Layout design and application of 4D-printing bio-inspired structures with programmable actuators

Four-dimensional (4D) printing is an advanced form of three-dimensional (3D) printing with controllable and programmable shape transformation over time. Actuators are used as a controlling factor with multi-stage shape recovery, with emerging opportunities to customize the mechanical properties of bio-inspired structures. The print pattern of shape memory polymer (SMP) fibers strongly affects the achievable resolution, and consequently influences several other physical and mechanical properties of fabricated actuators. However, the deformations of bio-inspired structures due to actuator layout are more complex because of the presence of the coupling of multi-directional strain. In this study, the initial structure was designed from closed-shell behavior and divided into a general unit and actuator unit, the latter responsible for driving the transformation. Mutual stress confrontation between the actuator and the general unit was considered in the layout thermodynamic model, in order to eliminate the transformation produced by the uncontrolled shape memory behavior of the general unit. Three critical and effective strategies for the layout design of actuators were proposed and then applied to achieve the desired accurate deformation of 3D-printed bilayer structures. Finally, the proposed approach was validated and adopted for fabricating a complex shell-like gripper structure. Graphic abstract