1. Collagen and Collagenous Tissues
Structure of collagen
Biomechanics of collagenous tissues
Testing soft collagenous connective tissues
2011: Combined Collagen and CollagenousTissues with Testing Connective Tissues
1 Feb 2001
This lecture could be improved by using pericardium and heart valves as examples of 2-D tissues and skin and cartilage as 3-D examples. I made a start on this restructuring and incorporated the heart slides into this lecture.
Could also add some slides on crosslinking from Keith’s research
And add a list of collagen/connective tissue diseases
Perhaps also something about collagen metabolism in pregnancy.

2. Collagen: Overview
Collagen is the primary structural protein in the body
Collagen is the most prevalent protein comprising ~30% of all proteins
Collagen is highly conserved between species (i.e. not undergone many evolutionary changes)
Molecules arranged in staggered pattern
X-ray diffraction or electron microscopy give rise to a banded pattern
Also relatively resistant to enzymatic breakdown
Collagen fibrillar structure

3. Collagen Molecular Structure
Triple Helix (Gly-X-Y)N
X=proline
Y=hydroxyproline
Triple helix + crosslinks: Structure give rise to a material that is very stiff and stable
Crosslinks (covalent bonds) occur between the ends (insert diagrams) of molecules

4. Collagen: Molecular Biology
>20 different collagen types have been identified
characterized by different α-chains each coded by a different gene
exons are often 54 bp long
3 bp in a codon
18 amino acids
6 sets of Gly-X-Y
Homotrimer Heterotrimer Type III=(a1(III))3 Type I=(a1(I))2a2(I) Type XI=a1(XI)a2(XI)a3(XI)

5. Collagen Types Classifications Examples
Fibrillar I Tendon, Skin, Ligament
II Cartilage
III Skin Vessels, Tendon
V Fetal Membranes - Assoc w/ Type I
VI Cartilage - Assoc w/ Type II
Fibril Associated IX Cartilage, Cornea
XII Embryonic Tendon
XIV Fetal Skin & Tendon
Network Forming IV Basement Membrane
X Hypertrophic Cartilage
VIII Descemets Membrane
Filamentous VI Vessels, Skin
Anchoring VII Anchoring Filaments
Fibrillar Collagen (I (mostly), III) has greatest stiffness

6. Material Properties Material Stiffness UTS Collagen 1000 MPa 100 MPa
Steel 200 GPa 1000 MPa
Wood 10GPa 100MPa
Rubber kPa
Bone MPa 125 MPa
Elastin kPa MPa
Silk MPa
But that's not enough information to predict behavior in tissues...
Tissues are composites
Complex organization
Complex boundary conditions

7. Ligaments and Tendon connect bones together (Ligament)
connect bones to muscle (Tendon)
Transmit forces
Aid in stabilizing joint motion
Absorb impacts/stresses
Prevent large displacements such as dislocations
Primarily uniaxial (1D) loading elements

8. Tendon Structure

9. Ligament and Tendon: Mechanical Properties

10. Ligaments Loading Fibers are parallel to load axis Organization
fascicular organization
Unloaded = crimped
loaded = straight
Composition
Collagen 75-80%
Elastin ‹5 %
Proteoglycans 1-2%

11. Knee Ligament Structure
Loaded
MCL
ACL
Unloaded

12. Age
Stress, MPa
Strain, %
Similar changes occur in collagenous tissues among individuals and most species.
Progressive increases in collagen which eventually becomes more organized and cross-linked until skeletal maturity.
This results in increased elastic stiffness and strength.
After skeletal maturity, properties begin to deteriorate.

13. Advancing Age
As a person becomes older, the maximal force their ACL can tolerate decreases, this is has as much to do with changes in geometry as it does changes in material properties

14. Immobilization
Immobilization of the knee causes deterioration of the MCL material properties, but not the ACL material properties.
MCL is metabolically more active, so as it remodels the tissue it lays down mechanically compromised material.
However, the ACL cannot produce new tissue, so it simply atrophies.

15. Skin Collagen: 65 - 70 % (more type III than ligament)
Elastin: %
Proteoglycan: %
collagen crimp decreases with age; stiffness increases
elastin crimp increases with age; decreasing recoil
A mechanical explanation for wrinkles?
Young
Adult
Old

16. Skin: Mechanical Properties
More compliant than ligament or tendon; needs to be for its functions.
orientation of coiled fibers change with load
collagen is stiffer that elastin but has greater hysteresis (absorbs more energy)

17. Ligament Tensile Testing
computer
Cross correlation
Strain computation

18. Connective Tissue Testing
Structural Properties describe the behavior of the actual tissue (e.g bone ligament bone complex)
Mechanical properties describe the behavior of the tissue as a general material

19. Clamping considerations
Device to hold tissue and clamping must be stiffer and stronger than the subject material. Otherwise the stiffness of the device contributes to what you measure.
Ligaments—have their own "built in" clamps -- bones. Usually drill holes in bone use steel rods.
Do not want to clamp too far away or elongation may include bone deformation.
Do not want it too close because may damage insertion (attachment) of ligament to bone. May weaken bone.
Tendons only have 1 "natural" clamp
Wherever you clamp, have to worry about inhomogeneities and edge effects.

20. Measurement of strain
Deformation of biological tissues is nonhomogeneous, i.e. the different regions can deform differently.
If we use the "clamp to clamp" strain the measurements would be average over the whole region
any slippage in the clamping system would also affect the measurement.
Some approaches to measuring regional strain in tissues
Imaging
Ultrasound
Strain gauges--invasive

21. Strain Gauges Sonomicrometry (piezoelectric crystals)
Mercury-in-rubber F Hg F V
Semiconductor (resistive, peizoresistive)

22. Strain Tensors: 1D example
Cauchy (infinitesimal)
Lagrangian
Eulerian

23. Stress-free state
How can we identify the best stress-free 'reference' state for the stress and strain calculations?
The soft tissues buckle under compression
Long toe region makes it difficult to identify transition from compressive to tensile forces
Solution:
Use a small tare load to repeatably identify the initial state

24. Anelastic Properties Hysteresis
Loading & unloading curves are different
Area between curves represents energy absorbed by material
Preconditioning
Apparent material properties are history dependent
Becomes repeatable with multiple cycles (in ligaments and tendons tested in vitro, this occurs between 4-7 cycles)

25. Viscoelastic properties
Stress-Relaxation
stress decreases with time but reaches an equilibrium for a step increase in strain
Creep
Strain gradually increases with time but reaches an equilibrium for a step load
Strain-Rate Effects
increased strain rate results in increased stiffness due to viscous forces
These effects are small in ligaments and tendons for the normal range of strain rates, but can be important in relation to prevention of injury

26. 6 DOF Knee Testing Rig

27. Collagenous Tissues: Key Points
Collagen is a ubiquitous structural protein with many types all having a triple helix structure that is cross-linked in a staggered array.
Some of the most common collagen types are fibrillar and the collagen can be organized in 1-D, 2-D or 3-D in different tissues to confer different material properties.
The 1-D hierarchical arrangement of stiff collagen fibers in ligaments and tendons gives these tissues high tensile stiffness
The 2-D arrangement of collagen fibers in tissues such as skin is often quite wavy or disordered to permit higher strains
Crimping, coiling and waviness of collagen matrix gives the tissue nonlinear properties in tension.
Collagen structure in tissues changes with disease & ageing.
Different tissue types require different testing configurations.