer gels due to fluid absorption.
QQ3: Poster Session I
Monday PM, November 30, 2009
Exhibit Hall D (Hynes)
9:00 PM - QQ3.1
Controlled Porous Structure of DNA Hydrogel by Salt and pH.
Sun Hee Lee 1 , Chang Kee Lee 1 , Su Ryon Shin 1 , Seon Jeong Kim 1 Show Abstract
1 , Hanyang University, Seoul Korea (the Republic of)
DNA hydrogels are currently of considerable interest in bio-applications due to the remarkable properties of DNA. DNA has a high sensitivity by salt and pH because of electrostatic repulsion of phosphate group and hydrophobic interaction of base pair. These DNA properties have influence on the characteristic of DNA hydrogel consisted of native DNA in which DNA strands were formed random entanglements to provide physically crosslinked networks. DNA hydrogel have response to change the volume transition by external stimuli such as temperature, pH, ionic strength, chemicals, surfactants, etc. Because of their significant swelling and deswelling in response to external stimuli, these polymeric networks are used for a variety of applications in biological such as system drug delivery and tissue engineering. In this work, DNA hydrogel have been fabricated without crosslinker using wet-spinning method. The DNA hydrogel showed interesting as response behavior, since drastic volume changes can be induced by change such as salt and pH. When salt concentration increased, DNA hydrogels appeared to be decreased. In pH condition, as pH value decreased, DNA hydrogels showed shrinking behavior. The swelling of the DNA hydrogel appeared to be reversible by controlling change of pH and concentration of salt. Thus DNA hydrogel is useful for bio-application.
9:00 PM - QQ3.10
Time-Resolved Fourier Transform Infrared Spectroscopy of Diblock-Copolymers Inspired by Spider Silk.
Wenwen Huang 1 , Sreevidhya Krishnaji 2 , David Kaplan 3 , Peggy Cebe 1 Show Abstract
1 Physics, Tufts University, Medford, Massachusetts, United States, 2 Chemistry, Tufts University, Medford, Massachusetts, United States, 3 Biomedical Engineering Department, Tufts University, Medford, Massachusetts, United States
We report the characterization of a new family of silk-based block copolymers. The block copolymers HAB3, HAB2, HBA, HBA2, HBA3 and HBA6 utilize protein sequences, A and B, found in native spider dragline silk (Nephila clavipes), where B = hydrophilic block, A = hydrophobic block, and H is a histidine tag. To assess structure we used time- resolved Fourier transform infrared spectroscopy and X-ray scattering. Thermal properties were determined by differential scanning calorimetry and thermogravimetic analysis. Results indicate that as the size of the hydrophobic A-block increases, the content of beta sheets increases, reaching a crystallinity of 30% in the block copolymer design that had the greatest hydrophobic content (A6 content). As temperature was slowly increased from room temperature to 280 °C (at 2 °C/min), bound water which served as a plasticizer evaporated during heating and the conformation of the block copolymers also changed. This process was monitored by Fourier transform infrared spectroscopy. In the Amide I region, a similar trend was shown in all block copolymers: as temperature increased to 180 °C which is above the copolymer glass transition temperature, Tg ~ 150 °C, content of Beta sheet and Alpha helices was reduced, and content of Beta turns was increased.Support was provided from the National Science Foundation, Division of Chemical, Bioengineering, Environmental, and Transport Systems, through CBET-0828028 and the MRI Program under DMR-0520655 for thermal analysis instrumentation.
9:00 PM - QQ3.11
The Synthesis of Poly(N-isopropylacrylamide) / Polyacrylamide Anisometric Hydrogel Particles by Photocrosslinking and Temperature Responsive Release Behavior.
Sona Lee 1 , Jonghwi Lee 1 Show Abstract
1 Chemical Engineering, Chung-Ang University, Seoul Korea (the Republic of)
Poly(N-isopropykacrylamide) (PNIPAAm) formed interpenetrating network (IPN) hydrogels with polyacrylamide (PAAm) in the preparation of anisometric particles by Ultra Violet (UV) irradiation under nitrogen. The PNIPAAm-based hydrogels were prepared by using N,N’-methylenebisacrylamide and clay of various concentrations as crosslinkers. 2-Hydroxy-1-[4-(hydroxyethoxy)-phenyl]-2-methyl-1-propanone (Irgacure2959) was used as a water soluble initiator. A freeze drying step was inserted into the two photocrosslinking steps of anisometric particles of IPN in order to photocrosslink PAAm in pre-crosslinked PNIPAAm hydrogels phases. Water soluble dyes and Green Tea Polyphenol (GTP) were infiltrated into the resultant IPN hydrogels as a model active pharmaceutical ingredient. The swelling ratio of PNIPAAm/PAAm hydrogels was examined with loaded model drugs at various temperatures. The anisometric properties related with drug release from the hydrogels were measured by optical microscopy, Fourier transform infrared spectroscopy (FTIR), UV-vis spectroscopy, and differential scanning calorimeter (DSC). Uniaxial compression tests revealed that the concentration of PNIPAAm and crosslinkers could control the morphological changes of the hydrogels as a function of temperature.
