Showing papers on "Self-healing hydrogels published in 1991"

Physiologically responsive hydrogels

Abstract: or by semipermanent physical crosslinks such as entanglements. They may be neutral, anionic, cationic or ampholytic in nature. Because of their nature, they show swelling behavior dependent on the external environment conditions. Polymer complexes can be broken or polymer networks can be swollen by changing the external environment. This process results in the drastic change in their swelling ratio. Parameters defining their swelling behavior are the weight equilibrium swelling ratio, q, which is defined [1] as the weight of the swollen

Amperometric glucose microelectrodes prepared through immobilization of glucose oxidase in redox hydrogels.

Abstract: Glucose microelectrodes have been formed with glucose oxidase immobilized in poly[(vinylpyridine)Os(bipyridine)2Cl] derivative-based redox hydrogels on beveled carbon-fiber microdisk (7 microns diameter) electrodes. In the resulting microelectrode, the steady-state glucose electrooxidation current density is 0.3 mA cm-2 and the sensitivity is 20 mA cm-2 M-1. The current density and sensitivity are 10 times higher than in macroelectrodes made with the same hydrogel. Furthermore, the current is less affected by a change in the partial pressure of oxygen. The higher current density and lower oxygen sensitivity point to the efficient collection of electrons through their diffusion in the redox hydrogel to the electrode surface. These results contrast with those observed for enzyme electrodes based on diffusing mediators, where loss of the enzyme-reduced mediator by radial diffusion to the solution decreases the current densities of microelectrodes relative to similar macroelectrodes.

Polymer gels : fundamentals and biomedical applications

Abstract: Fundamentals.- Characterization of Gel Structure by Means of SAXS and SANS.- Physical Gelation of Synthetic and Biological Macromolecules.- Effects of Shear Modulus of Polymer Gels.- Interaction between Solvent Molecules and Networks in Biogels -Viscoelasticity, Strength.- Synthesis and Properties of Inorganic/Organic Hybrid Gels by the Sol-Gel Process.- Biologicals.- Excitation Process and Swelling of Nerve Fibers.- Cytoskeletons Are Functionally Differentiated in the Squid Giant Axon.- Synthetic Vitreous Body of PVA Hydrogel.- Immobilization of Bioactive Substances with PVA Supports.- Electric Modulation of Polymer Gel Contraction and Animal Cell Proliferation.- Separation.- Separation of Organic Solvent/Water Mixtures by Acrylamide Gel Membrane Supported by Silica-Alumina Porous Membrane.- Permeation Mechanism for a Thermo-Sensitive Switching-Functional Composite Membrane of Porous Glass and Hydrogel.- Gel Dewatering Process for Biological Slurry.- Sensors and Actuators.- A Polymer Composite Showing Electrocontractile Response.- Conversion of Chemical into Mechanical Energy by Synthetic Polymer Gels (Chemomechanical System).- Amphoteric Polyvinyl Alcohol Hydrogel and Electrohydrodynamic Control Method for Artificial Muscles.- Deformation Behaviors of Polymer Gels in Electric Field.- Thermoresponsive Polymer Hydrogel.- Contraction Behavior of Poly(acrylonitrile) Gel Fibers.- Measuring Contact Stress with 19F-NMR Spectroscopy.- Drug Delivery System.- Conventional and Environmentally-Sensitive Hydrogels for Medical and Industrial Uses.- Thermo-Responsive Polymeric Hydrogels and Their Application to Pulsatile Drug Release.- Hydrophobic Weak Polybasic Gels: Factors Controling Swelling Equilibria.- Diffusion-Reaction through Coated Gel Containing Enzyme Effects of Asymmetry and Sol-Gel Transition.

Immobilized polyethylene oxide star molecules for bioapplications

Abstract: This invention pertains to a method for immobilizing polyethylene oxide (PEO) star molecules in the form of hydrogels. The PEO star molecules are biocompatible and demonstrate non-thrombogenic properties. As such, the PEO star molecules have numerous uses for biomedical applications. The hydrogels contain a high percentage of terminal hydroxyl groups for attachment of affinity ligands and can be used for separating and purifying therapeutic proteins.

"On-off" thermocontrol of solute transport. II. Solute release from thermosensitive hydrogels.

Abstract: Poly(N-isopropylacrylamide) (NIPAAm)/polytetramethylene ether glycol (PTMEG) interpenetrating polymer networks (IPNs) were synthesized and their feasibility as thermosensitive hydrogels for drug release was investigated. The release of indomethacin incorporated into these matrices showed pulsatile patterns in response to temperature changes and was sensitive to a few degrees of temperature fluctuation. The temperature inducing on–off release deviated from the gel collapse temperature of unloaded gel, possibly because of solute effects on network properties. The lag time and release profile of indomethacin in the low-temperature region (on process) of each temperature cycle were affected by the gel composition and applied temperature. The results of this study demonstrate that solute release can be regulated by rapid deswelling of the surface of the gels in response to temperature.

