4D printing of dual-stage curing ink for graded functional materials and direct, robust, integrated shape morphing

4D printing marks a significant leap beyond traditional 3D printing, enabling structures to morph and adapt dynamically to changing environmental conditions. Research efforts are increasingly being focused on this promising technology. Existing 4D printing methods, however, typically rely on shape-changing mechanisms that operate after material solidification in the 3D printing process, necessitating additional energy inputs or even manual mechanical interventions. In this study, we introduce a novel 4D printing approach that employs a specially designed dual-stage curing ink optimized for grayscale Digital Light Processing (g-DLP) 3D printing. This novel approach not only grants precise, on-demand control over the material properties of the printed structures but also seamlessly integrates shape transformation into the thermal curing stage of the 3D printing process, eliminating the need for extra energy or manual steps. Furthermore, these transformed shapes demonstrate exceptional robustness and permanence, as evidenced by our experimental results. We conduct a comprehensive investigation into the grayscale-dependent mechanical properties of this ink and highlight its potential applications through auxetic metamaterial lattice structures. The underlying mechanism behind this direct shape morphing during thermal curing is explored in depth, and effectively demonstrated in achieving desired shape transformations across various printed structures. The good agreement between our experimental results and simulations provides robust validation of our methodology. Our study provides a reliable framework for designing and optimizing 4D-printed structures with tailored properties and deformation behaviors, paving the way for future applications in various fields.