4D Printing of Ag Nanowire-Embedded Shape Memory Composites with Stable and Controllable Electrical Responsivity: Implications for Flexible Actuators

An electrical stimulus is convenient in locally controlling the deformation/recovery of electrically responsive structures and is therefore considered as one of the most promising stimulus methods in four-dimensional (4D) printing. However, it is extremely difficult to homogeneously blend large amounts of conductive particles in shape memory composites (SMCs) with sufficient electrical responsivity for UV-curing-based 4D printing. Herein, we develop an innovative 4D printing strategy of electrically responsive architectures with a sandwich structure consisting of a silver nanowire (AgNW) electrothermal layer between two multiwalled-carbon-nanotube-reinforced shape memory polyurethane acrylate (MWCNTs-SMPUA) layers. It is realized by tentatively stopping the UV-curing printing based on digital light processing and embedding the AgNW electrothermal layer between two printed MWCNTs-SMPUA layers. The electrically responsive shape recovery speed (E-SRS) is finely adjustable by varying the current flow, and a stable and impact-free shape recovery process is demonstrated. MWCNT reinforcement enhances the mechanical properties of the SMPUA matrix and improves its E-SRS by 25% without increasing the value of current (0.6 A). According to the dynamic mechanical analysis, the storage modulus of SMPUA is greatly enhanced, which well explains the positive effects of MWCNTs on the SRS of SMPUA. The demonstrated 4D printing strategy is advantageous in prototyping structures with deployable and extendible abilities and could promote the development of 4D printing and make it feasible to integrate printing strategy into the flexible actuators industry.