Environmentally Sensitive Polymers and Hydrogels

Abstract: Environmentally sensitive polymers and hydrogels exhibit sharp changes in behavior in response to relatively small changes in conditions such as temperature or pH. Typical changes include precipitation of the polymer in water or collapse of a hydrogel with expulsion of a large fraction of the gel pore water. These changes are depicted in Figure 1. If such a polymer is grafted or adsorbed on another polymer surface or within the pores of a porous membrane, then one observes sharp changes in wettability or permeability, respectively, when the polymer is environmentally stimulated. These changes are usually reversible, although the recdissolution or reswelling processes are often slower than the precipitation or deswelling steps. Other examples of environmental stimuli and polymer system responses are listed in Tables I and II. A large number of polymers display sharp responses to such stimuli. Many are based on vinyl monomers, such as anionic and cationic monomers for pH, specificion, solvent, or electrically responsive systems, and N-alkyl substituted acrylamides (e.g., N-isopropyl acrylamide, or NIPAAm) for temperature-sensitive systems. Other copolymer compositions may contain the responsive moiety as a pendant group on one of the monomers. Photoresponsive groups have been incorporated in such a way. The ratio in the responsive polymer of the responsive monomeric component to the “nonresponsive” comonomer (e.g., the pH-sensitive monomer, acrylic acid, versus acrylamide or the temperature-sensitive monomer, NIPAAm versus acrylamide) will control both the sharpness and intensity of the response as well as the specific environmental conditions where it begins.

Influence of a salt on some properties of hydrophilic methacrylate hydrogels

Abstract: Poly(2-hydroxyethyl methacrylate) (PHEMA) and poly [N,N-dimethyl-N-[(methacryloyloxy)-ethyl]-N-(3-sulfopropyl) ammonium betaine] (PSPE) hydrogels have been prepared by means of γ-irradiation. The variation of the swelling and mechanical properties of these gels with the concentration of potassium thiocyanate in the swelling medium has been examined. Comparison of these properties in terms of the nature of the hydrophilic group has been made. The results have been analyzed and quantified via the application of the Donnan-type equilibrium theory in conjunction with the equilibrium thermodynamics of ternary systems solvent/polymer/solvent. In this manner, relevant water/polymer, salt/water, and salt/polymer interaction parameters have been obtained

COLONIC-TARGETED ORAL DRUG-DOSAGE FORMS BASED ON CROSSLINKED HYDROGELS CONTAINING AZOBONDS AND EXHIBITING pH-DEPENDENT SWELLING

Abstract: Colonic-targeted oral drug dosage forms composed of a drug confined within a crosslinked hydrogel that exhibits pH-dependent swelling and is composed of an ionizable comonomer such as acrylic acid, a nonionizable comonomer such as an acrylamide, and a crosslinking agent that contains an aromatic azobond. Selective drug release in the colon occurs by a combination of pH-dependent swelling of the hydrogel and degradation of the hydrogel by enzymatic cleavage of the azobonds by azoreductases.

Conventional and Environmentally-Sensitive Hydrogels for Medical and Industrial Uses: A Review Paper

Abstract: Hydrogels are water-swollen polymer networks. They are of great interest both medically and commerically. Conventional hydrogels change little in swelling with environmental conditions while stimuli-responsive hydrogels may swell or deswell sharply with relatively small changes of temperature or pH. A general review of the principles and applications of these interesting materials is presented here.

In vitro characterization of a bone marrow stem cell-seeded collagen gel composite for soft tissue grafts: effects of fiber number and serum concentration.

Abstract: Cell-seeded collagen hydrogels have been used in the engineering of many tissue types, from skin and vasculature to spinal cord. One of the primary limitations of collagen-based hydrogels for use in tissue-engineered grafts is that cells seeded within the gel cause it to contract as much as 70%. By forming a composite gel by adding short collagen fibers, Gentleman et al. (Tissue Eng. 10, 421, 2004) determined that the contraction due to fibroblasts was decreased and permeability was increased. Before these composite hydrogels can be used to design soft tissue replacements, however, the effect of fiber number and serum concentration should be addressed. Consequently, short collagen fibers were included in adult rat bone marrow stem cell-seeded hydrogels for composite support. The mass of fibers was varied from 1.6 to 31.3 mg per gel, and the effect of serum concentration in the growth medium was examined. It was determined that increasing fiber mass and decreasing serum concentration significantly decrease...

Novel polyurethane hydrogels for biomedical applications

Abstract: Polyurethane hydrogels derived from UV-curable urethane prepolymer and hydrophilic monomers were prepared and their properties were evaluated. The urethane prepolymer used in this study contained well-defined hard segments centered with a polyether-based soft segment and end-capped with methacrylate groups. The hydrophilic monomers studied were 2-hydroxyethyl methacrylate (HEMA), N-vinyl pyrrolidone, and glycerol methacrylate. Methacryloxypropyl tris (trimethysiloxy) silane (TRIS) was also used in some cases to modify properties. All composition were UV-cured and formed hydrogels after hydration. The oxygen permeabilities of the hydrogels decreased as the water contents increased and increased as the TRIS content was increased. The tear strengths and moduli decreased as the water contents of the hydrogels increased. Most compositions studied have higher oxygen permeability and tear strength than poly(HEMA) because of the presence of urethane prepolymer in the composition.

