Science Concentrates MATERIALS
Polar bear hair inspires stealth fabric Hide your lettuce and lock up the carrots: Stealth rabbits are on the prowl. Researchers have woven a cloak that makes a bunny almost invisible to infrared cameras, thanks to fibers that mimic the structure of polar bear hairs (Adv. Mater. 2018, DOI: 10.1002/ adma.201706807). The hairs of a polar bear have hollow cores, which reflect back IR emissions from the animal’s body. By trapping those emissions, this structure helps prevent heat loss and keeps the bears warm in their Arctic environments. But the hairs have an added advantage: They can conceal the bears from thermal imaging cameras used in many night-vision devices. Textiles that can mimic polar bear hair’s IR-reflecting abilities might be useful in stealth applications, such as concealing soldiers. Previous attempts to make synthetic versions of the hairs have produced fibers that are too weak to be practically useful. A team from Zhejiang University has now used a freeze-spinning method to make fibers that are porous, strong, and highly thermally insulating. They consist of fibroin, a protein found in silk, along
A rabbit wearing a cloak with fibers that mimic polar bear hair (top left) is invisible to a thermal imaging camera (top right). Under a polyester cape (bottom left), the bunny’s cover is blown (bottom right). with a small amount of the polysaccharide chitosan. The researchers slowly squeezed a viscous, watery mixture of these materials through a cold copper ring, forming a frozen fiber that contained flat ice crystals. Freeze-drying the fibers removed the ice by sublimation to produce strong fibers about 200 µm wide with up to 87% poros-
ELECTRONIC MATERIALS
Liquid metal makes stretchy nanogenerator Wearable electronics such as activity trackers and biometric sensors demand power sources that can bend and flex as the body moves. A team at Soochow University has developed a triboelectric nanogenerator—which scavenges energy from static electricity produced during motion—using an electrode made from a liquid gallium, indium, and tin alloy. The device retains its function even when bent in half or stretched to three times its length (ACS Nano 2018, DOI: 10.1021/acsnano.8b00147). To see it in action, watch our video at cenm.ag/nanogenerator.—KERRI JANSEN
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C&EN | CEN.ACS.ORG | FEBRUARY 26, 2018
ity. After varying conditions such as the viscosity of the mixture and the temperature of the ring, they found that running the process at –100 °C produced pores about 30 µm across, which offered the best balance between strength and thermal insulation. “I was surprised to see the thermal conductivity of the biomimetic fiber was even lower than polar bear hair,” says Hao Bai, who led the team. It’s not the first time that this ice-templating method has been used to make porous fibers, says Sylvain Deville, research director of the Ceramic Synthesis & Functionalization Laboratory, who uses the method in his research. But, he says, the team demonstrated good control of the fiber structures. To show the thermal stealth potential of the fibers, the researchers wove them into a textile to make a little cape for a live rabbit. The critter’s body heat was all but invisible by thermal imaging, whether the background temperature was 40, 15, or –10 °C. Bai has patented the freeze-spinning technique and hopes to develop the fiber into a commercial product. However, Deville notes that the freeze-spinning process is currently quite slow. “I suspect they will never be able to go very fast, so they may not be able to use it for large-scale applications.”—MARK PEPLOW C R E D I T: A DV. M AT ER . ( B UN N I ES ) ; ACS NA N O/C &E N ( NA N O GE N E RATOR )
A cape made from porous fibers traps heat and hides a bunny from night-vision cameras
