9 Viscoelasticity and biological tissues

Viscoelastic behavior of biological tissues

1. Viscoelasticity and Biological Tissues BIOE 3200 Biomechanics

2. Define viscoelastic stress/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 Learning Objectives:

3. Review: Constitutive Relationships

4. Examples: Instant deformation under load Deformation is recovered σ = E ε Elastic behavior

5. Not instantaneous deformation Deformation not recovered σ = μ ἐ Viscous behavior

6. Viscoelastic = viscous + elastic behavior Instantaneous and delayed deformation Some deformation is recovered, some is not Viscoelasticity defined

7. Time-dependent behavior Viscoelasticity: Creep and Stress Relaxation Behaviors

8. Difference in creep behavior of rubber band and electrical tape demonstrates the concept Creep behavior can be demonstrated experimentally Rubber band Electrical tape Weights (apply constant force)

9. Force gauge (like a fish scale) applies constant displacement, measures resulting load Stress relaxation behavior can be demonstrated experimentally Rubber band Electrical tape Force gauge (apply constant displacement)

10. Stress Relaxation Behavior of Biological Materials (Soft Tissues)

11. All biological tissues 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 Comparing viscoelastic behaviors of different tissues

12. Different tissues contain different amounts or fractions of collagen and elastic fibers resulting in different mechanical properties (tendon, ligament, arteries) Tissue components that contribute to viscoelastic behavior

13. Triple-helical structure stabilized 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 Collagen

14. Collagen From http://cdn.intechopen.com/pdfs/22189/InTech-Biomechanics_and_modeling_of_skeletal_soft_tissues.pdf

15. 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 S kin Elastin From http://helpfromthedoctor.com/blog/2010/07/27/what-is-a-protein/

16. Combined effects in biological structures From http ://www.astarmathsandphysics.com/a_level_physics_notes/medical_physics/a_level_physics_notes_medical_physics_stress_and_strain_in_blood_vessels.html