Inspired by the "big thick sweaters" of polar bears, residents of the polar regions, Professor Bai Hao and Associate Professor Gao Weiwei of Zhejiang University came up with a new strategy. By imitating the "core-shell" structure of polar bear fur, they prepared an ultra-warm artificial fiber encapsulated with aerogel. A related paper was published in Science on December 22.
"It has the heat insulation function of traditional thermal insulation materials and can 'block' the infrared rays radiated by the human body. Compared with traditional materials, its mechanical properties such as stretch resistance are greatly improved, and it can be directly woven, truly realizing the ability to wear airgel through "On the body." Bai Hao, the corresponding author of the paper, said that in the near future, people are expected to wear only a thin shirt and be able to brave the cold like polar bears.
Inspiration from polar bears
Polar bears are born with a super warm "sweater" , making it adaptable to an environment of minus 40 degrees Celsius. Polar bear fur is a hollow structure that encapsulates a large amount of "still" air, which reduces heat loss by inhibiting heat conduction and convection. Nowadays, the design of many warm clothing uses this principle. For example, the wool and down used in clothing can inhibit heat conduction and heat convection.
In order to make clothes thinner and warmer at the same time, people naturally think of using less material to encapsulate more air. Aerogels, which are extremely porous and less dense than air, are an ideal choice. Over the past few decades, attempts have been made to coat aerogels on fabric surfaces or to directly "spun" fibers containing aerogels.
However, the airgel coating is easy to fall off, the airgel content of the material is limited, and the mechanical properties such as wear resistance and stretch resistance are poor, which limits the space for improving the performance of the next generation of warm clothing.
Against this background, in 2018, Bohao’s research team produced the first generation “Polar Bear Sweater”. However, several existing solutions cannot achieve warmth, thinness and durability at the same time.
With new challenges, Bohao's research team once again opened their "textbook" - polar bear hair. Sure enough, they noticed an overlooked detail: the polar bear's fur is not only hollow, but also has a shell! Under an electron microscope, this shell is about 20 microns thick, accounting for nearly 1/4 of the diameter of the hair. It was this discovery that inspired the team to develop "Polar Bear Hair 2.0".
"Lock" infrared radiation
Drawing from polar bear fur With a "core-shell" structure, after nearly 6 years, the team developed a new type of fiber with a polymer aerogel in the center and long pores with a diameter of about 10 to 30 microns distributed inside. They are arranged in the same direction. Like "warehouses" for storing air. At the same time, a TPU (thermoplastic polyurethane elastomer) shell wraps the aerogel inside.
One core and one shell, each with its own function, among which the "core" is responsible for achieving super warmth. "To a certain extent, keeping warm means preventing heat loss." Wu Mingrui, the first author of the paper and a doctoral student, said that the main forms of heat dissipation in the human body include thermal radiation, thermal convection, heat conduction, and sweat evaporation. Among them, thermal radiation has the greatest impact, and heat is loss in the form of infrared radiation. However, existing thermal clothing has limited capabilities in regulating heat radiation.
The research team believes that by regulating the direction and size of the small holes inside the fiber, it is possible to "lock" infrared radiation. "Infrared rays radiate outward from the skin. By orienting the holes perpendicular to the direction of radiation, and adjusting the size at the same time, it is expected to match the wavelength of the infrared rays, thereby achieving the purpose of 'locking' infrared radiation." Wu Mingrui said.
In order to verify the thermal insulation effect, the researchers turned the minus 20 degrees Celsius constant-temperature cold storage in the school cafeteria into a temporary "fitting room" and held a "warmth preservation challenge" here. Zhang Zibei, the author of the paper and a doctoral student, served as a model and tried on down jackets, wool sweaters, cotton sweaters and "polar bear sweaters" with the same initial temperature, and recorded the rise in surface temperature of the clothing.
After a few minutes, the surface temperature of the cotton sweater rose to 10.8 degrees Celsius, and the surface temperature of the down jacket rose to 3.8 degrees Celsius. The surface temperature of the "polar bear sweater", which is close to the thickness of a wool sweater and only 1/3 to 1/5 of a down jacket, only rises to 3.5 degrees Celsius. Less warming means less heat loss from the human body, so the "polar bear sweater" beats other competitors.
"The new aerogel fiber can deal with thermal radiation, heat convection, and heat conduction in a multi-pronged manner, and the thermal insulation performance has been greatly improved." Gao Weiwei said, "At present, all thermal insulation materials rely on encapsulation of as much air as possible or vacuum suppression Heat conduction and heat convection. Our fibers have an ordered pore structure that suppresses thermal radiation."
Can be directly woven into fabrics
"Good comprehensive performance is the key to the application of bionic fibers. Fiber fabrics have more stringent requirements than static insulation, and a series of problems such as tensile strength, pressure resistance, washability, and thinning need to be solved," said Bo Hao.
The team designed a TPU shell for the fiber. In experiments, the new fiber with a shell can be stretched to twice its length without breaking, which satisfies the tensile resistance requirements of clothing fibers. After testing, the bionic fibers prepared in large quantities from the laboratory can be directly woven into fabrics on commercial textile machines.
"The shell is like a skeleton, providing good mechanical support to the fiber, making it durable.�, stretch-resistant and washable. " Bai Hao said.
But the stronger the shell, the better. Wu Mingrui said that an overly thick shell will affect the thermal insulation performance of the fiber. Therefore, the team selected an optimal value that takes into account the thermal insulation performance and mechanical properties of the material.
"Polar bear hair allows us to see how nature solves problems through 'decoupled' design. The core and shell each perform their own duties and are indispensable, and together they create one of the most cold-resistant materials in nature." Bo Hao said, "The new woven airgel fiber is designed following this idea."
When discussing the wording of the paper with the team, the reviewer suggested that "it may be said that artificial polar bear hair 'surpasses' natural polar bear hair." However, in Bohao's view, the word "beyond" is not necessarily appropriate.
"The essence of bionics is to learn from nature how to solve problems. Bionics is an endless learning process. It is too early to say 'transcendence'. New problems will prompt us to continue to learn from nature and constantly reveal the secrets of nature. Discovering new knowledge and creating new materials that improve people's lives is the mission of bionic research and our pursuit for many years," said Bohao.
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