Thermo-responsive hydrogels based on acryloyl-L-proline methyl ester and their use as long-acting testosterone delivery systems.

Abstract: New thermo-responsive hydrogels were synthesized by copolymerizing acryloyl-L-proline methyl ester (A-ProOMe) with minor amounts of 2-hydroxypropyl methacrylate (HPMA) or polyethylene glycol 600 dimethacrylate (14G), using gamma-rays from a 60Co source. In water, extensive swelling of the hydrogels occurred at 10 degrees C, but there was marked deswelling as the temperature was raised to 37 degrees C. The poly(A-ProOMe-co-HPMA) hydrogel was characterized by an initial rapid shrinkage at the surface in the deswollen state; this shrinkage arose because of the formation of a rigid membrane barrier devoid of micropores. The system is therefore 'surface regulated'. In contrast, no such a barrier formed in the deswollen poly(A-ProOMe-co-14G) hydrogel. The whole matrix shrunk without the disappearance of micropores, and it is therefore a 'matrix pumping' system. Testosterone was incorporated into both these types of hydrogels, and the drug-loaded hydrogels were implanted subcutaneously into the backs of castrated rats. The daily dose of testosterone released in vivo from the poly(A-ProOMe-co-HPMA) hydrogel was constant at approximately 30 micrograms/day throughout an experimental period of 54 weeks. In contrast, drug release from the poly(A-ProOME-co-14G) hydrogel reached a maximum after one week and then decreased linearly with time down to the 7th week, when it was undetectable. These conclusions were supported by the changes in weight of the ventral prostates and right-side seminal vesicles of the rats, which were restored to normal when delivery of the testosterone was sustained.

Effect of crosslinking on the properties of poly(2-hydroxyethyl methacrylate) hydrogels

Abstract: By means of gamma-irradiation 2-hydroxyethyl methacrylate has been copolymerized in bulk with two different tetrafunctional monomers, ethylene glycol dimethacrylate and 1,3-bis(acryloxymethyl) tetramethyl disiloxane and with a hexafunctional monomer, 1,1,1-trimethylolpropane trimethacrylate. The resultant xerogels were subsequently swollen to equilibrium in water. In order to allow a comparison among the three crosslinkers, their molar masses and functionalities were taken into account, the theoretical crosslink concentrations being varied from 0.032 to 0.644 mol dm-3. The relative hydrophobicity of the three crosslinkers was found to have a significant effect on both the equilibrium swelling and elastic modulus of the swollen gels. Non-freezing water in several gels crosslinked with 1,1,1-trimethylolpropane trimethacrylate was analysed by differential scanning calorimetry (DSC). The proportion of non-freezing water in the gel relative to the overall water content was found to increase with increasing crosslink density.

Polymerized lyotropic liquid crystals as contact lens materials

Abstract: When an aqueous solution of a hydrophilic monomer such as 2-hydroxyethylmetharylate (HEMA) is mixed with a surfactant at the correct proportions, the components self-assemble into a lytropic liquid crystal comprising a highly ordered periodic microstructure, and polymerization of the monomer and removal of the surfactant results in a macroporous hydrogel of potential importance as a material for contact lenses and other prostheses. The two most important properties of these structured polymers as contact lens materials are: (1) unlike simple hydrogels which have irregular pores with a highly polydispersed size distribution, these new materials have an extremely well-characterized-in fact, triply periodic—network of pores of identical size, which can be preselected so as to allow for the transport of proteins, mucins, and other components of the pre-ocular tear film; and (2) this pore size can be selected independently of the polymer concentration and crosslink density, unlike simple hydrogels where large pores can only be obtained by reducing the crosslink density and along with it the shear modulus. The pore diameter can be selected in the range 20–400 A and possibly higher. A wide variety of surfactant systems that have been shown to yield materials of this degree of precision and versatility is reviewed.

Controlled Release of Herbicides Supported on Polysaccharide Based Hydrogels

Abstract: Polymeric hydrogels based on polysaccharides, hydroxyethylcellu lose and dextran, were attained by cross-linking with epichlorohydrin. The cross-linked polymer beads were loaded with 2,4-dichlorophenoxyacetic acid by direct esterification in the presence of carbonyldiimidazole. Loads ranging be tween 0.5 and 2.2 mol of 2,4-dichlorophenoxyacetyloyl groups per glucose unit were obtained. The release of 2,4-D herbicide was investigated in buffered aqueous solution at different pH values (4, 7 and 9) and evaluated with respect to different structural parameters, such as nature of the polymer matrices, cross-linking extent and herbicide loading. A fairly slow release, ranging from 10 to 25% after four months, was recorded under neutral and acid conditions, whereas at pH 9 an initial burst in the release profile, reaching almost 90% release of 2,4-D loading, was observed. In the majority of the cases, the release kinetics are reproduced by the combination of two exponential decay processes, that differ by about...