Programming time-dependent behavior in 4D printing by geometric and printing parameters

Smart structures realize sequential motion and self-assembly through external stimuli. With the advancement of four-dimensional (4D) printing, the programming of sequential motions of smart structures is endowed with more design and manufacturing possibilities. In this research, we present a method for physically programming the timescale of shape change in 4D-printed bilayer actuators to enable the sequential motion and self-assembly of smart structures. The effects of the geometric and printing parameters on the time-dependent behavior of 4D-printed bilayer actuators are investigated. The results show that the thickness of the active layer directly affects the timescale of motion, and increasing the thickness leads to faster motion until the thickness ratio is close to 4:6. Similarly, a higher printing speed results in faster motion. Conversely, a higher printing temperature and a greater layer height result in a slower shape change. The effects of the length-width ratio, line width, and filling ratio on the timescale of motion are not as straightforward. Finally, we demonstrate several smart structures that exhibit sequential motion, including a labyrinth-like self-folding structure that is choreographed to achieve multi-step self-shaping and a flower-shaped structure where each part completes its movement sequentially to avoid collisions. The presented method extends the programmability and functional capabilities of 4D printing.