Chemical and Stimulus Driven Morphogenesis in Soft and Hybrid Materials: Structures for Energy Storage and Soft RoboticsPublic Deposited
Living organisms undergo morphogenesis as they develop and change shape to fit their evolved niche in natural ecosystems. The biological processes underlying morphogenesis involve sophisticated feedback loops between spatiotemporal release of morphogenic molecules that diffuse and signal cell differentiation, as well as contextual interaction with the physical surroundings of the propagating cells. Analogous to biological morphogenesis, the development of morphology in synthetic materials influences many of the properties required for their function. Both synthetic pathways and external stimuli can be used to control the shape of structures in materials across scales. For example, designed molecular precursors and synthetic conditions such as temperature can enable morphogenesis in soft and hybrid materials across the nano- to microscale, impacting properties such as ionic and electronic conductivity and surface area that are critical to energy applications. Introducing external stimuli such as light and magnetic fields can induce morphogenesis at the macroscopic scale, resulting in geometries that allow locomotion and other robotic functions. In this work, morphogenesis is studied in several soft and hybrid materials, with structural evolution observed at the nanoscale in some systems and at macroscopic scales in others. First, the formation of hybrid nanostructures is studied for the synthesis of materials for energy storage materials. A conjugated aromatic surfactant molecule was found to template the formation of oriented nanotubes consisting of concentric layers of cobalt hydroxide and organic surfactant. Energy storage electrodes made with electrodeposited films of these nanostructured hybrids exhibited enhanced charge storage capacity and lower electrochemical impedance due to their high surface area and oriented structure. A similar class of metal hydroxide/organic surfactant hybrid was used in another project to template metallic nanowires on macroporous nickel foam. The resulting hierarchical electronically conductive structures combine both the large surface area and bulk electronic conductivity important for electrochemical energy storage and conversion applications. The increased surface area provides the intimate contact with the surrounding liquid solution required for high rate of electrochemical reactions, while the bulk electronic conductivity contributed by the macroporous nickel foam reduces ohmic losses in high current applications. While the synthesized hierarchical electrodes exhibited high surface area, the long synthetic route and fragility of metallic nanowire networks limited extensive study of their use in catalysis and energy storage. The synthetic expertise in forming nanowires gained in this project was translated to scaling up production of nickel nanowires for use in soft artificial actuators that could exhibit stimulus driven morphogenesis at macroscopic scales using external magnetic fields. The ferromagnetic nickel nanowires were encapsulated within photoactive hydrogels to create hybrid materials that respond to both light and external magnetic fields. When nickel nanowires were aligned using external magnetic fields during polymerization of the hydrogel matrix, they formed a ferromagnetic skeleton with macroscopic nematic order. The photoactive spiropyran moieties within the hydrogel allow the material to macroscopically change shape in response to light as gradients of hydrophobicity and physical contraction are formed along the path of the photons. Flat cross-shaped films were found to bend into curved arches during this light-activated process and rotating external magnetic fields introduced torques that produced extension, contraction and rotation of the hydrogel objects. The synergistic response was found to enable fast walking motion of macroscopic objects in water and delivery of cargo through rolling motion and light-driven shape changes. The theoretical description and finite element simulation of the material’s response to external energy input allowed for programming of specific trajectories of hydrogel objects that were verified experimentally, paving a path for fast and rational design of new geometries and material configurations for light and magnetic field activated robots with additional functionality and modes of locomotion. Finally, direct in-situ observation of nano- to micro-scale morphogenesis in supramolecular soft materials was enabled by coupling a custom-designed variable temperature sample stage with confocal laser scanning microscopy. The sub-micrometer resolution of this technique allowed for real-time observation of temperature-dependent morphogenic behavior of supramolecular peptide amphiphile nanofibers and supramolecular photocatalytic chromophore amphiphile nanoscale ribbons. The technique enabled direct monitoring and quantification of the kinetically-limited disassembly and breakdown of long peptide amphiphile nanofibers to short nanofibers, as well as the discovery of large microscale curl formation during thermally-driven crystallization of chromophore amphiphiles. The technique demonstrated in this work can sample large sample volumes and provides real-time information on thermally induced morphological changes in solution. Across this body of work, both synthetic inputs and external stimuli were studied as ways to control morphogenesis in soft and hybrid materials from the nano- to macroscale. These approaches are important for controlling the morphology and resulting materials properties that enable function and practical performance. Future studies expanding synthetic design inputs and additional external stimuli may enable morphogenesis of new materials and geometries for enhanced performance in energy storage applications and soft robotics.
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