Production of Stimuli-Responsive Microlens Arrays by Photografting Surface-Assembled Hydrogel Prepolymers

Hydrogels garner much interest due to their broad applicability to a wide range of fields such as soft robotics, biomaterials, and adaptive optics. Current manufacturing procedures for hydrogels, specifically those at the micro- and nanoscale, require the use of time consuming and expensive equipment. Moreover, often these microsctructured hydrogels (microgels) lack strong interfacial adhesion to support materials limiting their applications. We developed a manufacturing scheme for hydrogel prepolymers that: 1) uses mechanically-driven coalescence to assemble prepolymer droplets into well-defined geometries and morphologies and 2) creates robust bonds with an elastomer support substrate using a “graft from” polymerization scheme. We used this technique with several hydrogel systems to manufacture large arrays of highly ordered microgels in minutes. The size and periodicity of the hydrogels was easily controlled by changing the surface chemical template. Hydrogel arrays demonstrated robust adhesion to the surface versus hydrogels assembled on surfaces without the necessary moieties for covalent attachment. Despite the strong adhesion the attached hydrogels maintained their stimuli responsive nature. We demonstrated optically dynamic hydrogel arrays whose refractive index could be adjusted by external stimuli, such as solvent or heat. We quantified the change in refractive index by measuring the magnification of an image projected through a hydrogel array. These microlens arrays can be applied as optical sensors in soft robotics or lens elements in wearable electronic devices.