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Plenary Speakers
Atom Transfer Radical Polymerization for Stimuli Responsive Polymers
Krzysztof Matyjaszewski-Carnegie-Mellon University
Atom transfer radical polymerization (ATRP) and other controlled radical polymn techniques have been used to successfully graft polymeric brushes from various inorganic surfaces and also from polymeric backbones. The substrates include flat silicon, silica and gold wafer as well as various spherical nanoparticles that were functionalized with ATRP initiators of a variable density. Homopolymers, block and gradient copolymers were subsequently grown to control thickness of the layers. Polymethacrylate backbones were also functionalized with variable density bromoesters groups, used as ATRP initiators to generate bottle-brush structures. The resulting polymeric brushes and thin films respond to external stimuli such as pressure, solvents (and their vapors), temperature, pH, etc., depending on their molecular structure, block composition, density of grafting and other parameters.
Aqueous RAFT Polymerization: Stimuli-Responsive Systems Relevant to Biomedicine
Charles McCormick - The University of Southern Mississippi
Controlled/’living’ radical polymerization methods, including the versatile reversible addition fragmentation chain-transfer (RAFT) polymerization process, are rapidly moving to the forefront in construction of drug and gene delivery vehicles. The RAFT technique allows unprecedented latitude in the synthesis of water soluble or amphiphilic architectures with precise dimensions and appropriate functionality for attachment and targeted delivery of diagnostic and therapeutic agents. This presentation focuses on the chemistry of the RAFT process and its potential for preparing well-defined block copolymers and conjugates capable of stimuli-responsive assembly and release of bioactive agents in the physiological environment. Recent examples of block copolymers with designed structures and segmental compositions responsive to changes in pH or temperature are reviewed and hurdles facing further development of these novel systems are discussed.
Smart Bioconjugates and “Sweet Tooth” Micelles
Brent Sumerlin - Southern Methodist University
Combining the utility of controlled radical polymerization with the versatility of precise chemical transformations allows facile tailoring of new polymers with enhanced macromolecular features. We have prepared a variety of responsive materials by employing reversible addition-fragmentation chain transfer (RAFT) polymerization and highly efficient postpolymerization functionalization via copper-catalyzed azide-alkyne cycloaddition, Diels-Alder reactions and Michael additions. Capitalizing on the utility of these synthetic tools facilitates the investigation of polymeric materials with unique stimuli-responsive behavior in aqueous solutions. This presentation will discuss our recent advances in the investigation of novel temperature-responsive polymer-protein bioconjugates, cancer-targeting micelles, and sugar-responsive “sweet tooth” micelles.
Controlling Drug Release with Well-Defined Polyethylene Prodrugs
Kenneth Wagener - University of Florida
A polymer prodrug is a low molecular weight pharmaceutical immobilized on a polymeric carrier that in itself has no biological activity, thus any therapeutic effect.1,2 Polymer prodrugs require the transformation of itself to the active drug in order to elicit the desired therapeutic action. The synthesis of polyethylene based polymeric prodrugs using Acyclic Diene Metathesis polymerization (ADMET) afford controlled polymer architectures via exact ethylene run lengths between drug branches. These non-steroidal anti-inflammatory (NSAID) drug branches are spaced every 21st carbon along a polyethylene backbone having a number average molecular weight (Mn) of 30,000 and can be hydrolyzed via an ester linkage from the polymer backbone. These hydrolysable branches can be linked to the diene monomer either directly or with a variety of different spacers. Tetraethylene glycol (TEG) and decanediol are two potentially useful spacers that impart hydrophilic or hydrophobic environments between the drug and polymer backbone. With the primary structure of our polymers perfectly known, making subtle changes in the structure can have a dramatic influence on its physical properties.3,4 The polymers discussed herein are characterized with NMR, IR, TGA and DSC, along with their abilities to degrade in a controlled manner via enzymatic or chemical hydrolysis as monitored by HPLC. Tailoring the type of spacer and hydrolysable linker to a particular drug offers a new class of polyethylene with controllable features that can be used in the application of a new drug delivery material.
