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.