Showing papers on "Self-healing hydrogels published in 1995"

Comb-type grafted hydrogels with rapid deswelling response to temperature changes

Abstract: MANY polymeric hydrogels undergo abrupt changes in volume in response to external stimuli such as changes in solvent composition1, pH2, electric field3 and temperature4–6. For several of the potential applications of these materials, such as 'smart' actuators, a fast response is needed. The kinetics of swelling and de-swelling in these gels are typically governed by diffusion-limited transport of the polymeric components of the network in water, the rate of which is inversely proportional to the square of the smallest dimension of the gel7–9. Several strategies have been explored for increasing the response dynamics10–14, such as introducing porosity14. Here we show that we can induce rapid de-swelling of a polymer hydrogel by tailoring the gel architecture at the molecular level. We prepare a crosslinked hydrogel in which the polymer chains bear grafted side chains; the latter create hydrophobic regions, aiding the expulsion of water from the network during collapse. Whereas similar gels lacking the grafted side chains can take more than a month to undergo full de-swelling, our materials collapse in about 20 minutes.

Synthesis and Characterization of Thermo- and Chemomechanically Responsive Poly(N-isopropylacrylamide-co-methacrylic acid) Hydrogels

Abstract: Random copolymer hydrogels of methacrylic acid (MAA) and N-isopropylacrylamide (NIPAAm) were synthesized by free-radical polymerization in the presence of a cross-linking agent. The gels were characterized for their temperature- and pH-responsive behavior by equilibrium swelling experiments, differential scanning calorimetry, and thermal mechanical analysis. Depending upon composition, the gels showed sharp swelling transitions with small changes in temperature or pH, with initial time-dependent response reaching equilibrium on a time scale of hours.

Dielectric relaxation spectroscopy in poly(hydroxyethyl acrylates)/water hydrogels

Abstract: The electrical and dielectric properties of poly(hydroxyethyl acrylate), PHEA, hydrogels were studied by means of dielectric relaxation spectroscopy in wide ranges of frequencies (5–2 × 109 Hz), temperatures (173–363 K) and water contents (0.065–0.46, g of water per gram of dry material). The secondary dipolar mechanisms (γ and βsw) and the dc conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism, and the complex impedance formalism. For both mechanisms molecular mobility was found to increase with increasing temperature or increasing water content (T-f-h superposition principle). The energy parameters and the shape parameters of the response were determined for both mechanisms at several water contents and temperatures. The temperature dependence of dc conductivity was found to change from Vogel-Tamman-Fulcher (VTF) type to Arrhenius type at water contents of ca. 0.30. At water contents lower than about 0.30 the hydrogels are homogeneous whereas at higher water contents a separate water phase appears. In terms of the strong/fragile classification scheme our results suggest that the PHEA hydrogels are fragile systems. ©1995 John Wiley & Sons, Inc.

Influence of Freely Mobile Grafted Chain Length on Dynamic Properties of Comb-Type Grafted Poly(N-isopropylacrylamide) Hydrogels

Abstract: Amino semitelechelic poly(N-isopropylacrylamide)s (PIPAAm) with three different molecular weights were synthesized by telomerization of IPAAm monomer with 2-aminoethanethiol as a chain transfer agent, changing the molar ratio of monomer to chain transfer agent. Macromonomers of thermosensitive PIPAAm were synthesized by condensation reaction of amino semitelechelic PIPAAm with N-acryloxysuccinimide. The molecular weights of macromonomers determined by titration of the terminal amino groups were 2900, 4000, and 9000, respectively. The comb-type grafted PIPAAm hydrogels having different lengths of graft chains were synthesized by radical copolymerization of IPAAm monomer with PIPAAm macromonomer in the presence of N,N'-methylenebisacrylamide as a cross-linker. An important aspect of the graft-type gels is the construction of a molecular architecture different from PIPAAm normal type ofgel even though the composition is same. Higher equilibrium swellings at lower temperatures were observed in graft-type gels in contrast to the normal-type gel, and longer graft chains resulted in higher equilibrium swelling due to the freely mobile grafted chains. Both the normal-type and the graft-type gels exhibited reversible swelling-deswelling changes in aqueous milieu in response to an alteration of temperature. The deswelling kinetics at 40 °C changed from equilibrium swelling states at 10 °C, however, exhibited remarkable differences, and rapid responses were observed for graft-type gels. The rapid dehydration of graft chains during gel shrinking was confirmed by analysis of DSC measurements. These dehydrated graft chains strongly aggregated with hydrophobic intermolecular forces, inducing the rapid deswelling of the gels. The attractive forces operating between dehydrated chains were larger in the gel having longer grafted chains, resulting in faster deswelling. A deswelling mechanism distinct from polymer network collective diffusion was demonstrated with these comb-type grafted hydrogels having freely mobile grafted chains in the network.

