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Microgels are hydrogel particles with micron and sub-micron diameters. They have beendeveloped, studied, and exploited for a broad range of applications because of their uniquecombination of size, soft mechanical properties, and controllable network properties. We havebeen using microgels to modulate the properties of surfaces to differentially control theirinteractions with tissue cells and bacteria. The long-term goal is to create biomaterials thatpromote healing while simultaneously inhibiting infection. Because poly(ethylene glycol) [PEG]is used in a number of FDA-approved products and has well-known antifouling properties, wework primarily with PEG-based microgels. We render these anionic either by copolymerizationwith monomeric acids or by blending with polyacids. Both methods produce pH-dependentnegative charge. Surfaces, both planar 2-D surfaces as well as topographically complex 3-Dsurfaces, can be modified using a hierarchy of non-line-of-sight electrostatic depositionprocesses that create biomaterials surfaces whose cell adhesiveness is modulated by a submonolayerof microgels. Average inter-microgel spacings of 1-2 microns exploit naturaldifferences between staphylococcal bacteria and tissue cells, which open the opportunity todifferentially control surface interactions with them based on length-scale effects. Afterdeposition, the microgels can be loaded with a variety of small-molecule, cationic antimicrobials.The details of loading depend on the relative sizes of the antimicrobials and the microgelnetwork structure as well as on the amount and spatial distribution of electrostatic charge withinboth the microgel and on the antimicrobial. The exposed surface between microgels can befurther modified by the adsorption of adhesion-promoting proteins such as fibronectin viaelectrostatic interaction. This approach combines a rich interplay of microgel structure andchemistry as a key component in a simple and translatable approach to modulate the surfaceproperties of next-generation biomaterials.
Elastmeric materials are of great importance in both academic and industrial field due to the soft and highly stretchable properties. Thus, many theories and models are proposed to correlate the physical properties and structural parameters. However, in general, it is difficult to validate these models experimentally. Thus, to this day, we do not know the requirement conditions for each model or even the validity of each model. The validation of these models has been inhibited by the inherent heterogeneity of polymer networks.
Recently, we, for the first time, succeeded in fabricating polymer network with extremely suppressed heterogeneity with a novel molecular design of prepolymers. The homogeneous polymer network, called Tetra-PEG gel, is prepared by AB-type crosslink-coupling of mutually reactive tetra-arm prepolymers. In this study, we examined the models of elastic modulus and fracture energy using Tetra-PEG gel as a model system. We controlled the structural parameters with tuning the molecular weight and concentration of prepolymers, and reaction conversion of the reaction. This series of controlled network structures, for the first time, enabled us to quantitatively examine these models. We performed the stretching and tearing measurements for these polymer gels. As for the elastic modulus, we observed the shift of the models from the phantom to affine network models around the overlapping concentration of prepolymers. As for the fracture energy, we confirmed the validity of the Lake-Thomas model, which is the most popular model predicting fracture energies of elastomers.
In the previous investigation, piezoelectric properties of the ‘Aligned-type’ in which the piezoelectric-ceramic particles are formed in linear aggregates in the rubber, remarkable piezoelectric properties were confirmed. In this investigation, to further enhance the piezoelectric properties of the Aligned-type, the influence of the matrix properties was investigated. The properties on which we focused were the dielectric constant and the Young’s modulus. Four kinds of matrix materials whose dielectric constant and Young’s modulus are different from each other; Silicone gel, Silicone rubber, Urethane rubber and Poly-methyl-methacrylate were investigated. As a result of measurement of the piezoelectric strain constant d33 of the Aligned-Type, it was confirmed that though the influence of the dielectric constant of the matrix material was small, the lower the Young’s modulus of the matrix was, the higher d33 was.
