When walking under the blazing midsummer sun, many people avoid black or dark-colored shirts in favor of white ones. Everyone has experienced how dark fabrics absorb solar energy intensely and rapidly convert it into heat. This everyday intuition aligns with the engineering logic behind traditional architecture in the Mediterranean and tropical regions, where buildings are coated in white plaster. For years, the freedom to choose vibrant colors and the coolness of fabric have been regarded as an unavoidable trade-off.
A new material developed by a research team from the University of Adelaide and Zhengzhou University, published in the journal Small, directly challenges this assumption. The team synthesized a fiber that is vividly purple in color yet keeps the wearer cooler than commercial white cotton under direct sunlight. Professor Jun Ma of the University of Adelaide explains, "We can create a fabric that appears purple to the eye while reflecting most solar energy and, at the same time, releasing heat extremely efficiently." By using materials precisely engineered at the molecular level, the team overcame optical constraints that had long seemed insurmountable.
The physics of releasing heat toward near-absolute-zero space through an atmospheric window
Every object on Earth emits electromagnetic waves at wavelengths corresponding to its temperature—a phenomenon known as thermal radiation. Under normal conditions, infrared radiation emitted by an object collides with gas molecules in the atmosphere, such as nitrogen, oxygen, water vapor, carbon dioxide, and methane, and is absorbed. Because these gas molecules, having absorbed thermal energy, re-emit infrared radiation in all directions, heat becomes trapped within the atmosphere rather than directly lowering the temperature near the ground.
However, the atmosphere surrounding Earth contains a specific wavelength range that is nearly transparent to electromagnetic waves: the mid-infrared band spanning 8 to 13 micrometers (). Electromagnetic waves in this narrow band pass straight through the thick atmospheric layer without being absorbed by gas molecules, traveling directly into outer space.
The background radiation temperature of deep space is approximately 3 K—just 3 degrees above absolute zero, an extremely cold environment. If an object placed on the ground has strong radiative properties in the 8–13 band, it becomes effectively linked to this ultra-cold outer space through a radiative heat pathway. If such an object can block or reflect direct sunlight while efficiently radiating heat into space through this band, it can cool below the ambient air temperature without consuming any external power. This is the basic principle of passive radiative cooling.
As global temperatures rise, the surge in electricity consumption from mechanical air conditioning and the resulting greenhouse gas emissions pose a serious environmental burden. Passive radiative cooling, which consumes no electricity, has long attracted attention as a technology for reducing air-conditioning loads. However, conventional radiative cooling materials had to reflect the entire solar spectrum like a mirror, limiting their appearance to inorganic-looking silver or pure white sheets. This lack of aesthetic versatility has been a major barrier to their use as everyday clothing.
Why purple was the hardest color to achieve in radiative cooling
Among efforts to give radiative cooling materials arbitrary colors, achieving purple has been considered one of the most academically challenging. For the human eye to perceive a material as purple, it must strongly absorb green light in the 500–550 nm range of the visible spectrum while reflecting or scattering blue and red light.
The intensity of solar radiative energy peaks in the green wavelength region of visible light. Absorbing green light means directly capturing the densest thermal energy within sunlight into the material. When ordinary commercial organic dyes are used to dye fabric purple, the molecules that absorb green light vibrate intensely and store heat, disrupting the thermal radiation properties in the 8–13 mid-infrared region as well. This is the physical reason why dark-colored fabrics heat up rapidly under direct sunlight.
To overcome this optical contradiction, the research team adopted a porous crystalline material called ZIF-67, a type of metal-organic framework (MOF). The Nobel Prize in Chemistry was awarded to pioneering researchers who established the foundational theory and synthesis methods for metal-organic frameworks. This class of materials, in which metal ions and organic ligands bond together in a regular jungle-gym-like structure, can be given specific optical properties depending on how the molecular structure is designed.
ZIF-67 strongly absorbs green light in the visible spectrum, producing a vivid purple color, while maintaining high thermal emissivity and appropriate reflective properties in the 8–13 mid-infrared region. The research team uniformly embedded these ZIF-67 microparticles along with zinc oxide () nanoparticles inside fine polymer fibers.
