biomaterials 1

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BIOMATERIALS UNIT 1 LECTURE NOTES PREPARED BY V.MYTHILY,ASST.PROFESSOR,JCE BIOMATERIALS- INTRODUCTION A > Definition of Biomaterials (2 Marks) Bio material- It is a non viable material used in a medical device, intended to interact with biological system. A biomaterial is defined as any systemically, pharmacologically inert substance or combination of substances utilized for implantation within or incorporation with a living system to supplement or replace functions of living tissues or organs. Materials used to safely replace or interact with biological systems A biomaterial is a synthetic material used to replace part of a living system or to function in intimate contact with living tissue A biomaterial is a systemically and pharmacologically inert substance designed for implantation within or incorporation with living systems.” A biomaterial is material of synthetic as well as of natural origin in contact with tissue, blood, and biological fluids, and intended for use for prosthetic, diagnostic, therapeutic, and storage applications without adversely affecting the living organism and its components

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Page 1: BIOMATERIALS 1

BIOMATERIALS UNIT 1 LECTURE NOTES

PREPARED BY V.MYTHILY,ASST.PROFESSOR,JCE

BIOMATERIALS- INTRODUCTION

A > Definition of Biomaterials (2 Marks)

Bio material- It is a non viable material used in a medical device, intended to interact with biological system.

A biomaterial is defined as any systemically, pharmacologically inert substance or combination of substances utilized for implantation within or incorporation with a living system to supplement or replace functions of living tissues or organs.

Materials used to safely replace or interact with biological systems

A biomaterial is a synthetic material used to replace part of a living system or to function in intimate contact with living tissue

A biomaterial is a systemically and pharmacologically inert substance designed for implantation within or incorporation with living systems.”

A biomaterial is material of synthetic as well as of natural origin in contact with tissue, blood, and biological fluids, and intended for use for prosthetic, diagnostic, therapeutic, and storage applications without adversely affecting the living organism and its components

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A biomaterial is any substance (other than a drug), natural or synthetic, that treats, augments, or replaces any tissue, organ, and body function.

Examples- sutures, needles, catheters, plates, tooth fillings, etc

Need for biomaterials (2 Marks)

replacement of body part that has lost function (total hip, heart)

correct abnormalities (spinal rod)

improve function (pacemaker, stent)

assist in healing (structural, pharmaceutical effects: sutures, drug release)

HISTORY OF BIOMATERIALS

– 1860's: Lister develops aseptic surgical technique

– early 1900's: Bone plates used to fix fractures

– 1930's: Introduction of stainless steel, cobalt chromium alloys

– 1938 : first total hip prosthesis (P. Wiles)

– 1940's: Polymers in medicine: PMMA bone repair; cellulose for dialysis; nylon sutures

– 1952: Mechanical heart valve

– 1953: Dacron (polymer fiber) vascular grafts

– 1958: Cemented (PMMA) joint replacement

– 1960: first commercial heart valves

– 1970's: PEO (polyethyleneoxide) protein resistant thin film coating

– 1976: FDA ammendment governing testing & production of biomaterials /devices

– 1976: Artificial heart (W. Kolff, Prof. Emeritus U of U)

Early biomaterials:

– Gold: Malleable, inert metal (does not oxidize); used in dentistry by Chinese, Aztecs and Romans--dates 2000 years

– Iron, brass: High strength metals; rejoin fractured femur (1775)

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– Glass: Hard ceramic; used to replace eye (purely cosmetic)

– Wood: Natural composite; high strength to weight; used for limb prostheses

– and artificial teeth

– Bone: Natural composite; uses: needles, decorative piercings

– Sausage casing: cellulose membrane used for early dialysis (W Kolff)

– Other: Ant pincers. Central American Indians used to suture wounds

GENERATION OF IMPLANTS

First Generation Implants

• “ad hoc” implants

• specified by physicians using common and borrowed materials

• most successes were accidental rather than by design Examples — First Generation Implants

• gold fillings, wooden teeth, PMMA dental prosthesis • steel, gold, ivory, etc., bone plates • glass eyes and other body parts • dacron and parachute cloth vascular implants

Second generation implants

• engineered implants using common and borrowed materials

• developed through collaborations of physicians and engineers

• built on first generation experiences

• used advances in materials science (from other fields) Examples — Second generation implants

• titanium alloy dental and orthopaedic implants • cobalt-chromium-molybdinum orthopaedic implants • UHMW polyethylene bearing surfaces for total joint replacements • heart valves and pacemakers Third generation implants

• bioengineered implants using bioengineered materials

• few examples on the market

• some modified and new polymeric devices

• many under development

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Example - Third generation implants

• tissue engineered implants designed to regrow rather than replace tissues • Integra LifeSciences artificial skin • Genzyme cartilage cell procedure • some resorbable bone repair cements • genetically engineered “biological” components (Genetics Institute and Creative

Biomolecules BMPs) APPLICATIONS OF BIOMATERIALS (5 MARKS)/16 MARKS

EXAMPLES OF USES OF BIOMATERIALS

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MATERIAL ATTRIBUTES FOR BIOMEDICAL APPLICATIONS/ BASIC REQUIREMENT OF MATERIAL FOR USE AS IMPLANT (4 marks)

Biocompatibility Noncarcinogenic, nonpyrogenic, nontoxic, nonallergenic, blood compatible, non-inflammatory

Sterilizability Not destroyed by typical sterilizing techniques such as autoclaving, dry heat, radiation, ethylene oxide

Physical characteristics Strength, elasticity, durability

Manufacturability

Biofunctionality

Bioinert

longevity

Machinable, moldable, extrudable

Note: Biocompatibility is the ability of material to perform within an appropriate host response in a specific application.

