Downloaded 157 times
More Related Content
Similar to 9 Viscoelasticity and biological tissues
9 Viscoelasticity and biological tissues
- 1.
- 2. Define viscoelasticstress/strain and time – dependent relationships, and compare for different materials Define viscoelastic behaviors (creep and stress relaxation) and compare for different biologic materials (muscle, ligament, tendon, cartilage) Identify tissue structures and components that contribute and/or explain viscoelastic properties for different biologic materials
- 4. Examples: ◦ Instantdeformation under load ◦ Deformation is recovered σ = E ε
- 5. Not instantaneousdeformation Deformation not recovered σ = μ ἐ
- 6. Viscoelastic =viscous + elastic behavior ◦ Instantaneous and delayed deformation ◦ Some deformation is recovered, some is not
- 7.
- 8. Difference increep behavior of rubber band and electrical tape demonstrates the concept Rubber band Electrical tape Weights (apply constant force)
- 11. Force gauge(like a fish scale) applies constant displacement, measures resulting load Rubber band Electrical tape Force gauge (apply constant displacement)
- 17. All biologicaltissues exhibit viscoelastic behaviors (hysteresis, creep, stress relaxation) ◦ Elastin fibers ◦ Collagen ◦ Smooth Muscle Different tissues contain different amounts or fractions of collagen and elastic fibers resulting in different mechanical properties ◦ Tendon ◦ Ligament ◦ Intestinal wall
- 18. Different tissuescontain different amounts or fractions of collagen and elastic fibers resulting in different mechanical properties (tendon, ligament, arteries)
- 19. Triple-helical structurestabilized by hydrogen bonds (see Fig 1.8 in textbook) Individual fibers surrounded by gel- like ground substance (mainly water) ◦ Combination results in viscoelastic behavior Fibers are crimped; crimp stretches out under load
- 20.
- 21. Elastin +microfibrillar proteins = elastin fibers Behave like rubber ◦ Low modulus (lower than collagen) ◦ Elastic behavior: very extensible and reversible deformation even under high strains Found in ◦ Blood vessels ◦ Lungs ◦ Skin From http://helpfromthedoctor.com/blog/20 10/07/27/what-is-a-protein/
- 22.
Editor's Notes
- #4 Stress/strain curves for metal, soft tissue and rubber/elastomers (from pp. 81-87 in text); strains are orders of magnitude higher for soft tissues and elastomers
- #5 An instantaneous increase in applied force is known as a step-function (See force-time curve for step function above); resulting deformation curve is also a step function for elastic materials Mechanical analogy for elastic material: spring Characterized by straight stress/strain curve, elasticity (constant elastic modulus, or slope of stress/strain curve), no energy loss when loaded and unloaded Metal is linear (direct relation between stress and strain) and elastic – it responds almost instantaneously under load and there is no energy dissipated when it deforms. Bone and teeth are linear under small strains (within physiologic ranges); there are no changes in microstructure, so no change in properties.
- #6 Force-time curve for step function on top; below is resulting deformation curve for viscous material (linearly increasing deformation from t0 to t1, flat line after t1) Mechanical analogy for viscous material: dashpot Curved stress/strain curve; typically shown as stress and strain rate (change in strain with time); viscous materials have a constant strain rate under constant load
- #7 - Soft tissues and elastomers are non-linear under stress with no permanent change in structure; this is due to their long-chain polymeric structure. Specific behavior of different tissue types depends on underlying conformations of molecules (inherent order or disorder). Elastin and collagen dominate soft tissue behavior; soft tissue is “nearly elastic” or “pseudoelastic”, meaning it responds almost instantaneously under load, but exhibits hysteresis (different loading and unloading curves due to energy loss). Hysteresis is due to movement of the structural proteins within the viscous ground substance (dominated by proteoglycans). Bones experience small strains (less than .001 during walking) which are necessary for balanced osteoblast/osteoclast activity (compressive strain); higher strain = higher bone production and lower removal of bone, eventually leading to microdamage, which may stimulate growth/healing response. Extremely low strains (unloading) leads to net loss of bone. Strain response to applied stress depends on strain rate (similar to loading rate); strain rate sensitivity is a characteristic of time-dependent behavior of viscoelastic materials.
- #8 Refer to Humphrey and Delange Ch 11.4 for a thorough explanation of creep and stress relaxation curves. Creep: apply a constant force, and resulting deformation is time-dependent. Stress relaxation: apply a constant deformation, and resulting stress within the material is time-dependent.
- #9 Test set up for measuring creep (known load, measure deformation over time); different for electrical tape vs. rubber band Apply a constant stress and measure resulting strain Strain (creep) will grow with time Modulus is time-dependent Unlike elastic materials - under fixed stress elastic materials will reach a fixed strain and stay at that level
- #10 Creep – time-dependent deformation under a constant load; (Sort of the inverse of stress relaxation); refers to the general characteristic of viscoelastic materials to undergo increased deformation under a constant stress, until an asymptotic level of strain is reached
- #11 Creep –constant load (step function for force-time curve); increased deformation to asymptotic level of strain, as shown in stress/strain figure
- #12 Test set up for measuring stress relaxation (known deformation, measure change in force over time) Apply constant strain and measure resulting stress Stress decreases with time (material relaxes) Modulus is time-dependent Unlike elastic materials - under fixed strain, elastic materials will reach a fixed stress and stay at that level with no relaxation
- #17 Stress relaxation – time-dependent decrease in load at a constant deformation; refers to the behavior of stress reaching a peak and then decreasing or “relaxing” over time under a fixed level of strain, as shown in stress/strain figure
- #18 The more viscous a material, the more it relaxes and creeps; more strain rate dependency Different relative fractions of collagen and elastin in each tissue Tendons (muscle – bone) – more collagen than elastin by weight compared to ligament Ligaments (bone – bone) – more elastin than collagen by weight compared to tendon
- #19 Collagen in tendon, ligament, skin, cartilage, bone, cornea, blood vessels – most abundant protein in the body (~25-30%) Not all collagen is the same (29 types – or more!); most common are I, II, III and IV I and III – fibers – structural support in tension; found in tendons, skin, bone, hear, arteries and cornea II – fibrils – cartilage (which also has lots of proteoglycans) IV – sheet; porous network that forms basement membrane – scaffolding for epithelial and endothelial cells – inner layer of blood vessels
- #21 Three distinct regions of stress-strain curve for collagen 1 – Toe region – non-linear, physiological level loading 2 – Linear elastic 3 – Yield 4 - Failure At rest: randomly oriented molecules – high disorder, higher entropy Under stretch: more oriented – lower disorder, lower entropy, increased stress (like a spring – energy is stored in stretched fiber, which is released to restore crimp when load is removed)
- #22 - Damage to elastin: - in blood vessels – aneurysm - in skin – wrinkles (often due to sun damage) - in lung - emphysema
- #23 Elastin contributes to toe region Collagen contributes to stiffer slope of stress-strain curve