Researchers have succeeded in programming the thermal conductivity of a material for the first time. The work, which was carried out on a squid-inspired protein containing multiple DNA string repeats, could help in the development of better thermal switches, regulators and diodes to solve thermal management problems in modern technologies such as refrigeration, data storage, electronics and textiles.
“From bitcoins to cloud servers, sportswear to protective suits, medical devices to vaccine stability, thermal management is a key challenge in our modern world,” says Melik Demirel of Penn State University, who co-led this research effort with Patrick Hopkins at the University of Virginia. “The synthetic squid-inspired biomaterials that we are working on have low thermal conductivity under ambient humidity conditions. We can engineer them, however, so that their thermal conductivity increases dramatically by increasing the number of tandem repeats (repeating strings of DNA) in the protein when hydrated.”
Network topology
Conventional solid-state devices conduct or control thermal energy by phonons (vibrations of the crystal lattice) or by scattering of random vibrations in an amorphous material, he explains. “In soft matter, there is a third parameter – network topology – that comes from interactions of long polymeric chains.
“Structural proteins like the ones we are studying contain both crystalline domains that are linked by amorphous ones. Together with colleagues at the University of Virginia, the University of Maryland and NIST, we discovered that we could programme the thermal conductivity between these domains by controlling their network topology.”
The researchers did their experiments on synthetic proteins that mimic squid ring teeth. “We patterned these proteins on tandem repeating sequences and were able to choose the number of repeats to study how the proteins react under different experimental conditions.”
This is not the first time that squid proteins have been used to inspire new technologies, he says. They have already led to the development of camouflage coatings, self-healing materials, soft actuators and renewable bioplastics, to name but a few.
Programming the amount of thermal conductivity
“We found that under ambient conditions – less than 35% humidity – the thermal conductivity of films made from these proteins do not depend on the number of repeat units and have similar thermal conductivities to disordered polymers and water-insoluble proteins,” says Demirel. “However, when we engineer the materials to have an increased number of tandem repeats, their conductivity jumps when they become wetter. Indeed, the greater number of repeats, the greater the thermal conductivity.
“Since the thermal conductivity is linearly related to the number of repeats, we can programme the amount of thermal conductivity into the materials,” he tells Physics World.
The researchers measured the thermal properties of the materials using sub-picosecond optical pump-probe and inelastic neutron scattering techniques.
The material returns to its original level of thermal conductivity under normal humidity conditions. “Such a switch could be used to make better regulators and diodes, similar to high-performance solid-sate devices, to solve the thermal problems in modern technologies, such as refrigeration, data storage, electronics and textiles,” says Demirel.
“For example, the material could become more thermally conducting when it absorbs the sweat produced by an athlete and so remove excess heat from her or his body,” he says. “We are indeed now in the process of developing some small and large prototypes to test this concept in sportswear textiles.”
The researchers, reporting their work in Nature Nanotechnology 10.1038/s41565-018-0227-7, say they are also testing out the technology for heat dissipation applications in electronics devices.