Biocompatibility in other words is the quality of not having toxic or injurious effects on biological systems

The biomaterials/devices must not degrade in its properties within the body (unless this is wanted).

The biomaterials/devices (and any degradation product) must not cause any adverse reaction within the host´s body.

BIOCOMPATIBILITY

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. Biocompatibility testing include procedures designed to evaluate: � cytotoxicity; � acute, subchronic, and chronic toxicity; � irritation to skin, eyes, and mucosal surfaces; � sensitization; � hemocompatibility; � short-term implantation effects; � genotoxicity; � carcinogenicity; and effects on reproduction, including developmental effects.

BIOACTIVITY VS. BIOCOMPATIBILITY

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Biocompatibility :

Objective is to minimize inflammatory responses and toxic effects

Bioactivity - Evolving concept:

The characteristic that allows the material to form a bond with living tissue (Hench, 1971)

The ability of a material to stimulate healing and trick the tissue system into responding as if it were a natural tissue (Hench 2002).

Advantages: Bone tissue – implant interface, enhanced healing response, extends implant life

Biodegradability:

Breakdown of implant due to chemical or cellular actions

If timed to rate of tissue healing transforms implant to scaffold for tissue regeneration

Negates issues of stress shielding, implant loosening, long term stability

FUNCTIONAL PERFORMANCE / BIOMEDICAL APPLICATIONS (16 MARKS)

Load transmission and stress distribution

(e.g. bone replacement)

– Articulation to allow movement

(e.g. artificial knee joint)

– Control of blood and fluid flow

(e.g. artificial heart)

– Space filling

(e.g. cosmetic surgery)

Electrical stimuli

(e.g. pacemaker)

– Light transmission

(e.g. implanted lenses)

– Sound transmission

(e.g. cochlear implant)

-Improved wound healing (e.g., sutures, wound dressings)

-Enhanced performance of medical devices (e.g., contact lenses, pacemakers)

-Correct functional abnormalities (e.g., spinal rods)

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-Correct cosmetic problems (e.g., reconstructive mammoplasty, chin augmentation)

-Aid in clinical diagnostics (e.g., probes and catheters)

-Aid in clinical treatments (e.g., cardiac stents, drains and catheters)

-Design biodegradable scaffolds for tissue engineering (e.g., dermal analogs)

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Skeletal system

• Joint replacements -Titanium, stainless steel, polyethylene

• Plate for fracture fixation - Stainless steel, cobalt-chromium alloy

• Bone cement - Poly(methyl methacrylate)

• Artificial tendon and ligament-Teflon, Dacron

• Dental Implants-Titanium,alumina,calcium phosphate

Cardiovascular system

• Blood vessel prosthesis - Teflon, Dacron, Polyurethane

• Heart valve -Reprocessed tissue, Stainless steel, carbon

• Catheter - Silicone rubber, Teflon, polyurethane

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B> CLASSIFICATION OF BIO-MATERIALS (16 Marks)

1.Based on medical uses:

2.Based on the interaction of material and tissue

Bioinert -stainless steel, titanium

Bioresorbable- tricalcium phosphate , calcium carbonate

Bioactive- synthetic hydroxyapatite

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Inert biomaterials: Implantable materials with little or no counter reaction from the body. Interactive biomaterials: Implantable materials designed to elicit a specific benign tissue reaction, such as integration, adhesion, etc. Living biomaterials: Implantable materials that possibly contain living cells at time of implantation, regarded by the host tissue as tolerable tissue, and are actively resorbed and/or remodeled. Replacement biomaterial: Implantable materials made of living tissue that has been cultivated from the patient own cells outside the body.

2.1. BIOINERT BIOMATERIALS

The term bioinert refers to any material that once placed in the human body has minimal interaction with its surrounding tissue.

Examples of these are stainless steel, titanium, alumina, partially stabilized zirconia, and ultra high molecular weight polyethylene.

Generally a fibrous capsule might form around bioinert implants hence its biofunctionality relies on tissue integration through the implant.

2.2. BIOACTIVE BIOMATERIALS

Bioactive refers to a material, which upon being placed within the human body interacts with the surrounding bone and in some cases, even soft tissue.

This occurs through a time –dependent kinetic modification of the surface, triggered by their implantation within the living bone .

An ion-exchange reaction between the bioactive implant and the surrounding body fluids-results in the formation of a biologically active carbonate apatite (CHAp) layer on the implant that is chemically and crystallographically equivalent to the mineral phase in bone.

Prime examples of these materials are synthetic hydroxyapatite [Ca 10 (PO4)6(OH)2], glass ceramic and bioglass.

2.3. BIORESORBABLE BIOMATERIALS

Bioresorbable refers to a material that upon placement within the human body starts to dissolve and slowly replaced by advancing tissue (such as bone).

Common examples of bioresorbable materials are tricalcium phosphate [Ca3(PO4)2] and polylactic- polyglycolic acid copolymers.

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Calcium oxide, calcium carbonate and gypsum are other common materials that have been utilized during the last three decades.

3. Material-classification based on wettability and uptake of water

4. Based on material properties

Biomaterials can be divided into three major classes of materials:

Metals

Ceramics (including carbons, glass ceramics, and glasses).

Polymers

Composites

4.1. METALS

Exhibit metallic bonding in the solid state. Mixtures or solutions of different metals are alloys. 85% have one of these crystal structures:

Metals Are Crystalline

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Every atom (ion) is surounded by 12 (FCC, HCP) or 8 (BCC) neighbors. Body-Centered Cubic

Metal Bonding The electrons in metals are mobile and surround a core of cations. This gives rise to their high electrical conductivity

Crystals and Grain Formation

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There are 4 main methods of metal product manufacture:

machining

melt casting

forging

hot isostatic pressing

Grain or Crystalline Structure

Crystals

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4.2.POLYMERS

Carbon atoms are usually joined in a linear chainlike structure and substituted with a great variety of atoms, molecules or functional groups. Material behavior is more a factor of molecular features than interatomic bonds.

