Thermomorphic pneumatic metamaterials: numerous and robust shape-locking under temperature modulation through 4D printing

Four-dimensional (4D) printing represents a groundbreaking advancement in manufacturing, yet a persistent challenge is the limited number of stable configurations achievable through spontaneous shape reconstruction. Herein, we present a novel 4D printing mechanism that utilizes self-adjustable gas pressure to facilitate a wide range of spontaneous and stable multi-shape transformations. The gas is precisely released at designated spatial locations through strategic temperature-controlled degradation of a solid material, which is printed and distributed as needed at the voxel level within a specially designed multi-material structure, consisting of a low degradation temperature material (LDTM), a high degradation temperature soft material (HDTSM), and a high degradation temperature hard material (HDTHM). Each shape configuration is determined and locked in by the maximum temperature experienced during its thermal history. Notably, this shape retains its form robustly, independently of subsequent temperature changes, until a higher temperature threshold is reached, at which point a new shape configuration is triggered. These shapes exhibit a remarkable temperature memory effect, permanently recording the peak temperature reached in their thermal history. Our study comprehensively investigates the underlying principles and key parameters that influence deformation. We present a series of examples demonstrating complex multi-shape transformations modulated by temperature, supported by finite element simulations. This advance in 4D printing has the potential to significantly enhance its functional capabilities, performance, and applicability, opening up new horizons in additive manufacturing and design.