Quadruple Shape-Memory Organohydrogels with Adjustable Trigger Temperatures

Organohydrogels (OHGs) are a class of soft materials with a biphasic structure consisting of hydrophilic and hydrophobic domains that interact with both water and organic solvents. This gives them unique properties and various applications, e.g., in biomedicine, antifreeze, soft robotics and environmental engineering, where adaptability and resilience are crucial. In the present work, we present OHG systems consisting of a continuous hydrogel phase based on silk fibroin (SF) in which single, binary or ternary combinations of semicrystalline poly-(n-tetradecyl acrylate) (PC14A), poly-(n-hexadecyl acrylate) (PC16A) and poly-(n-octadecyl acrylate) (PC18A) micro-organogels with side chain lengths between 14 and 18 are dispersed. The OHGs withstand 90-94% compression without failure and have a high Young's modulus (up to 2.3 MPa) at room temperature. They exhibit thermosensitive viscoelastic and mechanical properties, and an effective shape-memory effect with finely adjustable trigger temperatures. We also show that the dimer or trimer combinations of hydrophobic poly-(n-alkyl acrylates) exhibit cocrystallization that hinder the multishape-memory behavior in OHGs. To overcome this difficulty, we developed a "gluing method" in which the three hydrophobic layers are bonded together without mixing to create microinclusions with three hydrophobic layers. In this way, we were able to produce OHGs with quadruple shape-memory behavior, which have trigger temperatures of 40, 30, and 15 °C.