High-resolution microscopy image of a penguin feather.
High-resolution microscopy image of polar bear hair.Feathers, wool, hair, and fur keep birds and animals warm even in extreme temperatures. By comparing the microstructure of penguin feathers and polar bear fur, two species that can withstand some of the coldest temperatures on Earth, researchers from AGH University of Science and Technology in Poland have outlined the blueprint for the design of thermal insulating materials.
Both penguin feathers and polar bear fur are, like many other natural materials, made from keratin. The material is produced by keratinocytes, which then die, giving rise to a densely layered structure of overlapping tile-like sheets. At the microscale, keratin comprises crystalline fibers in an amorphous keratin matrix. The combination of strong fibers reinforcing a more flexible substance gives rise to number of unique properties, including thermal insulation.
“I was looking at the thermally insulating materials and the geometry that nature uses to protect animals in winter,” explains Urszula Stachewicz, who led the research. “Both materials are made of keratin fibers and I was interested if this building block is similar for these two structures.”
The crystalline form of keratin comes in two types: the alpha form in which the keratin molecules take up a coiled-coil structure and the beta made of sheets of keratin. Polar bear fur uses the alpha form, while the penguin possesses the beta form.
“Both materials are thermally insulating and are known for their internal porosity but have not been compared in such a direct microscopy study before,” says Stachewicz.
Naturally porous thermal insulating materials trap animals’ body heat and help maintain a constant body temperature. Polar bear fur is also able to absorb UV light and turn it, via a photothermal conversion process, into thermal energy to provide warmth in sub-zero temperatures. The penguin’s feathers, meanwhile, prevent penetration of cold seawater onto the skin and stop ice formation.
Scanning electron microscopy analysis of polar bear fur and penguin feathers reveal many similarities between the two keratin-based materials, in particular a hierarchical porous structure, but there are some key differences. In feathers, the main shaft has a ‘foamy’ porous interior, with very small pores in the external surfaces. Polar bear hairs, by contrast, have a hollow core with a porous structure extending from the interior to the surface, which has not been observed previously.
“We assume that the interconnected porosity of polar bear hair, which extends to the surface of the hair, should help to explain the mechanism of converting UV to IR light,” suggests Stachewicz.
An artificial thermal insulating material inspired by penguin feathers and polar bear fur should include a main structure of aligned fiber bundles around a highly porous core with an exterior made of cross-laminated fibers, suggest the researchers.
Metwally et al., Acta Biomaterialia (2019), https://doi.org/10.1016/j.actbio.2019.04.031