4D printing of cellular silicones with negative stiffness effect for enhanced energy absorption and impact protection

Cellular silicone foams are renowned for their exceptional flexibility, ultra-elasticity, and durability, which have gained significant attentions in diverse applications. However, manufacturing porous silicone foams featuring substantial bending curvatures and customized mechanical properties remain a challenge. Herein, this work presents a new strategy for manufacturing curved porous silicone foams with both 4D printed bending curvature and tailored meta-mechanical properties. The foam is achieved by direct ink writing of composite silicone inks embedded with thermal expandable microspheres as foaming agents. The work studied in detail the thermal expansion of microspheres, foaming of composite silicone inks, as well as properties of the expanded foam. Utilizing the strain mismatch under thermal stimuli, the printed bilayer structures achieved shape-shifting. Additionally, silicones composed of stacked bilayer filaments exhibited negative stiffness properties under compression, leading to enhanced energy absorption capacity, which can be fine-tuned through different printing structural designs, demonstrating its potential in fields such as energy dissipation and shock absorption while protecting objects with curved shapes.