BIOMATERIALS- CLASSIFICATION

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 • three main types governed by the tissue response. BIOMATERIALS

2. BIOMATERIALS- CLASSIFICATION • Biomaterials are widely classified as • Bioinert Biomaterials • Bioactive Biomaterials • Bioresorbable Biomaterials BIOMATERIALS

3. 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. BIOMATERIALS

4. 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 . BIOMATERIALS

5. 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. BIOMATERIALS

6. 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. BIOMATERIALS

7. 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. BIOMATERIALS

8. SURFACE PROPERTIES • The surface properties for the biomaterials which are being considered for discussion are • Surface Energy • Contact Angle • Critical Surface Tension BIOMATERIALS

9. 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. BIOMATERIALS

10. 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. BIOMATERIALS

11. 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. BIOMATERIALS

12. 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  BIOMATERIALS

13. 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. BIOMATERIALS

14. 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. BIOMATERIALS

15. 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 BIOMATERIALS

16. CRITICAL SURFACE TENSION The l / g at which cos  =1 is defined as the critical surface-tension (c). BIOMATERIALS

17. 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. BIOMATERIALS

18. MECHANICAL PROPERTIES • The Mechanical Properties which will be considered are • Youngs and Rigidity Modulus • Poisson’s Ratio • Hardness • Isotropy • Creep and Viscous Flow • Fatigue BIOMATERIALS

19. 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 BIOMATERIALS

20. 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). BIOMATERIALS

21. 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. BIOMATERIALS

22. 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). BIOMATERIALS

23. 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. BIOMATERIALS

24. COMPARISON OF POISSONS RATIO BIOMATERIALS

25. COMPARISON OF YOUNGS AND SHEAR MODULUS BIOMATERIALS

26. COMPARISON OF HARDNESS BIOMATERIALS

27. 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 BIOMATERIALS

28. 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. BIOMATERIALS

29. 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. BIOMATERIALS

30. 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 • liga­ment 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). BIOMATERIALS

31. 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 dash­pot 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. BIOMATERIALS

32. CREEP AND VISCOUS FLOW A Elongation Vs Time at constant load of ligament B Load Vs Time at constant elongation for ligament BIOMATERIALS

33. CREEP AND VISCOUS FLOW C Dash pot or Cylinder and Piston model of viscous flow D Dash pot and spring model of viscoelastic material BIOMATERIALS

34. 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). BIOMATERIALS

35. CREEP AND VISCOUS FLOW • Thereafter, the addi­tional 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 • dash­pot system are viscoelastic. • One consequence of viscoelastic behavior can be seen in • tensile testing where the load is applied at some finite rate. BIOMATERIALS

36. 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. BIOMATERIALS

37. 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. BIOMATERIALS

38. 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. BIOMATERIALS

39. 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 micro­scopic cracks that • then propagate by small steps at each load. BIOMATERIALS

40. FATIGUE • Fatigue, then, is a process by which structures fail as a result of cyclic stresses that may be much less than the ultimate ten­sile stress. • Fatigue failure plagues many dynamically loaded structures, from aircraft to bones to cardiac pacemaker leads. BIOMATERIALS

41. 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 . BIOMATERIALS