Ultra-Strong and Fast Response Gel by Solvent Exchange and Its Shape Memory Applications

Solvent response gels were synthesized by copolymerization of hydrophobic butyl methacrylate (BMA) and hydrophilic methacrylic acid (MAA) with cross-linker N,N'-methylenebis(acrylamide) (BIS) in a mixed solvent of H2O and DMSO. Solvent exchange was carried out by immersing in H2O, which turned the gel from a soft-and-weak elastomer to a strong-and-stiff plastic material. A viscoelasto-plastic model was used to describe the stress-strain behavior of the gels, which provided yield stress up to 8 MPa and Young's modulus over 300 MPa. The reversible hydrophobic association domains and tunable mechanical strength endowed the BMA gel with excellent shape memory, reversible volume change, and other interesting properties. The temporary shape of the gel was fixed in H2O within 1 min, and shape recovery was realized in DMSO within 4 min with favorable repeatability at least 10 times. A bilayer soft actuator consisting of silicon and BMA gel sheets exhibited fast actuating and recovering within 30 s by the solvent exchange. Some creative applications of a solvent leakage warning system, a sustainable and erasable "gel paper", and a bioinspired flower were demonstrated as examples. The present facile preparation and functionalization method of solvent exchange and the ultrastrong gels provides a way to realize soft actuators and sensors, as well as artificial skin for interesting applications.