A new hydrogel uses simple molecular interactions to shift from transparent to opaque as temperatures rise or fall, offering a passive approach to energy‑efficient smart windows.
Smart windows can automatically adjust opacity and light transmittance in response to their surroundings, offering a way to manage heat, light, and privacy without manual intervention. But many existing systems respond in only one direction—darkening as temperatures rise, for example—which limits their usefulness across changing conditions. A new study reveals a hydrogel‑based window material designed to respond to temperature changes in two directions, becoming opaque at both high and low temperatures while remaining transparent in between.
Watch the hydrogel in action in the latest Headline Science video:
How do smart windows work?
Smart windows interact with environmental factors such as temperature and light to adjust transparency. In buildings, this can help regulate indoor temperatures, reduce glare, and provide privacy when needed. Hydrogels are especially appealing for these applications because they are soft, transparent, and responsive to subtle molecular changes. However, most hydrogel smart windows transition at a single temperature threshold, making it difficult to balance daytime heat control with nighttime privacy. At low temperatures, many hydrogels also freeze, losing function and risking mechanical damage.
What’s new in this study?
The material, reported in ACS Applied Materials & Interfaces, pairs hydroxypropyl cellulose (HPC) with a flexible hydrogel network, along with tiny micelles stabilized by cetyltrimethylammonium bromide (CTAB).
The design brings together two temperature‑dependent mechanisms. At higher temperatures, hydrogen bonds between HPC and water weaken, causing the polymer chains to collapse and scatter light. The window gradually turns opaque, limiting solar heat gain. At lower temperatures, CTAB‑stabilized micelles aggregate and grow large enough to scatter light as well, also producing an opaque appearance. Between these two regimes, the hydrogel remains clear.
Because these responses arise from reversible, noncovalent interactions rather than permanent chemical cross‑links, the temperature window for transparency can be adjusted by changing the hydrogel’s composition.
Performance and durability
In tests, the researchers built a model smart window by sandwiching a thin layer of the hydrogel between glass panes. When exposed to simulated sunlight, a small model house fitted with the hydrogel window warmed far less than houses with air‑filled or water‑filled windows. As the temperature rose, the hydrogel transitioned from transparent to opaque, blocking near‑infrared light and slowing heat buildup inside.
The material also showed practical durability. The physically cross‑linked network allows the hydrogel to self‑heal after being cut, restoring both mechanical integrity and optical performance over time. The deep eutectic solvent helps prevent freezing, enabling the gel to remain flexible and functional at subzero temperatures.
What's next?
Bidirectional, passive control over light and heat could make smart windows more adaptable to real‑world conditions, particularly in climates with large day–night temperature swings. While the current hydrogel performs best within a defined temperature range, the authors note that future designs could combine thermoresponsive behavior with other control strategies to broaden usability. The study demonstrates how carefully tuned molecular interactions can translate into building‑scale energy management, using chemistry that responds automatically without external power or moving parts.
Watch more Headline Science on YouTube!
The video above is brought to you by the ACS Science Communications team. To watch more exciting videos and shorts covering some of the latest research in ACS journals, visit the Headline Science page on YouTube.
Video credits:
Written by Anne Hylden
Produced by Vangie Koonce
Editing and animations by Janali Thompson
Hosted by Anne Hylden
Series produced by Vangie Koonce, Anne Hylden, Andrew Sobey, Elaine Seward, and Jefferson Beck
Executive produced by Matthew Radcliff
Research videos from Zao Cheng, Ruoqiong Wang, Zeyu Zhang, Ph.D., and Patrizio Raffa, Ph.D.
Explore related articles from ACS journals
High Solar Modulation and Fast-Response Thermoresponsive Poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) Hydrogel for Energy-Efficient Smart Windows
Kai Yao, Zhenhui Hu, Longhao Xiao, Hengshan Wei, Jiashuo Sun, Yueyue Guan, and Yongsheng Yang*
DOI: 10.1021/acsami.6c00874
Bidirectional Thermoresponsive Window via a Facilely Tailorable Double-Layer Hydrogel
Chen Wang, Meng Sun, Xiaohan Zha, Zhuoying Meng, Yiwen Ye, Hui Sun, Ying Liu, Benzhi Ju*, and Ye Tian*
DOI: 10.1021/acsami.5c19264
Multiband Light Absorption Hydrogel with Adjustable Light Transmission for Smart Window
Shirui Zhang, Peng Zhang*, Hongyu Wen, Junzheng Niu, Yi Huang, Xuhui Xu, Huaiqiang Ba, Yu Zhang, and Ru Zhao*
DOI: 10.1021/acsami.5c24383
Hydrogel-Coated Polydimethylsiloxane with Reversible Transparency for Advanced Optical Switching
Chenxu Liu, Lin Yang, Yongxiang Sun, Pan Huang, Yuan Yao, Yu Tian, and Hongbo Zeng*
DOI: 10.1021/acsnano.4c17403
Toughness and Thermoresponsive Hydrogel for Sandwich Smart Window with Adaptive Solar Modulation and Energy Saving
Huijie Guan, Yinghan Lu, Yijiang You, Shengxiang Gao, Li Liu*, and Guangfeng Wu*
DOI: 10.1021/acsami.4c13133v
Hydrogel-Based Stimuli-Responsive Radiative and/or Evaporative Cooling Materials for Carbon Neutrality
Jing Shang, Yihe Zhang*, Jiahe Zhang, Xinyue Zhang, and Qi An*
DOI: 10.1021/acsenergylett.3c02325
Get e-Alerts
Stay Connected With ACS Applied Materials & Interfaces
Never miss an issue again! Sign up now for email updates on future calls for papers, the latest articles, and other content from ACS Applied Materials & Interfaces.
