Self-healing quadruple shape memory hydrogels based on coordination, borate bonds and temperature with tunable mechanical properties

Quadruple shape memory hydrogels were prepared by one-pot in situ copolymerization using acrylamide, acrylic acid, agar, and poly(vinyl alcohol). The hydrogels have multiple reversible shape memory based on the coordination bonds of poly(acrylic acid) with Fe3+, borate bonds based on poly(vinyl alcohol), and hydrogen bonds of agar and poly(vinyl alcohol). The hydrogel demonstrated tunable mechanical properties when the hydrogels immersed in different solutions for various lengths of time. After immersion in the ferric chloride solution, tensile stress and elastic moduli of the hydrogels were enhanced with increasing soaking time. After immersion in the borax solution, tensile stress first increased and then decreased with increasing soaking time. Due to the reversible effect of the borate bond, the hydrogel achieved ultra-fast self-healing. The hydrogel after immersion in borax solution could begin healing in 24 h and healed at 44 h. The tensile stress and tensile strain of the self-healing hydrogel increased when soaking time increased from 48 to 96 h, and tensile stress at healing times of 96 h was nearly as the same as that of the original hydrogel when compared with it. The combination of tunable mechanical properties, efficient recoverability and self-healing abilities coupled with facile preparation endowed the developed hydrogel a high potential for use in biomedical applications.