4D Printed Stiffness-Tunable Actuator for Load-Bearing Soft Machines

4D printing is an advanced technology that integrates the mechanics design and active materials into 3D printing to create objects with time-evolving transformation. However, the commonly used active materials, such as shape memory polymers or hydrogels, make it challenging to simultaneously realize two-way shapeshifting and high stiffness in 4D printed structures, constraining its application in engineering areas such as robotics, morphing structures, etc. Herein, inspired by human musculoskeletal structure, a 4D printing strategy that integrates two-way shapeshifting liquid crystal elastomer and amorphous shape memory polymer to realize reversible actuation and tunable stiffness via sequential thermal control is proposed. Through numerical analysis and experimental investigation, the mapping relationship among structural deformation, material property, and geometric design is established, allowing to seek out the optimized thickness ratio and material properties that lead to high shape fixity (81%) and complete shape recovery (100%). To demonstrate the potential applications, a variable stiffness hook capable of wrapping and lifting heavy objects through helical transformation is realized. This design strategy can potentially inspire the development of 4D printing toward load-bearing soft machines.