9:00 PM - QQ3.2
A New Bio-inorganic Nanocomposite Membrane for Glucose-modulated Release of Insulin.
Claudia Gordijo 1 , Adam Shuhendler 1 , Xiao Wu 1 Show Abstract
1 , University of Toronto, Toronto, Ontario, Canada
Diabetes is a serious condition characterized by the body’s inability to adequately metabolize glucose. It affects more than 246 million people worldwide. The conventional way of controlling glycemia is the frequent self-administration of insulin, which often results in hypoglycemia. A more effective approach to delivering insulin in direct response to blood glucose levels mimicking a healthy human pancreas is thus highly desirable. Smart materials have been investigated for glucose-modulated insulin delivery. Herein, we propose the application of a new bio-inorganic nanohybrid glucose-responsive membrane as a single self-regulated platform for self-regulated insulin deliver. In preparation of such materials we have coordinated functionalities coming from organic, inorganic and biological components. We have conjugated multifunctional MnO2 nanoparticles with bovine serum albumin and the enzymes glucose oxidase (GOx) and catalase (CAT). A membrane was prepared by crosslinking these biomolecule-MnO2 conjugates in the presence of pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM/MAA) hydrogel nanoparticles.The membrane acts as a glucose sensor by the action of GOx, which catalyzes glucose oxidation and production of gluconic acid. This reaction creates a low pH microenvironment in the membrane, causing the hydrogel nanoparticles to shrink, leading to the formation of an interconnected porous framework that increases insulin permeability. In this system GOx turnover cycles produces hydrogen peroxide, an undesirable product which can leads to enzymes inactivation and unwanted toxicity. We demonstrated that the combination of CAT and MnO2 nanoparticles yields better results in terms of quenching of H2O2 and long-term GOx stability as compared to the membranes in which CAT or MnO2 were separately employed. SEM images of the nanohybrid membranes revealed a complex sponge-like structure with MnO2 and hydrogel nanoparticles homogeneously distributed in the thin cavity wall of the base membrane. The in vitro release of insulin was evaluated over time at glucose concentrations relevant to diabetic patients. The self-regulated system released very small amounts of insulin at normal blood glucose levels, while a four-fold increase in insulin release was observed when glucose concentration was increased to hyperglycemic levels. When the glucose level drops to a normal level the rate of insulin release is decreased. The reliability and reproducibility of the glucose-responsive insulin release profile of the system allows for self-regulated insulin release in response to glucose concentrations analogous to a healthy pancreas.
9:00 PM - QQ3.4
Chemically-Responsive Gels Prepared from Microspheres Dispersed in Liquid Crystals.
Santanu Pal 1 , Ankit Agarwal 1 , Nicholas Abbott 1 Show Abstract
1 Chemical & Biological Engineering, University of Wisconsin-Madison, Madiosn, Wisconsin, United States
Liquid crystalline materials are a promising class of stimuli-responsive materials that have been demonstrated to undergo surface-induced orientational ordering transitions that can be highly sensitive and specific to chemical species. However, past studies demonstrating surface-induced transitions in LC have employed thin films of low molecular weight liquid crystals (LCs) that are difficult to stabilize (due to dewetting of the LC on a surface). In this presentation, we report that it is possible to prepare liquid crystalline gels using mixture of polystyrene (PS) microspheres and nematic LCs which undergo changes in orientational order, and thus optical appearance, in response to exposure to specific chemical compounds. These colloid-in-liquid-crystal (CLC) gels are mechanically stable and can be molded on chemically functionalized surfaces into thin films containing micrometer-sized LC rich domains that span the two interfaces of the gels. In contrast to other reports of LC gels, where the presence of a polymeric or self-assembled small-molecule gelator network within a nematic LC frustrates ordering transitions from propagating through the gels over distances, we demonstrate that thin films of CLC gels, when supported on chemically functionalized surfaces, do undergo easily visualized ordering transitions upon exposure to organophosphonate compounds. Because these optically responsive CLC gels are mechanically robust and can be molded, we believe this class of composite LC material may be broadly useful for the design of chemically-responsive LC devices.
9:00 PM - QQ3.5
Microfluidics-assisted Synthesis of Micrometer Scale Heparinazed Hydrogels.
