Fractal-Based Stretchable Circuits via Electric-Field-Driven Microscale 3D Printing for Localized Heating of Shape Memory Polymers in 4D Printing

Thermally responsive shape memory polymers (SMPs) usedin 4D printing are often reported to be activated by external heat sources orembedded stiffheaters. However, such heating strategies impede thepractical application of 4D printing due to the lack of precise control overheating or the limited ability to accommodate the stretching during shapeprogramming. Herein, we propose a novel 4D printing paradigm byfabricating stretchable heating circuits with fractal motifs via electric-field-driven microscale 3D printing of conductive paste for seamless integrationinto 3D printed structures with SMP components. By regulating the fractalorder and printing/processing parameters, the overall electrical resistanceand areal coverage of the circuits can be tuned to produce an efficient and uniform heating performance. Compared with serpentinestructures, the resistance of fractal-based circuits remains relatively stable under both uniaxial and biaxial stretching. In practice,steady-state and transient heating modes can be respectively used during the shape programming and actuation phases. Wedemonstrate that this approach is suitable for 4D printed structures with shape programming by either uniaxial or biaxial stretching.Notably, the biaxial stretchability of fractal-based heating circuits enables the shape change between a planar structure and a 3D onewith double curvature. The proposed strategy would offer more freedom in designing 4D printed structures and enable themanipulation of the latter in a controlled and selective manner.