Multistable Thermal Actuators Via Multimaterial 4D Printing

3D printing of smart materials, called four-dimensional (4D) printing, adds active, responsive functions to 3D-printed structures. Shape memory polymers (SMPs) can be employed as an active material in 4D printing, and this could be useful for a wide range of potential applications. New design for 4D printing is presented here by introducing SMPs into rotational multistable structures. Two different digital SMPs are employed to enable large-angle, thermal actuation in a controlled manner. It is started with bistable structures and then extending them to quadristable ones. In this design, by adjusting the thickness of SMP beams, a balance is controlled between the energy barrier and shape memory force, and this can enable controlled thermal actuation. Especially, one can control the activation time for thermal actuation. Multistable structures can simplify actuation and motion control without complicated control systems. Therefore, by adopting multistable structures in 4D printing, one can potentially enable precisely controlled, large-magnitude, rapid actuation beyond the material capability of SMPs. These multistable structures do not require heating in the programming stage and this significantly simplifies the actuation procedure. This work provides new physical insights into 3D-printable, active structures, and could be useful for various smart actuators responding to the environmental stimuli.