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Metals I


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Metals I

  1. 1. Ceramics Lecture 4 January 29, 2009
  2. 2. Zirconia Data for HW1 N. Navruz, Phys of metals & metallography 105: 6 (2008) Monoclinic values a = 0.5181 nm b = 0.5200 nm c = 0.5365 nm  = 98.6 ° Tetragonal a = 0.5126 nm c = 0.5206 nm
  3. 3. Ceramics in Medicine <ul><li>Historically common in medical industry – glass beakers, slides, thermometers, eyeglasses, etc. </li></ul><ul><li>Ceramic materials exist in the body </li></ul><ul><ul><li>Bone and teeth </li></ul></ul><ul><li>Thus, they are useful in devices and implants </li></ul>
  4. 4. Ceramic vs Glass <ul><li>Ceramic : an inorganic, nonmetallic, typically crystalline solid, prepared by application of heat and pressure to a powder </li></ul><ul><ul><li>Most ceramics are made up of two or more elements. </li></ul></ul><ul><ul><li>Contain metallic and non-metallic elements, ionic and covalent bonds </li></ul></ul><ul><li>Glass : (i) An inorganic product of fusion that has cooled to a rigid condition without crystallization; (ii) An amorphous solid </li></ul><ul><li>Glass-ceramic : Product formed by the controlled crystallization (devitrification) of a glass-forming melt. Consists of two-phases: crystals in a glass matrix. </li></ul>
  5. 5. Other Definitions <ul><li>Amorphous : </li></ul><ul><ul><li>Lacking detectable crystallinity; </li></ul></ul><ul><ul><li>Possessing only short-range atomic order; also glassy or vitreous </li></ul></ul><ul><li>Bioactive material : A material that elicits a specific biological response at the interface of the material, (usually) resulting in the formation of a bond between the tissues and the material. </li></ul>
  6. 6. Crystalline vs Glassy (Amorphous) Ceramics <ul><li>Crystalline ceramics have long-range order, with components composed of many individually oriented grains. </li></ul><ul><li>Glassy materials possess only short-range order, and generally do not form individual grains. </li></ul><ul><li>The distinction is based on x-ray diffraction characteristics. </li></ul><ul><li>Most of the structural ceramics are crystalline and oxides. </li></ul>
  7. 7. Atomic Bonds <ul><li>Ionic </li></ul><ul><ul><li>Large differences in electronegativity </li></ul></ul><ul><ul><li>Non directional strong bonds </li></ul></ul><ul><li>Covalent </li></ul><ul><ul><li>Small differences in electronegativity </li></ul></ul><ul><ul><li>Strong, directional bonds </li></ul></ul><ul><li>All ionic, all covalent or covalent-ionic bonds possible </li></ul>Ceramic Name Melt Point °C % Covalent Char. % Ionic Char. Magnesium Oxide 2798° 27% 73% Aluminum Oxide 2050° 37% 63% Silicon Dioxide 1715° 49% 51% Silicon Nitride 1900° 70% 30% Silicon Carbide 2500° 89% 11%
  8. 8. Properties <ul><li>High melting temperature – bond type (ionic-covalent) </li></ul><ul><li>Low thermal conductivities and thermal expansion coefficients </li></ul><ul><ul><li>Strong ionic - covalent bonding </li></ul></ul><ul><ul><li>Imperfections (grain boundaries, pores) </li></ul></ul><ul><li>High heat capacity and low heat conductance – good thermal insulators </li></ul><ul><li>Low density </li></ul><ul><li>High strength, compressive strength usually ten times > tensile </li></ul><ul><li>Very high elastic modulus (stiffness greater than metals) </li></ul><ul><li>Very high hardness </li></ul><ul><li>Brittle – due to ionic bonds </li></ul><ul><li>Wear resistant – because of high compressive strength and hardness </li></ul><ul><li>Corrosion resistant and/or unreactive– oxides do not oxidize further </li></ul><ul><li>High melting point, chemical inertness, high hardness and low fracture strength can make it difficult to make ceramic components </li></ul>
  9. 9. Ceramics as Biomaterials <ul><li>Advantages </li></ul><ul><ul><li>Inert in body (or bioactive in body); chemically inert in many environments </li></ul></ul><ul><ul><li>High wear resistance (orthopedic & dental applications) </li></ul></ul><ul><ul><li>High modulus (stiffness) & compressive strength </li></ul></ul><ul><ul><li>Esthetic for dental applications </li></ul></ul><ul><li>Disadvantages </li></ul><ul><ul><li>Brittle (low fracture resistance, flaw tolerance) </li></ul></ul><ul><ul><li>Low tensile strength (fibers are exception) </li></ul></ul><ul><ul><li>Poor fatigue resistance (relates to flaw tolerance) </li></ul></ul>
