Silk Fibroin-Based Multiple-Shape-Memory Organohydrogels

Organohydrogels (OHGs) are intriguing materials due to their unique composition of both hydrophilic and hydrophobic domains. This antagonistic nature endows the OHGs with several remarkable properties, making them highly versatile for various applications. We present here a simple and inexpensive approach to fabricate silk fibroin (SF)-based OHGs with multistage switching mechanics and viscoelasticity. The continuous hydrophilic phase of the OHG precursor consists of an aqueous SF solution, while the hydrophobic droplet phase consists of a crystallizable n-octadecyl acrylate (C18A) monomer and several long-chain saturated hydrocarbons (HCs) with various chain lengths between 14 and 32 carbon atoms, namely, n-tetradecane, n-octadecane, n-docosane, n-dotriacontane, and 1-docosanol. After the addition of a C18A/HC mixture containing Irgacure photoinitiator into the continuous aqueous SF phase under stirring, a stable oil-in-water emulsion was obtained, which was then photopolymerized at 23 ± 2 °C to obtain nonswelling OHGs with multiple-shape-memory behavior. By changing the chain length and mass proportion of HCs, a series of OHGs with tunable transition temperatures could be obtained, meeting various applications. OHGs containing dimer, trimer, and quadruple combinations of in situ-formed poly(C18A) and HC microinclusions exhibit effective triple- or quintuple-shape memory whose shape-recovery temperatures could be adjusted over a wide range, e.g., between 7 and 70 °C.