UV-induced coumarin-based spiropyran gradient photodimerization and photoisomerization to construct near-infrared responsive gradient hydrogel actuators in one-step

In recent years, the fabrication process of near-infrared (NIR) light-driven gradient hydrogels remains complex. Furthermore, NIR light-driven hydrogels based on the photothermal conversion mechanism have continued to face challenges in achieving spatio-temporal control of the internal structure due to the diffusive nature of heat. Therefore, it is sensible to develop NIR light-driven gradient hydrogels based on the near-infrared upconversion luminescence mechanism through a simple method. In this study, a novel photoresponsive coumarin-based spiropyran monomer (SPCMA) was designed and synthesized, followed by the fabrication of a series of double gradient hydrogels (MHMs) with rapid NIR light-driven capability and reusability through fast in situ photopolymerization of the precursor comprising SPCMA, upconversion nanoparticles (MUCNPs), and hydroxyethyl acrylate (HEA). MHMs exhibit excellent mechanical properties with fracture stress of 4.4 MPa and fracture elongation of 1282 %. Compared with hydrogel using N, N′-Methylenebis(2-propenamide) as a crosslinker, MHMs possess outstanding NIR light-driven capability and reversible cycling characteristics. MHMs rapidly transform from a three-dimensional spiral tubular to a sheet-like morphology within 30 s of NIR light irradiation and achieve a larger actuating angle of 450° within 120 s. Additionally, MHMs maintain similar driveability to the original samples after 10 cycles of NIR light-driven and UV light-recovery processes. More importantly, cargoes with seven times the mass of the "gripper" are firmly grasped and easily lifted by the "gripper" prepared using MHMs within 60 s of NIR light irradiation, demonstrating its powerful gripping ability. This work offers a new strategy for the development of biomimetic smart gradient materials.