Hydrogel-based three-dimensional matrix for neural cells

Abstract: The ability to organize cells in three dimensions (3D) is an important component of tissue engineering. This study sought to develop an extracellular matrix (ECM) equivalent with a physicochemical structure capable of supporting neurite extension from primary neural cells in 3D. Rat embryonic day 14 striatal cells and chick embryonic day 9 dorsal root ganglia extended neurites in 3D in agarose hydrogels in a gel concentration-dependent manner. Primary neural cells did not extend neurites above a threshold agarose gel concentration of 1.25% wt/vol. Gel characterization by hydraulic permeability studies revealed that the average pore radius of a 1.25% agarose gel was 150 mm. Hydraulic permeability studies for calculating average gel pore radius and gel morphology studies by environmental and scanning electron micrography showed that the average agarose gel por size decreased exponentially as the gel concentration increased. It is hypothesized that the average gel porosity plays an important role in determining the ability of agarose gels to support neurite extension. Lamination of alternating nonpermissive, permissive, and nonpermissive gel layers facilitated the creation of 3D neural tracts in vitro. This ability of agarose hydrogels to organize, support, and direct neurite extension from neural cells may be useful for applications such as 3D neural cell culture and nerve regeneration. Agarose hydrogel substrates also offer the possibility of manipulating cells in 3D, and may be used as 3D templates for tissue engineering efforts in vitro and in vivo.

Biomedical membranes from hydrogels and interpolymer complexes

Abstract: Biomedical applications of hydrogel membranes require understanding of their structural characteristics and diffusive behavior. Thus, the subject of this review is the analysis of the response of such biomembranes to their surrounding environment. This responsive behavior may be due to the presence of certain functional groups along the polymer chains or specific interactions between polymer chains (complexation). This behavior is particularly important in the use of these physiologically responsive materials in membrane applications. We begin with an introduction to the structural characteristics and behavior of hydrogel membranes followed by a discussion of the types of environmentally responsive behavior seen with hydrogels. The subject of interpolymer complexation is then treated with emphasis on complexation due to hydrogen bonding and how this type of behavior may be used to produce responsive membranes. Finally, the theories of transport in membranes are reviewed.

Hydrophilic, highly swellable hydrogels

Abstract: The present invention relates to hydrophilic, highly swellable hydrogels which are coated with non-reactive, water-insoluble film-forming polymers.

Preparation and characterization of pH-sensitive, interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid)

Abstract: Hydrogels with varying cross-linking ratio and ionic content were prepared from interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid). Equilibrium swelling studies were conducted and the average molecular weight between cross-links, Mc, and mesh size were determined. Hydrogels with large Mc, values were found to swell to a greater extent than those with small Mc values. It was also observed that an increase in Mc yielded faster swelling and deswelling rates, as the rates for membranes with Mc = 18,000 were about twice as fast as were the rates for membranes with Mc = 34,000. Oscillatory swelling behavior was investigated in response to changes in the pH and ionic strength of the swelling medium. A change in pH from 3 to 6 caused an ionization of the hydrogels and an increase in the weight swelling ratio, with a greater increase exhibited by IPNs with a higher ionic content. Increase in pH also caused an increase in the average mesh size. © 1995 John Wiley & Sons, Inc.

Hydrophilic hydrogels having a high swelling capacity

Abstract: In a hydrophilic, highly swellable hydrogel (A) based on (a) (co)polymerised hydrophilic monomers or (b) graft (co)polymers, the new feature is that it has been crosslinked with a polyamidoamine (I) in aq. medium.

