The document discusses soft tissue mechanics, specifically pseudoelasticity, collagen, ligaments, tendons, and viscoelasticity. It defines pseudoelasticity as large, recoverable strains without a temperature change. Collagen is the main structural protein, with 28 types identified, though type I composes 90% of the human body. Ligaments and tendons are composed of crimped collagen fibers within a ground substance. Both ligaments and tendons exhibit nonlinear, viscoelastic stress-strain behavior and hysteresis.

1. BANGLADESH UNIVERSITY OF HEALTH SCIENCE
Department of Biomedical Engineering
BIOMECHANICS
Course Code: 04041110
Lecture 08: SOFT TISSUE MECHANICS
Date: 13/08/2023
Course Teacher: Tohfatul Jinan
Lecturer
Department of Biomedical Engineering and Medical Physics
BUHS
1

2. Pseudo Elasticity
Pseudo elasticity refers to a situation where large strains,
in excess of the elastic limit, are completely recovered
upon unloading at a constant temperature. When
mechanically loaded, a super elastic alloy deforms
reversibly to very high strains (up to 10%) by the creation
of a stress-induced phase. When the load is removed, the
new phase becomes unstable and the material regains its
original shape. Unlike shape-memory alloys, no change in
temperature is needed for the alloy to recover its initial
shape.
• A shape-memory alloy (SMA) is an alloy that can be deformed
when cold but returns to its pre-deformed ("remembered") shape
when heated. It may also be called memory metal, memory alloy,
smart metal, smart alloy, or muscle wire

3. Soft connective tissues
■ Skeletal system consists
of -
Soft connective
tissue
Bones

4. Biomechanical constituents of
soft connective tissues

5. Collagen
■ Main structural protein
■ Most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content.
■ Depending upon the degree of mineralization, collagen tissues may be rigid (bone), compliant (tendon),
or have a gradient from rigid to compliant (cartilage).
■ Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendons, ligaments
and skin. It is also abundant in corneas, cartilage, bones, blood vessels, the gut, intervertebral discs and
the dentin in teeth.
■ So far 28 types of collagen have been identified. However, 90% of the collagen in human body is type I.

6. Common types of collagen
Type I: skin, tendon, vascular ligature, organs, bone (main
component of the organic part of bone)
Type II: cartilage (main collagenous component of cartilage)
Type III: reticulate (main component of reticular fibers),
commonly found alongside type I.
Type IV: formsbasal lamina, the epithelium-secreted
layer of the basement membrane.
Type V: cell surfaces, hair and placenta

7. Structure of collagen
10 um
1 um
300 nm
1 nm

8. Structure of collagen
(Sub-unit; stabilized
by hydrogen
bonding)
(Covalently crosslinked; 50-500 nm
in diameter; consists of five basic
unit)
(Basic unit)
Collagen fiber – 50-300 um in
diameter

9. Collagen fiber
■ SEM image of a collagen fiber showing the characteristics crimp
behaviour.

10. Ligaments

11. Ligaments - examples
ACL – Anterior cruciate
ligament MCL – Medial
collateral ligament

12. Tendon

13. Fiber alignment in ligament and
tendon

14. Structural properties
■ Typical force
displacement curve
for a rabbit ligament

15. Mechanical Properties_ Tendon
■ Stress-strain curve for tendon and ligaments

16. Mechanical Properties _ Ligaments
The mechanical role of ligaments is to transmit forces from one bone to another. Ligaments
also have a stabilizing role for the skeletal joints. The composition and structure of ligaments
depend upon their function and position within the body. Like tendons they are composite
materials containing crimped collagen fibers surrounded by ground substance. As compared
to tendons, they often contain a greater proportion of elastic fibers that accounts for their
higher extensibility but lower strength and stiffness. The mechanical properties of ligaments
are qualitatively similar to those of tendons. Like tendons, they are viscoelastic and exhibit
hysteresis, but deform elastically up to strains of about €y= 0:25 (about five times as much as
the yield strain of tendons) and stresses of about σy = 5 MPa. They rupture at a stress of
about 20 Mpa
Tendons and ligaments are tough materials and do not rupture easily. Most common
damages to tendons and ligaments occur at their junctions with bones.

17. Viscoelasticity
■ In materials science and continuum mechanics, viscoelasticity is the
property of materials that exhibit both viscous and elastic
characteristics when undergoing deformation.
■ Viscous materials, like water, resist shear flow and strain linearly with time
when a stress is applied. Elastic materials strain when stretched and
immediately return to their original state once the stress is removed.
■ Viscoelastic materials have elements of both of these properties and, as
such, exhibit time-dependent strain. Whereas elasticity is usually the result
of bond stretching along crystallographic planes in an ordered solid,
viscosity is the result of the diffusion of atoms or molecules inside an
amorphous material.