LECTURE 7 (Elastic properties of Biomaterials) Flashcards
What is Biomechanics?
Studies the development of mechanical processes in living organisms at various levels
What is Elasticity?
The property of a body that tends to return the body to its original shape after the force is removed
What happens when a force is applied to a body and when the force is sufficiently large?
FORCE APPLIED TO A BODY:
shape + size of body change -> depending how force is applied, body is stretched, compressed, bent or twisted
FORCE SUFFICIENTLY LARGE:
body is distorted beyond its ELASTIC LIMIT -> original shape not restores after removal of force
What is a force applied to the body resisted by?
The cohesive force that holds the material together
What is Stress?
The internal force per unit acting on the material
S = F/A
[F = force, A = area]
What is Strain?
The fractional change in length
ST ≡ ΔL/ L
[ΔL = elongation, L = length]
What does Hooke’s law state?
While the body remains elastic, the ratio of stress to strain is constant
S/ST = Y
[Y = Young’s modulus]
What is Young’s modulus?
The Young’s modulus (E) is a property of the material that tells us how easily it can stretch and deform and is defined as the ratio of tensile stress (σ) to tensile strain (ε)
Describe the Young’s modulus graph
- X axis = Strain
- Y axis = Stress
- Elastic limit = point where graph starts to curve
- Breaking point = point where graph starts to plateau
- Elastic behaviour = where graph is a straight line
Describe the equation of Elasticity of spring
F = K Δ L
Force (F) required to stretch/compress the spring is directly proportional to the amount of stretch
What is E in E = 1/2 K (ΔL)2?
The amount of energy stores in a stretched or compressed body
What is the equation of an elastic body under stress
F = YA/ L * Δ L
What energy considerations do we make to determine bone fracture?
- Maximum energy that parts of the body can safely absorb (allows to estimate the possibility of injury under various circumstances)
- Assume that bone remains elastic until fracture
What does a material’s deformation depend on?
- Elasticity
- Viscosity
- Plasticity
- Solidity margin
- Stretching
What does a material’s elasticity determined by?
The rate of thermal energy which is utilised during deformation-relaxation
What is Elasticity used in medicine for?
Strength calculations of skeleton and cartilage structures of tendons, muscles, blood vessels and alveoli
What is the equation for Muscle Elasticity?
ΔL/L= -1/Y·F/S
ΔL = elongation
L = muscle initial length
F = force causing deformation
S = muscle transverse cutting area
F/S = muscle mechanical tension
What is Tonicity?
Muscle tension in resting condition
Explanation: The maintenance of tonicity of the resting muscle provisos stabilisation of bones and cartilages
What is the difference between Isotonic and Isometric?
Isotonic = muscle contracts and changes length
- concentric - shortening of muscle
- eccentric - lengthening of muscle
Isometric = muscle contracts but does not change length
- stabilises posture + holds body upright
How is skeletal muscle attached to bones?
Through tendons to create a lever system -> lever allows a given effort to move a heavier load or to move a load farther and faster
Describe the three types of levers
FIRST CLASS:
- resistance on LEFT (down)
- pivot in MIDDLE
- effort in RIGHT (down)
[mechanical advantage for strength + mechanical disadvantage for speed & distance)
SECOND CLASS:
- pivot on LEFT
- resistance in MIDDLE (down)
- effort in RIGHT (up)
[levers of strength]
THIRD CLASS
- pivot on LEFT
- effort in MIDDLE LEFT (up)
- resistance on RIGHT (down)
[speedy + always operate at a mechanical disadvantage]
Explanation: effort farther than load from fulcrum = lever operates at a mechanical advantage, effort nearer than load from fulcrum = lever operates at a mechanical disadvantage
What are the 4 main stages of the Respiratory process?
1) Ventilation - mechanical movement of air in and out of lungs
2) Gas exchange between lungs and blood
3) Transport of O2 and CO2 by blood
4) Tissue respiration
(gas exchange between tissues and blood)
What is the gas movement in respiratory system determined by?
