Researchers have developed a flexible, self-cleaning smart window that combines optical tuning, thermal control, and low energy use, offering a multifunctional solution to energy loss in buildings.
Study: All-flexible self-cleaning hydrogel smart window with multifunctionality based on an electro-thermal manipulator. Image Credit: Erman Gunes/Shutterstock.com
Published in the International Journal of Extreme Manufacturing, the study addresses a key challenge in sustainable infrastructure: conventional glass windows contribute to nearly 40 % of energy loss in buildings due to poor insulation and heat regulation. This new smart window integrates multiple capabilities, including tunable transparency, thermal management, defogging, and a pixelated display, into a single, adaptable device.
Introduction
Traditional glass windows offer little control over solar heat gain or loss, making buildings less energy efficient year-round. Smart windows aim to solve this by regulating light and heat transmission, using either passive or active systems.
Passive systems rely on materials like thermochromic hydrogels, which change opacity in response to temperature changes. While energy-efficient, these lack user control and often don't offer additional features like self-cleaning. Active systems, by contrast, respond to external stimuli such as electric current or mechanical force, allowing users to adjust transparency on demand. However, they often struggle with limited flexibility, reduced transparency, or surface maintenance issues.
To bridge these gaps, the research team developed an all-flexible, multifunctional hydrogel-based smart window. At its core is a self-cleaning electro-thermal actuated (SETA) device that combines a thermo-responsive hydrogel with a hydrophobic silver nanowire (AgNW) heater. This design enables precise control over light transmission while adding benefits like low power consumption and easy cleaning, all in a flexible format suitable for both flat and curved surfaces.
Methodology and Approach
The SETA smart window was constructed using a multilayered sandwich structure, designed for durability, responsiveness, and optical clarity. It consists of three main layers: a hydrophobic AgNW heater, a thermo-responsive hydrogel, and PET encapsulators.
The AgNWs were synthesized using a modified polyol method to produce ultrathin (~20 nm) wires, which were then evenly coated onto PET film. A hydrophobic SiO2 nanoparticle layer was applied on top, mimicking the lotus leaf effect to promote self-cleaning.
The hydrogel layer was synthesized through free radical polymerization of poly(N-isopropylacrylamide) (pNIPAM) combined with hydroxypropylmethyl cellulose (HPMC). The inclusion of HPMC enhanced the hydrogel’s thermal stability, mechanical strength, and optical clarity, which is crucial for maintaining performance over repeated cycles.
The layers were bonded using a femtosecond-laser spacer, creating a sealed, flexible chamber. When voltage is applied, the AgNW heater warms the hydrogel, prompting it to shrink or expand depending on temperature. This reversible process alters the window’s opacity, offering precise control with minimal power.
To evaluate performance, the researchers used a range of characterization tools, including FTIR, UV-Vis-NIR spectroscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and high-speed imaging.
Results and Discussion
The SETA window stood out for its ability to switch seamlessly between transparent and opaque states. Using just 5 volts, it shifted from a clear mode (with 84.9 % light transmittance at 550 nm) to a much dimmer, more private setting (just 11.0 % transmittance). This change was driven by interactions within the hydrogel, specifically between its hydrophilic and hydrophobic components, which respond to heat by altering the material’s structure.
Tests using FTIR and DSC confirmed these phase transitions, and the switching itself took just 36 seconds. Better yet, it required very little power (about 8 × 10-3 watts per square centimeter per degree Celsius), making it a practical option for everyday, low-energy use.
In addition to managing light, the window kept itself clean with surprising efficiency. Its surface is coated with a layer of hydrophobic silica nanoparticles, which mimic the water-repelling behavior of lotus leaves. As a result, liquids like water, milk, coffee, even ethylene glycol, rolled right off, without leaving behind smudges or reducing visibility.
Water droplets bounced off in what researchers call a “pancake” effect, and after thousands of droplets, the surface still showed no signs of wear. That kind of durability is especially useful in places where windows are regularly exposed to rain, dust, or pollution.
Thermal control was another area where the window performed well. When installed in a model house exposed to artificial sunlight, it helped lower the indoor temperature by as much as 6.5 °C, enough to take pressure off air conditioning systems. It also cleared fog quickly, becoming fully transparent in under a minute when powered at just 2.5 volts. This could be especially useful for vehicles, greenhouses, or aircraft cabins, where visibility is critical and condensation is a constant issue.
The researchers even added a display function by incorporating a pixel array into the design. This allowed different sections of the window to be turned on or off independently, creating patterns or encrypted visuals on demand. This opens up a whole new range of potential applications, from dynamic privacy screens to interactive signage.
What makes the SETA window especially promising is how it brings all these features together—optical control, thermal regulation, self-cleaning, defogging, and display—in one lightweight, flexible system.
Its ability to bend and adapt to curved surfaces makes it a good fit for everything from vehicle windshields to architectural facades. The only major limitation flagged in the study is scratch resistance, which may need to be addressed before the technology is ready for widespread use in rougher environments.
Conclusion
The SETA smart window represents a significant advance in multifunctional building materials. By integrating optical regulation, self-cleaning, thermal management, defogging, and display capabilities, it overcomes key limitations of conventional smart windows. Its high transparency, ultra-low energy consumption, and adaptability to curved surfaces highlight its potential for applications in greenhouses, intelligent vehicles, architectural facades, and consumer electronics.
Although challenges remain in scratch resistance, the system’s scalability and multifunctional integration highlight its transformative potential. Beyond material innovation, this research presents a conceptual framework for sustainable, energy-efficient, and self-maintaining infrastructure, demonstrating the value of cross-disciplinary research that merges nanotechnology, hydrogel chemistry, and electro-thermal engineering.
Journal Reference
Chen, C., Guo, S., et al. (2025). All-flexible self-cleaning hydrogel smart window with multifunctionality based on an electro-thermal manipulator. International Journal of Extreme Manufacturing, 8(1), 015501. DOI: 10.1088/2631-7990/AE00FE. https://iopscience.iop.org/article/10.1088/2631-7990/ae00fe
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