Session P47: Biopolymers: Molecules, Solutions, Networks, and Gels

Sub-nm porous membrane based on cyclic peptide nanotubes

Porous thin films containing subnanometer channels oriented normal to surface exhibit unique transport and separation properties and can serve as selective membranes for different applications. Generating flexible nanoporous films with densely packed vertical channels over large areas remains a significant challenge. We developed a new approach where the growth of cyclic peptide nanotubes can be directed in a structural framework set by the self-assembly of block copolymers. Conjugating polymers to cyclic peptides enables the nanotube subunits be selectively solubilized in one copolymer microdomain. Conjugated polymers mediate nanotube-polymer interaction to guide nanotube growth. This led to subnanometer porous membranes containing high-density arrays of through channels. In parallel, we also studied how to modify the interior of nanotubes with controlled geometry. Artificial amino acid is introduced in the primary sequence of cyclic peptide with a functional group presented in the nanotube interior without disrupting the high aspect ratio nanotubes. The new design of such a cyclic peptide enables one to modulate the nanotube growth process to be compatible with the polymer processing window, hence opening a viable way of fabricating polymeric membranes for different application   

Gel-on-Brush Model of Airway Surface of the Lung: its Predictive Role in Chronic Pulmonary Disease

Clearance of mucus is the primary defense mechanism that protects airways from inhaled infectious and toxic agents. The current two-layer Gel-on-Liquid model in which a gel-like mucus is propelled on top of a ``watery'' periciliary layer (PCL) surrounding the cilia does not adequately describe efficient mucociliary clearance in health nor properly predict failure of mucus clearance in disease. We propose and provide evidence for a qualitatively different Gel-on-Brush model with a gel-like mucus layer in contact with a ``brush-like'' periciliary layer, composed of macromolecules tethered to the airway surface. The relative osmotic moduli of the mucus layer to the ``brush-like'' PCL layer explain both the stability of mucus clearance in health and its failure in airway disease. Our Gel-on-Brush model of airway surface layer opens important new directions for treatments of airway disease.   

Experimental and Computational Analysis of Structural and Mechanical Properties of Fibrin Gels

An important area in coagulation research is the study of the structural stability of a blood clot, which has important medical consequences. The stability of clots is closely related to the fibrin networks, which provides the structural support for a blood clot. We synthesized, studied, and compared fibrin networks, with and without cells, formed under wild type and hemophilic conditions. The 3D structure of each fibrin network was reconstructed from z-stacks of 2D confocal microscopy sections by implementing novel image analysis algorithms. These 3D images were utilized to establish microstructure-based models for studying the relationship between the structural features and the mechanical properties of the fibrin networks. The mechanical properties were assessed by analyzing the networks' responses to uniaxial tensile and shear stresses, simulating the impact of blood flow on the fibrin network. We will show in this talk that the elasticity of the fiber network predicted by the model agrees well with prior experimental data for small networks. We will also show that a nonlinear worm-like chain model correctly predicts both the alignment of the fibers and the elastic properties of the networks as the clot sample is stretched.   

Block copolymer self-assembly for the responsive reinforcement of injectable protein hydrogels

Shear-thinning injectable protein hydrogels are emerging as important biomaterials for the minimally-invasive implantation of scaffolds for tissue engineering and drug delivery. In this work, responsive block copolymer self-assembly is employed to trigger nanostructure formation in hydrogels made from artificial associative proteins, producing hydrogels with resistance to shear-thinning post injection, reduced erosion rate, higher toughness, and dramatically reduced creep compliance. Polymer-protein-polymer triblock copolymers have been synthesized by conjugating poly(N-isopropylacrylamide) to either end of a protein midblock decorated with self-associating pentavalent sticker domains. Self-assembly at elevated temperatures introduces a second physical network into the protein hydrogel with an independent and tunable relaxation time. The phase behavior of these dual-network hydrogels has been explored, revealing the ability to access nanostructured morphologies, and the effect of self-assembled polymer domains on the linear mechanics and toughness of injectable hydrogels has been investigated.   

Electrophoresis of DNA-protein conjugates: hydrodynamic end effects and electrostatic interactions

DNA fragments can be separated by length in free solution by attaching them to neutral or positively charged ``drag-tags'' (e.g., proteins), a technique known as end-labeled free-solution electrophoresis (ELFSE). We first extend a previous theory of ELFSE for neutral drag-tags to the case of weakly charged drag-tags. The simplest variant of the theory assumes that all parts contribute equally to the mobility (no end effects) and that both the DNA and the drag-tag are fully flexible and do not interact. We analyze the influence of these assumptions. We obtain the exact (within the Kirkwood-Riseman approximation) form of the function describing the end effects for flexible polymers. The main significance of the end effects is the $N^{-3/4}$ (instead of $N^{-1}$) form of the correction to the mobility for large DNA lengths $N$. We also show that the end effects are weaker for semiflexible and stiff polymers. Using a simple model, we study how the conformation of the drag-tag changes due to the electrostatic interaction with the DNA and how this influences the mobility.   

