Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U4: Interfaces between Synthetic and Biological Polymers |
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Sponsoring Units: DPOLY DBP Chair: Christine Ortiz, Massachusetts Institute of Technology Room: Colorado Convention Center Korbel 2B-3B |
Thursday, March 8, 2007 8:00AM - 8:36AM |
U4.00001: Design Rules for Thermally Responsive Polymer Brushes Invited Speaker: Thermally responsive polymers such as poly(N-isopropylacrylamide) (PNIPAM) are extensively used to thermally tune the interfacial properties of thin polymer films. Above a lower critical solution temperature (LCST) of 32C, PNIPAM becomes insoluble in water and the chains collapse. Below the LCST the polymer chains are swollen. Yet such dramatic changes are not observed in all cases. The molecular weight and grafting density may also influence the phase behavior. This talk describes the systematic investigation of the thermally driven collapse of end-grafted PNIPAM as a function of the chain grafting density, molecular weight, and temperature. The polymer was grafted from the surface of an alkanethiol monolayer on gold containing a brominated alkanethiol initiator. The chains were synthesized by Atom Transfer Radical Polymerization (ATRP). Extensive chain collapse occurred at the highest grafting density and molecular weight, but the change in the film thickness decreased with decreasing density and molecular weight. The LCST was unchanged within 1C for all films. The force profiles measured between the PNIPAM brushes and a second surface at T below the LCST further suggest a one-dimensional phase separation within the polymer brush. These findings are compared with theoretical models of water-soluble polymers. We further discuss design criteria that impact the ability to thermally tune the interfacial properties of grafted PNIPAM films. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U4.00002: Studying Polymer Transport on Soft and Hard Surfaces Invited Speaker: We have employed experiments and simulations to understand the factors controlling the transport of polymers on surfaces. From an experimental viewpoint we have focused on the transport of DNA (single stranded) on lipid bilayers. We show that this behavior is slaved to the mobility of the lipids. More surprisingly, it appears that the transport of molecules adsorbed on surfaces follows the same dependence on lipid mobility as for molecules incorporated into the lipid layer. The ability to control this surface diffusion through the introduction of posts or varying the strength of adsorption (by the use of an AC field normal to the surfaces) will also be studied. Theoretically we have used molecular dynamics simulations of a polymer chain of length N dissolved in explicit solvent and adsorbed as a pancake at the solid-liquid interface to discriminate between respective influences on surface diffusion of hydrodynamics and adsorption energetics. Only for analytically-smooth surfaces do we observe a strong influence of hydrodynamics; the polymer lateral diffusion constant, D, scales as $D \propto 1/N^{3/4}$, more weakly than for implicit solvent. For atomistic surface corrugation with uniform surface chemical makeup, $D \propto 1/N$ instead. This suggests that while we can understand the results for diffusion on lipid surfaces, more recent experimental observations of stronger N dependence for diffusion on hard solid surfaces originate not in hydrodynamic interactions but in spatially patchy energetic interactions. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:48AM |
U4.00003: Design of dendrimer-based drug delivery nanodevices with enhanced therapeutic efficacies Invited Speaker: Dendrimers and hyperbranched polymers possess highly branched architectures, with a large number of controllable, tailorable, `peripheral' functionalities. Since the surface chemistry of these materials can be modified with relative ease, these materials have tremendous potential in targeted drug delivery. They have significant potential compared to liposomes and nanoparticles, because of the reduced macrophage update, increased cellular transport, and the ability to modulate the local environment through functional groups. We are developing nanodevices based on dendritic systems for drug delivery, that contain a high drug payload, ligands, and imaging agents, resulting in `smart' drug delivery devices that can target, deliver, and signal. In collaboration with the Children's Hospital of Michigan, Karmanos Cancer Institute, and College of Pharmacy, we are testing the \textit{in vitro} and \textit{in vivo} response of these nanodevices, by adapting the chemistry for specific clinical applications such as asthma and cancer. These materials are characterized by UV/Vis spectroscopy, flow cytometry, fluorescence/confocal microscopy, and appropriate animal models. Our results suggest that: (1) We can prepare drug-dendrimer conjugates with drug payloads of greater than 50{\%}, for a variety of drugs; (2) The dendritic polymers are capable of transporting and delivering drugs into cells faster than free drugs, with superior therapeutic efficiency. This can be modulated by the surface functionality of the dendrimer; (3) For chemotherapy drugs, the conjugates are a factor of 6-20 times more effective even in drug-resistant cell lines; (4) For corticosteroidal drugs, the dendritic polymers provide higher drug residence times in the lung, allowing for passive targeting. The ability of the drug-dendrimer-ligand conjugates to target specific asthma and cancer cells is currently being explored using \textit{in vitro} and \textit{in vivo} animal models. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:24AM |
U4.00004: Ligand-receptor binding in the presence of polymeric spacers Invited Speaker: Ligand-receptor binding is of fundamental importance in many biological processes. Examples include cell-cell adhesion and cell-surface interactions among others. In several biomimetic materials as well as in some biological systems the ligand is attached to the surface by a spacer. In this talk we address the role that spacers play in ligand-receptor binding. More specifically, we present a series of theoretical studies in which we systematic study the role of polymeric spacers on the efficiency of ligand-receptor binding. The systems of interest correspond to the ligand chemically bound at the free end of polymers tethered to the surface, while the receptor is part of proteins free to move in the solution. Our theoretical approach is based on a molecular theory that has been shown to predict thermodynamic and structural information for tethered polymer layers in excellent agreement with experimental observations. We have generalized the theory to include the equilibrium between the bound and unbound species. We find that the presence of spacers increases the amount of binding as compared to the case in which the ligands are directly on the surface. The maximal binding is obtained at a relatively low surface coverage of spacer and it increases as the spacer chain length increases. The maximal binding is found to correspond to the cases in which the bound proteins can accommodate at different distances from the surface while bound to the ligand. We will show how the binding depends upon the size of the protein, the free energy of binding of the bare ligand-receptor pair, the polymer surface coverage and molecular weight. The predictions of the theory will be compared with recent experimental observations on the interactions between protein coated surfaces and surfaces with ligands at the end of polyethylene oxide spacers. Finally, we will show the use of mixed tethered layers to optimize ligand- receptor binding and at the same time to minimize non-specific adsorption of proteins. Throughout the presentation the interplay between different interactions in determining the binding will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 11:00AM |
U4.00005: Using Liquid Crystallinity to Design Interfaces between Synthetic and Biological Materials. Invited Speaker: This presentation will discuss the spontaneous assembly of amphiphiles and biological macromolecules at interfaces between thermotropic liquid crystalline phases and aqueous phases. This assembly process gives rise to patterned orientations of the liquid crystals that reflect the spatial and temporal organization of the amphiphiles and macromolecules. Strong and weak specific binding events involving proteins at these interfaces drive the reorganization of phospholipids and trigger orientational transitions in the liquid crystals. Because these interfaces are fluid, processes involving the lateral organization of proteins (e.g., formation of protein and phospholipid-rich domains) are also readily imaged via the orientational response of the liquid crystal, as are stereospecific enzymatic events. These results suggest new principles for designing interfaces between synthetic and biological polymers. [Preview Abstract] |
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