Computational Design of Soft Functional Composite Hydrogel Structures and Devices

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, 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.

Bio

Thao (Vicky) Nguyen, the Marlin U. Zimmerman, Jr. Faculty Scholar and a professor of mechanical engineering, is known for her research on the mechanics of soft engineering and biological materials.

Her lab uses an integrated experimental and modeling approach to study adaptive materials that can exhibit dramatic changes in microstructure, mechanical properties, and macroscopic shape in response to an environmental stimulus. Her research focuses on the biomechanics of fibrous soft tissues, constitutive modeling of shape memory polymers and polymer composites, and the fracture and failure of rate dependent materials.

Nguyen's work has contributed to the development of innovative experimental tools and models to investigate the fundamental microscale mechanisms and microstructural origins of the behavior of soft adaptive materials. Her research has provided design tools and guidelines for the creation of shape-shifting biomedical devices, including materials that expand in the body to secure tendons to bone and "smart" medical devices that function without wires or batteries.

She also is an expert on the complex mechanics of the eye. Nguyen has worked collaboratively to investigate the role of the sclera and cornea in the development of glaucoma, and to develop a biomechanical model of the sclera and its effects on glaucoma. Her research on the eye has been funded by the Department of Defense, National Eye Institute, BrightFocus Foundation, and the National Science Foundation.

Nguyen's awards include an NSF CAREER Award, a Johns Hopkins Catalyst Award, and the Thomas J.R. Hughes Young Investigator Award and Sia Nemat-Nasser Early Career Award, both from the American Society for Mechanical Engineers (ASME). She serves on the Board of Directors for the Society of Engineering Science and has chaired multiple committees for ASME. She is also a member of the Society for Experimental Mechanics, the Association for Research in Vision and Ophthalmology, and the International Society of Eye Research. She is associate editor for two ASME journals: Applied Mechanics Review and J. Biomechanical Engineering.

She received her bachelor's degree in mechanical engineering from Massachusetts Institute of Technology in 1998, and MS and PhD degrees from Stanford University in 2000 and 2004, respectively. Prior to joining Johns Hopkins in 2007, Nguyen worked for three years as a senior member of the technical staff at Sandia National Laboratories in Livermore, California. She holds a secondary appointment in the Department of Materials Science and Engineering.