Formation Of Physical Hydrogels Research Articles

Mechanically initiated gelation in solutions of chitosan with low degree of deacetylation

Mechanical activation of solutions of chitosan with low degree of deacetylation under the conditions of combined action of ultrasound and high shear stresses in a rotary pulse apparatus leads to the formation of physical hydrogels. Films capable to retain at repeated swelling up to 5000% water without dissolution were prepared by drying of the hydrogels.

A hydrophobically-modified alginate gel system: utility in the repair of articular cartilage defects

Alginate is a family of natural polysaccharides, widely used in industry and medicine for many applications, with its non toxic nature, gentle sol/gel transition procedure and low cost, alginate inferior biomechanical properties have limited its utility especially in tissue engineering. Additionally, ionically cross-linked alginate hydrogels generally lose most of their initial mechanical and swelling properties within a few hours in physiological solution. In order to overcome these limitations, the referenced alginate was treated by covalent fixation of octadecyl chains onto the polysaccharide backbone by esterification. In semi dilute solution, intermolecular hydrophobic interactions of long alkyl chains result in the formation of physical hydrogels, which can then be reinforced by the addition of calcium chloride. FTIR studies clearly showed the presence of ester bonds at 1612 and 1730cm(-1) indicating that the alkyl groups are incorporated in the backbone of resulting polymer. The endothermic peak and exothermic peak present in the DSC thermogram of Alg-C18 had shifted to lower temperatures comparing to native alginate (from 106 to 83°C and from 250 to 245°C, respectively) due to the esterification reaction that leads to high hydrophobic nature of the modified sample. From rheological experiments, it can be inferred that the combination of both calcium bridges and intermolecular hydrophobic interaction in the treated alginate leads to enhanced gel strength accompanied by more stable structure in physiological solution comparing to native sodium alginate hydrogel. Finally, the modified alginate tended to have no toxic effects on mesenchymal stem cell culture, rather it supported MSC chondrogenic differentiation.

Formation of Physical Hydrogels via Host−Guest Interactions of β-Cyclodextrin Polymers and Copolymers Bearing Adamantyl Groups

In this paper, we would like to present a novel system of host−guest hydrogels. By polymerizing adamantyl containing acrylamide monomers (12−15) with water soluble comonomers, we synthesized various copolymers bearing guest components. As host system we applied easily accessible β-cyclodextrin polymers with Mn values of about 30K−100K g mol−1. The viscosity of the resulting hydrogel could be influenced by the concentration of both substances, by the length of the carbon spacer chain of the guest monomer, by the pH value or by the conformation of the β-cyclodextrin polymers. Moreover, the hydrogels stayed stable in solutions of starch hydrolyzing enzyme taka-diastase from Aspergillus oryzae.

Poly(ethylene imine) hybrids containing polyhedral oligomeric silsesquioxanes: Preparation, structure and properties

Both octaglycidyletherpropyl polyhedral oligomeric silsesquioxane and hepta(3,3,3-trifluoropropyl)glycidyletherpropyl polyhedral oligomeric silsesquioxane were synthesized via the hydrosilylation reactions between octahydrosilsesquioxane [and/or hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane] and allyl glycidyl ether. The polyhedral oligomeric silsesquioxane (POSS) macromers were characterized by means of Fourier transform infrared and nuclear magnetic resonance spectroscopy. The inter-component macromolecular reactions between the POSS macromers and poly(ethylene imine) (PEI) were employed to prepare the POSS-containing organic–inorganic PEI hybrids. The inclusion of octaglycidyletherpropyl POSS into PEI results in the formation of the organic–inorganic hybrid networks whereas the introducing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS to PEI affords the linear POSS-grafted PEI copolymers. Differential scanning calorimetry and thermogravimetric analysis show that the POSS-containing PEI hybrids displayed increased glass transition temperatures ( T g’s) and enhanced thermal stability compared to the plain PEI. These PEI hybrid composites can be significantly swollen with water without dissolving, suggesting the formation of hydrogels. The PEI hydrogels containing octaglycidyletherpropyl POSS is in reality the chemically-crosslinked hydrogels whereas the those containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS displayed the behavior of physical hydrogels. The formation of physical hydrogels is ascribed to the microphase-separated morphology in the hybrids. In addition, the hybrids containing hepta(3,3,3-trifluoropropyl)glycidyletherpropyl POSS exhibited the typical amphiphilicity as evidenced by the increase in surface hydrophobilicity.

Physical alginate hydrogels based on hydrophobic or dual hydrophobic/ionic interactions: Bead formation, structure, and stability

Hydrophobically associating alginate (AA) derivatives were prepared by covalent fixation of dodecyl or octadecyl chains onto the polysaccharide backbone (AA-C 12/AA-C 18). In semidilute solution, intermolecular hydrophobic interactions result in the formation of physical hydrogels, the physicochemical properties of which can be controlled through polymer concentration, hydrophobic chain content, and nonchaotropic salts such as sodium chloride. The mechanical properties of these hydrogels can then be reinforced by the addition of calcium chloride. The combination of both calcium bridges and intermolecular hydrophobic interactions leads to a decrease in the swelling ratio accompanied by an increase of elastic and viscous moduli. Beads made of hydrophobically modified alginate were obtained by dropping an aqueous solution of alginate derivative into a NaCl/CaCl 2 solution. As compared to unmodified alginate beads, modified alginate particles proved to be stable in the presence of nongelling cations or calcium-sequestering agents. However, evidence is presented for a more heterogeneous structure than that of plain calcium alginate hydrogels with, in particular, an increase in the effective gel mesh size, as determined by partition and diffusion coefficient measurements.

Formation of physical hydrogels with terpyridine-containing carboxylic acids

Terpyridine-, diphenylbipyridine-, and triphenylpyridine-containing carboxylic acids were prepared, and the physical hydrogels formed from their sodium salts were studied. When terpyridine-containing carboxylic acids were dissolved by heating in aqueous NaOH solutions and then cooled, thermally-reversible transparent hydrogels were formed. The gelation ability was characterized by minimum gel concentrations. The structurally related diphenylbipyridine and triphenylpyridine-containing carboxylic acids failed to form hydrogels. Critical micelle concentrations (CMCs) of the sodium salts were determined according to emission spectra. Under an appropriate condition, terpyridine- and triphenylpyridine-containing carboxylic acids formed highly viscous fluids instead of hydrogels. The kinematic viscosity, determined with a Ubbelohde viscometer, increased with an increasing concentration of carboxylic acids. The increase in viscous fluids was associated with the formation of rod-like micelles. Transmission electron micrographs revealed numerous fibrous aggregates with diameters of several nanometers. The formation of hydrogels occurs on the following process: an increase in the concentration beyond CMCs brings about the transformation of spherical micelles to rod-like micelles, then the elongated rod-like micelles become juxtaposed and interlocked, which immobilizes the liquid.