Ecm And Biomechanics

Front 1

Principles of functional tissue engineering (2)

Back 1

(1) understand structure function relationships
(2) the mechanical properties of the Native tissues must be established for subfailure and failure conditions

Front 2

Epithelial Tissue function

Back 2

Skin epidermis, thyroid follicles. Lines, covers, protects, absorbs, and secretes

Front 3

Connective tissue

Back 3

Skin dermis, tendon, bone marrow, cartilage, bone, blood cells. Scaffolds for cells, nerves, blood vessels, energy storage, defence mechanism, oxygen and nutrient transport, mechanical support

Front 4

Muscle tissue

Back 4

contraction. regulates blood flow, regulates flow in the GI tract, motor functions

Front 5

ECM

Back 5

represents a complex alloy of variable members of diverse protein families defining structural integrity and various physiological functions. The primary function of ECM is to endow tissues with their specific mechanical and biochemical properties and also effects cell functions.

Front 6

What does the ECM do to cell function? (4)

Back 6

(1) Promotes cell attachment and migration (2) regulate or promote cell differentiation and gene expression levels (3) establishment of tissue microenvironment (4) sequestering, storage, and presentation of soluble regulatory molecules

Front 7

What is the ECM made of?

Back 7

Fibrous proteins (collagen, elastin, fibronectin), large aggregating sugars (proteoglycans- cement material), water

Front 8

What does collagen do?

Back 8

forms the fibrillar and microfibrillar networks of ECM basement membranes. Type I is most abundant- in skin, tendon, bone, ligaments. Type II in cartilage.

Front 9

Collagen structure

Back 9

3 strands of procollagen form a helix made which is stabilized by hydrogen bonds. Hydroxylation and Glycosylation occurs to the procollagen. Telopeptides on the ends are cleaved and tropocollagen is formed. Multiple tropocollagen mols form collagen fibrils and are crosslinked by lysyl oxidase.

Front 10

Tendon structure

Back 10

The crimping pattern structure helps with shock absorption, Allows some facicles to move without moving others. endotendon binds facicles together and allows some sliding.

Front 11

Ligaments vs. Tendons

Back 11

Ligaments: Connects bones together, augment joint stability, constrain joint motion. Tendons: Attach muscle to bone. Transmit muscle force to bone. Absorb and release energy- important for protecting from injury

Front 12

Tendon & ligament biochemistry

Back 12

-70% water, dry mass is mostly collagen with some elastin and proteoclygan. The collagen is mostly made of type I collagen. The proteoglycans include decorin, which helps the assembly of collagen into larger fibers, and aggrecan, which mediates the viscoelastic response.

Front 13

Tendon vs. ligament biomechanics

Back 13

Tendons contain parallel collagen fibrils which provide high unidirectional strength. Parallel alignment of collagen fibers is less in ligaments which enables them to withstand loads in different directions.

Front 14

Tendons can behave like...

Back 14

Springs

Front 15

Stress, strain, and hooke's law

Back 15

stress = sigma = F/A;
strain = epsilon = delta L/L0;
Hooke's law for elastic materials = sigma = E * epsilon where E is the elastic modulus

Front 16

Stress strain curve of Tendons and Ligaments

Back 16

area under the curve is proportional to toughness. Stiffness is the slope of the force/deformation curve.

Front 17

Doubling the area leads to...
Doubling the length leads to...

Back 17

...2x the stiffness, aka greater slope.
...1/2 the stiffness, smaller slope.

Front 18

Mechanical traits of tendons (4)

Back 18

(1) mechanics are dependent on the strain rate. Faster loading leads to stiffer tendons with higher capacity for energy storage.
(2) A small amount of energy is lost when they are strained, so different stress-strain curves for loading and unloading (hysteresis)
(3) The fluid within them reduces oscillations and shock by absorbing kinetic energy or dissipating it slowly (damping).
(4) if a tendon is held in a stretched position, the energy stored within the tendon will decrease over time (creep).

Front 19

the mechanical spring model

Back 19

a linear elastic solid; the stress is linearly proportional to the strain

Front 20

Mechanical dashpot model

Back 20

a linear viscous fluid element; the rate of deformation or flow is proportional to the force, where the proportionality constant is the viscosity. Also analogous to the shear stress in a viscous fluid where shear stress is linearly proportional to the rate of shear deformation and the constant is called the shear modulus (mu).

Front 21

Maxwell model of a viscoelastic fluid

Back 21

spring in series with a dashpot. This undergoes creep indefinitely- initially there is a lot of deformation but then there is none after a while.