Very long chain molecules of monomers. – The mer (monomer) is the basic unit. – Molecule containing one or more atoms that can each participate in two or more covalent bonds.

Covalent bonding forms the chain structure: – Fixed bond length. – Permits rotation of adjacent atoms about its axis. – When atoms (carbon) form multiple covalent bonds, the size of the angles between the bonds involving a particular atom is fixed.

Other covalent, hydrogen or van der Waals bonds link between chains: – Wide variety of conformations possible. Polymers Two (3) classes of polymers:

– Thermoplastic – Thermosetting – Degradable nd non biodegradable

Thermoplastic polymers: Basic chains with little or no branching; can be melted and remelted without a basic

change in structure.(=crystalline) • “Straight” - have little or no branching

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• Can be melted and remelted • Will reform into similar structure

Thermosetting polymers:

Side chains form covalent links between chains (threedimensional network); do not melt uniformly on reheating.

(= chemically cross-linked) • Side chains are present • Side chains form links between chains • Will not reform equally upon remelting

ex– Elastomers Polymers � Non-degradable � Biodegradable

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4.3.CERAMICS-CRYSTALLINE Ceramics are refractory polycrystalline compounds

Usually inorganic Highly inert Hard and brittle High compressive strength Inorganic, non-metallic materials: Generally good electric and thermal insulators Good aesthetic appearance Requires specific organization of covalently bonded atoms. Leads to complex crystal or mixed structures. Combined cation anion structure results in mechanical and thermal/electrical

properties: Resistant to high temperatures and severe environment. Those materials that aren’t metals or polymers Most contain one or more metallic oxides along with other compounds. May have varied phases: Crystalline, single or poly crystalline, or amorphous Ceramics are: Stiff, Hard Chemically stable Wear resistant

Applications:

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orthopaedic implants dental applications

compromise of non-load bearing for bioactivity Ceramics

Classification of Ceramics 1. Bioinert Bioceramics

(nearly inert crystalline ceramics) Elicit minimal response from host tissue Forein body response = encapsulation Undergo little physical/chemical alteration in vivo

Example Alumina (Al2O3 >99.5% pure)

Carbon (diamond) – Partially stabilised zirconia (ZrO2) – Silicon nitride (Si3N4) Functional properties

» High compressive strength » Excellent wear resistance » Excellent bioinertness

High density , high purity (>99.5%) alumina Used for >30 years Very chemically inert Excellent corrosion resistance High wear resistance, but Low fracture toughness and tensile strenght (high

elastic moculus) Used in compression only (to reduce encapsulation thickness)

– Femoral head of total hip replacements – Orthopedic implants in general – Dental implants

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Smaller the grain size and porosity, higher the strength

E = 380 GPa (stress shielding may be a problem) High hardness: Low friction Corrosion resistance

Friction: surface finish of <0.02 um Wear: no wear particles generated – biocompatible

Applications » orthopaedics: » femoral head » bone screws and plates » porous coatings for femoral stems » porous spacers (specifically in revision surgery) » knee prosthesis » dental: crowns and bridges

Advantages Bioinertness -Results in biocompatibility – low immune response Disadvantage:

» Minimal bone ingrowth » Non-adherent fibrous membrane » Interfacial failure and loss of implant can occur

2. Bioactive Ceramics: Glass Ceramics & Calcium phosphate

Refractory compounds/materials.

Usually some combination of metal and nonmetal in general AmXn structural form (A = metal; X = nonmetal)

Relative size of ions (radius ratio) and degree of covalent/ionic bonding determine atomic arrangements.

High oxidized state and ion/covalent bonding in ceramics makes them: – Resistant to oxidation and increases stability – Nonconducting – High melting temps – Hard and brittle

Common characteristics: – time-dependent modification of the surface → formation of a biologically active carbonated HA layer (hydroxy apatite) that provides the bonding interface with tissue.

Generally used to repair or replace skeletal hard connective tissue. No one material is suitable for all biomaterial applications.

– Their success depend upon achieving a stable attachment to connective tissue.

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– Tissue attachment is directly related to the type of tissue response at the implant-tissue interface.

` –No material implanted is inert; all materials elicite a response from the tissue - Highly reactive:

– Can be resorbed by exposure to biological environment: Example: Glass:

Glass-ceramic is a polycrystalline solid prepared by controlled crystallization of glass

an inorganic melt cooled to solid form without crystallization an amorphous solid Possesses short range atomic order Brittle! Glass ceramics were the first biomaterials to display bioactivity (bone

system): • Capable of direct chemical bonding with the host tissue • Stimulatory effects on bone-building cells

Composition includes SiO2, CaO and Na2O • Bioactivity depends on the relative amounts of SiO2, CaO and Na2O • Cannot be used for load bearing applications • Ideal as bone cement filler and coating due to its biological activity

Calcium phosphate Surface reactive ceramics: Sites for oxidation or protein bonding on surfaces.

