(77c) Stimuli-Responsive Soft Materials Containing Artificial Opal Micropatterns

A variety of smart soft materials such as hydrogels and organogels containing artificial opal microstructures have gained significant attention due to their potential as sensing platforms owing to user-friendly optical signals in the visible range. Although there exists a variety of fabrication routes including photolithography and microfluidics to prepare these materials, they are mostly time-consuming procedures or need delicate control of microflows involving harsh polymerization conditions. Our proposed solution to overcome such existing limitations is a simple and reliable micromolding-based technique to prepare soft materials containing opal micropatterns in multiple formats such as films and microparticles.

First, poly(ethylene) glycol (PEG) based hydrogel films containing opal micropatterns were prepared based on evaporative deposition-photopolymerization technique. Intense and uniform opalescent colors were achieved by a simple evaporative deposition of polystyrene (PS) bead solution in patterned micromolds and captured into PEG hydrogels via photopolymerization. Incorporation of carboxylate groups via use of comonomers such as acrylic acid (AA), methacrylic acid (MAA), 2-hydroxyethylmethacrylate (HEMA) to prepare poly(HEMA-co-AA-co-MAA) films yielded responsiveness towards water content and pH via large reversible optical shifts of 88 nm (green to red color). Although synthetic polymers such as PEG-based poly(HEMA-co-AA-co-MAA) films are readily responsive to stimuli, they lack ready degradability, and are limited by harsh multi-step processing and polymerization preparation conditions.

In contrast, biopolymers such as gelatin and aminopolysaccharide chitosan are readily biodegradable, easy to handle and biocompatible, and naturally stimuli-responsive making it ideal to prepare hydrogel films and microparticles. Next, we prepared gelatin hydrogel film containing chitosan-based opal micropatterns via evaporation-thermal gelation technique, where chitosan plays a key role in holding the assembled PS beads together through electrostatic attraction. The as-prepared gelatin hydrogel films show high tunability as well as responsiveness to various environmental cues such as humidity, pH and ionic strength readily manifested via large shifts in color. We then extended this chitosan-enabled robust fabrication approach to produce shape-encoded opal microparticles that possess a variety of potent functionalities, including chemical reactivity and responsiveness to electric signals that enable spatially controlled electroassembly onto electrode surfaces. Finally, embedding the opal micropatterns in resilient eutectogels via simple stamping led to optically responsive gels to mechanical stimuli in a tunable and reversible manner.

Combined, our integrated deposition-polymerization approach offers a simple, reliable and facile route toward controlled fabrication of multifunctional soft materials in a broad range of formats. We envision that the results and the facile approach reported here can be readily extended to multiple applications including environmental and healthcare monitoring and biosensing applications.

References

1. A Robust Fabrication Technique for Hydrogel Films Containing Micropatterned Opal Structures via Micromolding and an Integrated Evaporative Deposition-Photopolymerization Approach. Maurice Bukenya, Jun Hyuk Lee, Subhash Kalidindi, Michael DeCortin, Lauren Tice, Pil J. Yoo, and Hyunmin Yi, Langmuir, 2021, 37(4) 1456–1464.
2. Robust and Reliable Fabrication of Gelatin Films Containing Micropatterned Opal Structures via Evaporative Deposition and Thermal Gelation. Subhash Kalidindi and Hyunmin Yi, ACS Applied Materials and Interfaces, 2022, 14(51) 57481-57491
3. Micromolding-Based Fabrication of Chemically Functional and Stimuli-Responsive Opal Microparticles via Evaporative Deposition–Neutralization. Subhash Kalidindi and Hyunmin Yi, ACS Applied Engineering Materials, 2023, 1(8): 2217-2227