Structural colors found in nature—such as those observed in chameleons, butterfly wings, and octopus skin—have long inspired researchers seeking to develop responsive optical materials capable of dynamically adapting to external stimuli. Unlike pigment-based coloration, structural color arises from the interaction between light and periodic nanostructures, offering advantages including high brightness, environmental stability, and reversibility.
Among various photonic materials, cellulose nanocrystals (CNCs) have attracted growing attention because of their natural abundance, sustainability, and ability to self-assemble into chiral nematic structures capable of selectively reflecting visible light. However, despite rapid progress in cellulose-based photonic hydrogels, a persistent challenge has limited their broader application: materials with highly ordered structures often exhibit vivid color but poor mechanical durability, while mechanically reinforced systems frequently lose optical quality due to structural disruption. To address this contradiction, researchers designed a biomimetic triple-network hydrogel inspired by the reflective iridophore structures found in octopus skin. In these biological systems, reflective protein platelets dynamically rearrange within a soft matrix to regulate optical appearance. Translating this concept into a synthetic material, the team integrated a glucose-modified cellulose nanocrystal photonic scaffold with a covalently crosslinked polyacrylamide (PAAm) network and an elastic waterborne polyurethane (WPU) network rich in dynamic hydrogen bonds. The resulting CNC/polyacrylamide/waterborne polyurethane (CPW) hydrogel combines rigid photonic ordering with soft, energy-dissipating polymer networks. In the system, the cellulose nanocrystals preserve long-range chiral nematic order responsible for structural coloration, while the PAAm and WPU networks distribute stress and dissipate energy during deformation. Dynamic hydrogen bonding among the three components allows the material to stretch extensively without catastrophic structural failure.
Mechanical testing demonstrated substantial improvements compared with conventional photonic hydrogels. The optimized CPW hydrogel achieved a tensile strength of approximately 420 kPa and elongation exceeding 821%, while maintaining stable photonic behavior under repeated deformation. The researchers observed that introducing WPU created reversible physical crosslinking sites that acted as sacrificial bonds during stretching, effectively protecting the covalent network and improving toughness.
At the same time, the material retained highly responsive optical behavior. In the relaxed state, the hydrogel appeared nearly colorless because its reflection wavelength resided in the near-infrared region. Under tensile strain, compression of the helical pitch within the CNC structure induced a continuous blueshift in reflected light, producing reversible color transitions spanning the visible spectrum. Reflection peaks shifted from 670 nm to 467 nm as strain increased, corresponding to visible transitions from red and orange to green and blue. The process remained highly reversible over repeated stretching-release cycles, with minimal optical degradation after 20 cycles. Microscopic analysis further confirmed that the CNC chiral nematic architecture remained structurally intact within the interpenetrating polymer network. Polarized optical microscopy, SEM imaging, and two-dimensional wide-angle X-ray diffraction collectively showed that stretching induced progressive orientation of CNC domains without destroying the underlying photonic structure. This structural stability enabled consistent mechanochromic response even under large deformation.
Beyond dynamic coloration, the hydrogel also demonstrated potential in intelligent information encryption. By embedding CNC domains with different helical pitches into selected regions, researchers created encoded hydrogel patterns that remained visually concealed in the unstretched state. Upon mechanical deformation, only designated regions generated visible structural colors, revealing hidden information. Once the external force was removed, the encoded information disappeared again as the material returned to its initial translucent appearance.
The study additionally reports that the hydrogel maintained functionality after prolonged storage at −60 °C. After returning to room temperature, the material rapidly recovered its stretchability and structural color response, indicating strong environmental adaptability. By resolving the long-standing conflict between optical responsiveness and mechanical robustness, the work provides a new design strategy for bio-based mechanochromic materials. The authors suggest that such systems may find applications in adaptive optical devices, anti-counterfeiting technologies, flexible sensors, wearable photonics, and soft robotic systems operating under complex environmental conditions.
See the article:
DOI
https://doi.org/10.1016/j.jobab.2026.100265
Original Source URL
https://www.sciencedirect.com/science/article/pii/S236996982600037X
Journal
Journal of Bioresources and Bioproducts
Experimental study
Not applicable
Decoupling Optical and Mechanical Trade-offs in Photonic Hydrogels via Bioinspired Multi-Network Architecture
12-May-2026