Plastic clothing material developed to keep people cool

Researchers at Stanford University have engineered a low-cost, plastic-based textile that, if woven into clothing, could become the basis for garments that keep people cool in hot climates without air conditioning.

In addition to letting perspiration evaporate through the material, something ordinary fabrics already do, the new textile cools human body by providing a revolutionary cooling mechanism: allowing heat that the body emits as infrared radiation to pass through the plastic textile.

Wearers of the textile, known as nanoporous polyethylene (nanoPE), show skin temperature about 4 degrees Fahrenheit, or 2.2 degrees Celsius, lower than when they are covered with cotton.

"If you can cool the person rather than the building where they work or live, that will save energy," said Yi Cui, an associate professor of materials science and engineering at Stanford and of photon science at SLAC National Accelerator Laboratory, and one of the authors of a paper in the September 2 issue of the journal Science.

The work is based on understanding that all objects, including our bodies, throw off heat in the form of infrared radiation, an invisible and benign wavelength of light. Blankets warm us by trapping infrared heat emissions close to the body.

"Forty to 60 percent of our body heat is dissipated as infrared radiation when we are sitting in an office," explained Shanhui Fan, a professor of electrical engineering who specializes in photonics, which is the study of visible and invisible light.

"But until now there has been little or no research on designing the thermal radiation characteristics of textiles."

The researchers blended nanotechnology, photonics and chemistry to give polyethylene, namely the clear, clingy plastic we use as kitchen wrap, a number of characteristics desirable in clothing material: it allows thermal radiation, air and water vapor to pass right through, and it is opaque to visible light.

First, they found a variant of polyethylene commonly used in battery making that has a specific nanostructure that is opaque to visible light yet is transparent to infrared radiation. Then, they modified the industrial polyethylene by treating it with benign chemicals to enable water vapor molecules to evaporate through nanopores, allowing the plastic to breathe like a natural fiber.

To make the thin material more fabric-like, they created a three-ply version: two sheets of treated polyethylene separated by a cotton mesh for strength and thickness. And to test the cooling potential of the construct versus a cotton fabric of comparable thickness, they placed a small swatch of each material on a surface that was as warm as bare skin and measured how much heat each material trapped. The comparison showed that the cotton fabric made the skin surface 3.6 degrees Fahrenheit, or 2 degrees Celsius, warmer than their cooling textile.

Noting that this difference means a person dressed in their new material might feel less inclined to turn on a fan or air conditioner, the researchers are continuing their work on several fronts, including adding more colors, textures and cloth-like characteristics to their material.

Fan believes that the research opens up new avenues of inquiry to cool or heat things, passively, without the use of outside energy, by tuning materials to dissipate or trap infrared radiation.

"In hindsight, some of what we've done looks very simple, but it' s because few have really been looking at engineering the radiation characteristics of textiles," he said. Endit