Scaling law and microstructure of alginate hydrogel

•At a given concentration of alginate, the critical gel concentration increased linearly with alginate concentration.•At the gel point, the critical relaxation exponent decreased and the critical gel strength increased with alginate concentration.•The plateau modulus Ge depended on Ca2+ concentration to obey a power-law scaling, Ge = kɛ1.5.•The FESEM images verified for the first time the fibrillary microstructure of the alginate gel.

The gelation of alginate in aqueous solution was studied as a function of Ca2+ concentration. At each given concentration of alginate, a critical gel concentration cg, Ca2+cg, Ca2+, was successfully determined for the first time using the Winter–Chambon criterion. The critical gel concentration cg, Ca2+cg, Ca2+ was found to increase linearly with alginate concentration. At the same time, the critical relaxation exponent n decreased and the critical gel strength Sg increased linearly with alginate concentration. An improved egg-box model was proposed to describe the change in gel junction and gel network. In the stable gel state, the plateau modulus Ge of alginate gel depended on Ca2+ concentration according to a power-law scaling, Ge = kɛ1.5, where ɛ is the relative distance of a gelling variable (Ca2+ concentration in this case) from the gel point (cg, Ca2+cg, Ca2+). The FESEM images verified the microstructure of alginate gel in which alginate chains associated into fibrils in the presence of Ca2+ ions. The fibrillar diameter and network density increased with increasing Ca2+ ion concentration while alginate concentration had a weak influence on fibrillar diameter.