W. Jeong 1 2 , J. Lim 1 2 , J. Choi 1 2 , S. Yang 1 2 Show Abstract
1 Chemical & Biomolecular Eng., KAIST, Daejeon Korea (the Republic of), 2 National Creative Research Initiative Center for Integrated Optofluidic Systems, KAIST, Daejeon Korea (the Republic of)
Over the past decades, considerable efforts have been devoted for the fabrication of polymeric gels at nano-to-micrometer scale with uniform size and functional property. Micrometer scale gels, cross-linked and swellable hydrophilic polymer particles, especially offer significant advantages as drug delivery vehicles compare to conventional submillimeter scale gels. The small size of the micrometer scale gels can improve drug or gene effect in target tissue, and the stability of therapeutic agents against chemical/enzymatic degradation. Microfluidic flow-focusing devices made of PDMS provide a versatile approach to prepare monodisperse droplets at high frequencies. However, they exhibit significant problems as the size of channel decreases. Leakage can occur at the interface between PDMS and glass due to the high pressure originated from the fluid injection. Continuous-flow microfluidics tends to undergo chaotic oscillations in which flows vary uncontrollably. In addition, the dust and debris often block the channels after only a few uses. To overcome these problems, we introduce additional sheath flow in the flow-focusing regime. The dispersed and continuous phases are hydrodynamically focused into a narrow stream, and the width of stream determines the size of the droplets. Three-inlet streams are controlled pneumatically to offer fast response time and stable flow. Few-micron sized emulsion droplets can be generated at steady state by using the pneumatic pumping system. The evaporation of solvent leads to further shrinkage of droplets, therefore submicron-sized hydrogels can be generated. We use biodegradable heparin, a highly sulfated natural glycoaminoglycan, as model polymeric materials. The photo-crosslinkable heparin is prepared by substituting the carboxylic groups of heparin with acrylate groups using N-(3-aminopropyl) methacrylamide hydrochloride. Aqueous emulsion droplets comprising of heparin, photoinitiator, crosslinker and growth factor are generated via hydrodynamic focusing and in-situ crosslinking by UV light. Hydrogels via these strategies suggest opportunity for two-stage degradation. Non-covalent heparin-growth factor interaction is capable of receptor-mediated gel erosion for targeted drug delivery. The left fragments of covalently bonded hydrogel can degrade via hydrolytic processes under physiological condition. To determine optimum conditions for growth factor delivery, we investigate swelling and degradation behavior for hydrogel at various heparin concentrations.
9:00 PM - QQ3.6
Depth Dependence of the Osmotic Properties of Bovine Cartilage.
Candida Silva 1 , Iren Horkayne-Szakaly 1 , David Lin 1 , Peter Basser 1 , Ferenc Horkay 1 Show Abstract
1 NICHD, NIH, Bethesda, Maryland, United States
Articular cartilage is composed of highly hydrated extracellular matrix synthesized by chondrocytes. Cartilage is an inhomogeneous and anisotropic tissue that covers the ends of bones. Healthy cartilage allows bones to glide over one another and absorbs energy from the shock of physical movement. Osteoarthritis is one of the most frequent causes of physical disability among adults. It is a joint disease that mostly affects the cartilage. In osteoarthritis the surface layer of cartilage breaks down and wears away.The major structural elements of cartilage (collagen fibers, proteoglycan assemblies and chondrocytes) are not randomly organized but divided into zones that differ in molecular organization. The orientation of collagen fibers within the tissue differs between the zones. They exhibit a tangential orientation at the surface, random configuration in the middle layer, and radial orientation close to the bone surface. The superficial layer contains flattened chondrocytes aligned parallel to the surface. In the middle layer the chondrocytes are spherical, while in the deep layer they form columns oriented in the vertical direction. The proteoglycan aggregates are composed of aggrecan subunits bound into very large aggregates with hyaluronic acid. (This interaction is stabilized by a third component, the link protein.) The proteoglycan aggregates occupy the pores of the collagen matrix. Articular cartilage exhibits high water content, ranging from 65% to 90% of the tissue weight, depending on age and location. This complex morphology provides the basic structural framework for the cartilage matrix, and the load-bearing ability of the tissue strongly depends on this structure. The collagen fibers contribute to the tensile properties and the proteoglycans provide resistance and resilience to deformation. When cartilage is compressed, the interstitial fluid is pressurized and supports the applied contact stress.Although several biomechanical models have been proposed to explain the biomechanical properties of cartilage, none of these models provides a complete understanding of the relationship between the structure and the functional properties. In this work we use osmotic stress techniques to investigate the osmotic response of cartilage layers as a function of the depth from the articular surface. The hydration of thin cartilage slices from different zones is measured under controlled osmotic conditions. The osmotic compression modulus, which quantifies the load-bearing capacity of the tissue, is determined from measurement of the osmotic swelling pressure as a function of the swelling degree. The atomic force microscope is used to map the local elastic properties of the extracellular matrix and the cells. The experimental results reveal significant depth-dependent variations in the compressive properties.
9:00 PM - QQ3.8
Rheology of Highly Packed Ionic Microgels.
