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BIOMECH - Biological Composites (natural composites 1 - keratin (designs…
BIOMECH - Biological Composites
problems with silks
crystals are short, silks deform mostly in the matrix
to stiffen a material better the crystals should be longer
The crystals particularly stiffen along the fibres (voigt model)
a composite with very long fibres -HAGFISH SLIME
exudate is used to clog gills of predators
structure
99.996% seawater
0.002% threads 1-3 um wide
0.0015% mucin
coiled threads and mucin vesicles are secreted from gland into the sea water which cause them to rupture and expand
mechanics
stiffness ~= 40Pa
really compliant
thread stiffness ~= 6MPa
rubber like
mucin stiffness ~= ZERO
from the voigt model expected stiffness ~= 120pa, so only 30% of threads are stressed. the material works because the threads are as long as the slime, the mucins merely delay it falling apart
Tendon: a simple composite
triple helix (tropocollagen)
formed by joining 3 molecules each with a shallow helical structure
either: gly-x-pro or gly-xhypro
hycine allows hydrogen bonding between chains
pack approximately in quarter stagger every 67nm
collagen molecules are 280nm long
tensile properties
very stiff E ~=2-3GPa
very strong breaking stress ~= 60 -100MPa
reasonable breaking strain ~= 0.03 or 3%
good energy storage 3MJm-3
excellent resilience 93%
flexible toe region
use of tendons
to transmit forces
from large muscles & store energy
act in high stress
and movements from small muscles
act at low stresses
advantages of a compliant matrix
very notch resistant because compliant matrix does not transmit stresses to fibre -> crack is blunted
energy is absorbed as material around the end deforms so the material is toughened
toughening in rigid composites
to resist compression fibres should be in a stiff glassy matrix
cracks expected to be common but are stopped by cook-gordon mechanism
as the crack aproaches a weak interface, the two material debond and crack tip is blunted
fracturing across the fibre therefore involves extensive fibre pull out which uses lots of energy and toughens the composite
fibres are protected from being scratched by the matrix
disadvantages of a complient matrix
tendon cant be used to take compressive or bending forces
natural composites 1 - keratin
mammalian hair, horn, hoof, claws and nails
low sulfur proteins containing mostly helix forming AA - forms fibres made from a-helix molecules (stabilised by Hbonds)
the molecules are actually arranged in a strict hierarchy
ALTERNATIVELY...
high sulphur proteins with lots of crosslinks cystine S-S bond form the rubbery matrix at the scale above the microfibril
many macrofibrils are laid within a cell
mechanical properties
Extremely still parallel to fibres E~= 2-5GPa
very strong breaking stress ~= 200-300MPa
Yield & high breaking strain = 20-50%
why??
the molecules are stretched out from a a-helix to b-sheets form this uses a lot of energy
results in a conformational change
keratin have low resilience
but they are extremely tough (around 10-15KJm-3) across the fibres
designs of keratin structures
1 - horns, hooves, claws and nails use the high strength & toughness of keratin to take force and impacts
2- fibres in horn and claw follow stress lines
3-fibres in hoof direct crack away from pastern
4- nails have a sandwich structure
5- hair doesnt need to be strong
perms put keratin into b-sheet form
reptile and bird keratin
made of b-keratin (scales and feathers)
contains lots of serine and glycine like silks
twice as stiff as mamalian keratin
it only stretches around 6-15%before breaking
it has lower hysteresis but still tough (around 10-15KJm-3)
insect cuticles
composite of chitin with a protein matrix
fibres are 3nm in diameter, 0.3um long, E~= 150GPa
matrix is a protein with variable amount of water and long with fibre content and direction controls the mechanical properties
soft insect cuticle
found in membrane hinges
contains 40-75% water, e.g locust intersegmental membranes
fibre direction and high water content make the membrane very soft - E~=1KPa with high breaking strain allowing insects to lay eggs far underground
stiffening of the cuticle
stiffened by tanning - dihydroxyphenols are secreted into it. 3 possible mechanism have been suggested
1- forms crosslinks between protein
Evidence - phenol concentration is proportional to stiffening (but maximum stiffening expected by rubber theory = 10MPa while it can be 2GPA)
UNLIKELY
2 - exclusion of water gives more plass like properties
Evidence - water lost in tanning process and untanned cuticles are stiffened by drying
3 - phenolics polymerase to form melanin "filter molecules"
evidence - heavily tanned cuticles are black and hard, similar melanin fillers are seen in feather keratin (black feathers are harder and more wear resistant )
stiff cuticles
body cuticle
has cuticle arranged in many ways to give isotropic properties
E~=2-5GPa
these helicoidal & "plywood (OSB board)" arrangements also gives good toughness ~= 2-3KJm-3
body design of insects
insects make a mobile exoskeleton from a single coating of cuticle by varying both thickness and tanning
