4D Printing of Glass Fiber-Regulated Shape Shifting Structures with High Stiffness.

4D printing allows 3D printed structures to change their shapes overtime under external stimuli, finding a wide range of potential applications in actuators, soft robotics, active metamaterials, flexible electronics, and biomedical devices. However, most 4D printing uses soft polymers to accommodate large strain shape-changing capability at the price of low stiffness, which impedes their engineering applications. Here, we demonstrate an approach to design and manufacture self-morphing structures with large deformation and high modulus (∼4.8 GPa). The structures are printed by multimaterial direct ink writing (DIW) using composite inks that contain a high volume fraction of solvent, photocurable polymer resin, and short glass fibers as well as fumed silica. During printing, the glass fibers undergo shear-induced alignment through the nozzle, leading to highly anisotropic mechanical properties. The solvent is then evaporated, during which the aligned glass fibers enable anisotropic shrinkage in the parallel and perpendicular directions to the fiber alignment for shape shifting. A final postphotocuring step is applied to further increase the stiffness of the composite from ∼300 MPa to ∼4.8 GPa. A finite element analysis (FEA) model is developed to predict the influence of the solvent, fiber contents, and fiber orientation on the shape shifting. We demonstrate the anisotropic volume shrinkage of the structures can be used as active hinges to transform printed two-dimensional structures into complex three-dimensional structures with large shape-shifting and outstanding mechanical properties. This strategy for fabricating composite structures with programmable architectures and excellent mechanical properties shows potential applications in morphing lightweight structures with load-bearing capabilities.