Solution, Surface Assembly, and Interactions of Architecturally Complex Block Copolymers
Michael Kilbey – University of Tennessee/Oak Ridge National Laboratory
Manipulating the sequence, connectivity and composition of block copolymers provides the means to alter the way chemical information is encoded in the macromolecules. In turn, these changes imparted by synthesis impact the assembly, structure, dynamics and interactions of the macromolecules, providing an intrinsic link to systems that exhibit responsive behaviors. The role of block size, composition and architecture on supramolecular aggregates formed in solution and adsorbed onto surfaces has been investigated using sets of well-defined block copolymers in the form of star block copolymers and mikto-arm (mixed-arm) copolymers. Specifically, a series of amphiphilic star-block copolymer based on polystyrene and polyvinylpyridine and having 26 and 40 arms have been synthesized and characterized, as well as mikto-arm copolymers having a linear polystyrene block connected to a three-arm, “star-like” poly(isoprene) branched block. Results from static and dynamic light scattering are consistent with the idea that branched polymers have more compact structures compared to their linear analogs, influencing their dynamics and interactions when adsorbed onto surfaces. Our results also show the importance of surface reorganization events on adsorption, and that in some cases, self-assembly of these highly-branched materials onto surfaces proceeds by a random sequential adsorption process. The implication of macromolecular topology on interfacial structure and interactions will also be highlighted.
Multicompartmental Nanocolloids and Fibers
Joerg Lahann - University of Michigan
Nanoparticles are excellent candidates for drug delivery or biomedical imaging, because they often exhibit superb tuneability of critical properties, such as size, surface characteristics, degradation rate and, therefore, drug release rates. We have recently developed a route towards fabrication of sub-micron particles that relies on electrohydrodynamic co-jetting. In this process, fluid manipulation in an electrical field is used to fabricate large quantities of multi-compartment particles, where individual compartments can be independently loaded with different drugs or selectively surface-modified. In this presentation, aspects of multifunctional particles for biomedical applications are reviewed and a specific focus is given to recent progress with compartmentalized, multiphasic nanocolloids in our laboratory.
Stimuli-Induced Micelle Formation of Block Copolymers
Kenichi Nakashima - Saga University
It is well known that amphiphilic (i.e., hydrophobic-hydrophilic) block copolymers form micelles in aqueous solutions. The micelles have a core of the hydrophobic block and a corona of the hydrophilic block. On the other hand, if all of the blocks consist of hydrophilic units, the block copolymer does not form a micelle, but molecularly disperses in water. “Double hydrophilic block copolymers (DHBCs)” is a typical example of such block copolymers. However, DHBCs also can form micelles if one of the blocks is insolubilized by stimuli such as the change of temperature and the binding of a counter ion to the polyelectrolyte block. Micelle formation of DHBCs have recently attracted much attention because it is easy to control the micellization and demicellization of DHBCs. Another feature of DHBCs micelles is that they can incorporate ionic species into the core domain if one of the blocks is polyelectrolyte. This property is especially advantageous when DHBC micelles are employed as nano-reactors of ionic species and carriers for ionic drugs, because amphiphilic block copolymer micelles cannot incorporate ionic species into the core domain. In this paper, we will demonstrate our recent studies on the physicochemical properties of the micelles of DHBCs and “triple hydrophilic block copolymers.
Polymeric Nanomedicines: Biorecognition and Efficacy
Jindřich Kopeček - University of Utah
The concept of targeted water-soluble polymer-drug conjugates was developed to address the lack of specificity of low-molecular weight drugs for malignant cells. Features needed to design an effective conjugate include a polymer-drug linker that is stable during transport and able to release the drug in the endosomal/lysosomal compartment of the target cell at a predetermined rate, adequate physicochemical properties of the conjugate (solubility, conformation in the biological environment) and the capability to target the diseased cell or tissue by an active or a passive mechanism. Various advantages of polymer-bound drugs have been demonstrated when compared to low-molecular weight drugs. However, to move the field forward, numerous areas need to be pursued, including: i) design of biocompatible long-circulating conjugates, ii) further studies on combination therapy, iii) new targeting strategies, iv) relationship between detailed structure of conjugates and their physicochemical and biological properties, v) mechanism of internalization and subcellular trafficking, vi) differences in the mechanism of action of free and polymer-bound drugs, and vii) subcellular targeting. In addition, conjugates should be designed for the treatment of diseases other than cancer. The lecture will highlight recent results in polymer therapeutics: the design of long-circulating N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer – drug conjugates, use of combinatorial chemistry for identification of targeting peptides, the relationship between the conformation of polymer conjugates and their biological properties, possibility to manipulate cell’s stress response by attaching drugs to polymeric carriers, the use of cytosolic receptors to manipulate the subcellular fate of drugs, and the design of novel bone-targeting polymer therapeutics combining anticancer and antiangiogenic drugs.
