Literature Review
Structures & Mechanical
Properties of Keratins
Yasuaki Seki
1
Overview
Biological Materials
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Integument of Animals
(keratins)
The Formation of the Keratin
The Classification of Keratin
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Structure of Keratins
Mechanical Properties
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α-keratin
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β-keratin
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Wool
Skin
Nail
Hoof
Beak
Feather
Claws
Reptilian scale
Summary
2
Biological materials
 Biological materials have cell structure and protein.
 The feature of biological materials is hierarchal
structure that optimized for use of the properties.
(Abalone shell, Bone,Hair)
 Biomineralization: biological controls over
crystallization of inorganic salt in living organism. (Ca,
K, Mg, Cl)
3
Hierarchical structure
Hierarchal structure of merino wool
M. Feughelman, Mechanical properties and structure of αkeratin, USW press, 1997 .
4
Biomineralization
Nanostructure of biological materials
B.Ji, H.Gao, Journal of Mechanical & Phys Solid, 52, 1963-1990, 2004.
5
Integuments
Hoof
Fingernail
wool
Horn
Scale
Beak
6
Keratins and their functions
Foam
Hollow
Functions
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Keratin (sulfur
containing
protein)
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Prevention of egress & ingress of fluids
Prevention of ingress of micro-organism,
parasites ,and foreign matter
Protection against mechanical injury and
attack from predators
Food gathering
Temperature regulation
Locomotion, including flight, climbing, and
floating
7
Formation of the keratin
Structure of mammalian skin
D.J Tomlinson et al, Journal of Dairy Science,87,797-809,2004,
8
Keratinization process
Electron micrograph of horn cell
in final stage
Formation of epidermal cell
D.J. Tomlinson et al, Journal of Dairy Science, 87,797-809,2004.
9
Classification of keratins
Keratins
Hard
α-keratin
(mammal)
Soft
β-keratin
(avian)
β-keratin
(reptilian)
α-keratin
(mammal)
Wool
Beak
Claw
Hair
Feather
Scale
nail
Claws
Skin
R.D.B. Fraser, T.P. Macrae, The Mechanical Properties of Biological Materials, 211-246, 1980.
10
Structure of α-keratin (coil-coil)
Schematic of human hair fibre
α-helix (coil-coil structure)
R.C Marshall,et al, Electron Microscope Rev, Vol 4, 47-83.
11
Structure of β-keratin
The structure of β-keratin (based on the
β-sheet)
(a) the central framework
of the filament.
(b) Model for the arrangement
of the β-sheet portions of
the protein molecules in the
filaments
β-sheet conformation
R.D.B. Fraser, T.P. Macrae, Int J Bio Macro, 207-211 19, 1996.
12
Young’s modulus–density chart
Keratin
Wool
Skin
U.G.K Wegst, M.F.Ashby, Philo. Mag, 84,21,2167-2181, 2004.
13
α-Keratin
14
Structure of wool
M. Feughelman, Mechanical properties and structure of αkeratin, USW press, 1997.
15
Mechanical properties of α-keratin
filament
Two-phase model
S-S curves of wool
Wortmann F. J, et al,Textile Res, 64 737, 1994.
M. Feughelman, Mechanical properties and structure of αkeratin, USW press, 1997.
16
Humidity sensitivity of wool
Stress (MPa)
300
200
100
0
0
0.2
0.4
Strain
0.6
J. Vincent, Structural Biomaterials, Princeton University Press,1990.
17
Stratum corneum (soft keratin )
Structure of mammalian skin
Outermost layer of the skin
R.D.B. Fraser, T.P. Macrae, The mechanical properties of biological materials, 211-246, 1980.
18
Mechanical properties of stratum
corneum
Fracture mechanics
specimen geometry
Untreated SC
K.S.Wu et al, Biomaterials, 27, 785-795, 2006.
19
Comparison of dilipidized and
untreated stratum corneum
Delamination of Stratum
corneum
K.S.Wu et al, Biomaterials,27,785-795,2006.
20
Fracture surface of stratum
corneum from double cantilever
beam
100% RH
45% RH
K.S.Wu et al, Biomaterials,27,785-795,2006.
21
Fingernail
Free or distal edge
Lateral
nail fold
Lunula
Proximal nail
Eponychium fold
Three layers model
L.Farren et al, The J Exp Bio,207,735-741, 2004.
Y.Kobayashi et al, J.Pharm.Pharmacol,51,271-278,1999.
22
Scissors cutting test
B.P. Pereira, et al J Biomech, 1997
L. Farren, et al, J Exp Bio,207,735-741, 2004,
23
Fracture surface of fingernail
200µm
200µm
Central area
Edge of the nail
100µm
Lateral edge
L.Farren, et al, J Exp Bio, 207,735-741, 2004.
24
Structure of hoof
M.A.Kasapi et al, J Exp Bio 202,377-391,1999.
25
Stress-Strain curve of hoof wall
M.A.Kasapi, J.M.Gosline, J. Exp. Bio, 200, 1639-1659,1997.
26
Strain rate dependent of keratin
(Hoof)
M.A.Kasapi, et al, J. Exp. Bio, 199,1133-1146 ,1996.
27
Strain rate dependent of keratin
(Hoof)
M.A.Kasapi, et al, J. Exp. Bio, 199,1133-1146 ,1996.
