Beyond 3D: Smart Material Systems for Microscale 4D Printing

Abstract 4D printing combines advancements in 3D printing with smart materials that change properties in response to external stimuli, enabling the fabrication of functional devices directly from the printer. Microscale 4D printing is rapidly growing, driving the discovery and applications of new functional materials and fabrication strategies. The interest in microscale 4D printing comes from its potential to propel industrial transformation across different sectors, ranging from advanced healthcare to cryptography. However, this requires rational design and a good understanding of the material response. This review starts by describing smart materials for microscale 4D printing. 3D printing technologies with micrometer resolution and their use in 4D printing are then discussed. The focus then shifts to the different stimuli that microscale smart materials respond to, the mechanism behind the obtained shape changes with respect to the applied stimuli, and the methods implemented so far to realize complex shape changing behaviors. Examples of applications within sensing, microactuation, data encryption, and cargo transport are shown. After discussing current challenges and mitigation strategies, suggestions on the direction the field of microscale 4D printing can take are given, starting from a systematic and iterative approach for developing functional 4D microstructures.