  10. 10. Applications <ul><li>Orthopedics: </li></ul><ul><ul><li>bone plates and screws </li></ul></ul><ul><ul><li>total & partial hip components (femoral head) </li></ul></ul><ul><ul><li>coatings (of metal prostheses) for controlled implant/tissue interfacial response </li></ul></ul><ul><ul><li>space filling of diseased bone </li></ul></ul><ul><ul><li>vertebral prostheses, vertebra spacers, iliac crest prostheses </li></ul></ul><ul><li>Dentistry </li></ul><ul><ul><li>dental restorations (crown and bridge) </li></ul></ul><ul><ul><li>implant applications (implants, implant coatings, ridge maintenance) </li></ul></ul><ul><ul><li>orthodontics (brackets) </li></ul></ul><ul><ul><li>glass ionomercements and adhesives </li></ul></ul><ul><li>Other </li></ul><ul><ul><li>inner ear implants (cochlear implants) </li></ul></ul><ul><ul><li>drug delivery devices </li></ul></ul><ul><ul><li>ocular implants </li></ul></ul><ul><ul><li>heart valves </li></ul></ul>
  11. 11. Attachment <ul><li>Four types of ceramic-tissue attachment are related to the tissue response to a material </li></ul><ul><li>Morphological fixation – dense, inert, nonporous ceramics attach by bone (or tissue) growth into surface irregularities, by cementing the device into the tissues, or by press fitting into a defect </li></ul><ul><li>Biological fixation – porous, inert ceramics attach by bone ingrowth (into pores) resulting in mechanical attachment of bone to material </li></ul><ul><li>Bioactive fixation – dense, nonporous surface-reactive ceramics attach directly by chemical bonding with bone </li></ul><ul><li>Resorbable – dense, porous or nonporous resorbable ceramics are slowly replaced by bone </li></ul>
  12. 12. Types of Ceramics <ul><li>Nonporous, nearly inert materials are very strong and stiff </li></ul><ul><li>Porous, inert materials have lower strengths, but are useful as coatings for metallic implants </li></ul><ul><li>Nonporous, bioactive materials establish bonds with bone tissue </li></ul><ul><li>Resorbable materials may be porous or nonporous and degrade with time </li></ul>
  13. 13. 1. Nonporous, Nearly Inert Ceramics <ul><li>Alumina (Al 2 O 3 ) and Zirconia (ZrO 2 ) </li></ul><ul><ul><li>The two most commonly used structural bioceramics. </li></ul></ul><ul><ul><li>Primarily used as modular heads on femoral stem hip components. </li></ul></ul><ul><ul><li>Wear less than metal components, and the wear particles are generally better tolerated. </li></ul></ul><ul><li>Pyrolytic Carbon </li></ul><ul><ul><li>Coatings for heart valves, blood contacting applications </li></ul></ul>
  14. 14. Processing of Ceramics <ul><li>Compounding </li></ul><ul><ul><li>Mix and homogenize ingredients into a water based suspension = slurry or, into a solid plastic material containing water called a clay </li></ul></ul><ul><li>Forming </li></ul><ul><ul><li>The clay or slurry is made into parts by pressing into mold (sintering). The fine particulates are often fine grained crystals. </li></ul></ul><ul><li>Drying </li></ul><ul><ul><li>The formed object is dried, usually at room temperature to the so-called &quot;green&quot; or leathery state. </li></ul></ul><ul><li>Firing </li></ul><ul><ul><li>Heat in furnace to drive off remaining water. Typically produces shrinkage, so producing parts that must have tight mechanical tolerance requires care. </li></ul></ul><ul><ul><li>Porous parts are formed by adding a second phase that decomposes at high temperatures forming the porous structure. </li></ul></ul>
  15. 15. Solid State Sintering <ul><li>Sintering is a diffusional process that combines distinct powdered grains below the melting point into one cohesive material </li></ul><ul><li>Powder particles are pressed together forming a compacted mass of powder particles </li></ul><ul><ul><li>Powders are milled or ground to produce a fine powder (d ≈ 0.5 – 5.0  m) </li></ul></ul><ul><ul><ul><li>Smaller grain size = greater strength </li></ul></ul></ul><ul><li>Powder compact is then heated to allow diffusion to occur and the separated powder particles become fused together </li></ul><ul><ul><li>Usually T > ½ T m in Kelvin </li></ul></ul><ul><ul><ul><li>Higher temperature = smaller pore size </li></ul></ul></ul><ul><li>Final product consists of grains with boundaries containing a mixture </li></ul><ul><li>of atoms from two separate particles </li></ul><ul><li>Material also becomes denser as it is sintered </li></ul>
