BIOMATERIALS- CLASSIFICATION

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LECTURE 2 BIOMATERIALS 1 BIOMATERIALS- CLASSIFICATION When a synthetic material is placed within the human body, tissue reacts towards the implant in a variety of ways depending on the material type. The mechanism of tissue interaction depends on the tissue response to the implant surface. Biomedical materials can be divided roughly in to

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BIOMATERIALS- CLASSIFICATION. When a synthetic material is placed within the human body, tissue reacts towards the implant in a variety of ways depending on the material type. The mechanism of tissue interaction depends on the tissue response to the implant surface. - PowerPoint PPT Presentation

Transcript of BIOMATERIALS- CLASSIFICATION

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BIOMATERIALS- CLASSIFICATION

When a synthetic material is placed within the human body, tissue reacts towards the implant in a variety of ways depending on the material type.

The mechanism of tissue interaction depends on the tissue response to the implant surface.

Biomedical materials can be divided roughly in to three main types governed by the tissue response.

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BIOMATERIALS- CLASSIFICATION

Biomaterials are widely classified as

Bioinert Biomaterials

Bioactive Biomaterials

Bioresorbable Biomaterials

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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.

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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 .

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BIOACTIVE BIOMATERIALS

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.

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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.

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

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COMPARISON OF PROPERTIES

The wide application of biomaterials in medicine depends on the properties of these materials.

Different biomaterials should have different properties depending on the high end applications.

The surface properties, mechanical properties and the thermal properties which are important are discussed.

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SURFACE PROPERTIES

The surface properties for the biomaterials which are being considered for discussion are

Surface Energy

Contact Angle

Critical Surface Tension

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SURFACE ENERGY Surface energy quantifies the disruption of

intermolecular bonds that occurs when a surface is created.

In other words surface energy is a measure of the extent to which bonds are unsatisfied at the surface of material.At the surface, there is an asymmetric force field, which results in an attraction of atoms which are there on the surface in to the bulk.

This tends to deplete the surface of atoms putting the surface in tension.

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SURFACE ENERGY

Metals and ceramics have surfaces with high surface energies ranging from 102 to 104 ergs/cm2.

In contrast, most polymers and plastics have much smaller surface energies, usually <100 ergs/cm2.

The surface energy values are subject to much experimental variation due to adsorption of gases or organic species.

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CONTACT ANGLE

The contact angle is the angle at which a liquid/vapor interface meets the solid surface.

The contact angle is specific for any given system and is determined by the interactions across the three interfaces.

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CONTACT ANGLE

When a liquid drop is placed on to the surface of a solid or the surface of the liquid, the processes which occur are: 1.The liquid may sit on the surface in the form of a droplet or 2. It may spread out over the entire surface depending on the interfacial free energies of the two substances.

At equilibrium contact angle or Young Dupree equation is given by s / g = s / l + l / g cos

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CONTACT ANGLE

Where s / g, s / l and l / g are the interfacial free energy between the solid and gas; solid and liquid, liquid and gas respectively and the contact angle.

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CONTACT ANGLE

The wetting characteristic can be generalized as = 0, complete wetting ; 0 900, partial wetting ; > 900 , no wetting.

The contact angle can be affected greatly by the surface roughness and adsorption of polar gases or organic species or contamination by dirt.

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CRITICAL SURFACE TENSION

The critical surface tension is defined as that value of surface tension of a liquid below which the liquid will spread on a solid and is expressed in dynes/cm.

The critical surface tension of a material is determined by measuring the different values of contact angle formed by liquids with different values of l / g.

A plot of cos versus l / g is usually a straight line

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CRITICAL SURFACE TENSION

The l / g at which cos =1 is defined as the critical surface-tension (c).

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CRITICAL SURFACE TENSION

Blood compatibility of material surfaces has been shown to vary in the same order as the critical surface tension.

It is found that the amount of thrombus formation increases and blood clotting time decreases as c increases.

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MECHANICAL PROPERTIES

The Mechanical Properties which will be considered are

• Youngs and Rigidity Modulus• Poisson’s Ratio• Hardness• Isotropy• Creep and Viscous Flow• Fatigue

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YOUNGS AND RIGIDITY MODULUS

By using the definitions of stress and strain, Hooke’s law 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

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YOUNGS AND RIGIDITY 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).

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YOUNGS AND RIGIDITY MODULUS 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.

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POISSON’S RATIO 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).

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HARDNESS

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.

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COMPARISON OF POISSONS RATIO

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COMPARISON OF YOUNGS AND SHEAR MODULUS

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COMPARISON OF HARDNESS

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ISOTROPY The two constants, E and G, are all that are needed to

fully characterize the stiffness of an isotropic material (i.e., a material whose properties are the same in all directions).

Single crystals are anisotropic (not isotropic) because the stiffness varies as the orientation of applied force change relative to the interatomic bond directions in the crystal.

