Most of us are accustomed to the comfort of an air-conditioned or heated shelter, which provides temporary relief from an often undesirable outdoor climate. In fact, such heating and cooling dominates home and business energy use, accounting for a whopping 12.3 percent of total U.S. energy use. As a result, the emissions from the maintenance of our indoor environment affect the global outdoor climate.
But what if we heated or cooled people instead of spaces? Researchers investigating “personal thermal management” focus on delivering heating or cooling directly to the human body. This approach reduces the energy consumption that is largely lost when providing climate control for an entire building, resulting in higher energy efficiency. Recently, scientists have developed a cost-effective textile that, when incorporated into clothing, can 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 (usually 34 degrees Celsius), the human body emits infrared (IR) radiation with a peak emission at a wavelength of 9.5 µm. Dissipation of this radiant heat accounts for more than 50 percent of total body heat loss indoors.
Ideally, personal thermal management systems should improve radiant heat output in summer to help people stay cool. The easiest way to do this is by using clothing, but developing a textile system that can handle this is no small feat. Not only do these materials need to be transparent to IR, but they also need to be opaque to visible light so the sun doesn’t heat people up even more (and, you know, so they don’t look naked).
Unfortunately, the most common textiles do not meet these requirements; in fact, they often strongly absorb the radiant heat of the human body. But new polyethylene-based textiles provide the tight control over infrared radiation needed for effective personal thermal management systems.
An inverted box
The scientists behind the new research developed a nanoporous polyethylene textile that can promote radiant cooling while maintaining other critical characteristics, including proper air permeability, water drainage rates and mechanical strength. Because polyethylene is composed only of carbon-carbon and carbon-hydrogen bonds, it has narrow absorption peaks centered far away from the peak of the human body’s radiation.
Nanoporous polyethylene differs from regular polyethylene because it contains interconnected pores with a diameter of 50 to 1000 nm. Due to the similarity between the pore sizes and the wavelengths of visible light, the material strongly scatters visible light, resulting in an opaque appearance to the human eye. However, the pore size is much smaller than the IR wavelength, resulting in a material that is highly transparent to IR. In addition, these pores provide critical air permeability and water-draining properties.
The team analyzed the transmission of visible light and infrared radiation from a nanoporous polyethylene film and found that the material allowed more than 90 percent total IR transmission (for wavelengths above 2 µm).
The developers of the material also evaluated the cooling effect using hardware that simulated skin temperatures. The non-porous 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 the IR transmission using thermal imaging of an H-shaped piece of metal placed behind the fabric. The H pattern can be clearly seen through nanoporous polyethylene. However, the metallic pattern is not visible when covered with 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 could 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 from 50 to 1000 nm with about 50 percent pore volume. In fact, the cost is comparable to traditional textiles, about $2 per square foot.
Wearable fabrics typically exhibit moisture wicking, mechanical strength and air permeability. Through processing, the team was able to turn the nanoporous polyethylene into a suitable textile for clothing.
The scientists used a microneedle punch to make 100 µm holes spaced 500 µm apart. Due to the small feature size of the holes, the visual properties of the fabric remained unchanged. The team then coated this material with a water-friendly polymer, polydopamine. The final material was made by sandwiching a cotton gauze between two sheets of polydopamine-coated perforated nanoporous polyethylene.
The team found that all textiles containing nanoporous polyethylene are capable of permeating water vapor, regardless of punching or hydrophilic treatment. This would mean that the fabric can allow moisture from sweat to evaporate. Tests showed that the nanoporous polyethylene actually outperformed cotton and traditional fibrous polyethylene.
Next, the team evaluated the materials’ air permeability, which reflects the fabric’s ability to allow moving air to extract heat from the body. Both the perforated, coated nanoporous polyethylene and the three-layer fabric had an air permeability comparable to that of cotton. Traditional fibrous polyethylene, non-porous polyethylene and normal polyethylene do not provide sufficient air permeability.
The wicking rate, which measures how efficiently liquid water is transported in the textile, was also good. The researchers found that the three-layer construction performed about as well as cotton. The other substances could not compete with these two.
Finally, the researchers evaluated the mechanical properties of the materials. The three-layer construction was made into a strip of material 2 cm wide, which had a strength comparable to that of cotton and could withstand a tensile force of 45 N. The scientists think that the strength of this material comes from the innermost layer, the cotton mesh.
This research shows that this type of portable personal thermal management system has the potential to address some of the world’s most pressing problems. However, there are still questions about the lifecycle impact of these materials, as well as the feasibility of mass-producing such a multi-layered architecture.
Science2016. DOI: 10.1126/science.aaf5471 (About DOIs).