Bunny Blankets

8 Apr

A bunny in bear’s clothing? Yes, it is. A team of researchers in China developed a fabric that mimics the fur coat of a polar bear. A live rabbit was wrapped in a ‘blanket’ made img_6854-1of this prototype fabric and used infrared imaging (thermal imaging camera) to measure the difference between the rabbit’s body and ambient temperatures. The body of the rabbit under the blanket was almost completely undetected. Where the body was exposed to the changing temperatures (14-104 degrees Fahrenheit), thermal imaging detected only those exposed areas. This demonstrates good thermal insulation.

Like the fur of polar bears, the fabric also reflects infrared light. The blanket fabric reflected light up to twice that of commercial textiles and can’t be detected by night-vision scopes and cameras, just like polar bears.

What makes polar bear’s fur so special? A fiber from the fur of a polar bear is a

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Polar bear fiberS

simple cylinder. The interior core of the fibers are hollow, which helps traps their body heat and insulate them from Arctic winters. This hollow trait of the bear’s fibers also reflect back infrared light, but the transparency of those same fibers scatter UV light to neighboring hairs, via light scattering. Their black skin absorbs and stores heat from that UV light. All these factors help insulate the polar bear from cold, even when wet. Think of passive solar energy and storage.
Despite being so porous, the fibers are strong because a thick layer of keratin surrounds the hollow core. Keratin is a protein common in nails and hair. Keratin protein molecules form a helix, and two keratin helixes wind around each other conferring both strength and elasticity. You can think of animal fiber as a thick rope containing many stings of these entwined keratin filaments. In the case of the polar bear, these keratin layers surround the hallow core.

So how did those scientists mimic the fur of a polar bear? They did so through a very complex process of spinning a mixture of compounds found in silk and crustaceans (like shrimp and crabs). A solution of fibroid, a protein in silk, and chitosan, a structural carbohydrate made from chitin, was spun in freezing conditions. Ice particles separated the fibers leaving air spaces when warmed. These resulting fibers, which had both properties of thermal insulation and strength, were aligned and then woven into a fabric, and tested on the bunny.

Being a fiber artist, I wondered how fibers from sheep compare to the properties of polar bear fibers. The variability of fiber between breeds and individual sheep is immense. So many factors influence the characteristics of wool fibers: breed, age, nutrition, and weather. However, the primary composition of wool fibers is fairly consistent.

Because I raised sheep in Oregon (decades ago) and spun their wool, I was familiar with the overall structure – overlapping scales in varying lengths – of wool fibers. However, I wanted more details on its morphological structure. I can quickly draw and label the structure of muscle fibers and fibrils, but not a wool fiber! So I searched online for a diagram and electron micrographs, longitudinal and cross-sectional,  of a typical wool fiber. From these we can see how polar bear fibers are structurally different, but they both have insulating properties.

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Mammal, silk, and plant fibers

Most animal fibers contain exterior scales (except polar bears,  remember). You can see the difference in scales, or lack of, in the image to the right, in both animal, silk and plant fibers. Also apparent are the sizes of the scales, and their distance apart. These are the images most of us are familiar with when we think of fiber structure. But it’s more complex than this.

If we compare the fibers in this image to that of the polar bear fiber, we can see that fiber structure of wool is more complex. You don’t see slightly rough exteriors and hollow centers! Instead, wool has irregular and overlapping scales. But that’s not where the differences end.

Let’s look at the interior morphological structure of wool. Wool fiber contains three

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Morphological structure of a wool fiber.

primary parts: the outer cuticle, the cortex (divided into two sections), and the medulla. The cuticle includes the scales and several layers that surround and protect the cortex.

The cortex is the bulk of a fiber. In that cortex are many macro- and microfibrils, similar to a muscle fiber. A similarity to the polar bear fiber, and that of most mammals, is keratin. The medulla is not really distinct, depending on the mammal species. It is empty space and arranged in a honeycomb fashion within the cortex. That space can be tiny, as in sheep breeds with fine fiber, or larger, which is more typical in medium and coarse fiber breeds.

The structure of the wool fibers, and the fiber ‘community’ (meaning the collective grouping of fibers in what we sheep folks call ‘fleece’), contribute to the insulating properties. Another structural component is fiber crimp. Wool fiber can be long and straight, or short with many crimps. The number of crimps, how they are spaced along the length of the fiber, and how close they are to neighboring fibers, greatly influence how much air a fleece can hold within it. That air also contributes to its insulating properties.

But it doesn’t stop there! At least for humans that shear those fleeces, wash and prepare them to spin into yarn that is eventually knitted or woven into fabric to wear or for other domestic purposes. The natural structure and characteristics of those fibers determine how we process the fleeces, how we spin them, use the yarn for a final product.

We could even knit a blanket for a bunny.

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