Synthetic hydrogels have poor mechanical properties that limit their use in load bearing applications. In contrast, biological hydrogels are tough and strong due to reinforcement with nano to micron size fibers. Our work is focused on designing new class of hydrogel assemblies based on fiber reinforced hydrogel composites. In analogy to the spinning of a spider web, a pultrusion system was developed to spin micron-diameter polymer fibers from solution in order to build predefined three dimensional patterned fiber-reinforced hydrogel structures. The gel chemistry is based on epoxy-amine crosslinking. We have formulated various epoxy-amine gels with swellability ranges from 0% to 1000% in water. The fibrous reinforcement affects the swelling and mechanical properties of the gel. For mechanical characterization flat punch probe indentation technique was performed. The effect of fiber density and extent of swelling on the reinforcement of epoxy matrix is investigated. Based on a better understanding of fiber reinforcement of gels, we hope to be able to design strong gels for biomedical devices and soft machines.

A “smart” hydrogel is a crosslinked polymer network that reversibly swells and absorbs water in response to an external stimulus such as change in pH or in the concentration of some analyte such as glucose. Microscopically-thin smart hydrogels can be combined with microfabricated piezoresistive pressure transducers to obtain “chemomechanical sensors” that serve as selective and versatile wireless biomedical sensors. Proof-of-concept is shown here using glucose- and pH-responsive hydrogels.

Conducting polymers and hydrogels are two classes of polymers that currently receive an increasing attention in the field of biomaterials, particularly for their application in the assembly of artificial muscles. In this paper we present the development of Polyacrylamide (PAAM) microfibers and polyaniline (PANI) - poly vinyl alcohol (PVA) conductive gel membrane.

The fabricated PAAM microfibers have diameters between 1 and 12μm depending on the preparation parameters. These microfibers respond instantaneously to 100mV electrical stimulation, which solves the problem of time response of the hydrogels. On the other hand, we showed that the inclusion of conducting chains within a crosslinked gel matrix allows combining the conductivity of the PANI with the mechanical flexibility of PVA in order to provide flexible gel membranes that can adhere to the PAAM microfibers to ensure their electrical stimulation.

The mechanics of systems of non-cross-linked fiber networks is studied in this work using a computational model inspired from the bead-spring models of polymeric melts. The fibers have random orientation and distribution in space, interact via stiff repulsive potentials and are characterized by their axial and bending stiffness. Fiber-fiber Coulombian friction is considered. The system is subjected to isostatic compression (strain control) and various statistical measures are evaluated. As the system is compacted, a critical density is reached at which stiffness develops. At this stage there is in average one fiber-fiber contact per fiber and the fiber free segment length has a uniform probability distribution function. Upon further compaction, the number of contacts per fiber increases and the segment length distribution becomes exponential. The respective cross-over densities depend on the fiber length and the friction coefficient. Significant hysteresis is observed upon loading-unloading in the total energy and the number of contacts per fiber. It is also observed that the distribution of contact energies in the range of densities where the system forms a topological network is a power law.

Polyelectrolyte (PE) gels exhibit complex phase behavior that includes the existence of nanostructures in poor-solvent conditions. The formation of these inhomogeneous structures is made possible by the competition between the short-range hydrophobic, elastic, and entropic interactions and the long-range electrostatic forces. We develop a theoretical framework that describes the effect of monomer and charge inhomogeneities in PE gels. Numerical calculations performed on a salt-free PE gel with one-dimensional heterogeneities demonstrate the presence of nanophases for a finite range of physical parameters.

Simplified models of flexible chain and stiff fiber networks are introduced to address how the network elasticity becomes modified when the cross-linking is thermoreversible in nature and changes in the stability of the network with deformation. These idealized models apparently able to capture many aspects of the elastic properties of real networks.

In our study, thermosensitive core-shell microgel particles have been used as the carrier system for the deposition of metal nanoparticles, in which the core consists of polystyrene (PS) whereas the shell consists of poly(N-isopropylacrylamide) (PNIPA) network crosslinked by N, N'-methylenebisacrylamide (BIS). Silver, gold and palladium nanoparticles have been homogeneously embedded into thermosensitive PNIPA-networks, respectively. We demonstrate that the catalytic activity of the microgel-metal nanocomposites can be tuned by the volume transition within the microgel of these systems by using the catalytic reduction of 4-nitrophenol as the model reaction. Moreover, following the concept of a “green chemistry”, the oxidation of alcohols to the corresponding aldehydes or ketones can be carried out in aqueous solution under aerobic conditions at room temperature by using microgel-metal nanocomposites as the catalyst. The influence of temperature on the catalytic activity has been also investigated, which will be affected both by the volume transition of the microgel and by the change of polarity of the microgel in this case.