Stimuli-Responsive Polymer Nanocomposites
Christoph Weder - Case Western University
We recently developed a new family of stimuli-responsive nanocomposites that mimic the defense mechanism at play in the skin of sea cucumbers: these sea creatures can switch the modulus of their soft connective tissue on a physiological time scale. To realize this effect Nature relies on an active nanoscale structure, where rigid collagen fibrils are embedded in a viscoelastic matrix. Regulatory proteins vary the degree of bridging and stress transfer between adjacent fibrils and thus control the macroscopic mechanical properties of the material. Our adaptive nanocomposites closely mimic Nature’s architecture and are comprised of a low-modulus ethylene oxide matrix copolymer and high-stiffness, high-aspect ratio cellulose nanofibers. The non-covalent interactions between the percolating cellulose fibers can be mediated by chemical stimuli. Through modest aqueous swelling (20 %), the reinforcing cellulose network can be disrupted, resulting in a dramatic modulus reduction from 800 to 20 MPa. Exploitation of the intrinsic thermal properties of the polymer allows for amplification of this contrast, and mechanical switching over several orders of magnitude (5 GPa to 2 MPa) has been achieved. These chemo-responsive mechanically-dynamic nanocomposites are currently being investigated in in-vivo rat studies to evaluate the tissue response as well as chronic inflammatory response for their potential to serve as ‘smart’ materials for biomedical applications.
Biofilm Resistant Polymer Materials
Brian Cavitt - Abilene Christian University
Four novel, biofilm resistant monomers were synthesized and formulated into a commercially available ultraviolet (UV) curable coating for metals at varying weight percents (i.e., five, 10, 15 and 20). The formulations were applied to and UV-cured on uncoated, polished stainless steel plates to a thickness of two mils whereupon the crosshatch adhesion, pencil hardness, chemical resistance, impact resistance and flexibility were evaluated for each formulated coating based on the commercially available UV curable coating for metals. In most cases, the addition of the biofilm resistant monomers improved the performance of the coating relative to the unaltered commercially available UV curable coating for metals. Using photo-differential scanning calorimetry (photo-DSC), the monomers were observed to not initiate photopolymerization evidenced by the absence of an enhanced polymerization rate upon addition of the monomers at varying weight percents. Finally, the resistance to biofilm formation of several bacterial strains was also examined relative to two standards: 1) the unaltered commercially available UV curable coating for metals and 2) a structurally similar monomer lacking the biofilm resistant moiety and then formulated into a commercially available UV curable coating for metals at identical weight percent incorporation compared to the biofilm resistant monomers. As expected, biofilm formation occurred on both standards as observed from the standard nucleic cell staining using a solution of methylene blue dye. The use of the same cell staining procedure for formulations incorporating the biofilm resistant monomers demonstrated varying degrees of biofilm resistance with that generally increased with higher levels of monomer incorporation.
Stimuli-Responsive Surfaces by Electrografting
Christine Jerome - University of Liege
The design of stimuli-responsive coatings on a variety of solid surfaces is the topic of increasing researches. In this area, the adhesion of a stimuli-responsive polymer to the inorganic support whatever the conditions of pH and temperature, remains a concern. In this field, we have been studying an electrochemical process able to initiate the polymerization of acrylic derivatives together with promoting their chemisorption to the substrate used as electrode. We have demonstrated the versatility of this method that can be applied to a variety of acrylics leading to the possible generation of responsive coatings to either pH or temperature. Reversible anchoring of molecules of interest has also been achieved recently by this method based on the grafting of dienes able to bind a dienophile and release it at higher temperature by retroDiels-Alder reaction. The electroinitiation has been found compatible with a variety of surfaces going from conventional metals (iron, steel, gold) to carbon (fibers and nanotubes) and semi-conductors (doped-silicon), which spreads the applications to various fields such as medical devices, biosensors, catalysis, electronics and photovoltaics. Particularly, the application of this process to AFM tips allows the straightforward functionalization of these devices allowing not only the handling of polymers at the molecular level but also the elaboration of various temperature, pH or analyte sensitive sensors at the nanometer scale.