Measurement of the porous microstructure of hydrogels by nuclear magnetic resonance

Abstract: A method is described for using measurements of nuclear magnetic resonance (NMR) relaxation times in hydrogels to obtain pore radius distribution profiles in a rapid and noninvasive fashion. Data for spin-lattice relaxation times of the water in the gel are interpreted by using the "magnetization-diffusion" equation that has been used previously for similar measurements in porous glass, sandstones and wood pulp. To account for the fibrous microstructure of hydrogels, a fiber-cell model is introduced and used to demonstrate the existence of a "fast-diffusion" limit, in which NMR relaxation is exponential with a single time constant. It is demonstrated that agar, agarose, and polyacrylamide gels easily satisfy the criterion for fast diffusion, and the model is used to calculate pore-size distributions for those materials. This technique will prove very valuable for real-time, noninvasive monitoring of variations in microstructure occurring over length scales of approximately 100 μm in both synthetic gels and many foods.

Methods for making composite hydrogels for corneal prostheses

Abstract: Methods for forming implantable composite keratoprostheses are provided. The methods provide keratoprostheses designed to provide a suitable substrate for corneal epithelial cell growth while maintaining the desirable characteristics of hydrogels, i.e., clarity, flexibility and the ability to allow diffusive flow of nutrients. In a preferred embodiment the method includes placing corneal tissue in a mold having a corneal implant shape and crosslinking a polymeric solution to chemically bond a biocompatible hydrogel having a thickness between approximately 50 and 100 microns to the corneal tissue to form the keratoprosthesis. Upon implantation, the corneal tissue abuts epithelial cells surrounding the keratoprosthesis. In another preferred embodiment, a polymer solution is placed between the corneal tissue and a pre-formed hydrogel and then polymerized so that the polymer solution couples to both the hydrogel and the corneal tissue.

Self-oscillating swelling and deswelling of polymer gels

Abstract: Novel gel systems demonstrating rhythmically pulsatile mechanical motion similar to that of a heartbeat were developed. Self-oscillations of swelling and deswelling for the polymer hydrogels were realized by coupling pH and temperature sensitive hydrogels with a non-linear chemical reaction in the external solution media. The novel gel dynamics exhibiting cyclic and rhythmical oscillations may establish a new concept for functional materials that work under dynamic oscillating states.

Conductive hydrogels and physiological electrodes and electrode assemblies therefrom

Abstract: A highly conductive hydrophilic gel comprising a uniform aqueous solution of a crosslinked water-soluble polymer, an amount of a water-soluble electrolyte effective to reduce the transverse electrical resistance of said aqueous mixture to an impedance at 60 Hz less than about 1,000 ohm, which hydrophilic gel also contains a humectant in an amount effective to retard the drying of the conductive hydrophilic gel when it is exposed to the atmosphere while being used. A physiological electrode adapted for providing electrical contact with a surface of a sentient creature and comprising a sheet of the conductive viscoelastic hydrophilic gel.

'Smart' polymers in biotechnology and medicine

Abstract: 'Smart' water-soluble polymers and hydrogels are capable of responding reversibly to slight changes in the properties of the medium (pH, temperature, ionic strength, the presence of certain substances, illumination, electric field), the response of the system being readily seen with the naked eye (the formation of a new phase in a hitherto homogeneous solution, sudden swelling or contraction of the hydrogel). The properties of such polymers and hydrogels are examined. The use of 'smart' polymers and hydrogels for the concentration of protein solutions and the dehydration of suspensions, for the creation of membranes with a controllable permeability, for the isolation and purification of biomolecules, for the immobilisation of biocatalysts, and for the creation of sensor systems and systems for the controlled release of medicinal drugs is discussed. The bibliography includes 261 references.

Behaviors of Acrylamide/Itaconic Acid Hydrogels in Uptake of Uranyl Ions from Aqueous Solutions

Abstract: In this study, adsorptions of uranyl ions from two different aqueous uranyl solutions by acrylamide-itaconic acid hydrogels were investigated by a spectroscopic method. The hydrogels were prepared by irradiating with γ-radiation. In the experiment of uranyl ions adsorption, Type II adsorption was found. One gram of acrylamide-itaconic acid hydrogels sorbed 178–219 mg uranyl ions from the solutions of uranyl acetate, 42–76 mg uranyl ions from the aqueous solutions of uranyl nitrate, while acrylamide hydrogel did not sorb any uranyl ion. For the hydrogel containing 40 mg of itaconic acid and irradiated to 3.73 kGy, swelling of the hydrogels was observed in water (1660%), in the aqueous solution of uranyl acetate (730%), and in the aqueous solution of uranyl nitrate (580%). Diffusions of water onto hydrogels were a non-Fickian type of diffusion, whereas diffusions of uranyl ions were a Fickian type of diffusion.