Programmable 4D-Printed Soft Actuators: Harnessing Bending Strain Distribution for Embedded Topological Functionality

Embedded programming plays a crucial role in 2D to 3D shape transformation by providing on-demand, precise, and localized control for applications ranging from soft robotics to biomedical devices. In this work, as a first of its kind, bending strain (or stress) distribution is applied on the TPU-BIFA composite to embed topology information in 2D, which transforms into a 3D structure with the corresponding topology when exposed to light. The internal bending strain distribution characteristics of the composite structure govern the poststimulant-triggered actuation or shape morphing. The internal strain distribution can be effectively varied through the magnitude and position of external loads and the boundary conditions. The prebending approaches of embedding topology are more useful for soft and planar structures with complex architecture. We also report an embedded composite ink 3D printing strategy to fabricate such complex structures by applying the principle of liquid–liquid phase separation. In addition to enabling programmability within the system, light as a stimulus offers great control over the actuators by tuning the light intensity and point of light irradiation. Upon irradiation with blue light (455 nm), the fabricated actuator performs well with omnidirectional light-triggered bending actuation of 20°/s and response rate of 60° in 8 s. As a proof of concept, phototunable curvature was demonstrated in complex architectures such as grids and human hand models by manipulating the prebending strain distribution. This work presents a comprehensive catalog of data generated by tuning various parameters to unveil a fresh horizon, which can be leveraged for programming complex geometries and opening avenues for smart polymers in biomimicry and biomedical applications.