Gradient Polydopamine Coating: A Simple and General Strategy toward Multishape Memory Effects.

Multi shape memory polymers (multi-SMPs) exhibit many potential applications such as aerospace, soft robotics, and biomedical devices because of their unique abilities. Although many works are done to broaden the preparations of multi-SMPs, the desire to a simple and versatile strategy as well as more complex shapes still exists. Moreover, a light-induced SMP shows more advantages than a thermal-induced one in many practical working circumstances. Herein, inspired by strong adhesion and efficient photothermal conversion of polydopamine (PDA) coating, we report a more simple and facile approach to prepare light-induced multi-SMPs by introducing a gradient PDA coating on a dual-SMP through time-controlled dipping. The photothermal converting properties with varying thicknesses of PDA under the tunable near-infrared light source are investigated. Then, light-induced multishape memory effects based on gradient PDA coatings are illustrated, where three designs of multi-SMPs - rectangle, triangle, and cross are prepared and demonstrated. Also, the evolutions of coating morphology during shape shifting are carefully studied. Finally, we present few complex designs of patterns and shapes as well as a design of potential application for the highly controllable smart devices. This strategy demonstrates a very simple and general strategy to design and prepare the light-induced multi-SMPs with complex shapes based on any thermal-responsive dual-SMPs.