Keratin mechanics

Bone mechanics

Terrestrial locomotion

Education

Full publications list

Mechanical properties of keratin

Keratin is a familiar material to all of us as our hair and nails are composed of it. We also use it extensively as insulation materials in clothing, for example wool and down feathers, and also in decorative items and other artifacts. One of my ongoing research interests has been quantifying the mechanical performance of keratin from birds, where it is the principal component of feathers and the horny coverings of both claws and beaks.

Feather keratin occurs in a 'b-sheet' configuration which is differs from the a-helices that occur in mammalian keratins. If mammalian keratins are stretched in steam, then they develop a b-sheet configuration, so imagining a very stretched spring gives one some idea of the form of avian keratin.

The first step in exploring the mechanics of feathers in birds was to investigate how variable the properties of keratins are between species. When we examined the stiffness (Young's modulus) properties of a wide range of bird species, we found very little evidence of systematic differences in properties between species. We concluded that the mechanical performance of feathers was therefore controlled more by shape than material properties. More recently, we have explored the failure properties of keratins and the modulating influence of the environment on properties.

As feathers are replaced only infrequently at moult, they should be robust enough to withstand the rigours of their environment. We have used microhardness testing as a method of gauging the competence of keratins at resisting abrasive wear. It has long been known that feathers containing the pigment melanin, which gives rise to black and dark brown colouration, suffer less wear than adjacent, paler areas of feathers. Microhardness tests confirmed that this observation had a basis in differences in microhardness as melanic keratin is significantly harder than non-melanic keratin.

A scanning electron micrograph of penguin down feathers

Feathers have a commercial value too. Duck and goose down are still extensively used as high-grade insulation materials in both clothing and bedding as for a given level of warmth, they are lighter and more compressible. It's not all good news, however, as they suffer serious disadvantages when exposed to damp- they coalesce and lose much of their insulative value. We have measured the properties of individual down feather from ducks, geese and penguins and found that their properties are similar to flight feathers and, indeed, the man-made polymers used in artificial insulation fibres. The message is that the architecture of down feathers is probably more important than material properties in determining their advantages over synthetic materials.

Recently, we have begun to explore the toughness of feather keratin by using instrumented clippers and scissors. The fracture toughness of β-keratin has proved to be very high, around 10 kJ m-2.

We are using instrumented clippers to explore the toughness of small samples of keratin.

Key Publications

1. Hedges, V.J., Green L.E., Blowey, R.W., Packington, A.J. & Bonser, R.H.C. (2004) Testing white line strength in the dairy cow. Journal of Dairy Science 87: 2874-2880.

2. Bonser, R.H.C., Saker, L. & Jeronimidis, G. (2004). Toughness anisotropy in feather keratin. Journal of Materials Science. 39: 2895-2896.

3. Taylor, A.M., Bonser, R.H.C. & Farrent, J.W. (2004) The influence of hydration on the tensile and compressive properties of avian keratinous tissues. Journal of Materials Science 39: 939-942.

4. Bonser, R.H.C. Farrent, J.W. & Taylor, A.M. (2003) A novel method of measuring friction and wear of biological materials. Biosystems Engineering 86: 253-256.

5. Cameron, G., Wess, T.J. & Bonser, R.H.C. (2003) Molecular packing and the mechanical performance of feather keratin. Journal of Structural Biology 143: 118-123.

6. Bonser, R.H.C. (2002) The hydration sensitivity of ostrich claw keratin. Journal of Materials Science Letters 21: 1563-1564.

7. Bonser, R.H.C. (2001) The mechanical performance of medullary foam from feathers. Journal of Materials Science Letters 20: 941-942.

8. Bonser, R.H.C. & Farrent, J.W. (2001) The influence of hydration on the mechanical performance of duck down feathers. British Poultry Science 42: 271-273.

9. Bonser, R.H.C. (2001) The elastic properties of wing and contour feather keratin from ostriches, Struthio camelus. Ibis 143: 144-145.

10. Bonser, R.H.C. (2001) Mites of birds. Trends in Ecology and Evolution 16: 18-19.

11. Dawson, A, Hinsley, S.A., Ferns, P.N. Bonser, R.H.C. & Eccleston, L. (2000) Rate of moult affects feather quality: a mechanism linking current reproductive effort to future survival. Proceedings of the Royal Society B. 267: 2093-2098.

12. Bonser, R.H.C. & Dawson, C. (2000) The mechanical properties of down feathers from Gentoo penguins. Journal of Zoology (London) 251:545-547.

13. Bonser, R.H.C. (2000) The Young's modulus of ostrich claw keratin. Journal of Materials Science Letters. 19: 1039-1040.  

14. Bonser, R.H.C. & Dawson, C. (1999) The structural mechanical properties of down feathers and biomimicking natural insulation materials. Journal of Materials Science Letters 18: 1769-1770.  

15. Bonser, R.H.C. (1996) The mechanical properties of feather keratin. Journal of Zoology (London) 239: 477-484.

16. Bonser, R.H.C. (1996) Comparative mechanics of bill, claw and feather keratin. Journal of Avian Biology 27: 175-177.

17. Bonser, R.H.C. (1995) Melanin and the abrasion resistance of feathers. Condor 97: 590-591.

18. Bonser, R.H.C. & Purslow, P.P. (1995) The Young's modulus of feather keratin. Journal of Experimental Biology 198:1029-1033.

19. Bonser, R.H.C. & Witter, M.S. (1993) Indentation hardness of the bill keratin of the European Starling. Condor 95: 736-738.