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.
occurs in a 'b-sheet'
configuration which is differs from the
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
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.
Hedges, V.J., Green
L.E., Blowey, R.W., Packington, A.J. & Bonser, R.H.C.
Testing white line strength in the dairy cow. Journal of Dairy Science
Saker, L. & Jeronimidis, G. (2004). Toughness anisotropy in feather keratin.
Journal of Materials Science. 39: 2895-2896.
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.
Farrent, J.W. & Taylor, A.M. (2003) A novel method of measuring friction
and wear of biological materials.
Cameron, G., Wess, T.J. &
Bonser, R.H.C. (2003) Molecular packing and the mechanical
performance of feather keratin.
Journal of Structural
(2002) The hydration sensitivity of ostrich claw keratin. Journal of Materials Science Letters 21: 1563-1564.
(2001) The mechanical performance of medullary foam from feathers. Journal
of Materials Science Letters 20: 941-942.
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.
R.H.C. (2001) The elastic properties of wing and contour feather keratin
from ostriches, Struthio camelus. Ibis
(2001) Mites of birds. Trends in Ecology
and Evolution 16: 18-19.
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.
Bonser, R.H.C. & Dawson,
C. (2000) The mechanical properties of down feathers from Gentoo penguins. Journal
of Zoology (London) 251:545-547.
Bonser, R.H.C. (2000) The
Young's modulus of ostrich claw keratin. Journal of Materials Science Letters. 19: 1039-1040.
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:
Bonser, R.H.C. (1996) The
mechanical properties of feather keratin. Journal of Zoology (London) 239: 477-484.
Bonser, R.H.C. (1996)
Comparative mechanics of bill, claw and feather keratin. Journal
of Avian Biology 27: 175-177.
Bonser, R.H.C. (1995) Melanin
and the abrasion resistance of feathers. Condor 97: 590-591.
Bonser, R.H.C. & Purslow,
P.P. (1995) The Young's modulus of feather keratin. Journal
of Experimental Biology 198:1029-1033.
& Witter, M.S. (1993) Indentation hardness of the bill keratin of the
European Starling. Condor 95: 736-738.