Design, fabrication, and application of stimuli-responsive hydrogel actuators

In this thesis, the topics around stimuli-responsive hydrogel actuators were discussed. In each project, a specially designed, stimuli-responsive, hydrogel was fabricated. By initiating a corresponding stimuli, swell-deswell changes were triggered within the hydrogels, and certain mechanical movements can be created and controlled by manipulating the patterns and structures of the hydrogels. With these projects, we are targeting improvement of the fabrication of biomimetic actuators and soft robotics. Among the 5 projects that will be discussed in this thesis, the first 3 systems are chemically initiated stimuli-responsive actuators (chemomechanical actuators), and the other two systems are non-chemically initiated. In the chemomechanical systems, we explored relationships between different chemical systems and hydrogels, as well as the conversion of chemical energy into mechanical movements in a biomimetic fashion. For the first chemomechanical project, we described a new planar processing chemistry that allows the synthesis and patterning of dynamically self-actuating gels of diverse form. The chemical reaction covalently incorporates a methacrylate-modified ruthenium trisbipyridine (Ru(bpy)32+) monomer within a poly acrylamide (PAAm) gel, modified to provide a flexible chemistry for UV-curable, self-oscillating Belousov-Zhabotinsky (BZ) gels. Then, inspired by the photosensitizing capability of Ru(bpy)32+ from the first project, we successfully used visible light to trigger macroscopic movements of polyacrylic acid (PAA) based pH sensitive hydrogels through a photo catalytic water-splitting reaction system, which contains Ru(bpy)32+ as the photosensitizer and iridium dioxide (IrO2) nanoparticles as the catalyst. Finally, inspired by the pH sensitive PAA materials from the second project, we developed electrochemically-induced micro and macro scaled chemomechanical hydrogel actuators. The actuation from pH sensitive PAA hydrogel was achieved through the electrochemically-induced oxygen reduction reaction (ORR), during which protons are produced at the anode and consumed at the cathode, forming a pH gradient between the two electrodes. For the non-chemically triggered actuator systems, we exploited the fast-responsive and vast volume change of the hydrogel due to thermal change or solubility change and aimed to explore the hierarchical programmability of matter through strain in a 3D fashion. In the fourth project, we show the controlled nonuniform swelling and de-swelling of one 3D hydrogel based on integrating electronic meshes, utilized as a local heating agent, into thermally responsive poly N-isopropyl acrylamide (PNIPAAm). Such hydrogel yields from embedding electronic meshes with functionalities and controlling abilities to locally program the shape of the hydrogel. In the last project, we used light to “program” the folding mechanics of a flat, two-dimensional material. Mixtures of polydimethylsiloxane (PDMS) and SU-8 photoresist are exposed to different photomasks, creating a disparity in cross-linked SU-8 density between the exposed and unexposed portions. Upon immersion in nonpolar organic solvent, strain gradients are formed into the folding configuration, due to the swelling difference of PDMS and SU-8. The photomasks in the fabrication process can be varied to tailor the strain and direct folding into different 3D configurations upon immersion into nonpolar solvent. By changing the exposure pattern, different folding configurations can be generated from the same two-dimensional precursor