Silk Fibroin Cryogel Building Adaptive Organohydrogels with Switching Mechanics and Viscoelasticity

Abstract

In contrast to synthetic gels, their biological counterparts such as cells and tissues have synergistic biphasic components containing both hydrophilic and lyophilic phases, providing them some special abilities including adaptive biomechanics and freezing tolerance. Hydrogels containing both hydrophilic and lyophilic phases, referred to as organohydrogels (OHGs), are capable of mimicking the biological systems, and they might have great potential in various applications. Here, we present a facile strategy to obtain adaptive OHGs with tunable and programmable mechanics and viscoelasticity. We utilize a hydrophilic cryogel scaffold as the continuous phase of OHGs, while the pores of the scaffold act as the reaction loci for the formation of organogel microinclusions. Thus, we first prepared mechanically robust cryogels based on silk fibroin (SF) via cryogelation reactions at −18 °C. The cryogels with 94% porosity containing interconnected μm-sized pores were then immersed in an ethanolic solution of acrylic acid (AAc), n-octadecyl acrylate (C18A), N,N′-methylenebis(acrylamide), and a free-radical initiator. Polymerization reactions conducted within the pores of the cryogels lead to mechanically strong adaptive OHGs consisting of a SF scaffold containing semi-crystalline poly(AAc-co-C18A) organogel microinclusions. The mechanical strength of OHGs is much higher than that of their components due to the significant energy dissipation in the OHG networks. Depending on the amount of the crystallizable C18A monomer units, the melting temperature Tm and the degree of crystallinity of OHGs could be varied between 49 and 54 °C and 1.3 and13%, respectively. The crystallinity created in OHGs provided them switchable mechanics and viscoelasticity in response to a temperature change between below and above Tm. All OHGs exhibited shape-memory function with a shape-recovery ratio of more than 92%. The strategy developed here to obtain high-strength smart OHGs is suitable for a wide variety of combinations of hydrophilic scaffolds and organogels.