Researchers have developed a 'cool' textile that self-regulates its thermal properties based on how hot, sweaty or cold the wearer is, according to a new report. Made from infrared-sensitive yarn, which responds to changes in the temperature and humidity of a person's skin by dynamically collapsing or expanding the structure of its fibers, the textile shows great potential in the development of clothing systems capable of autonomously adapting to demanding environments. The human body absorbs and sheds much of its heat in the form of infrared radiation. Most textiles trap this energy, which keeps us warm in cold weather. However, the development of a material that is able to shed this energy, and thus passively cool the body, has remained a challenge. While other materials have achieved radiative cooling in various forms, through textiles that can reflect sunlight and also allow heat radiating from a person's body to escape, none are responsive to environmental changes or possess the ability to regulate both heating and cooling, according to the authors. Here, Xu Zhang and colleagues present an infrared-adaptive textile made using polymer fibers coated with a thin layer of carbon nanotubes. According to Zhang et al., when hot or wet, the adaptive yarn collapses, bringing the fibers in each strand closer together, increasing the spacing between each in the broader textile design. Not only does the wider spacing increase the textile's breathability, it also shifts its emissivity to better match the body's thermal radiation, enhancing radiative cooling. When the textile is cold or dry, the fibers expand, reducing gaps and preventing heat from escaping. The results of testing the material show that the textile was able to alter heat radiation by over 35% as it adjusted to the surrounding relative humidity. According to the authors, the heat-adapting meta-fibers can be knit, dyed and washed similarly to other performance fabrics and are compatible with current commercial processes.