Calcium (Ortho) Phosphate Structure resembles bone mineral; thus used for bone replacement

7 different forms of PO4 based calcium phosphates exist - depend on Ca/P ratio, presence of water, pH, impurities and temperature

• Powders • Scaffolds • Coatings for implants – metals, heart valves to inhibit clotting • Self-Setting bone cement

Uses

repair material for bone damaged trauma or disease void filling after resection of bone tumors repair and fusion of vertebrae repair of herniated disks repair of maxillofacial and dental defects ocular implants drug-delivery coatings for metal implants, heart valves to inhibit clotting

Advantages of Bioceramics:

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• Biological compatibility and activity • Less stress shielding • No disease transmission • Unlimited material supply

Disadvantage of Bioceramics: • Brittleness – not for load bearing applications

Material properties differ greatly dependent on (thermal) processing method, yielding 5 categories of microstructure: – Glass – Cast or plasma-sprayed polycrystalline ceramic – Liquid-sintered (vitrified) ceramic – Solid-state sintered ceramic – Polycrystalline glass-ceramic

Of the large number of ceramics known only a few are suitable biocompatible. Main problems: – They are brittle – Relatively difficult to process

3. Dental Ceramics � Excellent aesthetics-(opaqueness & color) � Very tough and hard material � But brittle; improvements of strength necessary (achieved by proper processing) � Expensive manufacture (dental labs) � Alteration of opaqueness & color possible 4. Porous Ceramics Advantage:

– Inertness, Mechanical stability of implant (bone ingrowth at poresize>100μm)

Disadvantage – Restricition to non-load bearing applications – Weaker, larger surface exposed

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� Microstructure of certain corals (hydroxyapatite) is almost ideal: – Machine the coral to desired shape. – Fire off CO2 (from CaCO3 → CaO) while microstructure is maintained. – Casting desired material into the pores (Al2O3, TiO2,..). – Removing of CaO by HCL.

5. Resorbable Ceramics � Chemically brocken down by the body and resorbed (ability to be processed through normal metabolic pathways) � Dissolution rate is controlled by composition and surface area.(ideally a composition should be used that is slowly resorbed by the body once new bone formation is complete) Example: Calcium phosphate ceramics – e.g., tri-calcium phosphate (TCP): Ca3(PO4)2 (lower Ca/PO4 ratio than HA) � Application: – Bone repair (maxillofacial and peridontal defects) – Temporary scaffold or space-filler, bone-cement which is gradually replaced by tissue.

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C> CORROSION – MECHANISM, TYPES (16 MARKS) Metals: Corrosion

• Corrosion is continued degradation of metals to oxide, hydroxide or other compounds through chemical reactions.

• The human body is an aggressive medium for inducing corrosion in metals: water, dissolved oxygen, proteins, chloride and hydroxide..

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• Corrosion is gradual degradation of materials by electrochemical attack

• Corrosion weakens the implanted material, changes the surface of the material and releases metal ions into the body fluids

• Passivation is the process by which a metal is surface coated by the oxide of the metal leading to a decrease in the corrosion rate.

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Corrosion: „Redox“-Chemistry Reduction and Oxidation:

(reduction of oxidation number)

„Redox“-Chemistry

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Corrosion: Basic Reactions � Ionization: Direct formation of metalic cations under acidic or reducing (i.e. oxygen poor) conditions.

M → M+ + e- � Oxidation: Direct reaction of metal with oxygen.

M + O2 → MO2 � Hydroxylation: The reaction of water under alkaline (basic) or oxidizing conditions to yield a hydroxide or hydrated oxide.

2M + O2(aq) + 2H2O → 2M(OH)2

Mechanism: – Materials have tendency to reach their lowest possible free energy (corroded state is preferred). – Most alloys, oxides, hydroxides, sulfides have negative free energy formation and they are thermodynamically favored over the pure metal ! – Metal atoms ionize, go into solution and combine with oxygen. – Metal flakes off Al → Al2O3 ΔG = -1576 kJ mol-1

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Corrosion Formation of rust: – Oxidation of iron to ferrous (2+) ion: Fe → Fe2+ + 2e- – Oxidation of ferrous ions to ferric (3+) ions: Fe2+ → Fe3+ + 1e- – Reduction of oxygen using electrons generated by oxydation: O2 (g) + 2H2O + 4e- → 4OH

� If two dissimilar metals are present in the same environment, the one that is most

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negative in the galvanic series will become the anode, and bimetallic (or galvanic) corrosion will occur. �Galvanic action can also result in corrosion within a single metal, if there is inhomogeneity in the metal or in its environment.The extra-cellular environment is a chemically aggressive space. Metallic biomaterials are good conductors in an electrolyte solution, leading to galvanic corrosion.

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TYPES OF CORROSION

1.Galvanic corrosion

• When two different metals are placed in contact in an electrolytic environment, one material gives up electrons to the other.Metals in an implant system should not be mixed.In case of Co-Cr & titanium alloy, passivation (TiO2) prevents formation of galvanic couple

2. Crevice corrosion.

• Intense localized corrosion within crevices on metal surface

• Isolated areas of restricted fluid conduction leading to accelerated accumulation of positive ions with influx of Cl- to maintain electroneutrality leading to corrosion

• Example: Beneath screw heads

3. Pitting corrosion

• Extremely localized corrosion similar to crevice corrosion, starting at the defect in the passive surface layer

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• Chromium, nickel & molybdenum are added to stainless steel to increase resistance to pitting corrosion

4. Fretting corrosion

• Corrosion occurring at contact areas between materials under load subjected to vibration and slip

5. Stress Corrosion

Unstressed region is more anodic than stressed region

Micro-corrosion cells. Left: grain boundaries are anodic with respect to the grain interior. Right: crevice corrosion due to oxygen-deficient zone in the metal's Environment To avoid corrosion:

– Consider the composition of the biological environment (ions, pH, oxygen pressure, etc.) – Use appropriate metals. – Avoid implantation of dissimilar metals. – Minimize pits and crevices.

– Avoid transfer of metal from tools to the implant during surgery.

