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

Abstract

ABSTRACT 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.