Polymer Electronic Memories: Materials, Devices and Mechanisms
E.T. Kang - National University of Singapore
Electrical switching and memory phenomena in polymers have emerged as an active research topic in organic electronics in recent years. Polymer electronic memories are potentially an alternative or a supplementary technology to the conventional memory technology facing challenges in miniaturizing from micro- to nano-scale. Based on the understanding of current memory technology and basic operating principles of electronic memories, the historical development of polymer electronic memories can be classified into three categories by drawing the mechanistic analog between the polymer memory element and one of the three primary circuit elements, viz., capacitor, transistor and resistor. Current effort and emphasis are centered on elucidating the relationships among polymer structures and properties, memory device performance and operating mechanisms. The existing electrical parameters of polymer memories and switching devices are yet to be critically compared with the leading edge memory technologies based on inorganic semiconductors. The immediate challenges facing polymer electronic memories are the fabrication of polymer thin film devices with reproducible switching and transport properties, validation of transport properties associated with electronic switching of the molecule and the exclusion of electrode reactions, formation and dissolution of filaments, and other artifacts.
Tailoring Metal Surfaces for Selective Biointeractivity with Bacteria and Bone Cells
Koon Gee Neoh - National University of Singapore
Orthopedic implants suffer occasional failures as a result of biomaterial-centered infection and/or poor bonding of the implant to bone tissue. Upon insertion, the implant presents a surface for colonization by both the host tissue cells and bacteria which may be present. In the competition for colonization of the implant surface, the probability of successful tissue integration would be greatly enhanced if tissue integration occurs before appreciable bacterial adhesion can take place since once bacterial adhesion has occurred, it is unlikely that tissue cells will be able to displace these primary colonizers. Infection in orthopedic implant surgery remains one of the most dreaded and major complications in orthopedic practice despite the use of modern antibiotic regimes and surgical measures. In this work, we report on some approaches for modifying titanium implant surfaces to develop a selective biointeractive surface that enhances bone cell (osteoblast) attachment and function while decreasing bacterial adhesion and growth. In our approach, we first inhibit bacteria colonization on the titanium surface by immobilizing an anti-adhesive biocompatible polymer such as dextran to inhibit bacterial adhesion or chitosan to provide bactericidal properties. The cell adhesive peptide, RGD or bone morphogenetic protein was then grafted on these antibacterial surfaces to promote osteoblast functions, while preserving the anti-bacterial properties. The functionalities immobilized on the titanium surface were stable even after prolonged immersion in aqueous medium. Such biointeractive surfaces with these dual functionalities are promising for orthopedic applications.
Functional Polymer Brushes and Macromolecular Grafting Strategies
Rigoberto Advincula - University of Houston
In this talk, we will focus on the chemistry of grafting polymer brushes and coatings primarily via surface initiated polymerization (SIP) approaches. The use of grafted initiators, macroinitiators and electrochemical deposition methods lend to a versatility and parameter control for the grafting of functional coatings. This includes ion-selective membranes with both pH and temperature control, hole-transporting materials for OLED devices, and solvent responsive block copolymer coatings. By making use of macroinitiators that can be deposited as polyelectrolytes or manipulated by Longmuir-Blodgett methods, it is possible to exhibit fine control on the brush density as well as layer architectures. On the other hand, SIP generated coatings of hole-transporting materials for OLED devices provide for greater charge-injection control and high performance display devices. The method also incorporates electropolymerization methods towards the grafting of polymer macroinitiators followed by SIP. These strategies have important implications for new coating protocols and functionality.
