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. A sequence of shape change can be designed to produce locomtion for soft robotics applications.   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 three 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. Finally, I will describe our efforts to design a 3D printed hydrogel robot.  The robot is composed of pNIPAM and polyacrylamide bilayers.  The thicknesses of the bilayers are varied to allow the different segments of the robot to swell at different rates. This assymetric actuation allows the robot to move forward when subjected to a heating and cooling cycle. The results demonstrates how kinetics can be used to program a sequence of shape change to produce locomtion,