Biomimetic, Programmable, and Part-by-part Maneuverable Single-body Shape Morphing Film

Recently, shape morphing films (SMFs) have been actively researched owing to their diverse applications such as soft robotics, soft gripper, and healthcare/wearable devices. Recent research about SMF concentrates on improving the morphing complexity, possibly widening the applicable area of the SMF. Their morphing complexity has been achieved by assembling multiple actuation elements in a single system. However, most of them are manually assembled, leading to unreliability in the assembled result, and are unsuitable for complex movements such as the gripping motion of the <i>Drosera Capensis,</i> which needs elaborate morphology and a highly ordered structure with multipTon order to resolve this problem, this study proposes a biomimetic, programmable, and part-by-part maneuverable single-body SMF, which may replace the conventional assembly methods.<br/><br/>An electrothermal actuator is adopted as the fundamental principle: multiple layers with diverse coefficients of thermal expansion (CTE) cause bending, originating from the difference in volume change induced by Joule heating. A strain-restricting layer and a strain-inducing layer were required, therefor polyimide and SU-8 with low CTE as the strain-restricting layer, and PDMS with high CTE as the strain-inducing layer, were adopted. Equivalently function to the assembled system, programmability and part-by-part maneuverability should be realized either. Programmability was achieved by a similar mechanism with <i>Bauhinia variegates</i>, which morphs by the hygroscopic volume change of overall tissue with restriction by the cellulose fibril structure. Similarly, the temperature-induced volume changes and anisotropy in elastic modulus by SU-8 microwall cause the bending of film in the intended direction. In the case of part-by-part maneuvering, it is achieved by controlling the voltage distribution of the underlying electrothermal heater.<br/><br/>The SU-8 wall pattern was optimized to maximize the bending sensitivity. Four variables (width (<i>w</i>), pitch/width ratio (<i>p</i>/<i>w</i>), height (<i>t</i>_SU-8) of the SU-8 micro-wall, and the relative thickness of PDMS concerning that of SU-8 (<i>t</i>_PDMS/<i>t</i>_SU-8)) are optimized to maximize the curvature (<i>k</i>) between the initial and heated states. Resultantly, <i>w</i> = 25 µm, <i>p</i>/<i>w</i> = 2, <i>t</i>_SU-8 = 75 µm, and <i>t</i>_PDMS/<i>t</i>_SU-8= 1.5 was selected. Based on the simulated result, over the NiCr nano-mesh-embedded PI film used as the electrothermal heater, the SU-8 wall with morphology that follows the optimization (<i>w</i> = 25.1 µm, <i>p</i>/<i>w</i> = 1.99, <i>t</i>_SU-8 = 80 µm, and <i>t</i>_PDMS/<i>t</i>_SU-8= 1.28) was fabricated as SMF.<br/><br/>For characterization, two samples of sizes 1x1 and 5x5 mm²were analyzed. The first experiment investigated the change in depends on power density (<i>P′ = </i>applied power/area of SMF). The results show a linear relationship between <i>P'</i> and <i>κ</i> for both samples, with a sensitivity (<i>S</i> = △<i>P'</i>/△<i>κ</i>) of 7.83 cm/W for the 1x1 SMF, and <i>S</i> = 6.26 cm/W for the 5x5 SMF. In the second experiment, 10–90% rise time (τ_R) and 90–10% fall time (τ_F) were measured for <i>P′</i><i> </i>= 0.6 W/cm². As a result, τ_R= 4.08 s, τ_F= 4.09 s for the SMF, and τ_R = 4.91 s, τ_F = 11.11 s for the SMF were measured. Finally, 1000 cycles of heating/cooling were performed with the 5x5 SMF, and no noticeable irregularity was observed.<br/><br/>Complex movements were demonstrated for two biomimetic applications: artificial inchworm (crawler) and artificial <i>Drosera capensis</i> (gripper). In the case of the artificial inchworm, five different motions were demonstrated (pose stabilization: front-to-back flip and back-to-front flip; basic movements: crawl, left turn, and right turn). In the case of the artificial <i>Drosera capensis</i>, the insect-gripping motion via hierarchical morphology was demonstrated. The proposed method for the fabrication of a biomimetic, programmable, and part-by-part maneuverable single-body SMF can successfully replace the conventional assembly process and achieve advanced SMF technology by enabling various complex movements toward practical applications.