- concentration gradient
- mechanical pressure of air
What is the Boil-Marriott law?
The relationship between pressure and volume
P1V1= P2V2 =const
Explanation: Internal volume of lung is increased -> pressure is reduced + atmospheric air gets into lung -> as pleural cavity is isolated from atmosphere chest extension influences air pressure value of INTRAPLEURAL (low) + INTERNAL SPACE of lungs
What is Transmural pressure?
Difference in pressure between two sides of a wall during the ventilation of lungs which depends on resistance of respiratory ways and lungs’ extensibility
What is resistance of respiratory ways determined by?
- Internal frication force between gas molecules
- Friction forces between molecules and walls of respiratory ways
- Airflow (laminar, turbulent or transitional)
What is Poiseuille’s law?
The relationship between flow intensity and change of air pressure for air laminar flow
When is resistance is the respiratory system determined?
- Sliding of lung and chest tissues towards each other
- Lung tissue extension
- Air movement in respiratory ways
[as surface area increases -> resistance decreases + smaller diameter -> laminar flow not turbulent flow]
When is Resistance in Respiratory ways increased?
- Tightening of airways during asthma, parasympathetic stimulation and smoking
- Pulmonary fibrosis (decrease in diameter)
- Density + viscosity of exhaled air contributes to conversion of air into turbulent flow (asthma, cystic fibrosis)
What is Tensility?
The size of elasticity of the respiratory system and is determined by the ability of respiratory system to change the volume by influence of pressure
C = ΔV/ΔP
Total tensility of respiratory system determined by
- tensility of lungs
- wall of chest
- alveolus
TENSILITY DEPENDS ON:
- characteristic of connective tissue
- reduce w/ age increase + fibrosis of lungs
- increase when EMPHYSEMA (extension of distal bronchi with destructive-alveolar changes in alveolar walls)
What is Air flow?
Change in volume per unit of time
J = ΔV/ Δt
Explanation: As lungs’ elasticity changes, air flow intensity is also changed
What is Surface tension measured in?
Force per unit length of Dyne per centimetre
What is Laplace equation?
Δp = 2γ /r
Explanation: The internal pressure of a vesicle is proportional to the surface tension of the liquid (γ) which forms it and is inversely proportional to its radius (r)
Explains why surface tension tends to decrease the surface of a droplet -> spherical shape -> decrease in inner volume -> generates pressure against included air
What is the importance of Pulmonary surfactant?
Without surfactant, pressure in smaller alveoli would be higher than in larger ones -> collapse of smaller alveoli in favour of larger one -> larger alveoli will inflate + burst -> decrease in surface area of alveoli -> decrease in gas exchange
Surface tension of water-air interface is strongly reduced by Pulmonary surfactant -> pressure is equalised in the large and small bubbles
Which law states that if two bubbles have the same surface tension, the smaller bubble will have higher pressure?
Laplace’s law
What are the properties of Surfactant?
- Consist of phospholipids, glycolipids, neutral lipids, cholesterol, glycoproteins and hydrophobic associated proteins
- Secreted by TYPE 2 alveolar cell
- Forms monomolecular layer on the surface of alveoli
What are the functions of Pulmonary Surfactant?
- Decrease surface tension value -> increases tensility
- Reduces minimum pressure in pulmonary cavity that is essential for lungs opening
- Provides alveoli stabilisation
- Prevents alveoli collapse under conditions of low air pressure
- Provides maintenance of dryness
- Reduces energy necessary for lung expansion during breathing
- Prevent pulmonary Oedema
What is the point called In Surfactants when surface tension does not get lower?
Critical Micelle Concentration
Describe how Surfactant deficiency leads to Respiratory Distress Syndrome in premature infants
Surfactant deficiency -> Surface tension induces fluid infiltration from capillaries into alveoli -> Pulmonary oedema