Poly(N-isopropylacrylamide) (PNIPAM) is a thermo-sensitive polymer that exhibits a lower critical solution temperature (LCST) around 305 K. Below the LCST, PNIPAM is soluble in water and above this temperature polymer chains collapse prior to aggregation. In the presence of methanol, electron paramagnetic resonance (EPR) spectroscopy suggests that, LCST of PNIPAM is depressed up to certain mole fraction of methanol (0.35 mole fraction) and it is speculated that addition of methanol affects the PNIPAM-water interactions. Above 0.35 mole fraction of methanol, LCST gets elevated to temperatures above ∼305 K (32°C) and cannot be detected up to 373 K (100 °C). The atomistic origin of this co-solvency effect on the LCST behavior is not completely understood. In the present study, we have used molecular dynamics (MD) simulations to investigate the effect of methanol-water mixtures on conformational transitions and the LCST of PNIPAM. We employ two different force fields i.e. polymer consistent force-field (PCFF) and CHARMM to study solvation dynamics and the PNIPAM LCST phase transition in various methanol-water mixture compositions (0.018, 0.09, 0.27, 0.5, and 0.98 mole fractions). Simulations are conducted at fully atomistic level for three different temperatures (260, 278, and 310 K) and radius of gyration (Rg) of PNIPAM chains was computed for determination of LCST behavior of PNIPAM.
The understanding of the physical properties of hydrogels has been controversial because hydrogels inherently have a substantial amount of heterogeneities in their structures. In this study, we focused on one of the simplest heterogeneities, heterogeneous distribution of strand length, and investigated its influence on physical properties. We prepared Tetra-PEG gels with bimodal distribution in strand length (Tetra-PEG bimodal gels) by combining Tetra-PEG prepolymers with different molecular weights and measured the physical properties including elastic modulus and ultimate deformation ratio. The physical properties of Tetra-PEG bimodal gels were well described by the models for conventional Tetra-PEG gels with the average polymerization degrees between cross-links. We conclude that the mechanical properties of hydrogels that have heterogeneous distribution in strand length can be predicted from those of hydrogels with the average strand length in the range tested in this study.
Physically crosslinked polyvinyl alcohol (PVA) hydrogels with high mechanical properties can be made by a low temperature crystallization method using a mixed solvent of dimethyl sulfoxide (DMSO) and water. Such hydrogels are studied as the artificial articular cartilage material. But DMSO shows cytotoxycity, and it is also have the effect of accelerating the absorption of harmful substances. Therefore completely elimination must be required for clinical application but the process is difficult.
However, PVA hydrogel made by water as a sole solvent by freeze-thawing method became cloudy because of micro-heterogeneous structure, and shows low mechanical properties.
Therefore, in this study, we developed the novel hot pressing method for preparing transparent and uniformly cross-linked PVA hydrogels without DMSO from highly concentrated aqueous solution. By this method, PVA hydrogels with high mechanical property and high transparency can be obtained without any harmful organic solvent because of the fast crystallization even at room temperature. The mechanical properties of PVA hydrogels were remarkably depended on their water contents after gelation, regardless of solution concentration.
It is biologically and clinically important to understand and explain the functional properties of cartilage, such as its load bearing and lubricating ability, in terms of the structure, organization, components and their interactions. Our approach tries to explain functional material properties of these tissues as arising from polymeric interactions between and among the different molecular constituents within the tissues at different hierarchical lengthscales. We treat the tissue effectively as a complex molecular composite containing highly charged polysaccharide microgels trapped within a fine collagen meshwork. We have been developing a multi-scale experimental and theoretical framework to explain key material properties of cartilage by studying those of its constituents and the interactions among them at a variety of length and time scales. We use this approach to address important biological questions. One novel application we highlight here is the use of non-invasive magnetic resonance imaging (MRI) methods to characterize different components and compartments within cartilage and the different water environments associated with each one, in an attempt to provide a comprehensive picture of the mechanical/chemical state of cartilage.
The mechanical characteristics of ionic-covalent entanglement hydrogels consisting of combinations of the biopolymers gellan gum and kappa-carrageenan, and the synthetic polymers polyacrylamide and an epoxy amine were investigated. Compression testing showed that these gels exhibited “double network” behavior, i.e. strong tough gels.