Paul Menut 1 2 , David A. Weitz 1 Show Abstract
1 , Harvard University, Cambridge, Massachusetts, United States, 2 , Montpellier SupAgro, Montpellier France
Microgels based systems exhibited a various range of rheological behavior depending on their particles volume fraction, the internal cross-linked density of each particle, and the properties of the solvent. One of their main interest for application purposes is their ability to respond quite rapidly and dramatically to the evolution of their surrounding medium. They can then swell or shrink in response to change in temperature, osmotic pressure, pH or ionic force, depending on their own chemical structure.At low volumic fraction, the viscosity of a microgel suspension increase with the concentration, following the usual Einstein’s rules for hard spheres.However, when the concentration increases, their ability to deform and to shrink allows the preparation of materials with microgel volume fraction higher than one, if taking their hydrodynamic diameter as equal to the one measured in dilute state. This behavior underline the ability of those particles to deform and to shrink. The study of the rheological properties of those highly packed systems has shown a paste-like behavior, characterized by aging and rejuvenation. In this study, we explore the rheological properties of charged microgel systems (polyNIPAM-co-AAc), in presence of counter-ions, while increasing packing ratio. Their behavior was characterized by a series of creep/relaxation procedures, on a stress-controlled rheometer (AR-G2). The effect of the electrostatic interaction evolution between particles during packing is describe, as the monitoring of counter-ion concentration allows the controlled screening of the charges.
9:00 PM - QQ3.9
Hybrid Gels from Self-Assembling Peptide Networks.
Sameer Sathaye 1 , Radhika Nagarkar 2 , Joel Schneider 2 , Darrin Pochan 1 Show Abstract
1 Department of Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 Department of Chemistry and Biochemistry, University Delaware, Newark, Delaware, United States
Hybrid Gels constitute a novel class of polymeric materials developed with an aim of combining and/or enhancing the diverse and complementary properties of their individual constituent networks. Self-assembling peptide hydrogels formed from aqueous solutions of β-hairpin forming peptides have been extensively reported. These hydrogels are interesting candidates as part components of hybrid gels due to their ability to retain their inherent physical properties in the presence of other hydrogel networks and other added functionality (e.g. an inorganic coating of the gel fibrillar nanostructure). Synergistic interactions of these peptidic networks with other added polymer co-networks with a range of tunable synthetic characteristics and properties have been explored by various characterization techniques such as Dynamic Mechanical Analysis (DMA), Transmission Electron Microscopy (TEM) and Small Angle Neutron Scattering (SANS). The ease of producing co-networks between a wide array of target polymer networks and β-hairpin peptides as the fundamental, core network will be discussed.
9:00 PM -
QQ3.12 TRANSFERRED TO QQ6.14
Ferenc Horkay National Institutes of Health
Noshir Langrana Rutgers University
Walter Richtering RWTH Aachen University
QQ4: Mechanical Properties of Gels and Tissues
Tuesday AM, December 01, 2009
Room 208 (Hynes)
9:30 AM - **QQ4.1
Some Minimal Models of Network Elasticity.
Jack Douglas 1 Show Abstract
1 Polymers Division, NIST, Gaithersburg, Maryland, United States
Classical network elasticity theories are based on the conception of flexible volumeless network chains fixed into a network in which there are no excluded volume interactions between the chains and where the chains explore accessible configurations by thermal fluctuations. The limitations of this approach are clear from the observation that unswollen rubbery materials are nearly incompressible, reflecting the existence of strong intermolecular interactions that restrict the polymer chains to an exploration of their local molecular environments. The imposition of a deformation to these solid rubbery materials then necessitates a consideration of how local molecular packing constraints become modified under deformation and the impact of these changes on the macroscopic elasticity of the material as a whole. The simple ‘localization model’ of rubber elasticity, introduced by Gaylord and Douglas (GD), provides an attractive minimal model for the network elasticity of rubbers having strong intermolecular interactions in the dense polymer state. The properties of this model are summarized and compared to observations on rubbery materials where both the cross-linking density and swelling are varied. The model is extended to describe networks of stiff chains and networks having junctions formed through reversible association.
10:00 AM - **QQ4.2
Mechanics of Active Biopolymer Networks.
David Weitz 1 Show Abstract
1 Physics & SEAS, Harvard University, Cambridge, Massachusetts, United States
This talk will present the results of measurements of the mechanics of reconstituted biopolymer networks, and will explore the effects of molecular motors within the networks.
11:00 AM - **QQ4.3
Nonlinear Elastic Response of Extracellular Matrices to Cell-generated Stresses.
Jessamine Winer 1 , Shaina Oake 1 , Paul Janmey 1 Show Abstract
1 Institute for Medicine and Engineering, Univ. Pennsylvania, Philadelphia, Pennsylvania, United States
Most native extracellular matrices composed of collagen or fibrin fibers exhibit nonlinear rheology characterized by strain stiffening and negative normal stress in response to simple shear deformation. Tension between matrix and cell membrane adhesion complexes can initiate signals that alter cell structure and function. Many cell types modulate their spread area, stiffness, motility, and protein expression in response to the substrate stiffness. Studies of stiffness sensing typically employ linear elastic materials whose stiffness is independent of the applied strain, whereas biological gels often stiffen in response to increasing strain. Fibroblasts and mesenchymal stem cells adherent to linearly elastic gels typically display a small, round phenotype on soft substrates and increase spread area as the elastic modulus of their substrate increases. On the strain-stiffening biopolymer gel fibrin, the same cell types are maximally spread even when the gel's low strain elastic modulus would predict a round morphology. Traction microscopy reveals that cells apply active displacements of several microns up to five cell lengths away, and atomic force microscopy shows that these displacements locally stiffen the gel by deforming it beyond its linear range. The magnitude of cell-applied strains is inversely related to the gel's low strain elastic modulus and results in long distance cell-cell communication and alignment. Non linear elasticity of both intracellular and extracellular biomaterials allows cells to alter their own stiffness as well as that of the extracellular matrix by applying tensions that locally strain the networks and cells appear to exploit local stiffness chances for long range mechanical communication.