The local dynamics of unfolded versus folded tRNA in comparison to synthetic polyelectrolytes and the role of electrostatic interactions

The local dynamics of RNA is strongly coupled to biological functions such as ligand recognition and catalysis. We have used quasielastic neutron scattering spectroscopy to follow the local motion of RNA and a synthetic polyelectrolyte as a function of Mg2+ concentration. We have observed that increasing Mg+2 concentration increases the picosecond to nanosecond dynamics of hydrated tRNA while stabilizing the tRNA folded structure. Analyses of the atomic mean-squared displacement, relaxation time, persistence length, and fraction of mobile atoms showed that unfolded tRNA is more rigid than in the folded state. This same behavior was observed for sulfonated polystyrene indicating that the increased dynamics in arises from charge screening of the polyelectrolyte rather than specific interactions. These results are opposite to what is observed for proteins for the relationship between the unfolded/folded states and the internal dynamics where the folded state is observed to be more rigid than the unfolded state. We conclude that the local dynamics for both bio- and synthetic polymers are strongly influenced by the electrostatic environment.   

Physical Structure of Methylcellulose Hydrogels

Methylcellulose (MC) is a chemically modified polysaccharide in which there is a partial substitution of hydroxyl groups with methoxy moieties. This results in a polymer that is water soluble at low temperatures and displays lower critical solution temperature (LCST) phase behavior at elevated temperatures. As such, aqueous solutions of MC have long been employed and studied due to their ability to form gels as temperature is increased. Currently, there is no consensus on the detailed mechanism of the gelation process, so a precise determination of the physical structure present in these materials may lead the way to new mechanistic understanding. Transmission electron microscopy (TEM) performed under cryogenic conditions is a powerful tool for the study of hydrogels as it allows direct imaging of the network while preserving the structure in the gel. Cryo-TEM investigations suggest that the hydrogel is composed of fibril-like aggregates comprising multiple polymer chains. Small-angle neutron scattering (SANS) provides a complimentary method to establish the detailed structure of the hydrogel network. We will report the effects of molecular weight, concentration, and temperature on the resultant physical structures within the gel.   

Mesoscopic Simulations of Free Solution Electrophoresis of Polyelectrolytes with Finite Debye Length

The mobility of charged oligomers results from a complex interplay between electrostatic and hydrodynamic interactions leading to a competition between counter-ion condensation and cooperative shearing within the diffuse layer. Simulations of polyelectrolytes that include explicit ions are computationally expensive, while algorithms that couple infinitely thin Debye layers to mesoscopic fluid models are only useful in the long chain limit because they fail to predict the rise and non-monotonicity of the mobility. We present a hybrid mesoscale simulation technique that utilizes multi-particle collision dynamics (MPCD) to simulate surrounding solvent molecules and the Debye-H\"{u}ckel approximation to assign effective charge to the MPCD particles. This hybrid scheme can capture the electro-hydrodynamics without having to explicitly include counter-ions or make costly electrostatic calculations. This MPCD-MD Debye-H\"{u}ckel method shows great potential for simulating electrophoretic behavior of polyelectrolytes in novel microfluidic devices.   

Interfacial adsorption and aggregation of amphiphilic proteins

The adsorption and aggregation on liquid interfaces of proteins is important in many biological contexts, such as the formation of aerial structures, immune response, and catalysis. Likewise the adsorption of proteins onto interfaces has applications in food technology, drug delivery, and in personal care products. As such there has been much interest in the study of a wide range of biomolecules at liquid interfaces. One class of proteins that has attracted particular attention are hydrophobins, small, fungal proteins with a distinct, amphiphilic surface structure. This makes these proteins highly surface active and they recently attracted much interest. In order to understand their potential applications a microscopic description of their interfacial and self-assembly is necessary and molecular simulation provides a powerful tool for providing this. In this presentation I will describe some recent work using coarse-grained molecular dynamics simulations to study the interfacial and aggregation behaviour of hydrophobins. Specifically this will present the calculation of their adsorption strength at oil-water and air-water interfaces, investigate the stability of hydrophobin aggregates in solution and their interaction with surfactants.   

Viscoelastic and poroelastic relaxations of polymer-loaded gels

Gel layers are prevalent in many applications including water purification, fuel cells, tissue engineering and drug delivery. In these materials, their performance is closely linked to controlling transport of solutes such as solvent or polymer. Thus, understanding the critical time- and length-scale that regulate solute transport will enable development of membrane materials with the desired performance. In this contribution, we present the Poroelastic Indentation Relaxation (PRI) approach in quantifying the viscoelastic and poroelastic relaxations of geometrically-confined hydrogel layers. We demonstrate this indentation-based measurement approach in characterizing several materials properties including diffusion coefficient, shear modulus, and average pore dimensions of the hydrogel independent of the extent of geometric confinement. Additionally, we present a relaxation model that accounts for the viscoelastic and poroelastic contributions to the total relaxation process. Finally, we show that the PRI approach can quantify diffusion of solvent and polymer solution in a single test simply by changing the extent of deformation.   