Colloids with anisotropic shape and properties can enable the assembly of advanced materials otherwise not attainable by microfabrication. In this study, we present a convenient method using common microfabrication tools to generate a diverse array of non-spherical microparticles with well-defined shapes, sizes, electromagnetic properties for self-assembly applications. Projection photolithography onto SU-8 photoresist enabled the production of large aspect ratio microparticles such as cubes, cuboids, cylinders, hexagonal prisms, and parallelepipeds. We characterized these particles to confirm their anisotropic shape and size monodispersity. Fluorescent stains (e.g., Nile red) were mixed into the photoresist prepolymer to enhance the visualization of particle orientation. Particles designed for passive self-assembly were prepared by conventional photolithographic techniques. Particles designed for active assembly were then decorated with metallic patches in precise locations along the surface (e.g., top, side or multiple sides) using electron beam metal evaporation. This metal deposition process can enable orientational control of particles during their assembly in directed fields. After fabrication, large particles (e.g., 1,000 µm3) were released from the substrate via gentle sheer forces, whereas small particles (e.g., 10 µm3) were released by the dissolution of a sacrificial layer underneath the SU-8. Suspending the particles in water with surfactant (or other suitable solvents) provided amenable conditions for their assembly in static or dynamic systems. These conventional methods have the potential to catalyze new research in the fabrication and assembly of anisotropic patchy particles with controllable properties for the hierarchical development of self-assembled micromirrors, biosensors, and photonic crystals as examples.
Cartilage is a complex biological tissue that exhibits gel-like behavior. Its primary biological function is providing compressive resistance to external loading and nearly frictionless lubrication of joints. In this study, we model cartilage extracellular matrix using a biomimetic system. We demonstrate that poly(vinyl) alcohol (PVA) hydrogels are robust biomaterials exhibiting mechanical and swelling properties similar to that of cartilage extracellular matrix. A comparison is made between the macroscopic behavior of PVA gels and literature data reported for cartilage.
The self-assembly of a hydrophobically modified biopolymer (chitosan) is described with particular reference to gelation of these systems. The hydrophobic modification consists of the attachment of long chain alkyl groups inserted randomly along the polysaccharide backbone. The attachment of these alkyl groups to hydrophobic surfaces or the insertion into nonpolar liquids provides a ubiquitous and versatile way to create hierarchical structures, particularly the formation of self-assembled gels. Such self-assembly can be used in a variety of new technologies relating to chromatography, lubrication and the environmental remediation of oil spills through gelation of surface layers.
The double-network (DN) hydrogel concept developed by J.P. Gong and Y. Osada builds upon interpenetrating networks by combining brittle and ductile components to have significantly enhanced fracture properties. The generality of the DN effect was tested by creating biopolymer-based hydrogels of methacrylated chondroitin sulfate (MCS) and polyacrylamide (PAAm) and extended upon creating DNs of MCS and poly(N,N dimethyl acrylamide) (PDMAAm), verifying that DNs were not limited to the original combination of poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS)/polyacrylamide (PAAm). Further, the mechanical properties were varied by changing the monomer concentrations, cross-linker concentrations and the addition of cross-linking groups through copolymerizations of MCS and poly(ethylene glycol) diacrylate (PEGDA). Overall, this work demonstrates that a broad range of mechanical properties achievable through DN effect under tension and compression, generally independent of the swelling degree, which is fundamentally different behavior than possible with single networks.
We apply a living polymerization theory to describe cooperative string-like particle rearrangement clusters observed in simulations of a coarse-grained polymer melt. The theory quantitatively describes the interrelation between the average string length L, configurational entropy Sconf, and the order parameter for string assembly Φ without free parameters. Combining this theory with the Adam-Gibbs (AG) model allows us to predict the relaxation time τ in a lower temperature T range than accessible by current simulations. In particular, the combined theories suggest a return to Arrhenius behavior near Tg and a low T residual entropy, thus avoiding a Kauzmann ‘entropy crisis’.
In this paper we show that a wide variety of composite structures can be obtained from structuring with multiaxial fields. The properties of these composites are highly responsive to field structuring and so significant increases in a variety of properties can be obtained. These composites have application as high-strain actuators, strain and temperature sensors, chemical sensors, and as thermal interface materials. We discuss these issues and provide a general summary of the research we have done in this area.