11:30 AM - QQ4.4
Theory of Load Support by Articular Cartilage.
Jeffrey Sokoloff 1 , J. Ruberti 2 Show Abstract
1 Physics, Northeastern University, Boston, Massachusetts, United States, 2 Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States
Articular cartilage is comprised of charged macromolecules, proteoglycans, trapped within a network of type II collagen fibrils. A proteoglycan can be modeled as a collection of charged polymers (in our case the chondroitin sulphate chains) each anchored to a straight line (to represent the backbone of the proteoclycan), which is essentially a cylindrical symmetry polymer brush. Analytic mean field, commonly used in the study of polyelectrolyte brushes, and scaling theory are used to determine the load support that arises as a proteoglycan is compressed. The calculations show that under a pressure of a MPa, the radius of each of these molecules is compressed to 14 % of its zero load value, which is in agreement with observation. The load carrying ability of cartilage based on this model for proteoglycan is studied as a function of degree of compression of the cartilage and salt concentration, and compared to experiment. Dissipation due to polymer chains protruding from these macromolecules getting entangled with polymer chains attached to neighboring proteoglycans is shown to be unlikely to occur because the number of monomers, belonging to a given polymer which gets entangled in a neighboring proteoglycan, is likely to be too small to produce a blob of radius as large as a mesh length.
11:45 AM - QQ4.5
Robustness-strength Performance of Alpha-helical Protein Filaments with Hierarchical Structures.
Zhao Qin 1 2 , Steven Cranford 1 2 , Theodor Ackbarow 1 3 , Markus Buehler 1 2 4 Show Abstract
1 Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Theory Division, Max Planck Institute of Colloids and Interfaces, Potsdam Germany, 4 Center for Computational Engineering, Massachusetts Institute of Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Hierarchical nanostructures are common for biological materials, ranging through atomistic, molecular to macroscopic scales. By utilizing the recently developed Hierarchical Bell Model (HBM), here we show that the use of hierarchical structures leads to an extended physical dimension in the material design space that resolves the conflict between strength and robustness which is a limitation faced by many synthetic materials. We report a case study in which we combine a large number of alpha-helical elements in various hierarchical combinations and measuring each performance in the strength-robustness space. We find that for a large number of constitutive elements, more than 98% random structural combinations of elements lead to either high strength or high robustness, reflecting the banana-curve performance in which strength and robustness are mutually exclusive properties. This banana-curve type behavior is common to most engineered materials. In contrast, only few (<2%) specific types of combinations of the elements in hierarchies is possible to maintain high strength at high robustness levels. We found that alpha-helical protein filaments with certain arrangement of two level hierarchies are optimal for strength and robustness. This behavior corresponds to the naturally observed material performance in biological materials, suggesting that some particular hierarchical structures enable to fundamentally change the material performance. The results suggest that biological materials may have developed under evolutionary pressure to yield materials with multiple objectives, such as high strength and high robustness, a trait that can be achieved by utilization of hierarchical structures. Our study indicates that both the formation hierarchies and the assembly of specific hierarchical structures play a crucial role in achieving these mechanical traits. Our findings may enable the development of self-assembled de novo bioinspired nanomaterials based on peptide and protein building blocks.
12:00 PM - QQ4.6
Physically Crosslinked Hyaluronic Acid-based Novel Hydrogels.
Divya Bhatnagar 1 , Miriam Rafailovich 1 Show Abstract
1 Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States
This work investigates the rheological properties of Hyaluronic acid(HA)-Clay hydrogels prepared by physically crosslinking non-modified HA with inorganic clay. Resulting hydrogels were transparent and mechanically stable. To investigate their mechanical properties the hydrogels were subjected to oscialltory shear rheometry which allows the evaluation and comparison of the shear storage moduli (G'), an index of the stiffness of the hydrogels. While the temperature sweep monitored the effect of temperature on G', the stress and frequency sweeps measured G' as a function of stress and frequency respectively. Results from frequency sweeps suggested the formation of a stable,three dimensional network while the stress sweep revealed the linear viscoelastic region and the breaking stress for the HA-clay hydrogels. Results from temperature sweep tests suggested the stability of the crosslinks before 130°C and that the effect of increasing temperature makes the hydrogels less rigid. Cell cultivation on the surface of a novel HA(Hyaluronic Acid)-clay hydrogel was studied using human dermal fibroblasts. It was found that the cells could becultured on the surfaces of HA-clay hydrogels for upto 4 days. Such a detailed rheological characterization of our HA-clay hydrogels with no chemical additives will aid in the design of biomaterials targeted for biomedical or pharmaceutical purposes, including rigid cell scaffoldstructures.
