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-C12/AA-C18). 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/CaCl2 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.

Production of microspheres based on hydrophobically associating alginate derivatives by dispersion/gelation in aqueous sodium chloride solutions.

A new "all aqueous" procedure for the preparation of stable polysaccharide microparticles was developed. The method consists of dispersing a water solution of an amphiphilic alginate derivative (in the current work, alginate substituted with low amounts of dodecyl chains) first fluidified under mechanical stress, into an NaCl solution. The procedure exploits the ability of amphiphilic associative derivatives to form strong hydrogels in the presence of nonchaotropic salts and their shear-thinning/thixotropic properties. Depending on the experimental conditions, the size of the microparticles can be varied from 10 microm to several hundred micrometers. Their mechanical properties can eventually be reinforced by addition of low concentrations of calcium chloride. The resulting microparticles exhibit a better stability than that of plain Ca(2+)-alginate particles, as they are not disrupted when nongelling cations or calcium-sequestering agents are added to the solution. In addition, the particles can be easily redispersed after being centrifuged or freeze-dried.