(113a) A Simple Evaporative Deposition-Thermal Gelation Approach for Facile Fabrication of Biopolymer Films Containing Micropatterned Opal Structures

Artificial opal structures have gained significant attention due to the ready tunability and control of colors in response to external stimuli. In specific, opal structure-based hydrogel platforms have shown great potential due to the reversible volume change in response to external stimuli (e.g., salt concentration, humidity, temperature, pH, & ionic strength) indicated by color change. Currently, majority of the opal structure-based hydrogel platforms utilize synthetic polymers such as poly (ethylene glycol) (PEG), N-Isopropylacrylamide (NIPAM) and SU-8 among many others. Existing techniques to prepare these synthetic polymer hydrogels suffer from long opal deposition times, harsh radical polymerization and involve complicated multi-step preparation procedures. On the other hand, natural biopolymers such as gelatin and agarose are easy to handle, only require simple thermal gelation for polymerization, and are biocompatible. Moreover, these natural biopolymers are stimuli responsive to conditions such as humidity, pH and salt concentration. Overall, these advantageous features make the natural biopolymers ideal candidates for preparing opal hydrogel platforms.

In this work, we exploit the features of these biopolymers to prepare gelatin-based hydrogel films containing micropatterned opal structures assembled via another natural biopolymer chitosan, a potent amino polysaccharide offering ample nucleophilic primary amine groups. First, polystyrene (PS) nanospheres are allowed to self-assemble into well-ordered face-centered cubic (FCC) lattices via evaporative deposition in an aqueous solution containing chitosan in the patterned polydimethylsiloxane (PDMS) micromolds. The uniformly deposited opal structures captured using chitosan exhibit intense color is examined via darkfield optical microscopy, and the wavelengths of the opal colors are confirmed using visible reflectance spectroscopy. Further, scanning electron microscopy (SEM) confirms uniform FCC packing. These chitosan-based opal structures are then captured into gelatin films via simple thermal gelation. Dark field microscopy and SEM confirm reliable and consistent capture of the opal structures and robust nature of the films. By tuning simple parameters like chitosan and gelatin concentrations and PS nanospheres size, wide color range in the visible spectrum is readily achieved, confirming rapid, reliable, and robust nature of our simple evaporation-thermal gelation technique. In this presentation, our latest results on potential stimuli responsive behavior of our micropatterned opal-biopolymeric films to humidity and pH will be presented. We envision that the simple evaporative deposition–thermal gelation technique can be readily extended to manufacture a variety of biopolymeric materials for facile monitoring of environmental conditions.

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