Fabrication and Characterization of Patterned, Stimulus-Responsive Polymer Brushes: From Particle Capture to Glucose Sensing
Stefan Zauscher - Duke University
Polymer brushes play an important role in the modification of surface properties and have attracted considerable interest because of their potential uses in many surface-based technologies. Here we show several, recent examples of stimulus-responsive brush micro- and nanostructures from our laboratory. (I) We rely on microcontact printing (µCP) as simple and cost-effective tool to pattern microscale initiator templates onto solid surfaces. Specifically we exploit the fact that bromo-undecyltrichlorosilane easily self-assembles onto an oxidized surface through hydrogen-bond formation between the hydrolyzed trichloro species and the -COOH functionality of a substrate. Using this reactive chemistry, we prepare bromine-functionalized, patterned templates as initiator platforms for surface initiated polymerization of polymer brushes. We demonstrate that our approach yields control over feature dimension, height, and shape primarily through varying the printing pressure. (II) By manipulation of the µCP process, we are able to fabricate stimulus-responsive polymer brushes with a height gradient across the circular features. We demonstrate that the resulting, stimulus-responsive polymer ‘anemone’ brushes can be used for reversible particle capture. (III) Furthermore, we functionalized pNIPAM-co-pAA brushes with phenylboronic acid (PBA) to be sensitive to glucose and to function as active surface coatings in microcantilever-based sensors.
Mixed Brush as a Responsive Substrate that Enhances the Polymer Adsorption
Alexander Chervanyov - Leibniz Institute of Polymer Research
We theoretically study the reversible adsorption of polymers onto selective mixed brushes. Mixed brushes are recently developed self-adoptive materials that reversibly change their morphology in response to altering external stimuli (e.g. quality of the solvent). The above changes in the morphology result in the formation of different patterns on the outer surface of the brush. We prove that thus achieved patterning of the adsorbing surface of the mixed brush drastically affects the adsorption of polymers, as compared to the adsorption onto the homogeneous brush surface. For this purpose, we develop self-consistent field theory of the polymer adsorption onto selective mixed brushes. This theory shows that the interplay between the depletion effect caused by the loss of the polymer entropy in the interior of the brush and the attraction of the adsorbed polymers to the brush surface leads to a reach adsorption-desorption behavior. The obtained results are presented in the form of dsorption-desorption diagrams that are calculated for different values of the degree of polymerisation of polymer species, Flory-Huggins interaction parameters, and the grafting density of the brush. By comparing the adsorption-desorption diagrams calculated for different microphases of the binary brush and gradient brushes, we discuss the main enthalpic and entropic mechanizms responsible for enhancing/reducing the adsorption of homo- and co-polymers onto the above brush morphologies. We also discuss how to enhance the adsorption ability of the binary brush with respect to polymers of certain properties (e.g. degree of polymerization, composition) by determining and switching to an appropriate brush morphology.
Responsive Materials Coupled with Enzyme Based Logic Operations
Sergiy Minko - Clarkson University
Nanostructured signal-responsive materials were coupled with information-processing enzyme-based systems to yield “smart” multi-signal responsive hybrid systems with built-in Boolean logic. The enzyme systems performed AND/OR logic operations, transducing biochemical input signals into macroscopic structural changes of the materials, thus resulting in the amplification of the biochemical signals. The information-processing hybrid materials could be scaled up to biocomputing networks composed of many logic gates mimicking biological systems and effectively processing complex biochemical information, resulting in reversible changes of macroscopic structures and allowing adaptation of materials to environment changes.
Kinesin Motors as Building Blocks for Active Surfaces and Probes for Non-Fouling Coatings
Henry Hess - University of Florida
A unique active coating is created by adhering kinesin motor proteins to a surface and utilizing these molecular motors to transport microtubules functionalized with specific linkers. The ability of the gliding microtubules to capture and transport a variety of particles can be exploited to self-assemble nanostructures and to improve the performance of sensors. Depending on the “magnification” of the designer’s perspective such an active coating can have the characteristics of a device or a material. Adsorbing kinesin motors to a surface not only creates a useful coating, but is in specific circumstances also a unique tool to characterize the surface. One of the biological functions of kinesin motors is to transport vesicles along microtubules. To facilitate this process, a long tail has evolved in the kinesin protein which efficiently probes the otherwise non-fouling surface of the vesicle for specific binding sites. Mimicking this approach, kinesin can be utilized to probe a non-fouling coating for adsorption sites and adsorption events can be detected by binding microtubules to the adsorbed kinesin. This strategy enables single molecule sensitivity in the detection of protein adsorption, which in turn enables us to differentiate the performance of classic and novel highly non-fouling coatings.