12:15 PM - QQ4.7
Ionic Crosslinking for Enhancing Mechanical Performance of Acrylic Triblock Copolymer Hydrogels.
Kevin Henderson 1 , Tian Zhou 1 , Kenneth Shull 1 Show Abstract
1 Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States
Symmetric, amphiphilic triblock copolymers provide a facile, self-assembled route toward the formation of physically crosslinked polymer hydrogels. Due to a well-defined network architecture and homogeneity of crosslinks, these materials are capable of withstanding much larger strains than chemically crosslinked materials of comparable stiffness. However, the mechanical properties of these materials are still incomparable to the robust characteristics of tough biological materials. The ability to tailor the molecular-scale dissipation mechanism of crosslinked polymer hydrogels is of significant interest for the fabrication of tough, synthetic alternatives to biological materials.Here, we present one such route through the extension of ionic crosslinking in an acrylic triblock copolymer hydrogel. The polymer architecture consists of symmetric, hydrophobic poly(methyl methacrylate) endblocks and a hydrophilic poly(methacrylic acid) midblock with a total polydispersity of 1.13. These materials spontaneously form bridged, spherical micelles upon appropriate solvent exchange with Young’s moduli on the order of 1 kPa. Upon ionic crosslinking with divalent cations (Zn, Ca), the solvated midblock of the polymer chain forms metallic complexes that serve as dynamic, pseudo-covalent linkages that significantly alter the mechanical response of the material. The modulus increases by 2-3 orders of magnitude depending on pH, solution concentration, and cation identity. Tensile testing and compressive indentation tests indicate not only a favorable increase in material toughness but also a considerable degree of recoverable energy dissipation upon successive loading not witnessed in many other toughening mechanisms.
12:30 PM - QQ4.8
Development of PEG-Based Hydrogels with Tunable Physical Crosslinks Mediated by Collagen Mimetic Peptide’s Triple Helical Propensity.
Patrick Stahl 1 2 , Nicole Romano 1 , Denis Wirtz 3 2 , S. Michael Yu 1 2 Show Abstract
1 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Institute for NanoBioTechnology, Johns Hopkins University, Baltimore, Maryland, United States, 3 Chemical and Biomolecular Engineering, Johns Hopkins University, Columbia, South Carolina, United States
Hydrogels are widely used in biomedical applications including tissue engineering and drug delivery due to their biocompatibility and biomimetic qualities. The most effective tissue engineering hydrogels need to mimic the natural extracellular matrix’s (ECM) dynamic mechanical properties. The complexity of the ECM suggests that engineering ideal scaffolds for specific tissue is an exceptionally challenging task, and the current inability to present mechanical signals in a spatially and temporally defined manner is considered one of the major obstacles to engineering complex tissue for organ replacement therapy. Our group has designed synthetic hydrogels featuring complexes of star-shaped poly(ethylene glycol) (PEG) and collagen mimetic peptides (CMPs). These PEG-CMP complexes form hydrogels via physical crosslinks mediated by the thermally reversible triple helical assembly of CMPs due to their characteristic (ProHypGly)x repeat sequence. Furthermore, by tailoring the peptides’ terminal bifunctionality, the PEG-CMP hydrogels can also feature covalently bonded network structure in addition to the triple helix physical crosslinks. Here we present the relative rheological properties of multiple PEG-peptide hydrogel architectures determined by particle tracking microrheology and circular dichroism studies that enable analysis of the temperature-sensitive triple helix crosslinks in the hydrogels. We are able to disrupt CMP-mediated physical crosslinks by altering the temperature of the gel or adding unbound CMPs that compete for triple helix formation. These results give the PEG-CMP systems potential as novel scaffolds with tunable mechanical properties.
12:45 PM - QQ4.9
Characterizing the Mechanical Response of DNA Crosslinked Hydrogels Under Physiological Conditions.