A number of drug carrier systems such as liposomes, polymeric-nanoparticles, microparticles, polymeric micelles have been investigated for intracellular delivery. Among these liposomes are the potential drug vehicles for efficient cytosolic delivery. They have an adhesive property for cell membrane to encapsulate the drug or protein effectively and showing the enhanced absorption rate. One of the problems could be the difficulty of incorporation of the drug or protein into cell. Therefore many studies of the drug carriers have been developed to enhance the intracellular delivery of materials. Here we propose the novel method to improve the intracellular uptaking by using freeze concentration. Solutes are excluded from ice crystallization and concentrated in the remaining solution during freezing by freezing concentration. We have reported that polymeric cryoprotectant which is carboxylated poly-L-lysine was adsorbed on to the cell membrane during freezing and caused effective freeze concentration. In this study we investigated that delivery of protein effectively taking place by liposome as a carrier agent. It was successfully delivered protein to L929 cells via freeze concentration using polymeric cryoprotectant as a novel drug delivery.
Stimuli-responsive materials are capable of reversibly altering their properties depending on the environmental conditions or external stimuli. External stimuli typically include thermal, pH, electric fields, optical, magnetic fields, mechanical forces and chemical interactions. There are many instances in nature where responsive surfaces have been observed. Temperature is the most widely used stimulus in environmentally responsive polymer systems. The change of temperature is not only relatively easy to control, but also easily applicable both in vitro and in vivo. Temperature responsive polymers exhibit a phase transition at a certain temperature, which causes a sudden change in the solvation state. Polymers that become insoluble upon heating have a so-called lower critical solution temperature (LCST). One example of these polymers is poly (N-isopropyl acrylamide), which shows LCST at about 32 °C, close to the physiological temperature. In this study, we report the developing of novel polyampholytes which shows thermo-, salt-responsive liquid-liquid phase separation in aqueous solution.
We report fluorescence correlation spectroscopy (FCS) measurements of the translational diffusion of two fluorescent nanoprobes, rhodamine (R6G) and carboxytetramethylrhodamine (TAMRA), embedded in poly(vinyl alcohol) (PVA) solutions and gels. The diffusion coefficient was measured as a function of the PVA concentration and pH. Furthermore, we designed and built an optical chamber to determine the diffusion coefficient of the nanoprobes within the PVA solutions and gels subjected to controlled dehydration. We find that 1) lowering pH causes an apparent slowing down of the diffusion of the nanoprobes, 2) increase of PVA concentration and crosslink density also induce slowing down of both nanoprobes, and 3) dehydration induces systematic decrease of the diffusion of TAMRA in both solutions and gels. Taken together, these results demonstrate that transient physical interactions between the nanoprobes and the PVA linear polymers have a significant effect upon nanoprobe diffusion.
A new peptide amphiphile (PA) called C16-W3K has hierarchical structures, presenting unique solution states, micelle structures, and secondary structures. In this work, the effects of salt (sodium dihydrogenorthophosphate) concentration on the hierarchical structural transitions of the C16-W3K solution due to its active hydrogen bonding in the peptide were discussed. In order to analyze the effects of salt on the structural transitions, the mechanical and structural analyses were conducted by viscosity measurements, transmission electron microscopy (TEM), and circular dichroic (CD) spectroscopy. It was found that the C16-W3K solutions with different salt concentrations presented different multi-scale structural transitions from spherical micelles with α-helix molecular conformations in the sol state to wormlike micelles with β-sheet conformations in the gel state. Additionally, we found that the speed of transition increased as the salt concentration increased and the conformational ratio of β-sheet to α-helix in the solutions increased with the increase in the salt concentration.
Researchers have investigated hydrogels as potential materials for biological monitoring. Hydrogel compositions have been designed to respond to changes in temperature, pH, glucose concentration and ionic strength concentration. Hydrogels designed to respond to changes in environmental conditions have demonstrated their ability to respond via a swelling or shrinking action. This swelling behavior can be exploited using a variety of signal transduction methods. While this technology shows promise, the degradation of hydrogel materials has not yet been characterized with respect to the shelf life of hydrogel samples or to their use in continuous testing. A series of experiments were performed to test hydrogels stored for extended periods of time then subjected to oscillatory testing, and the results have been analyzed to determine whether hydrogels can be used for extended periods of time for biological sensing applications.