  16. 16. Energy Minimization <ul><li>Sintering is driven by a reduction in surface energy </li></ul><ul><ul><li>Two surfaces are replaced by one grain boundary (s/g to s/s) </li></ul></ul><ul><ul><li>Atoms diffuse from the grain boundary to the void surface </li></ul></ul><ul><ul><ul><li>Fast diffusion occurs at grain boundary </li></ul></ul></ul><ul><ul><li>Voids are filled and the part is more dense with less surface energy </li></ul></ul>
  17. 17. Liquid Sintering <ul><li>Heat the compacted powder up just above the eutectic melting point </li></ul><ul><ul><li>Eutectic melting point is the minimum melting point of a combination of two or more materials </li></ul></ul><ul><li>On heating a small proportion of the ceramic material melts to form a highly viscous liquid </li></ul><ul><ul><li>Occurs at the periphery of the particles </li></ul></ul><ul><li>The liquid draws the ceramic particles together </li></ul><ul><li>On cooling the viscous phase transforms to either: </li></ul><ul><ul><li>Glass state (poor high-temp properties) </li></ul></ul><ul><ul><li>Crystalline state (better high-temp properties) </li></ul></ul>
  18. 18. Alumina <ul><li>Al 2 O 3 </li></ul><ul><li>Single crystal alumina referred to as “Sapphire” </li></ul><ul><li>Most used in polycrystalline from </li></ul><ul><li>Unique, complex crystal structure </li></ul><ul><li>Strength increases with decreasing grain size </li></ul><ul><li>Elastic modulus (E) = 360-380 GPa </li></ul><ul><li>Low friction and wear properties </li></ul><ul><ul><li>Good for joint bearings </li></ul></ul><ul><ul><li>Grain size must be very small, < 4  m </li></ul></ul>
  19. 19. Zirconia <ul><li>ZrO 2 (same compound as CZ, but a different crystal) </li></ul><ul><li>Good mechanical properties </li></ul><ul><ul><li>Stronger than alumina (2-3 times stronger) </li></ul></ul><ul><ul><li>Less stiff than alumina </li></ul></ul><ul><ul><li>Surface of the zirconia can be made smoother than that of an alumina </li></ul></ul><ul><ul><li>Zirconia-PE wear rates are ½ of alumia-PE wear rates </li></ul></ul><ul><li>Properties only good for tetragonal crystals </li></ul><ul><ul><li>Tetragonal form is unstable, may transform to other crystal structure with poor properties </li></ul></ul><ul><li>Must be stabilized to be useful, much to learn still </li></ul>
  20. 20. Fabrication with Al 2 O 3 and ZrO 2 <ul><li>Devices are produced by pressing and sintering fine powders at temperatures between 1600 to 1700ºC. </li></ul><ul><li>High purity alumina used in biomedical applications (>99.5%) </li></ul><ul><ul><li>Additives such as MgO added (<0.5%) to limit grain growth </li></ul></ul>a – Alumina sintered 180 minutes at 1580 °C b – Zirconia sintered 120 minutes at 1400 °C
  21. 21. Alumina & Zirconia Applications <ul><li>Orthopedics – femoral head, bone screws and plates </li></ul><ul><ul><li>Alumina at a bone interface: bone will grow right up to it, but will not grow in </li></ul></ul><ul><ul><li>Ceramic-UHMWPE contact used in hip and knee replacements </li></ul></ul><ul><ul><li>Ceramic-ceramics contact also used </li></ul></ul><ul><ul><li>Problem with stiffness of alumina… </li></ul></ul><ul><li>Dental restorations – crowns, bridges, brackets </li></ul><ul><ul><li>Good mechanical and aesthetic properties </li></ul></ul>
  22. 22. Elemental Carbon <ul><li>Elemental, non-metal, many forms possible </li></ul><ul><li>Properties depend on atomic structure </li></ul><ul><ul><li>Diamond, graphite, fullerenes, etc. </li></ul></ul><ul><li>Carbons generally have good biocompatibility </li></ul><ul><li>Forms used in bio-applications </li></ul><ul><ul><li>Graphite – lubricating properties </li></ul></ul><ul><ul><li>Diamond-like carbon – hard, wear-resistant </li></ul></ul><ul><ul><li>Glassy carbon – temp and chem resistant, low strength and poor wear resistance </li></ul></ul><ul><ul><li>Pyrolytic carbon – wear-resistant, fairly strong, brittle </li></ul></ul>
