Programming a shape memory polymer network with thermo- and photo-reversible bonds toward a single-component soft robot.

Advanced multifunctional devices increasingly rely on challenging complex shapes for their functions. 3D printing offers a solution but is often limited by the fabrication speed and/or material diversity. 4D printing based on digitally controlled 2D-to-3D transformation is advantageous in speed, but the accessible shapes are limited and integration of multiple materials is difficult. We report herein a concept that significantly extends the technological scope by combining 4D printing with modular assembly. Specifically, 4D photo-printed structures based on dynamically crosslinked polymers can be assembled in a modular fashion by interfacial bond exchange. Complex 3D objects with tailorable multiple materials can consequently be produced. This allows the fabrication of sophisticated shape-memory devices including a 3D Miura-patterned structure with zero Poisson's ratio and a Kresling-patterned cylindrical structure with superior mechanical stability. Our approach extends the possibilities for the future development of multifunctional devices with seamless integration of material, structure, and function.

A recently emerged reversible shape memory effect greatly extends the capability of shape memory polymers and their practical potential. Physical confinement and chemical fixation are individually known to be effective in introducing network anisotropy essential for reversible shape memory. Herein, we demonstrate that synergetic combination of these two mechanisms effectively diversifies the shape-shifting behavior. Specifically, we introduce a transesterification catalyst into a network containing two crystalline phases: poly­(ε-caprolactone) (PCL) and poly­(ω-pentadecalactone) (PPDL). The reversible shape memory behavior of the resulting system can be programmed via the physical confinement by the PPDL phase and the chemical plasticity by the dynamic ester exchange. We illustrate that the two programming mechanisms can operate in a noninterfering way that allows achieving a synergetic benefit, notably realizing a zero-set reversible shape memory behavior. Our study points to a direction in diversifying the behaviors of reversible shape memory polymers and expands the scope for potential engineering devices.