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Tunable thermal biopolymer relies on network of crystalline domains

By Prachi Patel October 24, 2018
A synthetic protein inspired by squid proteins is made of repeated polypeptide sequences that form a semicrystalline structure, with crystalline domains (blue) connected by amorphous segments (yellow). Credit: Nature Nanotechnology

A new biological polymer inspired by squid proteins conducts four times more heat when it is wet, but goes back to its original thermal conductivity when dry, researchers report in a recent issue of Nature Nanotechnology. Controlling the thermal properties of materials is a challenge, and the change in conductivity in the polymer is three times larger than in state-of-the-art tunable thermal materials.

Athletic garments made with this polymer could regulate body temperature, says Patrick Hopkins, professor of mechanical and aerospace engineering at the University of Virginia who led the work. When the wearer sweats, the wetness would increase the fabric’s thermal conductivity so that it wicks away heat and cools down the user. The tunable thermal material could also lead to novel devices for refrigeration, electronics cooling, and harvesting waste heat for power.  

Scientists and engineers have tried for years to develop a “‘thermal switch,’ a device whose physical state could be flipped between conducting heat efficiently or conversely blocking heat flow,” says Olivier Delaire, professor of mechanical engineering and materials science at Duke University who was not involved in the work. But while controlling the electrical and optical properties of materials is common in engineering, tuning thermal properties has proven extremely difficult. That is because heat transport is governed by phonons, which are the vibrations of an arrangement of atoms, and changing these atomic vibrations is difficult. 

Researchers have previously switched heat flow in materials, such as in piezoelectric materials by controlling the millimeter-sized magnetic domains, and in lithium battery cathode materials by introducing defects. This however only results in a very small changes in conductivity, of a few tens of a percent, Hopkins says.

To get larger changes, Hopkins, Melik Demirel—Pennsylvania State University professor of engineering and mechanics—and their colleagues took inspiration from the protein found in the teeth that circle the suction cups of squid tentacles. Squid ring teeth proteins are high-strength polymers with a semicrystalline structure made of sheet-like crystalline domains (where a few protein strands are connected laterally with hydrogen bonds) that are tied to each other with amorphous protein chains. Demirel has previously made self-healing materials with these proteins.

Natural squid ring teeth protein can have crystalline and amorphous domains of different sizes. So the researchers tweaked the DNA sequences to express proteins that contain repeats of identical crystalline and amorphous sections. They insert the DNA into bacteria that then produce these synthetic proteins. “We can play with the distance between the domains, and with how many domains we want to connect,” Demirel says. They made films from these proteins and tested the thermal conductivity of different samples under various humidity levels.

When the humidity was less than 35%, the thermal conductivity of all the films was the same, the researchers found. It does not depend on the number of repeated crystalline units. But when the films get wetter, their conductivity increases because adding water makes the crystalline domains fluctuate much faster, Demirel says, which helps phonons to transfer much better. And the conductivity jumps up linearly with the number of repeats and interconnects in the protein network. The protein film with the most number of repeat units showed nearly a factor of four increase in thermal conductivity.

“This is a breakthrough in the field of tunable thermal materials, realizing a more efficient thermal switch than ever done before,” Delaire says. “This opens a new direction in this research field. There could be a lot more progress based on this idea of reconfigurable bio-mimetic materials.”

Read the abstract in Nature Nanotechnology.