filament materials. The FDM printing process is shown in Figure 1a where melted thermoplastic polymeric filament is extruded from hot nozzle and later experiences rapid solidification at build platform. Zhang et al. [5] first reported that the extruded polylactic acid (PLA) filaments exhibited significant heat-induced shrinkage (Figure 1b) and attributed this deformation to the release of built-in tensile strain created upon printing. Since then, this mechanism attracts increasing attention in the realm of 4D printing. [6] One representative example is the selffolding design where flat patterns with anisotropic filament orientations are laminated to create interlayer strainmismatch. [4b] This idea is well adopted [7] and generalized into other strategies including multimaterial FDM printing, [8] paper-assisted FDM printing, [9] thermal gradient-controlled FDM printing, [10] and feeding-rate-controlled FDM printing. [11] Besides printing orientation, some researchers observed that the moving speed of printing nozzle may also affect the built-in strain of extruded filaments. [5,8a,d,11] Taking advantage of this feature, we propose a new 4D printing strategy that utilizes printing speed gradient to embed FDM-printed patterns with interlayer mismatch strain. By tailoring the combination of printing speed, we are allowed to handily regulate the heat-induced bending of laminated patterns. To broaden the range of achievable bending curvature, we further investigate the effects of nozzle temperature, build platform temperature, and layer thickness on the built-in strain of FDM extruded filaments. With these optimized parameters, we fabricate several flat precursory patterns that employ embedded bending elements to drive themselves into complex 3D geometries. Compared with manipulating filament orientation of different layers, our approach takes less efforts because it simplifies the tedious procedure of generating G-code. Overall, we provide an efficient and economical way of realizing 4D printing, rendering potential applications in deployable structures and biomedical devices.
Results and Discussion
Effects of FDM Printing Parameters on Built-In StrainPrior to investigating the effect of printing speed on heatinduced shrinkage, we first determine the optimized activation temperature that enables the 3D-printed PLA samples to fully release the built-in strain. Herein, the activation temperature 4D printing is an advanced technology that aims to endow the additively manufactured objects with time-dependent transformations. Despite the great progress in the last decade, however, there are still some technical barriers for the majority of existing 4D printing techniques including the dependency on customized or expensive 3D printers and the demand of sophisticated chemistry in the preparation of active materials. Herein, an efficient and economical 4D printing approach that adopts multispeed fused deposition modeling (FDM) technique to embed graded built-in strain into flat precursory pattern is proposed, allowing...