Most of us are accustomed to the comfort of an air-conditioned or heated shelter, providing temporary relief from an outdoor climate that's often less than desirable. Such heating and cooling actually dominates residential and commercial energy consumption, accounting for a whopping 12.3 percent of total US energy use. As a result, emissions from maintaining our indoor environment affect the global climate outdoors.

But what if we heated or cooled people rather than spaces? Researchers exploring “personal thermal management” focus on providing heating or cooling directly to the human body. This approach reduces energy consumption that is largely wasted when providing climate control for an entire building, resulting in higher energy efficiency. Recently, scientists have developed a cost-effective textile that, when made into clothes, could provide a personal thermal management system.

Basic system requirements

Personal thermal management systems require careful control of the process of heat dissipation from the human body. At normal skin temperatures (typically 34 degrees Celsius), the human body emits infrared radiation (IR) with a peak emission at a 9.5µm wavelength. Dissipation of this radiative heat accounts for more than 50 percent of total body heat loss indoors.

Ideally, personal thermal management systems should enhance the radiative release of heat in the summer to help keep people cool. The easiest way to do this is by using clothing, but development of a textile system that can manage this is no easy feat. Not only do these materials need to be transparent to IR, but they should also be opaque to visible light so that the sun doesn't heat people up even more (and, you know, so they don't appear naked).

Unfortunately, most common textiles do not fit these requirements; in fact, they often strongly absorb radiative heat from the human body. But new polyethylene-based textiles provide the tight control over infrared radiation required for effective personal thermal management systems.

An inverse greenhouse

The scientists behind the new research developed a nanoporous polyethylene textile that is able to promote radiative cooling while maintaining other critical features, including appropriate air permeability, water-wicking rates, and mechanical strength. Since polyethylene is only composed of carbon-carbon and carbon-hydrogen bonds, it has narrow absorption peaks centered far away from the peak of human body radiation.

Nanoporous polyethylene varies from regular polyethylene because it contains interconnected pores that are 50 to 1,000nm in diameter. Due to the similarity between the pore sizes and the wavelengths of visible light, the material scatters visible light strongly, resulting in an opaque appearance to the human eye. However, the pore size is actually much smaller than the IR wavelength, resulting in a material that is highly transparent to IR. Additionally, these pores provide critical air permeability and water-wicking properties.

The team analyzed the transmittance of visible light and infrared radiation of a nanoporous polyethylene film and found that the material allowed more than 90 percent total IR transmittance (for wavelengths over 2µm).

The material's developers also evaluated its cooling effect using hardware that simulated skin temperatures. The nonporous polyethylene kept the simulated skin temperature from rising by more than 0.8 degrees Celsius—significantly better than traditional textiles such as cotton (3.5 degrees Celsius) and fibrous polyethylene (2.9 degrees Celsius).

Next, the team evaluated IR transmittance by using thermal imaging of an H-shaped piece of metal placed behind the textile. The H-pattern can be seen clearly through nanoporous polyethylene. However, the metallic pattern is not visible when it's covered by cotton or traditional polyethylene. Side-by-side comparison of the nanoporous polyethylene and the traditional fibrous polyethylene revealed an increase in IR transparency of 9 to 14 percent.

The discovery that nanoporous polyethylene might be a suitable material for personal thermal management systems is particularly exciting because it is already commercially available. The stuff on the market has interconnected nanopores of 50 to 1,000nm size with about 50 percent pore volume. Even better, its cost is comparable to traditional textiles, at roughly $2 per square meter.

Wearable fabrics typically exhibit wicking, mechanical strength, and air permeability. Through processing, the team was able to make the nanoporous polyethylene a suitable textile for clothes.

Demonstrating wearability

The scientists used a microneedle punch to create 100 µm holes spaced 500 µm apart. Due to the small feature size of the holes, the visual properties of the fabric remained unaltered. The team then coated this material with a water-friendly polymer, polydopamine. The final material was made by sandwiching a cotton mesh between two sheets of polydopamine-coated, punched nanoporous polyethylene.

The team determined that all textiles containing nanoporous polyethylene were able to transmit water vapor, regardless of punching or hydrophilic treatment. This would mean that the fabric could allow moisture from sweat to evaporate off. Tests showed that the nanoporous polyethylene actually performed better than cotton and traditional fibrous polyethylene.

Next, the team evaluated the air permeability of the materials, which represents the ability of the fabric to let moving air pull heat from the body. Both the punched, coated nanoporous polyethylene and the three-layered fabric had an air permeability similar to that of cotton. Traditional fibrous polyethylene, nonporous polyethylene, and normal polyethylene don't provide adequate air permeability.

The wicking rate, which measures how efficiently liquid water transports within the textile, was also good. The researchers found that the three-layered construct performed about as well as cotton. The other fabrics were not able to compete with these two.

Finally, the researchers evaluated the mechanical properties of the materials. The three-layered construct was made into a strip of material 2cm wide, which had a strength comparable to cotton, able to endure 45N of tensile force. The scientists think that the strength of this material comes from the inner layer—the cotton mesh.

This investigation demonstrates that this type of wearable personal thermal management system has the potential to help address some of the world's most pressing issues. However, there are still questions regarding the impact of the life cycle of these materials, as well as the practicality of mass producing such a multi-layered architecture.

Science, 2016. DOI: 10.1126/science.aaf5471 (About DOIs).