Mechanics-Based Design of Stimuli-Responsive Hydrogel Structures and Devices

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Johns Hopkins University
Stimuli-responsive hydrogels undergo large shape change in response to a wide variety of stimuli, such as temperature, biochemical molecule, pH, electric or magnetic field. Programmable shape changing devices made from stimuli-responsive hydrogels have potentially wide-ranging applications, including drug delivery, soft robotics, and biomedical devices. The current main challenges of advancing stimuli-responsive hydrogel devices into real-life applications lie in both the design and fabrication stage. From the design perspective, it is challenging to construct a multi-physics constitutive model to accurately describe the material behavior of hydrogels and execute finite element modeling to guide the design of stimuli-responsive hydrogel structures. Most models that have been developed in the research community are computational with qualitative experimental validation. Thus, simple strategies based on mechanics are needed to guide the design of these devices with complicated shape changes. From the fabrication perspective, traditional fabrication methods such as lithography can only make 2D planar structures, limiting the number of achievable device geometries. Advanced manufacturing is needed to achieve the device designs with more complicated functionalities. The objective of this work is to develop an efficient design framework for stimuli-responsive hydrogels. I aimed to advance the field of stimuli-responsive hydrogels in both the design and fabrication aspects. In this work, I have developed a design methodology that covers from constitutive modeling, finite element simulation, mechanics-based design rules, to experimental validation. This hybrid modeling design methodology provides an efficient design framework for stimuli responsive hydrogel structures and devices of functional importance. In the first part of this work, I applied a chemo-mechanical constitutive model to describe the swelling and mechanical behavior of a novel DNA hydrogel (AAm-co-DNA). In order to estimate the elastic modulus of DNA hydrogel that cannot be measured by traditional experimental approach, I used finite element analysis to study the curving of bilayer beams composed of the DNA gel and a passive gel (polyacrylamide, pAAM), and compared the finite element simulations to the experiment result. To explore the design space, I further applied the finite element model to investigate the influence of bilayer geometric and material properties on the equilibrium curvature. I used the modified Stoney formula for the curving of film/substrate system to develop a simple design rule for predicting the equilibrium curvature of bilayer gel beams with different dimensions and material properties. In the second part of this work, I report the design, fabrication, and characterization of segmented 3D printed gel tubes composed of an active thermally responsive swelling gel (poly N-isopropylacrylamide, pNIPAM) and a passive thermally nonresponsive gel (pAAM). Using finite element simulations and experiments, I demonstrated a variety of primitive shape changes including uniaxial elongation, bending, buckling, and gripping based on different segment arrangements of two gels. The assembly of shape-changing primitives could be directly printed and used to achieve complicated tasks. In the final part of this work, I report on the unusual periodic buckling behavior of a 4D printed tubular structure, composed of alternating vertical strips of pNIPAM segments and pAAM segments. The tube design was inspired by the buckled surfaces observed in nature, such as on cacti and euphorbias. I found that the tubes show tunable periodic buckling modes in water at the room temperature, due to the development of compressive stresses in the soft swellable segments induced by the constraint of the stiff non-swellable segments. I developed finite element models to explore the design space and investigate the effects of geometric and material design parameters on the buckling mode. Modeling the swellable segments as the buckling of a bar on an elastic foundation, I derived a bucking parameter that combines the effects of geometry and material properties to predict the transition between different periodic buckling modes and constructed a phase diagram to guide the design of periodic buckling tubes for bioinspired functional gel structures.
Mechanics-based design, stimuli-responsive hydrogels, finite element modeling, 4D printing