High-Performance 4D Printed ABS/Conductive TPU Electrothermal Actuator Devices with SWCNT Segregated Structures: A Gripper Demonstrator toward Soft Robotics Applications

A high-performance U-shaped bimetallic polymer-based "soft" electrothermal actuator (ETA) device is reported utilizing a versatile fused filament fabrication (FFF) three-dimensional printing (3DP) process. A dual-head multimaterial 3D printer is employed to fabricate the ETA devices, consisting of an acrylonitrile butadiene styrene (ABS)/conductive thermoplastic polyurethane (cTPU) bilayer architecture. The cTPU layer is intentionally printed with a gyroid microporous structure, facilitating the infiltration of a single-walled carbon nanotube (SWCNT) aqueous ink, deposited through "direct ink writing" (DIW). The final 4D printed ABS/cTPU/SWCNT ETA could reach orders of magnitude lower internal resistance compared to the "cTPU only" layer, namely, from ca. 9 kΩ to ∼20 Ω. Scanning electron microscopy (SEM), Raman spectroscopy, thermogravimetric analysis (TGA), and electrical resistance measurements highlight the morphological and physicochemical properties of the obtained electrothermally active materials and structures. 4DP ETAs are characterized for their actuation bending performance upon being exposed to different applied bias voltages (V_bias) and "ON-OFF" alternating cycles, measuring in real time the tip displacement through a high-resolution camera. Finite element analysis (FEA) corroborates the ETA device performance for a specific V_bias. The force generated by the ETAs is quantified via a digital microbalance, while infrared thermography (IR-T) images are captured upon device operation to validate the electrothermal Joule-heating effect. Three ETA devices are electrically connected in parallel to a three-finger "soft" gripper demonstrator. Our 4DP ETAs could have a modular design for variable applications, while the fast and reliably responsive gripper prototype could open new avenues in the field of soft robotics.