This paper presents the results of our analysis of ferrogels prepared using uncoated and polyvinylpyrrolidone (PVP)-coated magnetite (Fe3O4) nanoparticles. The N-isopropylacrylamide (NIPAM) based hydrogel is used with 15-25 nm Fe3O4 nanoparticles different concentrations in the range 1.25 -14%. These samples were analyzed using ultra small angle x-ray scattering (USAXS), Transmission Electron Microscopy (TEM), and Direct-Current Superconducting Quantum Interference Devices (DC-SQUID) magnetometry. Samples prepared with polyvinylpyrrolidone (PVP) coated nanoparticles showed a better single particle distribution compared to uncoated samples. USAXS data indicated that the gels prepared using uncoated nanoparticles have a large two particle agglomerations. In both cases, the volume fraction of the particles in the gel is linearly proportional to the initial particle concentrations. The DC-SQUID magnetometry analysis indicated that the magnetic moment of the gel samples prepared with uncoated particles is ∼ 2emu/g compared to ∼1.5emu/g of those having coated particles.

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 surface topography. By tuning this slippage, we aim to direct the surface features presented by thin hydrogel films. Active control of surface morphology could have important implications in several applications ranging from sensors to controlled cell attachment and detachment.

In this work, we discuss a method of tuning the amount of slippage and surface topography of patterned poly(N-isopropylacrylamide) (poly-NIPAAm) films. The films were fabricated from photo-crosslinkable polymers comprising NIPAAm and photo-active methacroyloxy-benzophenone (MaBP) monomers. The patterned films were anchored to a substrate through a hydrogen-bonding self-assembled monolayer. The relationship between pattern geometry, pattern dimensions, and solvent were investigated with respect to the mechanical instability generated at the free surface. We observed that the wavelength, width, and amplitude of the instability always increased with the thickness of the polymer pattern. The morphology of the surface instability, however, was especially sensitive to thickness of the pattern, the solvent, and the spatial position with respect to the edge of polymer pattern. If the slippage of the film against the substrate is minimal, a wrinkle pattern in the form of bicusps was observed. If the compressive stress exceeded the hydrogen-bonding anchors, slippage of the film generated either a blister pattern or a honeycomb pattern.

This work focuses on the development of a new bio-inorganic nanocomposite glucose-responsive membrane to be applied as a single self-regulated platform for insulin delivery. Crosslinked bovine serum albumin (BSA)-based membranes were prepared containing impregnated pH-responsive poly(N-isopropyl acrylamide-co-methacrylic acid) nanoparticles (hydrogel NPs), glucose oxidase (GOx), catalase (CAT), with or without MnO2 NPs. The membrane acts as a glucose sensor and insulin release attenuator. In this system glucose is oxidized by GOx to produce gluconic acid, which regulates the permeability of the membrane to insulin. CAT and/or MnO2 NPs are introduced into the membrane in order to quench unwanted H2O2 produced by GOx turnover cycles, which can cause inactivation of GOx and toxicity. The glucose-modulated insulin release through the membrane is determined by alternating glucose concentration between 100 – 400 mg/dL (normal and hyperglycemic levels, respectively). The results show that the combination of CAT and MnO2 NPs in the membrane formulation leads to better efficiency in quenching the H2O2 and better long-term stability of GOx than using either alone. Very small amounts of insulin permeate though the membrane at the normal blood glucose level while a four-fold increase in the release rate is observed when glucose concentration is raised to a hyperglycemic level. The release rate of insulin drops when the glucose level is reduced to a normal value. These results demonstrate the self-regulated capability of the system.

Silica aerogels were synthesized via sol-gel processing followed by a two-step surface modification and ambient pressure drying, using methyltrimethoxysilane (MTMS) and trimethylchlorosilane (TMCS)/ethanol/n-hexane as surface modification agents. The transparent silica aerogels possessed the porosities, densities and specific surface areas in the range of 87.7–92.3%, 0.27–0.17 g·cm-3 and 852–1005 m2·g-1, respectively. The SEM and HRTEM analysis revealed the three-dimensional nanoporous structure of the silica aerogels. The presence of –CH3 functional groups on the surface of silica particles as indicated by the FTIR spectra was further confirmed by two visible exothermic peaks at 310 and 450–500 °C from the DTA curve. In addition, the silica aerogels were superhydrophobic with the contact angle as high as 160°.