Abstract 

Hydrogel-based photonic systems integrating luminescent emitters offer promise as soft, reconfigurable optical platforms, yet most designs lack internal optical engineering to control light propagation and confinement. Here, we present a lithographically programmable soft-photonic platform in which upconversion nanocrystals (UCNs) encapsulated within fluorocarbon nanoemulsion droplets are embedded in a poly(ethylene glycol) diacrylate (PEGDA) hydrogel microdome. Upon drying, strong refractive index contrast between the PEGDA matrix and fluorocarbon droplets creates a cooperative optical microenvironment that structures the near-infrared (NIR) excitation beam into a speckle-like field with localized hot spots while extending the photon dwell time within the microdome via internal reflection-based waveguiding. These effects yield a fully reversible, greater than sevenfold enhancement of upconversion luminescence—well beyond simple concentration or mechanical densification. This optical gain originates from multiple-scattering-assisted speckle excitation activated only in the contracted microdome state. Because UCNs are pumped by invisible NIR speckle illumination that rapidly varies in 3D across the microdome height, the incoherent sum of the photoluminescence manifests as a homogeneous filter-free visible brightness increase. The hydrogel microdomes, fabricated via a customized digital micromirror device (DMD)-based microlithography, enable high-resolution patterning of moisture-responsive displays, multicolor emission motifs, and reversible QR-code encryption, establishing a scalable route toward speckle-engineered soft photonic systems.' 

Researchers at UNIST have created a new material that dims when it absorbs moisture. This innovation could lead to water-sensitive security features, humidity sensors, and environmental-reactive displays. 

Led by Professor Jiseok Lee from the School of Energy and Chemical Engineering and Professor Jung-Hoon Park from the Department of Biomedical Engineering, the team developed a hydrogel with embedded upconversion nanocrystals (UCNs). When dry, the material glows more than seven times brighter than when wet. 

The core design involves oil droplets trapped inside a hydrogel dome. When near-infrared (NIR) light hits the nanocrystals, they emit visible light. In this structure, scattering within the oil droplets traps the light, boosting brightness. When the hydrogel absorbs water, its internal scattering drops, and the glow fades.

The team demonstrated how this material can hide and reveal information. In one test, a hidden pattern beneath the hydrogel becomes visible only when water is applied, as the glow weakens. They also created QR codes that are scannable when dry but vanish when wet, making them useful for anti-counterfeiting.

Durability stood out. The material retained consistent brightness over 100 wet-dry cycles, with less than 4% variation. It responds rapidly—within 0.1 seconds—fading visibly in seconds after contact with water.

Lead author Chaeyeong Ryu said, “We improved brightness by designing the light pathways inside the hydrogel, without changing the nanocrystals. This makes the material ideal for moisture-triggered devices.”

Professor Lee added, “The ability to program the color and pattern of the hydrogel microdome, combined with simple manufacturing, opens new paths for security, sensors, and displays across industries.”

The findings of this research have been published in Advanced Functional Materials on April 20, 2026. This work was supported by the National Research Foundation of Korea (NRF) and the Ministry of Science and ICT (MSIT).

Journal Reference 

Chaeyeong Ryu, Byungcheon Yoo, Seunghun Lee, et al., "Speckle-Engineered Upconversion Amplification in Nanoemulsion-Templated Hydrogel Microdomes," Adv. Funct. Mater., (2026).