Lecture 17 - Mechanical Properties of Biological Tissue Flashcards
Salt deposits covering collagen in bone
85% calcium phosphate
10% calcium carbonate
Bone structural properties 1) 2) 3) 4) 5)
1) Non-homogenous
2) Anisotropic
3) Non-linear
4) Viscoelastic
5) Thermorhelogically complex
Why is bone an anisotropic material?
Mechanical properties are different in different directions.
Mechanical response is contingent upon both direction of force and load
Is bone tensile or compressive strength higher?
Compressive
What is viscoelastic?
Material properties are time-dependent
Can bone resist rapidly- or slowly-applied loads better?
Rapidly-applied
Number of distinct regions in a bone stress-strain curve
Three
How is a bones ability to resist applied force measured?
Bone stress-strain curve
Regions of a bone stress-strain curve
1)
2)
3)
1) Initial region (elastic zone) where if stress stops, bone returns to original shape without deformation.
2) Non-linear elastoplastic zone, where yielding occurs. Results in bone deformation.
3) Plastic zone. Permanent damage, bone fractures after this.
Bone stress-strain curve
BONESTRESSSTRAIN
How can bone stress-stain curves be constructed?
Cadaver studies.
Apply high mechanical force to a joint.
How can bone fractures be examined?
Using micro-CT scan
Comparison of bone subject to fast loading and slow loading
FASTSLOW
Strain
Elongation/Original length
Are cancellous and cortical bone very different chemically?
No. Very similar chemical compositions
Main difference between cancellous and cortical bone
Porosity. Chemical composition is very similar
Difference between compressive stress-strain plots of cortical and cancellous bone.
Cortical bone can withstand much more stress, but quickly fractures (very low strain)
Cancellous can’t withstand as much stress, but is much more ductile (more strain)
Stress versus strain
Stress is the amount of force placed on a bone.
Strain is how much a bone deforms under stress.
Outcome of a material being bale to withstand more strain
Can deform more under stress.
Modulus
Stress:strength ratio
Concentrations of elastin and collagen in skin (% of dry weight)
Elastin: 0.6 - 2.1%
Collagen: 71.9%
Concentrations of elastin and collagen in tendons (% of dry weight)
Elastin: 4.4%
Collagen: 86%
Concentrations of elastin and collagen in ligaments (% of dry weight)
Elastin: 78.4%
Collagen: 17%
Difference in stress-strain plots of collagen and elastin
Collagen can withstand more stress, but elastin is more deformable (can withstand more strain)
Why is skin elastic, even though it is mostly composed of inelastic collagen?
Collagen fibres in skin are not aligned. Elasticity is a function of stretching out the collagen fibres so that they align with one another
Collagen fibre geometry in tendons
Fibres are aligned with tensile stresses
Collagen fibre geometry in ligaments
All fibres aligned
Name for process of collagen fibres straightening out from an unaligned position under tension
Crimping
What is ‘creep’?
When a load is applied and maintained to soft tissue, deformation continues increasing with time.
Preconditioning
In soft tissue, when a load is repeatedly applied, deformation increases, although at a decreasing rate, leading eventually to stabilisation.
Hysteresis
When a systems output depends on the history of the system.
EG: preconditioning of skin.
Prestress
Direction of stress applied to skin all the time. Need to cut with lines of prestress to avoid opening of wounds.