MECHANICAL PROPERTIES OF BIOMATERIALS (16 Marks) – Youngs and Rigidity Modulus

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– Poisson’s Ratio – Hardness – Isotropy – Creep and Viscous Flow – Fatigue

1.Stress • Internal reaction to externally applied force (equal to the applied force in magnitude but

opposite direction) • Distributed over the cross-sectional area of the specimen

• Stress () = Force / Area (N/m2) (Pa) • Resolved into three types

• Tension • Compression • Shear

• 2.Strain

• Stress produces deformation. The measurement of deformation, normalized by the original length is called strain

• Strain () = Change in length/Original length • (Deformed length – original length)/ original length • (DL-OL)/OL

• Strain is dimensionless 3. Stress – Strain Curve

• Plot of load versus deformation • Stress on ordinate (y axis) & strain on abscissa (x axis) • Indicate the material properties of specimen tested

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4. Modulus of Elasticity

• At low levels of stress there is a linear relationship between applied stress and the resultant deformation

• This proportionality is called modulus of elasticity or Young’s modulus • Modulus of Elasticity (E) = Stress/Strain • It is a measure of the stiffness of the material

4.1.YOUNGS AND RIGIDITY MODULUS By using the definitions of stress and strain, Hooke’s law

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can be expressed in quantitative terms:

=E , ( tension or compression )

= G , ( shear ) E and G are proportionality constants that may be likened to spring constants. The tensile constant, E is the tensile (or (young’s) modulus and G is the shear

modulus These moduli are also the slopes of the elastic portion of the stress versus strain

curve. Since all geometric influences have been removed, E and G represent inherent properties of the material.

These two moduli are direct macroscopic manifestations of the strengths of the interatomic bonds.

Elastic strain is achieved by actually increasing the interatomic distances in the crystal (i.e., stretching the bonds).

For materials with strong bonds (e.g., diamond, Al2O3, tungsten), the moduli are high and a given stress produces only a small strain. For materials with weaker bonds (e.g., polymers and gold), the moduli are lower.

Cobalt Chromium Alloy is found to have high young’s modulus whereas SS316L(class of Stainless Steel) and Cobalt Chromium Alloy is found to have high shear Modulus

Poisson's ratio is the ratio of the relative contraction strain, or transverse strain (normal to the applied load), divided by the relative extension strain, or axial strain (in the direction of the applied load).

It is found that Tantulum has higher poisson ratio than SS316L(class of Stainless Steel) ,Cobalt Chromium,Nitinol (alloy of Ni and Ti-designated as Shape Memory Alloy).

5. Plastic deformation

• Residual deformation remaining after all the initial stresses have been removed

• Yield strength of material is defined as the stress at which the material exhibits a specified deviation from linear proportionality between stress & strain (yield point)

• Plastic deformation represents change in the molecular structure of the material and often indicates change in its material properties

6. Elastic limit

• Maximum stress that a material can withstand without permanent deformation or failure

7. Ultimate Strength & Strain • The point at which the material fails

Stress-Strain curves of Idealized Elastic Materials

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8.Hardness

• Surface property • Ability to resist plastic deformation at the material surface • It is measured by pressing an indenter into a surface (Rockwell hardness scale) • This is an important consideration for articulating surface in an artificial joint (ceramic

on ceramic) The resistance of a material to permanent deformation of its surface is called

Hardness. The hardness of a material is very important property since in any way it decides the

life of a biomaterial. The hardnesss is generally tested by Vickers hardness test and is represented in terms

of Vickers hardness number. It has been found that the Cobalt Chromium Alloys have higher hardness number than

the other major implant counter parts like Stainless steel,Tantulum,Nitinol 9.Ductility

• Brittle material fracture or fail before they undergo any permanent deformation. They have a straight stress-strain curve.

Examples: Ceramic, bone • Ductile material reach a yield point and then undergo deformation before failure.

Examples: Aluminum, ligament, capsule

10.Toughness

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• The area under both the elastic & plastic portion of the stress-strain curve is the total energy required to stress the material to a point of failure. This is the measure of the material’s toughness.

Creep occurs when a viscoelastic material is subjected to the action of a constant load

• The material undergoes a rapid initial deformation followed by a slow (time-dependent), progressively increasing deformation know as creep, until an equilibrium is reached

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11.Fatigue

• Repeated loading and unloading (cyclical loading) of a material will cause it to fail, even if the loads are below the ultimate stress

• The fatigue life of a material is recorded on a curve of stress (σ) versus number of cycles, or S-N curve

12.Endurance limit of a material is a level of stress tolerated for extended period of time Fatigue failure

Two stages • Crack Initiation

Occurs at the site of structural or geometric weakness (surface stretch, sharp changes in cross section)

• Crack Propagation 13. Axial Load Sharing

• Two materials placed adjacent to one another in structural application (fracture fixation by plate) will carry part of the applied load

• The stress distribution will depend on the material properties, cross sectional areas of two material & the nature of bond between them

• The less stiff material will be “stress shielded”

Tribological Properties 1.Friction

• When two material in contact are in relative motion, the resistance to the movement is called the frictional force

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• The frictional force is directly proportional to the load across the interface • μ (coefficient of friction) = Ff / R (load)

2. Wear • The essential feature of wearing process is loss of material from one of the bearing

surfaces • Several types of wear mechanisms

• Adhesive wear • Abrasive wear • Transfer wear • Fatigue wear • Third body wear • Corrosive wear

2.1Adhesive Wear

• When two bodies slide against each other, small fragments of each surface adhere to the other surface. When subsequent movement occurs, the material breaks, not at the interface but through one of the material

2.2.Abrasive wear • When a rough material slides on a relatively soft surface, it can plow through the softer

material and produce needles or curls of loose debris 2.3.Fatigue wear

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• Local strains gradients in the softer material may cause sufficient subsurface stress concentration to produce fatigue failure after repetitive or cyclical loading

