Abstract: Examples of soft functional composite structures with periodic and symmetric arrangements of active and passive materials of dissimilar properties are widely observed in nature. Biological organisms exploit the swelling and growth mismatch and mechanical instabilities inherent in these structures to enable complex shape change and motion. Our work has been focused on developing computational models to explore how soft active and stiff passive material segments can be arranged in two-dimensional plate and three dimensional tube structures to achieve targeted shape changes. Also, we seek to investigate how these shape-changing structures can be combined to make functional devices.
In this presentation, I will describe the modeling investigation and experimental validation of two classes of composite hydrogel structures. The first is a thin bilayer plate with periodic arrangements of stiff SU8 epoxy segments in a poly(N-isopropylacrylamide) (pNIPAM) matrix. pNIPAM is a common thermoresponsive hydrogel that undergoes a transition from a hydrophilic state to a hydrophobic state when the temperature increases above the lower critical solution temperature (LCST), resulting in a dramatic change in volume. The composite plate can exhibit unusual biaxial and bidirectional bending in response to temperature change through the LCST. We utilized a chemomechanical material model to describe the equilibrium swelling and stress response of pNIPAM-AAc. We applied computational modeling to explain how the shape and spacing of the stiff SU8 segments and the crosslinking gradient of the pNIPAM matrix can be tailored to achieve biaxial and bidirectional bending. The second class of structures involves 3D-printed composite tubes with different symmetric arrangements of pNIPAM segments and a stiffer passive hydrogel. We applied computational modeling to design the geometry and arrangement of the active and passive segments to produce tubular structures that exhibit uniaxial axial elongation, radial expansion, buckling bending, and twisting through the LCST. The results are a set of shape-changing primitives that can be combined to produce more complicated motions for a functional device.
Dr. Thao (Vicky) Nguyen received her B.S. from MIT in 1998, and M.S. and Ph.D. from Stanford in 2004, all in mechanical engineering. She joined the Mechanical Engineering Department at The Johns Hopkins University in 2007, where she is currently a tenured associate professor and the Marlin U. Zimmerman Faculty Scholar. Dr. Nguyen’s research encompasses the biomechanics of soft tissues and the mechanics of active polymers and biomaterials. Dr. Nguyen has received the 2008 Presidential Early Career Award for Scientists and Engineers (PECASE) for her work on modeling the thermomechanical behavior of shape memory polymers. She received the 2013 NSF CAREER award and 2016 JHU Catalyst Award to study the micromechanisms of growth and remodeling of collagenous tissues. She was also awarded the inaugural Eshelby Mechanics Award for Young Faculty, the ASME Sia Nemat-Nasser Early Career Award both in 2013, and the ASME Applied Mechanics Division T.J.R. Hughes Young Investigator Award in 2015.
This event was published on November 13, 2019.