Adhesive Polymers Inspired by Marine Proteins
Phillip Messersmith - Northwestern University
Nature provides us with a great variety of interesting adhesive strategies that operate in wet and dry environments and that can serve to inspire the development of new materials. Marine and freshwater mussels, for example, have evolved sophisticated protein glues that serve to tether the mussel onto surfaces. After a brief description of the mussel adhesive proteins, I will describe our efforts to develop biologically inspired adhesive polymers. Liquid surgical adhesives are being developed that incorporate catechols, one of the chemical constituents found in mussel adhesive proteins. The presence of the catechol in the polymer lends both adhesive and cross-linking properties to the material. Temperature- and light-responsive systems will be described in which solidification of the adhesive is facilitated either by warming from ambient to body temperature or by application of light. Finally, a variety of surface modification strategies will be described, whereby biological inspiration has led us to design polymer coatings with adhesive or nonadhesive properties.
Applications of Self-Assembled Materials in Microfabrication Devices
Jaegab Lee - Kookmin University
We have applied a self-assembly approach to fabricate the nanocrystal flash memeory devices, taking advantages of a well-known ordered structures, for example an array of nanocrystals (NCs) provided by the self-assembly process. The self-assembled materials including layer-by-layer(LBL) self-assembled multi-structures, block-copolymer micelles, ferritin, and SAMs (self-assembled materials) template for Cu NCs were employed to deposit the array of Co (Au, Cu) NCs. Those methods were compared in terms of uniformity of size, distribution, and density of NCs, which are critical to the nanocrystal memory devices. Based on these results, the feasiblity of the self-assembly approach in microfabrication processes will be discussed.
Stimuli-Responsive Nano- and Microgel- Particles
Hans-Juergen P. Adler - Technische Universität Dresden
The fascinating feature of hydrogels is their ability to change reversibly their volume by more than one order of magnitude due to the small change of environmental parameters such as temperature, pH, solvent concentration, ionic strength etc. The preparation of the hydrogel particles requires use of heterophase polymerization techniques or self-assembly and crosslinking of pre-formed polymers. By using the above mentioned methods it is possible to vary the size of the hydrogel particles from tens of nanometers (nanogels) to micrometer range (microgels). In all cases the morphology, chemical structure and crosslinking degree of the hydrogel particles can be effectively controlled. Obtained hydrogel particles can operate as: stable colloidal systems (application in catalysis, protein or drug carriers); particle assemblies (application in flow-control devices, chemostats); particle-based layers (application in sensors; special coatings). The preparation of the hydrogel layers with well-defined geometry and dimensions can be achieved by photo polymerization of monomers or by photo crosslinking of prepolymers. The hydrogel-based thin films can be covalently grafted to different solid substrates providing an exiting possibility for the preparation of the hydrogel arrays and further design of functional devices such as microvalves and micropumps.
Synthesis of Smart Nano-Hydrogels
Dirk Kuckling - Universität Paderborn
Designing functionalized nanogels with specific properties is a challenging task by the fact that both the total number of functional groups and their distribution within nanogels play a crucial role in controlling its properties. In addition the distribution and accessibility of functional groups are critical in determining the types of applications. In order to obtain a high degree of control over these properties and the stimuli that induce property changes, complex polymer structures designed for supramolecular aggregation must be prepared. Along these lines, tailor-made block copolymers were synthesized to spatially localize chemical functionalities to a defined position, to improve colloid stability, and finally, to obtain specific physical properties of the particles.
Responsive Nanoporous Colloidal Materials
Ilya Zharov - University of Utah
This talk will describe our work on the preparation and study of responsive nanoporous colloidal films and membranes that form via self-assembly of nanoscale-sized silica spheres into a close-packed face-centered cubic lattice and contain highly ordered arrays of three-dimensional interconnected pores 5-100 nanometers in size. We modify the surface of colloidal nanopores with organic moieties that can non-covalently interact with ions and molecules, and whose charge and shape respond to external stimuli, such as pH or light. As a result of the surface modification, we obtain responsive porous materials that where the molecular transport through the nanopores can be controlled by either by tuning the nature and strength of the non-covalent interactions or by changing the environmental conditions. We believe that integrating nanoscale, functional organic moieties into the surface of colloidal materials will allow creating responsive nanoscale devices with a host of technological applications in drug delivery, separations and sensing.