2.4.Third Body Wear • Trapping of wear debris within the moving interfaces or the introduction of foreign

particles, such as bone or PMMA produce local stress concentrations results in abrasion of one or both moving surfaces

2.5. Corrosive wear • Loss of substance as a result of chemical attack • Corrosion accelerated by motion (fretting) • Example: Plates & screws, Morse taper

S-N curve

Material behavior based on the direction of the applied load Isotropic

• Material is called isotropic if its mechanical properties is identical in all the coordinates to an applied load

• Example: Tennis ball Anisotropic

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• Material that have different properties in different direction of the applied load • All biological tissues (bone, cartilage, ligaments) are anisotropic

Structural properties

– Surface Energy – Surface Charge – Critical angle – Surface Wettability

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– Contact angle

Structural Properties of Material Bending

Neutral Axis

When a beam is subjected to bending loads one side of the beam undergoes tensile load and the other side compressive. There are zero stresses in the center of the beam this is called as the neutral axis

Structural Properties of Material Bending

• The magnitude of the tensile or compressive stress varies linearly with distance from the neutral axis

• The resistance to bending (areal moment of inertia) is created by the shape of the beam. It is independent of the material factors

• In a solid rod it is directly related to the 4th power of the radius Structural Properties of Material Torsion

• Torsion applies shear load whose magnitude increases with the distance from the center of rotation

Axial Load Sharing

• Two materials placed adjacent to one another in structural application (fracture fixation by plate) will carry part of the applied load

• The stress distribution will depend on the material properties, cross sectional areas of two material & the nature of bond between them

• The less stiff material will be “stress shielded”

HOST REACTIONS TO BIOMATERIALS / AND WOUND HEALING MECHANISM(16 MARKS)

All implants interact to some extent with the tissue environment in which they are placed. Placing a biomaterial in the in vivo environment involves: injection, insertion, or surgical implantation, all of which injure the tissues or organs involved.

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The body responds to reestablish homeostasis.

The degree to which the homeostatic mechanisms are perturbed determine the biocompatibility of a biomaterial

The host reaction can be:Tissue-dependent,Organ-dependent and Species-dependent. Inflammation:Is a reaction of vascularized living tissue to local injury- Serves to: absorb, neutralize, dilute, or wall off the injurious agent or process.

Host reactions to biomaterials

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In addition, it induces a series of events to heal and reconstitute the implant site though replacement of the injured tissue by regeneration of native parenchymal cells, formation of fibroblastic scar tissue or a combination of these two processes. Immediately following injury there are changes in vascular flow and permeability. Fluids, proteins and blood cells escape from the vascular system into the injured tissue = „exudation“. Regardless of the tissue or organ into which a biomaterial is implanted, the initial inflammatory response is activated by injury to vascularized connective tissue.Blood and its components are involved in the initial inflammatory responses, blood clot formation and/or thrombosis also occurs. Blood coagulation and thrombosis may be influenced by other mechanisms:

o The extrinsic and intrinsic coagulation system o The complement system o The fibrinolytic system o The kinin-generating system o Platelets

The predominant cell type present in the inflammatory response varies with the age of the injury.

Neutrophils predominate during the first several days.Neutrophils are short-lived, disintegrate and disapear after 24-48h.Then they are replaced by monocytes.Following emigration from the vasculature, monocytes differentiate into macrophages (long-lived response; up to months).

The size, shape, and chemical and physical properties of the biomaterial may be responsible for variations in the intensity and duration of the inflammatory or wound healing process. Biochemical mediators of inflammation are quickly in- activated, suggesting that their

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action is local. Lysosomal proteases and oxygen-derived radicals are also important in the degradation of biomaterials.

Host Response to Material Implantation

Implantation of a biomaterial is an invasive procedure that initiates a series of events whose outcome ultimately determine the biocompatibility of the material

NORMAL WOUND HEALING wound healing is a dynamic cascade of events initiated by injury it may be divided into phases characterized by both cellular population and

cellular function » blood clotting » inflammation » cellular invasion and remodeling

Clotting or Thrombosis blood coagulation or clotting is the blood response to damaged blood vessels objective is to form a patch that isolates the leak and stops blood loss

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Coagulation Pathways (BELOW)

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Phagocytosis: Engulfing and degradation or digestion of fragments of tissue or material

1. long membrane evaginations, called pseudopodia. 2. Ingestion forming a "phagosome," which moves toward the lysosome. 3. Fusion of the lysosome and phagosome (phagolysosome), releasing lysosomal enzymes 4. Digestion of the ingested material. 5. Release of digestion products from the cell.

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1. Inflammatory Response

Pathogen recognition and tissue damage begin an inflammation response. This is characterized by :

swelling pain redness heat

Inflammation allows for neutrophil and plasma protein extravasation. Both of these effects aids the immune response.

2. Diapedesis Movie/CHEMOTAXIS

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3.Phagocytosis 4. Leukocyte Invasion at Wound Site 5. Granulation Tissue Deposition – Remodeling

Due to death of cells following injury, and their removal, there is a local decreased tissue mass

fibroblasts and vascular endothelial cells are recruited to site » Begin to form granulation tissue (ECM and new blood vessels)

Acute inflammation:

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Acute inflammation Is of relative short duration (minutes to days).The main characteristics are:Exudation of fluid and plasma proteins (edema) Emigration of leucocytes (predominately neutrophils).Leucocyte emigration is assisted by „adhesion molecules“ present on leucocytes and endothelial surfaces.

