Shape memory polymers with improved shape recovery properties in focused ultrasound fields

Abstract

Shape memory polymers (SMPs) have garnered significant attention for their remarkable ability to recover deformations upon external stimulation, making them highly suitable for biomedical devices, soft robotics, and deployable systems. Among the various activation mechanisms, focused ultrasound (FUS) has emerged as a promising non-invasive and spatially selective stimulus, capable of inducing localized heating via viscous polymer chain shearing. However, the material design strategies enabling efficient and rapid shape recovery under FUS remain underexplored. This dissertation aims to develop, characterize, and computationally model acrylate-based SMPs and their composites, focusing on enhancing actuation efficiency under FUS. To accommodate miniaturized biomedical systems, fibrous SMPs are fabricated via electrospinning, where fiber morphology is tuned through polymer concentration and flow rate. The electrospun webs exhibit 100% shape recovery and variable fixity ratios, with fiber diameter significantly influencing thermal and mechanical properties. We systematically design SMP networks with tunable glass transition temperatures and hydrophilic/hydrophobic properties to investigate the role of water uptake in FUS-induced shape recovery. Experimental results reveal that pre-immersion of hydrophilic SMPs facilitates faster and more complete recovery, attributed to water-induced plasticization and improved acoustic coupling. To achieve advanced actuation functions, a two-way shape memory polymer (2W-SMP) based on crosslinked poly(ethylene-co-vinyl acetate) is developed. This polymer demonstrates reversible motions, enabling applications such as gripping, self-rolling, and jumping in soft robotic systems. Furthermore, boron nitride (BN) nanoplatelets are incorporated into SMP matrices to enhance thermal conductivity and actuation performance. Under FUS, BN-filled composites exhibit a 75% improvement in recovery ratio compared to unfilled SMPs, owing to more efficient heat transfer and reduced activation thresholds. Complementing experimental studies, fully atomistic molecular dynamics (MD) simulations are employed to examine the role of crosslinking density on thermomechanical properties and shape memory behavior. The simulations demonstrate that increased crosslinking enhances stiffness, glass transition temperature, and recovery efficiency, while identifying the molecular-scale mechanisms driving ultrasound-induced shape change. Collectively, this work provides a comprehensive framework for the design and optimization of ultrasound-responsive SMPs through multi-scale experimental and computational approaches. The findings offer critical insights into the interplay between polymer structure, water interaction, filler reinforcement, and acoustic actuation, advancing the development of tunable SMP systems for biomedical and soft robotic applications.