A new way of thinking about thermal comfort on hot days

The skin is the main thermoregulatory organ of the human body, and clothing acts as a thermal resistance interposed between the body and the environment, representing a barrier to heat exchange by convection. Its resistance depends on the type of fabric, fiber and fit to the body. It is measured through selective dry exchanges relative to the user, and its unit of measurement is known as clo (from the English term clothes).

Thermal comfort is defined as the mental state that expresses a person's satisfaction with the environment that surrounds him – and is a very important factor when considering the energy efficiency of buildings. Comfort variables are divided into environmental and human: temperature, mean radiant temperature, speed and relative humidity make up the environmental variables, while the metabolism generated by physical activity and the thermal resistance offered by clothing are human variables.

If a room is cold, you have a few options: you can choose to put on a warmer shirt or turn on the heater. If the room is hot, however, the choice is not so simple. There is only so much clothing we can wear in the workplace, and wearing some lightweight garments, such as T-shirts, is frowned upon in some business circles. The default solution, therefore, is to turn on the air conditioning.

Fortunately, our adaptation to warmer temperatures could change if a discovery published in the journal Science this week is brought to market. Yi Cui and colleagues at Stanford University have discovered a fabric that keeps the skin 2°C cooler than a cotton garment.

In terms of comfort, that is a significant reduction – one that could be good not only for the wearer but also for the energy bill. If the new fabric for broad-based protection could avoid temperatures higher than they are today, it could save huge amounts of energy in the summer months.

The goal of Dr. Cui’s research was initially to lower people’s temperatures by adjusting the way their body heat radiates. More than half of body temperature is in the infrared part of the electromagnetic spectrum. This means that the wavelength is longer than the wavelength of visible light. Materials such as polyethylene allow both wavelengths to pass through, and are therefore useless for weaving clothes – they let both light and heat through and do not protect us from the cold or nakedness. On the other hand, fabrics such as wool, cotton and silk do not allow either wavelength to pass through, thus trapping infrared light and heating us up. The answer to Cui's investigations came from research on batteries. A material commonly used in modern batteries is called nanoPE. It is a type of polyethylene perforated with pores measuring 50 to 1,000 nanometers (billionths of a meter). These pores are used for the regular passage of ions within the battery – and are precisely sized to make the material opaque to visible light. These pores, however, do not affect the infrared part of the electromagnetic spectrum, blocking less than 10% of infrared rays.

When fresh from the factory, nanoPE resembles a sheet of plastic and is not very comfortable to wear. To get around this, the Stanford team made three changes. First, they punctured the sheet at regular intervals with a small needle, to allow air to flow in and out. Then, they added a substance called polydopamine, which makes the plastic more hydrophilic. This means that instead of repelling sweat and causing it to clump on the skin, the modified nanoPE absorbs perspiration and carries it to the outer surface of the fabric, where the sweat evaporates. Finally, to increase the mechanical properties of the material, the final product was made up of two sheets of nanoPE separated by a widely spaced cotton mesh.

The team then tested how the added material affected the performance of the nanoPE. In a room at 23.5°C, the temperature of bare skin was 33.5°C. Skin covered in cotton fabric was 37°C, while skin covered in nanoPE alone was 34.3°C. Variations with perforations, mesh and other modifications did not perform as well in terms of thermal performance, but they still reduced skin temperature by at least 2°C compared to cotton.

Of course, designers won't be the first to rush out and create fashionable designs with the new fabric, but the discovery has created a new way of thinking about reducing body temperature. Manipulating the waves emitted by the human body through fabrics is clearly an idea that works. In addition to being used indoors, Dr. Cui's new fabric has several applications outdoors – after all, there is no air conditioning in deserts, and the people who work there, such as soldiers, are probably very fashion-conscious. The next steps were to find materials that perform a task in a more comfortable way for the user than nanoPE.

Source: The Economist