Mechanically Unstable Hydrogel Sheets: Formation Of Stimuli-Responsive Surfaces And Structures

A hydrogel is a crosslinked network of polymer chains swollen by water. When immersed in an aqueous medium, a hydrogel will swell by taking up water until the osmotic pressure set by mixing between water and polymer is balanced by the free energy required to stretch polymer chains. When considered at a length scale greater than the sub-micrometer scale inhomogeneity of the gel network structure, the unconstrained gel swells isotropically and reaches a macroscopically stress free state. However, when a sheet of gel is attached to either a non-swelling rigid substrate or a gel that swells by a different amount, the resulting mechanical constraints generate stress within the gel, leading to the out-of-plane deformations of the gel. In this thesis, we study and harness the instabilities of these mechanically constrained hydrogels, especially thin hydrogel sheets with thicknesses of 10-100 micrometers that swell and deswell rapidly (less than 10 seconds for sufficiently hydrophilic gels). We create hydrogel based micro-systems, where we locally apply mechanical constraints on the swelling of hydrogel sheets, and therefore, the gels deform out-of-plane into 3D shapes. Next, we experimentally characterize and analyze the deformed hydrogels, elucidate the mechanisms underlying the observed deformation using finite element analysis, and finally utilize these methods to fabricate stimuli-responsive surfaces and structures. As the first example, we attach a thin film of hydrogel on a rigid substrate, inducing an elastic creasing instability in which the surface of the hydrogel locally folds against itself. Through the chemical modification of the hydrogel surfaces that undergo the creasing instability, we fabricate dynamic surfaces that hide and display biomolecular patterns in response to an external stimulus and show how these materials hold promise for applications in studying cell mechanics and creating lab-on-a-chip devices. Next, we use a grayscale gel lithography to two-dimensionally patterned discretely varying swelling ratios within a hydrogel sheet of ∼ 10 micrometer thickness. This finite-thickness, differentially growing hydrogel sheet undergoes out-of-plane deformation as it swells and adopts a configuration that is determined by the initially prescribed local swelling ratios and minimizes the overall elastic deformation energy, i.e. the sum of stretching and bending energies. Additionally, we introduce a halftone-style two-level grayscale gel lithography, which prescribes effectively continuous metrics on the hydrogel sheets by patterning hexagonal arrays of dots that locally vary in their sizes and swell less than the background. This platform, grayscale gel lithography, provides opportunities both for asking fundamental questions about the mechanics of non-Euclidean plates, as well as for designing stimuli-responsive micro-devices