Front 22

Kelvin model of a viscoelastic solid

Back 22

spring in series with a dashpot. The spring deformation is confined by the dashpot, there is 0 initial deformation until the limit of the spring is released.

Front 23

Stiffness mismatch

Back 23

Tendon has to overcome stiffness mismatch which is when two mechanically different tissues are joined, resulting in stress concentrations where injury is most likely to occur

Front 24

Musculo-tendinous junction

Back 24

the link between myofibers that generate the force in muscle and collagen fibers that transmit the forces to bone to effect locomotion. There isn't a clear cut change from muscle to tendon- increases SA toreduce stress

Front 25

Osteo-tendinous junction

Back 25

tendon inserts into bone across a fibrocartilaginous transition

Front 26

Which has a higher elastic modulous, bone or tendon?

Back 26

Bone (20 GPA). tendons or ligaments have about 200 MPa

Front 27

Functions of cartilage (5)

Back 27

Joint lubrication, low-friction surface, load distribution, joint congruity, shock absorption

Front 28

What is cartilage made of?

Back 28

water, Type II collagen, also Type 9 which is a crosslinker and type 11 which helps maintain spacing between type II collagens.

Front 29

Structure of articular cartilage

Back 29

Superficial zone- horizontal collagen fibers, high water content. Middle- mixed collagen fiber orientation, high proteoglycan content. Deep- closest to bone, vertical collagen fibers

Front 30

Mechanics of the different zones in articular cartilage

Back 30

Surface- high flow in & out, fluid pressure/tension, large compressive strains, tensile surface strains.
Transitional zone- low fluid flow, mainly fluid pressure, moderate compressive strains. Radial zone- little fluid flow, fluid pressure, small compressive strains. Noncompressible.

Front 31

Anisotropic

Back 31

There are different properties of the material in different directions. For cartilage, collagen fibers resist tension in the same direction but break and perpendicular tensile forces.

Front 32

Proteoglycans structure and functions

Back 32

Made up of a hyaluronan backbone, glycosaminoglycans are attached wit hlink proteins. They are branch like structures which are negatively charged and can trap water. The water binds with electrostatic charges and it takes a lot of force to push the water out.

Front 33

Properties of Articular cartilage (6)

Back 33

(1) resists tensile (2) compressive (3) shear stresses, (4) Permeability of water (5) swelling, and (6) lubrication which can be tested with friction tests

Front 34

Models of articular cartilage (4)

Back 34

(1) AC is elastic- time dependent properties (2) single phase viscoelastic- flow independent and time-dependent (3) Biphasic- accounts for the interactions between the solid and interstitial fluid phases of the ECM, flow dependent and time dependent behaviors. (4) multiphasic- biphasic effects plus: electrical effects, chemical swelling, and transport of nutrients

Front 35

3 biphasic testing configurations

Back 35

(1) (Confined- determines bulk properties- piston pushes down on a cartilage sample which is enclosed and water can only escape in one direction- makes the problem into 1-D. (2) Unconfined- cartilage sample in between two plates, water can escape from sides and bulging can occur- not representative of tissue. (3) Indentation- allows to measure localized properties of cartilage- can create a topographical map of properties

Front 36

Confined compression of cartilage

Back 36

Can measure creep. Force is held constant over time, monitor deformation. This relates to the permeability of cartilage and compressibility of tissue. Normal AC has low permeability, diseased has high . During creep, the load at the surface is equal to the compressive stress + frictional drag due to fluid exudation which is releated to permeability as water leaves. at equilibrium, this is only due to compressive stress, which is related to aggregate compressive modulus (Ha) which is representative of the elasticity of the material.

Front 37

Creep problem set up

Back 37

k * Ha * u''(z) = u'(t). where k is the permeability, Ha is the aggregate modulus, and u (z,t) is the creep deformation. Boundary conditions: (z=0 at the top, z=h at the bottom) fluid pressure is 0, u(h,t) = 0 (no deformation at the bottom) and u(z,0)=0. As you increase the depth (towards the bottom) creep decreases. creep is mostly due to permeability

Front 38

How do you calculate Ha from creep model?

Back 38

set z=0, t=infinity. u(0, infinity) is equal to F0 *h/ (A* Ha)

Front 39

As Ha increases...

Back 39

the material is stiffer, and deformation decreases

Front 40

As k (permeability) increases...

Back 40

...the ability of water to leave increases and creep increases.