Abstract
Hydrogels are three-dimensional, hydrophilic networks of cross-linked polymers that can retain water molecules in the structures and respond to physical/chemical stimulus by changing their phase and volume. Recent progress shows hydrogels with optical clarity and good processability to construct various microstructures/nanostructures, which are attractive for photonics-based environmental and biological sensing applications. Moreover, hydrogel-based photonic structures formed by biocompatible and biodegradable polymers are capable of delivering light into living tissues for diagnosis, therapy, and stimulus. We introduce recent studies of hydrogel-based photonic structures, including waveguides, optical fibers, photonic crystals, and plasmonic structures. Optically active photonic devices have been reported by doping luminescent agents into the hydrogel. In addition to introducing important demonstrations of the photonic components, the recent progress in new applications using the hydrogel-based photonic platforms is discussed. Development in the technology of hydrogel photonics will provide a solution to the convergence of scientific fields where the use of conventional photonic devices has been limited.
Highlights
Semiconductors, rigid dielectrics, and metals have played a prominent role in the development of photonic components, including waveguides,[1,2,3,4,5] optical fibers,[6,7] photonic crystals (PhCs),[8,9,10,11,12,13] and plasmonic structures,[14,15] due to their high refractive index and good processability to engineer the structures
Recently developed hydrogels have optical clarity and good processability, which are suitable to be adopted to the photonic structures engineered in nanometer/micrometer scales using various patterning methods, such as molding,[34] coating,[35] self-assembly,[36]
We find that hydrogels are incorporated into various types of optical structures to replace conventional materials, and the developments of hydrogel photonics are mainly aimed at biomedical and environmental sensing applications
Summary
Semiconductors, rigid dielectrics, and metals have played a prominent role in the development of photonic components, including waveguides,[1,2,3,4,5] optical fibers,[6,7] photonic crystals (PhCs),[8,9,10,11,12,13] and plasmonic structures,[14,15] due to their high refractive index and good processability to engineer the structures. In the 1990s, Weissman et al reported two fabrication methods for demonstration of hydrogel opals based on self-assembled CCA structures, utilizing the temperature-induced volume-phase transition of poly(Nisopropyl acrylamide) (PNIPAM) hydrogels.[53] The researchers[53] developed a method of synthesizing nanohydrogel particles, which were self-assembled in water to form the CCA of PNIPAM.
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