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.