The major role of neutrophils in acute inflammation is to phagocytose microrganisms and foreign material. The process of recognition and attachment is enhanced when the injurious material is coated by naturally occuring serum factors(„opsonins“).E.g.: IgG, C3b (also known to adsorb to biomaterials)

Biomaterials are not generally phagocytosed by neutrophils or macrophages (most

biomats are to big a „mouthfull“ for the cells).But: instead „frustrated phagocytosis“: release of leucocyte products occur in an attempt to degrade the biomaterial

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Chronic inflammation:

Chronic inflammation Is histologically less uniform than acute inflammation. Characterized by the presence of macrophages, monocytes, and lymphocytes, with the proliferation of blood vessls and connective tissue.The chemical and physical properties of the biomaterial may lead to prolonged chronic inflammation. Motion in the implant site by the biomaterial may also produce prolonged chronic inflammation.Monocytes and macrophages belong to the MPS (mono-nuclear phagocytosis system), also called RES (reticulo-endothelial system)-Consists of cells in the: Bone marrow,Peripheral blood, and Specialized tissues.

These cells (MPS) may be responsible for systemic effects in organs and tissues secondary to the release of components or products from implants: Corrosion products Wear debris and Degradation products.

The macrophage produces a great number of biologically active products: Neutral proteases Chemotactic factors, Arachidon acid metabolites,Reactive oxygen metabolites,Complement components,Coagulation factors,Growth promoting factors and Cytokines .

Granulation tissue: Within one day the healing response is initiated by the action of monocytes and macrophages. Fibroblasts and vascular endothelial cells proliferate and begin to form granulation tissue (specialized type of tissue that is the hallmark of healing inflammation). New small blood vessels are formed by budding or sprouting of preexisting vessels (neovasculation or angiogenesis). Fibroblast proliferate, synthesize collagen and proteoglycans.Some fibroblasts differentiate into smooth muscle tissue mediating wound contraction

Foreign body reaction is composed of foreign body giant cells and the components of granulation tissue. Foreign body giant cells are formed by the fusion of monocytes and macrophages in an attempt to phago-cytose material:

The form and topography of the surface of the biomaterial determines the composition of

the foreign body reaction. Flat and smooth surfaces (breast prostheses) have a foreign body reaction composed of a layer of macrophages one or two cells in thickness. – Rel. rough surfaces (Teflon vascular prostheses): macrophages and foreign body giant cells at the surface. The foreign body reaction may persist at the tissue-implant interface for the lifetime of the implant. It is unknown if the foreign body reaction cells remain activated, releasing their lysosomal constitutes, or become quiescent.Generally, fibrosis (fibrous encapsulation) surrounds the biomaterial or implant with its interfacial foreign body reaction, isolating it from the local tissue environment.

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Fibrosis and fibrous encapsulation: End stage of healing response Usually four or more weeks after implantation A relatively acellular fibrous capsule

» spindle shaped fibroblasts » small number of macrophages

Presence of neutrophils suggests persisting inflammatory challenge Presence of foreign body giant cell suggests production of small particles by

corrosion, depolymerization, dissolution or wear

The end-step of healing response to biomaterials is generally fibrosis or fibrous encapsulation. Exception: porous material inocculated with parenchymal cells or porous materials implanted into bone. Cell Regeneration After Injury

Possible outcomes for the injured tissue: » replacement of injured tissue with parenchymal cells of the same type » replacement by connective tissue that constitutes the fibrous capsule

The regeneration of cells in the body is tightly controlled There are essentially 3 categories of cell populations

» Renewing or labile » Expanding or stable

Static or permanent

Repair of implant site can involve two distinct processes: Regeneration: replacement of parenchymal tissue by parenchymal cells of the same type. Replacement by connective tissue that constitutes the fibrous capsule. Tissues with static cells (little/no potential to reproduce after birth) give rise to fibrosis and fibrous capsule formation.E.g.: nerve cells, skeletal and cardiac muscle cells Tissues consisting of cells with potential to reproduce may follow the pathway to fibrosis or may regenerate.E.g.: parenchymal cells of liver, kidney, pancreas; mesenchymal cells (fibroblast); vascular endothelial cells; epithelial cells; lymphoid and hematopoietic cells. Regeneration capacity is species dependent.Cells from the same organ/tissue but from different species may exhibit different regenerative capacities.

Following injury many cells/tissues undergo adaption of growth and differentiation:

Atrophy (decrease in cell size and function)

Hypertrophy (increase in cell size)

Hyperplasia (increase in cell number)

Metaplasia (change in cell type)

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Altered gene expression Local and systemic factors play a role in the wound healing response:

Local: tissue/organ of implantation, adequacy of blood supply, potential of infection Systemic: nutrition, glucocortical steroids, preexisting disease (atherosclerosis, diabetes, infection,)

Vroman Effect

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VISCOELASTICITY (10 MARKS) Definition: time-dependent material behavior where the stress response of that material depends on both the strain applied and the strain rate at which it was applied! (2 MARKS)

Examples • biological materials • polymer plastics • metals at high temperatures

Elastic versus viscoelastic behaviors For a constant applied strain

• An elastic material has a unique material response • A viscoelastic material has infinite material responses depending on the strain-rate

Viscoelastic Hysteresis Viscoelastic solid

• some energy is dissipated with dashpots (as heat)some energy is stored in springs. Area in the hysteresis loop is a function of loading rate

• For viscoelastic material, energy is dissipated regardless of whether strains(or stresses) are small or large

• Under repetitive loading, a viscoelastic material will heat up

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Viscoelasticity

• Mechanical properties of the material depends on the rate of loading or rate of strain

• At low strain rates the material flows like a viscous liquid. At high strain rates, the same material can behave as a elastic solid (Silly Putty)

• Exhibits properties of creep, stress relaxation & hysteresis

• Most biological tissues are viscoelastic Stress-Strain curves to illustrate viscoelastic behavior

Creep

• Creep occurs when a viscoelastic material is subjected to the action of a constant load