Uday Chippada 1 , F. Jiang 2 , D. Varma 2 , B. Yurke 3 , N. Langrana 1 2 Show Abstract
1 Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Department of Biomedical Engineering, Rutgers University, Piscataway, New Jersey, United States, 3 Department of Material Science and Engineering, Boise State University , Boise, Idaho, United States
Recent studies have shown that mechanical response of extracellular matrices significantly influences cellular properties, and hence, is an important parameter in designing biomaterials. Our group developed a DNA crosslinked hydrogel to examine cellular responses of spinal cord neurons to substrate compliances. Using DNA as crosslinker in polymeric hydrogel formation has given rise to a new class of hydrogels with a number of attractive properties (e.g., reversible gelation and controlled crosslinking). Here, it was demonstrated that by varying length of crosslinker, monomer concentration, and level of crosslinking, DNA gel properties can be controlled from ~100 Pa to 30 kPa, which is in the range of stiffness of spinal cord. Our studies on neurite outgrowth on functionalized DNA gels showed that although primary dendrite length is not significantly affected, spinal cord neurons extend more primary dendrites and shorter axons on stiffer gels. Additionally, a greater proportion of neurons had more primary dendrites and shorter axons on stiffer gels. In this work, we further characterize the mechanical response of DNA cross-linked gels under varying conditions i.e. temperature and crosslink density. It has been observed for 14-14-40 DNA gels, that the stiffness goes down from 4.9 kPa to 1.87 kPa when the temperature is increased from the room temperature to the incubator temperature (37°C), while for 20-20-40 gels, the stiffness goes down from 12.5 kPa to 9 kPa. The same gels when tested after 24 hours of swelling have yielded a stiffness which is almost 9 times less. This indicates that under physiological conditions, DNA hydrogels behave differently as compared to normal conditions. Thus characterizing the mechanical properties of the hydrogels at different conditions would give us a better understanding of the material to cell interactions.
QQ5: Dynamic Response and Pattern Formation
Tuesday PM, December 01, 2009
Room 208 (Hynes)
2:30 PM - **QQ5.1
Dynamics of Long-Chain Tracers Enclosed in Hydrogels.
Sebastian Seiffert 1 , Julia Gansel 1 , Wilhelm Oppermann 1 Show Abstract
1 , Clausthal University of Technology, Clausthal-Zellerfeld Germany
We investigate the dynamics of fluorescently labeled linear macromolecules that are enclosed in semidilute polymer matrixes and gels. The experiments were designed such that the transition from a semidilute solution to a permanent network could be covered. This was achieved by employing a matrix polymer, polyacrylamide, carrying pendent dimethylmaleimide groups. Stepwise irradiation of such samples in the presence of a triplet sensitizer causes successive dimerization of the maleimides leading to progressive cross-linking. The translational diffusion coefficients of the tracers were estimated by fluorescence recovery after photobleaching (FRAP), while fluorescence correlation spectroscopy (FCS) gave additional information on the tracer dynamics on a length scale shorter than the size of the tracers. Parameters varied were the concentration of matrix polymer (20–80 g/L) as well as the molar masses (200,000–1,300,000 g/mol) of the labeled probes. The experiments show that the chemical cross-linking of a semi-dilute polymer solution causes a moderate decrease of the translational diffusion of enclosed linear chains. When intramolecular dynamics are analyzed, the degree of labeling of the tracer chains has a significant impact on the particular features of subdiffusion observed on the sub-µm scale: Lower degrees of labeling amplify the visibility of intrachain dynamics. The effect of cross-linking of the matrix polymer on subdiffusion can be quite prominent, depending on the concentration range under study. Cross-linking can even result in an apparent increase of chain dynamics.
3:00 PM - **QQ5.2
Probing the Dynamics of Biomacromolecules in Polymeric Systems.
Hacene Boukari 1 , Candida Silva 2 , Ariel Michelman-Ribeiro 3 , Ralph Nossal 2 , Ferenc Horkay 2 Show Abstract
1 Dpt of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York, United States, 2 Lab. of Integrative Medical Biophysics, National Institutes of Health, Bethesda, Maryland, United States, 3 , National Institute Standards and Technology, Gaithersburg, Maryland, United States
Measurement of the transport properties of biomacromolecules moving within complex biopolymeric matrices is relevant for understanding many biological processes and for developing appropriate structures for biomedical applications related to drug delivery and tissue engineering. Major effort is being made to identify the basic biophysical and biochemical properties of the host matrix which control the behavior of dispersed molecules (e.g., drugs, proteins, DNA). We recently demonstrated how fluorescence correlation spectroscopy (FCS) can be used to extract information about the diffusion and polymer binding of various nanoparticles and biomacromolecules (~1-100 nm) in various polymeric solutions and hydrogels. We particularly focused 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 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. In this talk, we will review FCS measurements of the diffusion of various globular, linear, and branched fluorescent probes moving in non-fluorescent PVA or Ficoll70 solutions. Two basic lengthscales are relevant to the analysis of the data : the probe size and the characteristic mesh size of the host systems. Here we find that the data can be readily described by a model proposed by de Gennes et al. For small probes the decrease of the diffusion coefficient D(c) with increasing polymer concentration (c) can be readily fit with a stretched exponential exp(-Bcn), where n is related to the solvent quality. In the case of PVA solutions we find 0.73 ≤ n ≤ 0.84, showing that water is a good solvent. Moreover, there is a roughly linear relation between B and the size of the globular probes. In contrast, n=1 for the Ficoll samples, suggesting a theta-like behavior of the Ficoll solutions. Cross-linking of PVA chains to form gels can further slow down the diffusion of some small probes such as TAMRA and R6G. The more that the polymer chains are cross-linked, the slower the nanoparticles diffuse. Moreover, we observed a simple linear relation between the elastic modulus and the diffusion time. We also studied the effects of dehydration on the diffusion of probes in PVA samples, using a specially-designed optical chamber to measure in-situ changes of the diffusion of the probes while the samples were dehydrating. With our FCS setup we were able to monitor, continuously and simultaneously, changes in the concentration and diffusion of the probes in the dehydrating samples. Further, we used the systematic changes of the fluorescence in the dehydrating samples to calculate the volumetric changes of the samples with time, and related the changes in the diffusion of the probes with the volumetric changes of the host PVA system.
