Material systems used in 4D printing

4D printing allows a 3D printed structure to change its configuration or function with time in response to external stimuli such as temperature, light, water, pH, etc. The basic idea is to manipulate materials at nano and micro levels in order to produce, via 3D printing, materials that can modify their structures over time at the macro level. Based on 4D printing, 3D printed objects from nano- to macroscale can be fabricated as smart devices, metamaterials, and origami for various functional applications in prototype, aerospace, biomedicine, etc.