4D-Printed Spin Crossover Metamaterials with Giant Programmable Positive or Negative Thermal Expansion.

In the past decade, 3D-printed cellular materials have witnessed an impressive advancement affording a wealth of remarkable mechanical properties, such as negative Poisson's ratio, negative compressibility, and negative coefficient of thermal expansion (CTE). Recent efforts in this field have been increasingly considered 4D-printed metastructures, which leverage shape-morphing properties of stimuli-responsive materials. Here, we introduce a new class of 4D-printed metamaterials based on bistable spin crossover (SCO) molecular materials. These systems synergistically couple dissimilar materials at different size scales to harness mismatched thermomechanical properties-specifically differential thermal expansion and stiffness-to generate large directional deformations upon heating or cooling. Through a combination of theoretical modeling and experimental validation, we demonstrate that our SCO-based 4D-printed structures can achieve programmable motions, including positive and negative expansion. The associated CTE reaches peak values of ca. +14400 and -11400 ppm/°C, respectively, more than 10 times greater than those reported in the literature for 3D-printed analogues. This work establishes a versatile and generalizable conceptual strategy for engineering multilevel, hierarchical architectures with programmable functionalities, advancing the design of energy-efficient soft actuators and reconfigurable/adaptive material systems.