New thermal material could keep you hot and cold

As we all know rather well, living in the British weather can be fairly unpredictable. Be it travelling to campus, on a sports field, or climbing mountains, we’re all exposed to the elements, which include frequent changes in temperature. Considering that most student homes are freezing and funds are tight, versatile clothing could be an essential addition to all of our wardrobes. A recent paper published in Science Advances in November last year may however hold the answers.

In ‘A dual-mode textile for human body radiative heating and cooling’ Po-Chun Hsu et al. describe the work that they have done towards a new kind of material that can be used to either heat or cool the body, depending on which way its worn. The paper begins by explaining the demands of maintaining body temperature, “which often consumes a huge amount of energy to keep the ambient [body] temperature constant.” The body has several methods to do this, such as controlling blood flow to the skin, sweating, or shivering. Sweating, for example relies on evaporation off of the skin. Another way of losing heat is by the body radiating infra-red (IR) radiation.

Considering that most student homes are freezing and funds are tight, versatile clothing could be an essential addition to all of our wardrobes

This particular fact can be used in thermal detectors, as humans can then be detected by this IR signature; however it can also be harnessed by clothes, reflecting the heat back onto the body and stopping it from escaping. Naturally then, a cooling clothing item would need to be made of textiles allowing ‘as much IR radiating away from the human body, which has recently been demonstrated using nanoporous polyethylene (nanoPE)’. The challenge now comes from designing a fabric with heating and cooling capabilities, but of course is also opaque (otherwise that rather defeats the purpose of the clothes).

This new material developed by Po-Chun Hsu et al. utilizes this nanoPE as the base layer of the clothing. Sandwiched between this there is a bilayer, made up of one layer of copper and one of carbon. Carbon has a very high emissivity, which means that it is very good at radiating IR radiation. Copper on the other hand has a low emissivity, which absorbs far more IR radiation. Whichever layer is on top of the bilayer determines whether it is in cooling mode or heating mode. When the carbon is on top, more IR radiation is emitted, allowing the skin beneath to cool down. Naturally, when the copper is on top, far less radiation is allowed out, ensuring the skin maintains as much heat as possible.

Sandwiched between this there is a bilayer, made up of one layer of copper and one of carbon

The nanoPE has different thicknesses above and below the bilayer, which controls the temperature of the emitter from being closer or farther from the hot side. In plain English, that means that the bilayer emitter is further away from the skin when in ‘heating mode’, so as to reduce the thermal conductance as much as possible, as the further the bilayer is from the skin, the less heat can diffuse through it. The thicker nanoPE side is 24 µm, followed by 9 µm of carbon, 0.15 µm of copper and then another 12 µm of nanoPE to complete the sandwich around the bilayer.

After testing on artificial skin, the team demonstrated that the ‘heating mode’ could change the skin temperature by as much as 4°C, and the ‘cooling mode’ bring skin temperature down by 3°C. Numerical studies have shown a potential variation of around 16°C in the future. So, with reversible thermal clothing around the corner, having overflowing wardrobes could be a thing of the past.