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How is collagen formed?
- 1) Pro-colloagen molecules are constructed and the ends are cleaved by enzymes.
- 2) These form alpha-chains (Gly-X-Y)
- In Glycine the R-group is a Hydrogen
- X and Y are determined by the R-group
- 3) Three alpha-chains combine in a helix.
- Glycine holds chain together. We get brittle structures without it.
- 4) X and Y determine collagen type (29 different)
- 5) Micro fibrils are constricted by cross-linking alpha-helices.
- 6) Fibrils are then made from micro fibrils
Give some types of collagen and their purpose.
- Type 1 - Ligament, tendon, bone, skin
- Type 2 - Articular cartilage
- Type 11,5,3,9 are all connective tissue.
What three things cause the change in stress-strain curve of tissues?
- 1) Composition (Collagen and Elastin)
- 2) Cross linking
- 3) Architecture
Give two reasons why tissue is elastic.
- 1) Entropic Spring - Fibrils try to be in chaos but stretch out doe to load.
- 2) Hydrophobic folding - Polar side charges repel one another causing tangling.
Describe Stress Relaxation and Creep.
- 1) Stress Relaxation
- input: Strain
- -Initial stress is high and relaxes over time
- E(t) = sigma(t)/Eo
- 2) Creep
- input: Stress
- -Initial strain is low and increases asymptotically
- D(t) = strain(t)/sigmao
- **Note Eto = 1/Dto and Etinf = 1/Dtinf but E(t) does not equal 1/D(t).
- **Note Constant part is elastic and dissipating or increasing part is viscous.
How does hysteresis effect loading.
As tissue is stretch multiple times Aloop goes down, sigma|max goes down.
**Steady state is reached after approximately 20 loads (Pre-conditioning).
Explain dynamic loading.
- 1) The stress response is delayed from the strain input by delta.
- 2) The stress response can be manipulated to have an in phase (elastic) and out of phase(viscous) part.
- -Storage Modulus = (s/e)*cos(delta)
- -Loss Modulus = (s/e)*sin(delta)
- 3) Complex modulus = (s/e)
**Delta = 0 - Perfectly elastic, Delta = pi/2 - Perfectly viscous.
Briefly explain the linear models of vicoelasticity.
- 1) Maxwell Model - Spring and Damper in Series.
- - Bad creep, Mediocre SR
- 2) Kelvin Boide - Spring and Damper in Parallel
- - Good creep, bad SR
- 3) Standard Linear Solid - (Spring and Damper in series) in parallel with another spring.
- -Good agreement with both.