In polycrystalline materials (e.g., most metallic and ceramic specimens), a great multitude of grants (crystallites) are aggregated with multiply distributed orientations

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ISOTROPY• On the average, these aggregates exhibit isotropic behavior at the macroscopic level, and values of E and G are highly reproducible for all specimens of a given metal, alloy, or ceramic.• On the other hand, many polymeric materials and most tissue samples are anisotropic (not the same in all directions) even at the macroscopic level.• Bone, ligament, and sutures are all stronger and stiffer in the fiber (longitudinal) direction than they are in the transverse direction.• For such materials, more than two elastic constants are required to relate stress and strain properties.

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CREEP AND VISCOUS FLOW Creep is the term used to describe the tendency of a

material to move or to deform permanently to relieve stresses.

Material deformation occurs as a result of long term exposure to levels of stress (physics) that are below the yield strength or ultimate strength of the material.

Creep is more severe in materials that are subjected to heat for long periods and near melting point.

Creep is often observed in glasses.

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CREEP AND VISCOUS FLOW It this been assumed that when a stress is applied, the

strain response is instantaneous. For many important biomaterials, including polymers

and tissues, this is not a valid assumption. If a weight is suspended from an excised ligament, the

liga ment elongates essentially instantaneously when the weight is applied.

This is an elastic response. Thereafter the ligament continues to elongate for a considerable time even though the load is constant (Fig.A).

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CREEP AND VISCOUS FLOW• This continuous, time-dependent extension under load is called "creep."• Similarly, if the ligament is extended in a tensile machine to a fixed elongation and held constant while the load is monitored, the load drops continuously with time (Fig.B). The continuous drop in load at constant extension is called stress relaxation.• Both these responses are the result of viscous flow in the material.• The mechanical analog of viscous flow is a dash pot or cylinder and piston (Fig.C). Any, small force is enough to keep the piston moving. If the load is increased, the rate of displacement will increase.

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CREEP AND VISCOUS FLOW A Elongation Vs Time at constant load of ligament

B Load Vs Time at constant elongation for ligament

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CREEP AND VISCOUS FLOW C Dash pot or Cylinder and Piston model of viscous flow

D Dash pot and spring model of viscoelastic material

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CREEP AND VISCOUS FLOW Despite this liquid-like behavior, these materials are

functionally solids.

To produce such a combined effect, they act as though they are composed of a spring (elastic element) in series with a dashpot (viscous element) (Fig.C).

Thus, in the creep test, instantaneous strain is produced when the weight is first applied (Fig.A).

This is the equivalent of stretching the spring to its equilibrium length (for that load).

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CREEP AND VISCOUS FLOW Thereafter, the addi tional time-dependent strain is modeled

by the movement of the dashpot.

Complex arrangements of springs and dashpots are often needed to adequately model the actual behavior of polymers and tissues.

Materials that behave approximately like a spring and dash pot system are viscoelastic.

One consequence of viscoelastic behavior can be seen in tensile testing where the load is applied at some finite rate.

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CREEP AND VISCOUS FLOW• During the course of load application, there is time for some viscous flow to occur along with the elastic strain.

• Thus, the total strain will be greater than that due to the elastic response alone.

• If this total strain is used to estimate the Young's modulus of the material (E = /), the estimate will be low.

• If the test is conducted at a more rapid rate, there will be less time for viscous flow during the test and the apparent modulus will increase.

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CREEP AND VISCOUS FLOW If a series of such tests is conducted at ever higher loading

rates, eventually a rate can be reached where no detectable viscous flow occurs and the Modulus determined at this critical rate will be the true elastic modulus.

For all viscoelastic materials, moduli determined at rates less than the critical rate are "apparent" moduli and must be identified with the strain rate used.

Failure to do this is one reason why values of tissue moduli reported in the literature may vary over wide ranges.

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CREEP AND VISCOUS FLOW

Finally, it should he noted that it may be difficult to distinguish between creep and plastic deformation in ordinary tensile tests of highly viscoelastic materials (e.g., tissues).

For this reason, the total nonelastic deformation of tissues or polymers may at times be loosely referred to as plastic deformation even though viscous flow is involved.

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FATIGUE• It is not uncommon for materials, including tough and ductile- ones like 316L, stainless steel, to fracture even though the service stresses imposed are well below the yield stress.

• This occurs when the loads are applied and removed for a great number of cycles, as happens to prosthetic heart valves and prosthetic joints.

• Such repetitive loading can produce micro scopic cracks that then propagate by small steps at each load.

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FATIGUE Fatigue, then, is a process by which structures fail as a result of cyclic stresses that may be much less than the ultimate ten sile stress.

Fatigue failure plagues many dynamically loaded structures, from aircraft to bones to cardiac pacemaker leads.

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FATIGUE The stresses at the tip of a crack, a surface scratch, or a sharp corner are locally enhanced by the stress-raising effect.

Under repetitive loading, these local high stresses ally exceed the strength of the material over a small region.

This Phenomenon is responsible for the stepwise propagation of the cracks, Eventually, the load-bearing cross-section becomes so small that the part finally fails completely .