BIOMECH - hydrostatic skeletons
Pneumatic structures
very few examples
male frigate birds
hydrostatic more common
cartilage
osmotically stiffened structure
highly organised
made of: water 75%, collagen 16%, proteoglycans 7%, chondrocytes 2%
proteoglycans osmotically attract water to the balloon-like structure
takes the tension in an outer skin and improves internal stiffness
structure of articular cartilage
top layer provides a skin (horizontally)
middle layer resists shear (in 'X' conformation )
bottom layer attaches to bone (vertically)
properties
stiffness around 1-10 MPa
breaking strain around 0.25
makes excellent hinges and shock absorbers in the backbone
disadvantages of osmotic design
water gradually seeps out of cartilage over time, resulting in creep
people get 1-2cm shorter throughout the day
hydrostatic skeletons in plants
prestressed by pressures up to 1MPa
central parenchyma cells held in compression by the outer epidermis, that they frm 14 sided shape
also supported by fibres, plant stems have high rigidity but complex mechanics
hydrostatic skeletons in the mammalian penis
one of the few structures reinforced by orthogonally orientated fibres
properties
high bending resistance (but kinking a problem )
low torsional resistance
the strengthening structure is the corpus cavernosum
the tunica albuginea is reinforced by crimped collagen fibres that straighten as blood flows in and its pressure rises as exit is blocked
mechanical properties of the human penis unknown
properties of helically wound cylinders
v. common in nature and exhibit useful properties
low bending resistance (smooth when bent)
high torsional ration
however depends crucially on fibre angle
maximal fibre angle of 54.7
angles lower than 54.7
volume less than maximal, adding pressure causes angle to fall...
cylinder will shorten and thicken
angles higher than 54.7
cylinder lengthens and thins out
when at 54.7 - pressure further stiffens the cylinder
how are helical fibres are used in biological structures
as a means of controlling growth in plants and animals
as a muscle antagonist
as a way of limiting movement in worms
plants
growth of plant cells powered by tugor pressuer
expansins loosen ther fibres within the cell wall allowing shear
typically in primary walls θ is high so cell expands
shaken plants produce ethylene θ is lower so the cell expands
verterbrates
notocords
produced as a short curved structure
must legnthen and straighten as it grows
controlled by helical fibres and growth modeled by koehl
models made from nylon fabric embedded in PUE and subjected to air pressure
models behaved as predicted all straightened
fibre angle more than 55
nematodes
only have longitudinal muscles, but the high angle fibres act as muscle antagonists
muscle contraction raises pressure ...
...therefore nematodes need specialised valve mouthparts to suck up their food
unilateral contractions shorten one side and lengthen the other
:. fibres antagonize the muscles
squid mantle
squid only have circular muscles in their mantle but the low angle fibres in the outer tunic act as muscle antagonists
contractions narrows the mantle
fibres prevent lengthening, so the volume of the mantle cavity falls, expelling water for jet propulsion
nemertean and turbellarian worms both circular and longitudinal muscles
fibres just act to limit extensibility
at rest worms flattish and the fibres are held around 55
when worm lengthened or shortened the fibre angle changes and maximum volume falls
the limit depends on how flat to start with
the flatter more it can deform
this explains why leeches become unable to move after feeding , then feed and expand till maximum volume and fibres at 55
structures with the wall under compression - the gas vesicle
many cyanobacterial and holobacteria produce gas vesicles in their cells
put under compression because of hydrostatic pressure and tugor
made up of largely brick like protein in a b-sheet, laid down at 35 to the long axis
so at right angles to greatest stresses like bricks in a wall
mechanics and ecology of the gas vesicle
cyanobacterial constantly produce new gas vesicles...
regulates height in the water
:. rise to the surface
:. photosynthesis rapidly and produce sugars
:. their tugor pressure rises
:. gas vesicles will collapse
:. cyanobacteria will fall again
because the wall is constant thickness stresses will rise with diameter of the vesicle
dwellers in shallow lakes have harge efficient vesicles
dwellers of deep lakes have smaller safer vesicles
sea dwellers have no vesicles as currents are faster than vesicle powered movements