4:00 PM - QQ5.3
Dynamic and Reversible Switching of Micro- and Nano-scale Patterns by Smart Soft Materials: Toward Adaptable Nanoscale Architecture.
Philseok Kim 1 2 3 , Lauren Zarzar 1 2 , Xuanhe Zhao 1 , Joanna Aizenberg 1 2 3 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 3 Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts, United States
The fabrication and control of surface structures at a molecular, nanometer and micrometer length scale are of increasing interest in materials science; such research encompasses superhydrophobicity and wetting, biomimetic surface fabrication, microfluidics, tunable optics and photonic devices, controlled material deposition, drug delivery, tissue engineering, and the understanding of cell-surface interactions and biofilm formation.1-3 Recently, we have demonstrated nanoscale reversible pattern formation based on tensegrity nanostructures consisting of arrays of isolated, high-aspect-ratio rigid silicon structures (AIRS) and polyacrylamide hydrogel that exhibit switching of hydrophilicity of the surface upon humidity change.4,5 The aforementioned various applications require flexibility in the choice of materials with tunable mechanical properties, as well as chemical and biological functionalities. To achieve this goal, silicon-based AIRS can be easily replicated into a soft material by using an unconventional lithography method, in which a negative replica of the silicon AIRS is molded by polydimethylsiloxane (PDMS) and then back-filled with polymeric materials.6 Our preliminary results show that reversibly switchable soft polymeric micro/nanoscale surface patterns actuated by responsive materials such as hydrogels, electroactive polymers, and dielectric elastomers can be readily achieved. The movement and orientation of these soft actuator materials can be controlled and patterned by synthesizing them in contact with a pre-patterned, confining substrate. Such procedures enabled cooperative actuation of soft segments into complex patterns including microflorets and nanotraps. Modeling of the actuation based on a finite element method will be presented along with the experimental results to provide insights and understanding of the dynamic behavior of these materials. Fabrication of a wide variety of switchable nanoscale patterns and integrated architectures are currently under way using combinations of different geometric elements and different responsive soft materials to make these structures adaptable to various applications.1.C. Dorrer and J. Rühe, Adv. Mater. 20 159, 20082.M. T. Yang, N. J. Sniadecki, and C. S. Chen, Adv. Mater. 19 3119, 20073.X. Zhang, F. Shi, J. Niu, Y. Jiang, and Z. Wang, J. Mat. Chem. 18 621, 20084.A. Sidorenko, T. Krupenkin, A. Taylor, P. Fratzl, and J. Aizenberg, Science 315 487, 20075.A. Sidorenko, T. Krupenkin, and J. Aizenberg, J. Mat. Chem. 18 3841, 20086.B. Pokroy, A. K. Epstein, M. C. M. Persson-Gulda, and J. Aizenberg, Adv. Mater. 21 463, 2009
4:15 PM - QQ5.4
Self-regulated Folding Instability in Highly Confined Polymer Gel.
Srikanth Singamaneni 1 , Michael McConney 1 , Vladimir Tsukruk 1 Show Abstract
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Folding is an important and one of the most complicated events in organogenesis such as neurulation which involves in the folding of the notochord to from neural tube, and the folding of the brain in developed animals to increase the surface area. The mechanics involved in the folding of various biological structures during the growth in confined environment remains intriguing. We report a homogenous, soft polymer gel, much like the biological tissues, in a confined state that exhibits self-regulatory and scale-invariant folding upon swelling (growth). The system demonstrates long-range spatial self-awareness and pattern placement control under simple boundary conditions. These topological boundary conditions have a dramatic effect on the newly forming topological patterns, which indicates a reinforcing property available to mechanical stress-strain based systems. This robust demonstration of intricate pattern formation in polymer gel might serve as a model system to understand the mechanically-mediated morphogenesis in complex biological systems.
4:30 PM - QQ5.5
Active Surface Topographies in Constrained Hydrogel Films.
Ophir Ortiz 1 , Ajay Vidyasagar 2 , Ryan Toomey 2 Show Abstract
1 Electrical Engineering, University of South Florida, Tampa, Florida, United States, 2 Chemical and Biomedical Engineering, University of South Florida, Tampa, Florida, United States
Periodic wrinkles or corrugations can appear on the free surfaces of constrained hydrogel films. The constraint generates a residual compressive stress that is partially relieved through deviation of free surfaces from a planar configuration. The morphology, amplitude and wavelength of the surface instability are generally well-defined and depend on the strength of the constraint. If swelling of the hydrogel overcomes the constraint, the film can slip along its substrate, altering the resultant compressive stress and su