While conventional organic dyes act as impurities that block infrared heat dissipation pathways, ZIF-67's own crystal framework functions as the desired optical filter. This is how the team created a fiber structure that displays a deep purple color while efficiently releasing absorbed heat into space through the atmospheric window.
Cooling performance and practicality as a clothing textile demonstrated through experiments
In outdoor exposure experiments conducted under direct sunlight in Zhengzhou, Henan Province, China, precise surface temperature measurements were taken using 5 cm square test specimens. The new purple fabric recorded a surface temperature 4.2°C lower than commercial white cotton fabric under direct sunlight, and 6.2°C lower than commercial purple cotton fabric dyed with conventional dyes.
To verify practicality for wear on the human body, the research team also conducted measurements using simulated skin preheated to 37°C, corresponding to body temperature. Compared to bare, uncovered simulated skin, the simulated skin covered with the new purple fabric showed a temperature reduction of up to 8.0°C. This compares favorably to the 4.6°C reduction achieved by commercial white cotton fabric and the mere 2.8°C reduction achieved by commercial purple cotton fabric, confirming that the new material demonstrates superior cooling performance even in environments involving heat conduction from the human body.
| Material Type | Surface Temperature in Sunlight (Comparison) | Temperature Drop on Simulated Skin (Preheated to 37°C) | Key Material Properties |
|---|---|---|---|
| Developed Purple MOF Fabric | 4.2°C lower than white cotton, 6.2°C lower than commercial purple cotton | Up to 8.0°C reduction | Water-repellent, withstands 50 wash cycles, UV-resistant equivalent to 108 days |
| Commercial White Cotton Fabric | Baseline (2.0°C lower than commercial purple cotton) | 4.6°C reduction | Hydrophilic, relies on scattering/reflection of sunlight |
| Commercial Purple Cotton Fabric | 2.0°C higher than white cotton | 2.8°C reduction | Significant heat accumulation from green light absorption |
For practical use as outdoor clothing, thermal performance alone is not enough—physical durability and comfort are also essential. The developed fiber composite is lightweight and flexible, showing sufficient strength against the tension involved in everyday body movement. Furthermore, by optimizing the microporous structure, breathability that allows water vapor to escape has been ensured, designed to reduce the discomfort of sweating.
The composited zinc oxide nanoparticles play a role in imparting hydrophobicity to the fiber surface. The fabric repels water droplets and showed no decline in cooling performance even after 50 standard wash cycles. No structural degradation was observed even after continuous UV exposure testing equivalent to 108 days of outdoor use, demonstrating robustness capable of withstanding long-term outdoor use. Yangzhe Hou, a doctoral student at Adelaide University, notes the growing interest in textiles that can manage body temperature without consuming additional electricity.
The path to practical application and remaining engineering challenges
Engineering applications of passive radiative cooling have already been commercialized ahead of schedule in the architectural field. Companies like Skycool Systems in the United States have already reached the practical stage with attempts to reduce air-conditioning electricity consumption by installing specialized radiative cooling panels on the rooftops of commercial buildings. However, unlike flat panels fixed to large roof areas, mass-producing clothing textiles that simultaneously require flexibility, wash resistance, and safety still presents hurdles to overcome.
As pointed out in an academic review paper summarizing trends in radiative cooling technology (DOI: 10.1038/s44359-025-00041-5), whether radiative cooling materials can move beyond basic research and become widely adopted by society largely depends on the scalability of manufacturing processes. Verification toward mass production—including the cost of mass-synthesizing metal-organic frameworks, establishing industrial techniques for high-speed spinning while uniformly dispersing nanoparticles, and ensuring thread strength suitable for commercial high-speed looms—has only just begun.
Similar challenges lie ahead regarding expanding color options. In theory, it may be possible to develop other colors using framework complexes with different metal ions and ligands, such as brown-colored HKUST-1(Cu) or teal-colored MOF-74(Ni). However, at present, purple is the only color for which clear cooling performance has been experimentally demonstrated, and whether practical radiative cooling effects can be achieved with other colors awaits future follow-up research.
In real-world living environments, fibers are subjected to various external factors such as airborne moisture and oils, intense friction, and sebum stains. Preventing the risk of nanoparticle detachment through repeated wearing and washing over extended periods, and confirming whether the material fully meets human safety standards, will also be important items for future verification.