4D Printed shape memory polymers in focused ultrasound fields

4D Printing is a new area of additive manufacturing that extends the possibilities of 3D printing by including the dimension of time. This cutting-edge technique entails creating elaborate structures out of intelligent materials, specifically shape memory polymers (SMPs), which may dynamically change shape or functionality in response to external inputs. The purpose of this study is to conduct a rigorous spatiotemporal characterization into the potential of focused ultrasound (FUS) in actuating 4D-printed SMPs as well as to evaluate the impacts of different printing parameters on shape recovery. Experiments demonstrate that FUS is a unique and non-invasive method that can cause localized heating, activate several intermediate shapes, and accomplish full shape recovery in SMPs. Moreover, by optimizing sample size, ultrasound frequency, exposure time, intensity, and the location of ultrasound focusing, FUS possesses an enhanced capacity for temporal and spatial control of shape recovery. We determine the effects of various 3D printing parameters, including printing temperature, printing speed, infill density, and infill structures, on the thermo-mechanical shape recovery properties of a thermoplastic polyurethane. Shape recovery ratios ranged from 50% to 80% across different printing parameters. The study demonstrated that increasing acoustic field intensity can maximize shape recovery to over 95%, although this may cause to material degradation depending on sample thickness. The findings also revealed that these printing parameters significantly influence storage modulus, loss modulus, and glass transition temperature, highlighting their impact on thermo-mechanical properties. Furthermore, this study uses acoustical principles and thermo-mechanical experimental data to show a systematic relationship between additive manufacturing settings and SMP viscoelastic deformation properties. Lastly, a dynamic transition of a 4D-printed functional gripper-like structure, exhibiting both opening and closing motions upon exposure to FUS irradiation, was demonstrated using the optimized parameters. This research paves the way for FUS to accurately spatiotemporal and localized actuation of SMPs, particularly in medical applications.