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• The material undergoes a rapid initial deformation followed by a slow (time-dependent), progressively increasing deformation know as creep, until an equilibrium is reached

Stress relaxation

• SR occurs when a viscoelastic material is subjected to a constant deformation

• Typically there is a high initial stress followed by a slow (time-dependent) progressively decreasing stress required to maintain the deformation

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Hysteresis

• The elastic recovery of a viscoelastic material stressed below its yield point does not always coincide with the deformation curve

• The area between the two curves represents the energy that is dissipated (heat)

• This loss of strain energy is called hysteresis

Fluids & Viscosity

• Viscosity is the resistance of a fluid to flow. It characterizes the internal resistance of a fluid to shear deformation

• When viscosity of a fluid is independent of the shear rate it is said to exhibit Newtonian behavior

Example: Water, plasma

• Shear thinning fluids exhibit nonnewtonian behavior where the viscosity decreases with increasing shear rates (the faster they are loaded the easier they flow)

Example: Synovial fluid, whole blood

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• There is an inverse relationship between viscosity & shear rates Viscosity versus Strain Rate

Viscoelas ticity• B ehavior exhibited by a material that has both

viscous and elastic elements in its response to a deformation or load

• R epresented by:

– Dashpot for viscous element.

• F ollows Newtonian fluid cons titutive law

– S pring for elastic element• As s umed to linearly elas tic

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Maxwell Model

• R epres ented by a purely viscous damper and a purely elas tic s pring connected in s eries

• T he model can be repres ented by the following equation:

• P redicts that s tres s decays exponentially with time

• Model does n’t accurately predict creep (cons tant s tres s ). P redicts that s train will increas e linearly with time. Actually s train rate decreas es with time

S tres s relaxation experiment

K elvin-Voigt Model• R epres ented by a Newtonian damper

and Hookean elas tic s pring in parallel.

• T he model can be expres s ed as a linear firs t order differential equation:

• R epres ents a s olid undergoing revers ible, vis coelas tic s train.

• At cons tant s tres s (creep), predicts s train to tend to σ/E as time continues to infinity

• T he model is les s accurate with relaxation in a material

C reep and recovery res pons e

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B urgers Model• F our elements us ed to capture

“minimum” amount of behavior for a polymer which are:

• “Ins tantaneous ” elas ticity or elas tic recovery (G 1)

• Molecular “s lip” (η1)

• R ubbery elas ticity (G 2)

• “R etarded” elas ticity (η2)

• Modeled as a Maxwell in s eries with a K elvin-Voigt

C reep and recovery res pons e

Model C omparison• Maxwell

• G ood for predicting s tres s relaxation

• P oor at predicting creep

• Us ed for s oft s olids (materials clos e to the melting point

• K elvin-Voigt

• G ood for predicting creep

• Not accurate with predicting s tres s relaxation

• Us ed for organic polymers , rubber, wood when the load is not toohigh

• B urgers

• P redicts es s entials of polymer vis coelas tic behavior

• Us ed for polymers

• G eneralized

• Us ed for fitting of experimental data to an arbitrary accuracy

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****************************************************************************** BLOOD COMPATIBILITY (16 MARKS) Blood compatibility is often referred to as haemocompatibility and is one aspect of biocompatibility. Blood compatibility relates to the specific interactions between biomaterials and circulating blood.

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Biomaterial-Tissue Interactions

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MECHANICAL PROPERTIESMechanical Properties Mechanical Properties

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VISCOELASTICITY (10 MARKS) Definition: time-dependent material behavior where the stress response of that material depends on both the strain applied and the strain rate at which it was applied! (2 MARKS)

Examples • biological materials • polymer plastics • metals at high temperatures

Elastic versus viscoelastic behaviors For a constant applied strain

• An elastic material has a unique material response • A viscoelastic material has infinite material responses depending on the strain-rate

Viscoelastic Hysteresis Viscoelastic solid

• some energy is dissipated with dashpots (as heat)some energy is stored in springs. Area in the hysteresis loop is a function of loading rate

• For viscoelastic material, energy is dissipated regardless of whether strains(or stresses) are small or large

• Under repetitive loading, a viscoelastic material will heat up

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Viscoelasticity

• Mechanical properties of the material depends on the rate of loading or rate of strain

• At low strain rates the material flows like a viscous liquid. At high strain rates, the same material can behave as a elastic solid (Silly Putty)

• Exhibits properties of creep, stress relaxation & hysteresis

• Most biological tissues are viscoelastic Stress-Strain curves to illustrate viscoelastic behavior

Creep

• Creep occurs when a viscoelastic material is subjected to the action of a constant load

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• The material undergoes a rapid initial deformation followed by a slow (time-dependent), progressively increasing deformation know as creep, until an equilibrium is reached

Stress relaxation

• SR occurs when a viscoelastic material is subjected to a constant deformation

• Typically there is a high initial stress followed by a slow (time-dependent) progressively decreasing stress required to maintain the deformation

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Hysteresis

• The elastic recovery of a viscoelastic material stressed below its yield point does not always coincide with the deformation curve

• The area between the two curves represents the energy that is dissipated (heat)

• This loss of strain energy is called hysteresis

Fluids & Viscosity

• Viscosity is the resistance of a fluid to flow. It characterizes the internal resistance of a fluid to shear deformation

• When viscosity of a fluid is independent of the shear rate it is said to exhibit Newtonian behavior

Example: Water, plasma

• Shear thinning fluids exhibit nonnewtonian behavior where the viscosity decreases with increasing shear rates (the faster they are loaded the easier they flow)

Example: Synovial fluid, whole blood

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• There is an inverse relationship between viscosity & shear rates Viscosity versus Strain Rate

******************************************************************************