BIOMECH - Gels, Mucus, Silks
problems with rubbery analysis
it makes the assumption that the molecular chains move rapidy enough to allow reconfiguration
properties of polymers depend on the the time over which they are stressed
glass - stiff, strong and resilience but brittle
Transition - visoelastic behaviour with high hysteresis
Plateau & Equilibrium - rubbery behaviour
stiffer polymer molecules move curves to the left
higher temperatures move curve to the left
How to investigate time dependent behaviour
Silks
Stress relaxation test
repeated stretching
Creep test
apply load & see displacement
Anemone mesoglea: a time dependant polymer
over 10s mesoglea shows plateau behaviour E~=20KPa
over 20h meoglea is at equlibrium E~=0.2KPa
Arthopeura is stiffer and creeps less then metridium
mucus and gels
Adaptive advantage of creep behaviour
anemones withstand wave well, particularly the open water species arthopteura
mesoglea acts as an antagonist to muscle forces
small pressures developed by pumping of cilia can reinflate them
anemones mesoglea is actually an example of a biological gel. theses all contain small amounts of relatively rigid polymer in water
glycoprotein, polysaccharide, gelatin
in mucus the molecules are separate -> length matters
in gels the mucus are crossed linked -> links matter
Slug mucus
slugs and snails crawl using mucus covered feet, HOW?
Denny studied the shear behaviour of slug mucus
results
at shear strains up to 5 mucus behavior as a solid -> E~=100pa
at higher sheer strains mucus behaves as a viscous liquid
entanglements break & the material shear soffens
How slugs crawl
mucus acts as a ratchet
its solid under the extended stationary parts
its liquid under the shortened moving parts allowing them to slide
how slugs rest on walls
they pump salt solution into the mucus, which acts to form fibrous crosslines between molecules
this makes the mucus 50x stiffer
limpets & other bivalves have special glue proteins to crosslink their glycoproteins
plant gels
agar and carrageenan are examples of gels found between the cells of red seaweeds, they buffer water around the fibrous cells and protect them
attach water in cage like structures
helical sugar molecules wrap around each other & calcium ions, forming ridgid straight parts
pectin is a plant gels
animal gels
collagen - like (triple helix) fibres joined by crosslinks
collagen is the basis of gelatines and other man made glues
to get stiffer 2 things can be done
increase the polymer content
produce straight and stiff polymer fibres by bonding within or between molecules
silks are fringed micille materials with both single floppy legnths of polymer and rigid crystals
structure of moth silks
a typical fibrous part of the silk molecule is made with alternating
hydrophobic glycine...
...and alanine or serine
these pack together neatly to form pleated B sheets held together by van der walls forces
inbertween are legnths of protein with bulky side groups
structure and behaviour of silks
lengths of crystalline material with crosslinked strands in between
...at first the strands are rearranged , next crosslinks broken and finally straightend
mechanical properties
high stiffness, E~=1-10 GPa
high stregnth~= 1GPa
Moderately high breaking strain ~= 0.2-0.5
huge energy to break ~= 50 - 150MJm-3
High hysteresis~=65%
comparative structure and properties of spider silk
the viscid silk of the catching spiral is far less stiff but more extensible than frame or dragline silk
viscid silk has much more water surrounding the chains giving it rubbery behaviour, only when stretched out hugely does it get stiffer. it provides a softer landing for flying insects and makes escape difficult