Mechanical Characterization of Photo-crosslinkable Hydrogels with AFM

Stimuli-responsive hydrogel films formed from photo-crosslinkable polymers are versatile materials for controlled drug delivery devices, three-dimensional micro-assemblies, and components in microfluidic systems. For such applications, it is important to understand both the mechanical properties and the dynamics responses of these materials. We describe the use of atomic force microscope (AFM) based indentation experiments to characterize the properties of poly($N$-isopropylacrylamide) copolymer films, crosslinked by activation of pendent benzophenone units using ultraviolet light. In particular, we study how the elastic modulus of the material, determined using the Johnson, Kendall, and Roberts model, depends on UV dose, and simultaneously investigate stress relaxation in these materials in the context of viscoelastic and poroelastic relaxation models.   

Tough hydrogels from hydrophobically associating polymers

Physical gels can be formed by interchain associations involving hydrophobic interactions. The viscoelastic and mechanical behavior of physically crosslinked copolymer hydrogels synthesized from $N$, $N$-dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctane sulfonamido) ethyl acrylate (FOSA), with varying FOSA content, were studied by rheology and static tensile testing. The strong hydrophobic association of the FOSA moieties in an aqueous environment produced core-shell nanodomains (6 nm diameter) that provided the physical crosslinks. The PDMA-FOSA hydrogels exhibited excellent mechanical properties: modulus of 80 -- 130 kPa, elongation at break of 1000 -- 1600 {\%}, tensile strength of 500 kPa, and toughness of 4 --6 MPa, depending on the FOSA concentration. The physical hydrogels were much more efficient at dissipating stress than the chemical hydrogels. Dynamic viscoelastic and stress relaxation experiments of the physical hydrogels and a chemically crosslinked PDMA hydrogel showed that the physical gel was more viscous than chemical gel and displayed much greater stress relaxation. That result was attributed to the extra energy dissipation mechanism provided by the reversible, hydrophobic crosslinks.   

Characterization of the Interfacial Adhesion for Responsive Hydrogels on Substrates

In recent years a growing collection of responsive hydrogels have been developed which are capable of reversibly responding to a range of stimuli. These hydrogels operate in a hydrated environment and respond with significant volumetric reversible transformation through absorption or release of water within the polymeric network. Polymeric hydrogels in this class of shape memory materials have been successfully implemented in microfluidic and biomedical devices as components such as microchannels, micro-orifices, microvalves, micropumps and microcompressors. In practice, the hydrogel component is often polymerized on a substrate or within the confines of a channel in order to create responsive components. It is well known that the adhesion of a swelling hydrogel with its substrate varies with substrate properties. We show an experimental method to quantify the interfacial adhesion of responsive hydrogels with a substrate. This work provides a valuable method and theoretical basis for characterizing and optimizing the adhesion of responsive hydrogels.   

Resilient Synthetic PEG/PDMS Hydrogels Inspired by Natural Resilin

Novel synthetic hydrogels are developed by incorporating hydrophobic polydimethylsiloxane (PDMS) into hydrophilic poly(ethylene glycol) (PEG)-based network using thiol-norbornene chemistry. The properties of these hydrogel are comparable to natural resilin, which is an elastic protein, existing in many insects, such as the tendons of flea and the wings of dragonfly, with extraordinary ability of mechanical energy storage. The energy storage efficiency (resilience) of the hydrogels is more than 97{\%} even at tensile strains up to 170{\%}. In addition, the Young's modulus of the hydrogels ranges from 3 kPa to 300 kPa by increasing the volume fraction of the PDMS in the network. These unique properties are attributed to the well-defined network and negligible secondary structure, provided by the versatile and efficient chemistry.   

pH-dependent and pH-independent self-assembling behavior of surfactant-like peptides

Self-assembly of amphiphilic peptides designed during the last years by several research groups leads to a large variety of 3D-structures that already found applications in stabilization of large protein complexes, cell culturing systems etc. In this report, we present synthesis and characterization of two novel families of amphiphilic peptides KA$_{n}$ and KA$_{n}$W (n=6,5,4) that exhibits clear charge separation controllable by pH of the environment. As the pH changes from acidic to basic, the charge on the ends of the peptide molecule varies eventually leading to reorganization of KA$_{n}$ micelles and even micellar inversion. On contrary, the bulky geometry of the tryptophan residue in KA$_{n}$W limits the variation of the surfactant parameter and hence largely prevents assembly into spherical or cylindrical micelles while favouring flatter geometries. The studied short peptide families demonstrate formation of ordered aggregates with well-defined secondary structure from short unstructured peptides and provide a simple system where factors responsible for self-assembly can be singled out and studied one by one. The ability to control the shape and structure of peptide aggregates can provide basis for novel designer pH sensitive materials including drug delivery and controlled release systems.