From highly ramified, large scale dendrite patterns of drying “alginate/Au NPs” solutions to capillary fabrication of lab-scale composite hydrogel microfibers

The processes of complexation, aggregation and self-assembly involving natural polymers are not only ubiquitous in biological systems (proteins), but they are also crucial for environmental and technological applications (humic acids, polysaccharide-based hydrogels). In this paper, we address these issues through a model study of the drying structures of bi-component solutions of oppositely charged alginate polysaccharide and gold nanoparticles. Unlike single component alginate and nanoparticle solutions, we show that the sessile drops of the bicomponent mixtures lead, upon drying, to fibrillar dendrite patterns of rather unusual size and ramification density. We discuss the key parameters and the mechanisms which drive the formation of these highly ramified dendrite structures. These involve, on the one hand the composition (residual salt content), the drying mode and nanoparticle size, and on the other hand, the shear-drainage and concentration, the counterion condensation, and the co-assembly and co-crystallization of the polymer, the nanoparticles and the residual salts. Finally, inspired by these fibril-like drying patterns and the gel forming property of alginate, we have developed a microfabrication process of composite hydrogel microfibers, which we showed to be an effective and flexible tool for a lab-scale production, characterization and functional optimization of such nanomaterials, before large production. ©2012