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

Abstract

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 °C), and (4) high conductivity (1.5 S m⁻¹). Furthermore, we show a concept skin strain sensor by using the CaAG hydrogel.