Duke scientists have created a fabric that releases heat once you start to sweat.

One's perspiration, in fact, activates vents that send out thermal energy trapped by the lightweight material. Once dry, the vents close again to retain heat.

The innovative approach — more physics-based than electronics-based — offers potential as a clothing patch, to keep a wearer comfortable in a variety of temperatures.

“People who are skiing or hiking in colder weather usually wear layers so they can adjust how much heat their clothing is trapping as their body heats up,” said Po-Chun Hsu, Assistant Professor of Mechanical Engineering and Materials Science, in a recent Duke University news release . “But by strategically placing patches of a material that can let out heat when a person is sweating, one could imagine making a one-piece-fits-all textile.”

Details for the invention were provided on December 15 in the journal Science Advances . While the prototype at the moment only fits in your hand, Prof. Hsu and the team have plans to make the wearable bigger.

"Currently, it is a thin film the size of a palm, with arrays of flaps cut from it," Prof. Hsu told Tech Briefs. "Scaling up the patch and miniaturizing the flaps are works in progress."

The flaps are made from nylon and an added top-layer of silver. The carefully tested thickness of the silver — about 50 nanometers, or 2,000 times thinner than a sheet of paper — allows the flaps to curl back easily. Any thicker and the weight of the silver interferes with the vent opening.

“It’s surprising and counterintuitive, but adding something heavy on top of a polymer can actually make it bend and open more,” said co-researcher and Mechanical Engineering and Materials Science Professor at Duke Cate Brinson, “What it comes down to is that the silver is shrinking and the nylon is expanding.”

When wet, the bottom nylon layer expands like a sheet being pulled from its sides. The top silver layer prevents the nylon from stretching and leads to a curl-up of the two-layer material.

In experiments, the researchers created a hand-sized patch with fingernail-sized, millimeter-long flaps. Compared with an average traditional textile represented by a blend of polyester and spandex, the Duke-developed material is about 16% warmer when dry, with the flaps closed. When the flaps open, the material is approximately 14% cooler than the polyester-and-spandex combination, according to the team's study.

The nylon-silver approach has potential advantages to existing methods of venting heat through warm clothing, such as placing zippers beneath the armpits.

“We want the sweating parts of the body to be vented, which is not necessarily the underarms,” Prof. Hsu said in the Duke news release. “Our chest and back need more venting, but the effort to unzip these areas, if zippers are even available, is almost the same as simply taking off the clothing.”

Next, Hsu and the team are working on making the vents as small as possible, so the material looks more like conventional clothes.

“I expect that if we can find the right laser cutting method to create very small flaps and attach the patch to clothing, we can create this effect without looking like we’re wearing a costume,” said Hsu. “With enough work, this kind of material could look very similar to what we’re wearing today.”

In a Q&A with Tech Briefs below, Prof. Hsu explains how the team's achievement may inspire researchers to push the limits of wearable technology.

Tech Briefs: What inspired you and your team to make this?

Prof. Po-Chun Hsu: I have been working on radiative heat managing wearables for more than six years, and there are still many projects that we can do to revolutionize the conventional wisdom.

Take this project, for example. Moisture-responsive venting flaps have already been demonstrated by some research groups, but its tuning range has reached a limit. We aimed to push the boundary by introducing radiative heat transfer into the design. This "multimodal" heat management turns out to be very effective in boosting the thermoregulation range and enhancing the switching speed.

Tech Briefs: What needs to happen next for this type of material to look and feel like conventional clothing?

Prof. Po-Chun Hsu: The patch has an overall thickness of only a fraction of hair diameter, so the current form is more suitable to be used on existing garments. The final smooth appearance might feel a bit like a windbreaker jacket, which is actually quite nice for outerwear. To resemble other conventional clothing, we will be engineering the fiber version while maintaining the multimodal design.

There are several promising future directions. For example, reducing the size of the flaps and increasing the fabrication speed will help this technology become closer to a marketable reality. Adding sunlight controllability will enhance outdoor thermoregulation.

Tech Briefs: How small are the vents?

Prof. Po-Chun Hsu: The flaps are a few millimeters in size, which will be further reduced for aesthetic reasons in the future. We are optimistic that this proof-of-concept paper can lead to further development in robustness and wearability.

Tech Briefs: Where are the flaps placed on to the clothing?

Prof. Po-Chun Hsu: The flaps can be on anywhere of the clothing, but it will be most effective to be placed around areas such as the chest, back, and shoulders, which are covered with more sweat glands.

Tech Briefs: How are the flaps activated exactly?

Prof. Po-Chun Hsu: The flaps are made of nylon, which can absorb water vapor and slightly expand. On the side facing the skin, nylon absorbs more water vapor and wants to expand more than the other side facing the environment, which creates mechanical strain inside the nylon. As the result, the nylon film has to curve to the environment side to accommodate for the strain. This is known as bimorph actuation. When the human body stops sweating, the water escapes from the nylon and it flattens again. (Watch a demo below of the material as it curls.)

Tech Briefs: Why is this achievement an important one?

Prof. Po-Chun Hsu: This is the first sweat-responsive wearable that incorporates radiation, convection, and evaporation all together to accomplish large tunability — probably the largest one in research literature because of the multimodal approach (although we don't have a side-by-side comparison). We hope this work can inspire more researchers to push the limit of wearable thermoregulation that can become more and more important under climate change.

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