28
Toughness of hoof
Outer hoof wall
M.A.Kasapi et al, J. Exp. Bio, 199,1133-1146, 1996.
29
Fracture surface of hoof
(Higher strain rate)
100µm
1mm
M.A.Kasapi et al, J. Exp. Bio, 199,1133-1146, 1996.
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Fracture surface of hoof
(Lower strain rate)
100µm
Higher degree of pull-out
1mm
M.A.Kasapi et al, J. Exp Bio 199,1133-1146, 1996.
31
β-Keratin
32
Hardness of beak
(European Starling)
R.H.C. Bonser and M. S. Witter, The Condor, 95, 736-738 1993.
33
Bird feather
rachis
Rachis (Shaft)
barb
Calamus
R.D.B. Fraser, T.P. Macrae, The mechanical Properties of Biological Materials, 211-246, 1980.
34
Mechanical properties of feather keratin
Species
Young’s modulus (GPa)
Rock pigeon
2.42
Willow ptarmigan
2.71
Mute swan
2.39
Eurasian sparrowhawk
2.41
Black-headed gull
2.04
Tawny owl
2.76
Grey heron
1.78
Common Starling
2.67
Mean
2.50
R.C.H.Bonser and P.P.Purslow, J. Exp. Bio, 198,1029-1033,1995.
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Breaking stress (MPa)
Melanized feather barb
30µm
0
Proximal
0.5
Fractional distance
1
Distal
Cross-section of Feather barb
(medullary)
M. Butler, A.S. Johnson, J Exp Bio 107, 285-293, 2004.
36
Compressive behavior of
medullary foam
Swan (Cygnus olor)
Compressive Behavior of Medullary Foam
R.H.C.Bonser, Journal of Materials Science Letters, 20, 941-942, 2001.
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Mechanical behavior of foam
Relative Young's modulus
0.1
0.01
0.001
0.03
0.04 0.05 0.06 0.070.080.090.1
Relative density
E ρ 
=  
Es  ρ s 
*
*
2
L.J.Gibson, M.F Ashby, Cellular solids 2nd ,Cambridge, 1991.
38
S-S curve of feather & claw
(ostrich)
Tensile S-S curves (claw)
Stress (MPa)
Stress (MPa)
Tensile S-S curves (rachis)
A. M. Taylor, R.H.C. Bonser, J.W. Farrent, Journal of Materials Science, 39, 939-942 2004.
39
Summary of tension & compression
results
Feather rachis
Claw
Humidity
0% RH
50%RH
100%RH
Humidity
0% RH
50%RH
100%RH
Young’s
modulus
(GPa)
Tension
3.66
2.58
1.47
Young’s modulus
(GPa)
Tension
2.70
2.07
0.14
UTS(MPa)
Tension
221.03
Young’s modulus
(GPa)
Compression
2.98
1.83
0.23
UTS (MPa)
Tension
90.28
68.68
14.03
Strain at failure (%)
Tension
5.71
6.66
20.51
Strain at
failure(%)
Tension
9.2
129.99
10.4
106.27
16.3
A. M. Taylor, R.H.C. Bonser, J.W. Farrent, Journal of Materials Science, 39, 939-942 2004.
40
Snake scale
R.D.B. Fraser, T.P. Macrae, The Mechanical Properties of Biological Materials, 211-246, 1980.
41
Mechanical properties of
snake scale
100
200
100
Snake scale
50
0
60
40
Snake scale
20
0
1
2
3
4
5
Strain (%)
6
7
8
RH 100 %
Feather rachis
80
Stress (MPa)
Feather rachis
150
Stress (MPa)
RH 65%
0
0
1
2
3
4
5
Strain (%)
6
7
8
R.D.B. Fraser, T.P. Macrae, The Mechanical Properties of Biological Materials, 211-246, 1980.
42
Chameleon
J. Sarfati, Creation, 26, 4, 28-33, 2004.
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Chameleon skin
transparent
yellow
chromatophores
guanophores
melanophores
red
blue reflecting
white reflecting
dark brown
J. Sarfati, Creation, 26, 4, 28-33, 2004. & N.J. Alexander, Z.Zellforech,110,153-165,1970.
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Summary I
Young’s modulus, E (GPa)
1000
100
10
1.0
0. 1
0.01
Natural
polymers
and natural
composites
Feather
Nail
Beak
& Claw
Scale
Hoof
Natural
cellular
materials
0.001
0.03
0.1
Natural
ceramics
and ceramic
composites
Keratin
Wool
Skin
Natural
elastomers
0.3 1.0
3.0
Density (Mg/m3)
10
30
U.G.K Wegst. M.F.Ashby, Philo. Mag, 84, 21,2167-2181, 2004.
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Summary II
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Hard α & β keratins are categorized as natural
polymers and soft keratin (skin) is natural
elastomers in terms of mechanical properties.
The mechanical properties of keratin are
associated with the compositions (lipids).
The wide rage of mechanical properties of
keratin depends on the structures.
The mechanical properties of keratin depend on
the environmental conditions (humidity &
temperature).
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Acknowledgements
Adviser:
Professor Marc A. Meyers
Group members:
Hussam Jarmakani, Buyang Cao, Albert Lin,
Po-Yu chen, Anuj Mishra, Sara Bodde,
Bimal Kad, Glaucio, and Liliane.
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Thank you
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