Design of Functional Nanoparticle Interfaces and Polyelectrolytes Guided By Atomistic Simulation
Hendrick Heinz - University of Akron
The simulation of inorganic-organic interfaces and mineralization processes using biological templates offers guidance in the key question to design surfactants and polymers for selective binding to inorganic surfaces. We discuss examples of the interaction of small peptide building blocks with metal surfaces of different shape and curvature, as well as binding to silica surfaces in aqueous solution. The quantitative connection with experimental data such as NMR chemical shifts, X-Ray data, IR, and SFG measurements, binding constants and zeta potentials significantly advances the understanding of the binding mechanisms and enables the design of functional interfaces. A further challenge is the chemically realistic simulation of longer chain molecules and their responses to stimuli in solution. Rapid advances in availability of computational power now permit fairly accurate simulations of the phase behavior of polyelectrolytes in aqueous solution and their responses to external stimuli which is demonstrated for poly(ethylene oxide) (PEO) of ca. 2000 g/mol weight. Earlier atomistic models for PEO have failed to reproduce the solubility and radii of gyration known from experiment by leading to chain collapse or excessively outstretched conformations under standard conditions. We show that the adjustment of atomic charges and corresponding dipole moments on the PEO backbone to close agreement with experimental data and consistency with available models for water (SPC and TIP5P) leads to reliable molecular-level representations of the chain dynamics. Preferences of the O-C-C-O dihedrals for gauche conformations (a bias towards helical conformation of the chains) and lower eclipsed torsion barriers (~4 kcal/mol) according to experimental data are now also included. The new models reproduce densities, interfacial properties, and radii of gyration in very good agreement with experiment, and can be employed using biomolecular and materials oriented force fields such as AMBER, CHARMM, GROMACS, CVFF and PCFF to explore a range of properties.
Raman Determination of Molecular Origin of the Poly-N-Isopropylacrylamide Hydrogel Volume Phase Transition
Sanford Asher - University of Pittsburg
We examined the volume phase transition of monodisperse poly-N-Isopropylacrylamide (NIPAM) nanogels by using UV and visible wavelength steady state and kinetic Raman spectroscopy. The objective is to determine the molecular origin of the volume phase transition process and to describe a reaction coordinate for the process. We find that the amide carbonyl significantly dehydrates and that the N-H linkage is much less dehydrated. The strength of water hydrogen bonding to amides decreases with temperature. At ~35 ºC hydrophobic collapse occurs because the entropic unfavorability of water hydrophobic interactions surmounts the enthalpic favorability of amide hydrogen bonding. The volume phase transition occurs in such a way to leave the N-H hydrogen bonded to water. The C=O find themselves in a crevice where hydrogen bonding to water is restricted. There is little hydrogen bonding of amides to one another. The unique volume phase transition properties of NIPAM result from the restricted crevices that self assemble around the carbonyls of the amide groups.
Responsive Polymeric Assemblies
Yunfeng Lu - University of California Los Angeles
Biological systems like cell membranes and chameleons adapt to their surroundings by undergoing reversible structural and functional changes in response to external stimuli. Translation of such natural responses to synthetic systems is of interest for sensors, responsive camouflage, drug-delivery and, more generally, for the development of robust engineering materials with life-like qualities. To date environmentally adaptive synthetic materials have been limited mainly to hydrogels, which, at a critical solubility temperature, ionic strength or pH, undergo reversible phase separation resulting in a large change in volume. In this talk, I will present the design of responsive supramolecular assembly through balancing covalent and noncovalent interactions. This design principle allows the successful synthesis of a new class of adaptive and responsive materials that are potential important for sensing, drug delivery, and other applications.
Biological Applications of Hydrogel Nanoparticles
Andrew Lyon - Georgia Tech
Ovarian carcinoma is the leading cause of death from gynecologic cancer. Despite high initial tumor response rates of 80% to surgical debulking and taxane and platinum-based chemotherapy, most women with advanced ovarian carcinoma will ultimately develop drug-resistant disease. Because second-line chemotherapeutic agents provide a response rate of only 15-25%, there is clearly a need to develop better therapeutic strategies. In this presentation, the use of peptide-targeted hydrogel nanoparticles in combination chemotherapy applications is presented. Specifically, the nanogel has proven to be a highly effective method for the delivery of silencing RNA, which when targeted to the appropriate gene product, can result in the recovery of drug sensitivity in vitro. The outlook for such approaches is discussed in the context of these results.
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