Role of a high calcium ion content in extending the properties of alginate dual-crosslinked hydrogels

Calcium ions (Ca2+) are extremely important for efficiently improving the mechanical properties of alginate dual-crosslinked hydrogels through the synergy of crack bridging of covalent crosslinking and hysteresis of ionic crosslinking, but it is hard to achieve a high content of Ca2+ (>5 wt%) in these hydrogels for the following reasons. The low solubility and poor dispersion of CaSO4 in the gelling solution lead to a low Ca2+ content (<0.1 wt%). The rapid formation of an alginate-Ca2+ egg-box structure during CaCl2 diffusion results in heterogeneous crosslinking, thus inhibiting further diffusion of Ca2+. Increasing the Ca2+ content, therefore, is a neglected strategy to extend the properties of hydrogels, for example, self-healing (dynamic ionic bonding of alginate-Ca2+), adhesion (dynamic ionic bonding of Ca2+ and carboxyl groups), anti-freezing (freezing point depression in the presence of Ca2+) and high conductivity (basic characteristics of Ca2+), particularly opening up a range of applications in low-temperature environments. Here we develop a highly Ca2+ crosslinked ald-alginate-gelatin imine-based (CaAG) hydrogel prepared by mixing a solution of ald-alginate and gelatin to form an ald-alginate-gelatin (AG) hydrogel, followed by immersing the AG hydrogel in CaCl2 solution with high solubility and good dispersion of Ca2+. Ald-alginate is covalently crosslinked in the network that retards the formation of alginate-Ca2+. By that, 7 wt% of Ca2+ (the highest Ca2+ content in alginate dual-crosslinked hydrogels as we know) of Ca2+ could be brought into hydrogels to extend their properties: (1) rapid self-healing (heals within 5 min), (2) strong and reversible adhesion to metal, skin, glass, and plastic, (3) anti-freezing (stretchable at -20 degrees C), and (4) high conductivity (1.5 S m(-1)). Furthermore, we show a concept skin strain sensor by using the CaAG hydrogel.