  23. 23. Pyrolytic Carbon <ul><li>Most successful and commonly used form </li></ul><ul><li>“ pyrolysis” – thermal decomposition </li></ul><ul><ul><li>Occurs at high temp, with an inert gas (N or He) </li></ul></ul><ul><ul><li>Instead of “burning,” the carbon “polymerizes” due to the absence of oxygen </li></ul></ul><ul><li>Often used as a coating material </li></ul><ul><ul><li>Preforms are coated, then machined and polished before assembly </li></ul></ul><ul><ul><li>Diamond plated grinders and tools are needed because PyC is very hard </li></ul></ul><ul><ul><li>Finish can be made very smooth </li></ul></ul>
  24. 24. Applications <ul><li>Very good blood-contacting properties </li></ul><ul><li>Used to coat </li></ul><ul><ul><li>Heart valve components </li></ul></ul><ul><ul><li>Stents </li></ul></ul><ul><li>Compatibility not perfect </li></ul><ul><ul><li>Anticoagulants needed </li></ul></ul><ul><ul><li>Blood compatibility not completely understood </li></ul></ul><ul><li>Other applications </li></ul><ul><ul><li>Joint components </li></ul></ul><ul><ul><li>solid PyC parts are possible </li></ul></ul>
  25. 25. 2. Porous Ceramics <ul><li>Porous ceramics have very limited properties due to the porosity (reduced solid volume) </li></ul><ul><ul><li>Generally restricted to non-load bearing applications </li></ul></ul><ul><ul><ul><li>Coatings for metal or other materials </li></ul></ul></ul><ul><ul><ul><li>Structural bridge for bone formation </li></ul></ul></ul><ul><ul><li>Increasing porosity results in </li></ul></ul><ul><ul><ul><li>Bone ingrowth to fix the component to tissue </li></ul></ul></ul><ul><ul><ul><li>Decreased mechanical properties </li></ul></ul></ul><ul><ul><ul><li>Increased surface area (more environmental effects) </li></ul></ul></ul><ul><ul><li>Pore size is critical to tissue growth & angiogenesis </li></ul></ul><ul><li>Calcium hydroxyapatite is the most common </li></ul><ul><ul><li>Converted from coral or animal bones </li></ul></ul>
  26. 26. Calcium Hydroxyapatite (HA) <ul><li>Ca 10 (PO 4 ) 6 (OH) 2 </li></ul><ul><li>HA is the primary structural component of bone. </li></ul><ul><ul><li>consists of Ca 2+ ions surrounded by PO 4 2– and OH – ions. </li></ul></ul>HA microstructure
  27. 27. HA <ul><li>Gained acceptance as bone substitute </li></ul><ul><li>Repair of bony defects, repair of periodontal defects, maintenance or augmentation of alveolar ridge, ear implant, eye implant, spine fusion, adjuvant to uncoated implants. </li></ul><ul><li>Properties </li></ul><ul><ul><li>Dense HA (properties are similar to enamel – stiffer and stronger than bone) </li></ul></ul><ul><ul><ul><li>Elastic modulus = 40 – 115 GPa </li></ul></ul></ul><ul><ul><ul><li>Compressive Strength = 290 MPa </li></ul></ul></ul><ul><ul><ul><li>Flexure Strength = 140 MPa </li></ul></ul></ul><ul><ul><li>Porous HA not suitable for high load bearing applications </li></ul></ul>
  28. 28. Bioceramic Coating <ul><li>Coatings of hydroxyapatite are often applied to metallic implants (most commonly titanium/titanium alloys and stainless steels) to alter the surface properties. </li></ul><ul><li>In this manner the body sees hydroxyapatite-type material which it appears more willing to accept. </li></ul><ul><li>Without the coating the body would see a foreign body and work in such a way as to isolate it from surrounding tissues. </li></ul><ul><li>To date, the only commercially accepted method of applying hydroxyapatite coatings to metallic implants is plasma spraying. </li></ul>
  29. 29. Bone Fillers <ul><li>Hydroxyapatite may be employed in forms such as powders, porous blocks or beads to fill bone defects or voids. </li></ul><ul><li>These may arise when large sections of bone have had to be removed (e.g. bone cancers) or when bone augmentations are required (e.g maxillofacial reconstructions or dental applications). </li></ul><ul><li>The bone filler will provide a scaffold and encourage the rapid filling of the void by naturally forming bone and provides an alternative to bone grafts. </li></ul><ul><li>It will also become part of the bone structure and will reduce healing times </li></ul>
  30. 30. <ul><li>Certain types of ceramics have been shown to bond to bone </li></ul><ul><ul><li>Bioactive glass </li></ul></ul><ul><ul><li>Bioactive glass-ceramics </li></ul></ul><ul><ul><li>Bioactive crystalline ceramics and bioactive composites exist also </li></ul></ul><ul><li>Have relatively high melt temperatures are (1300 – 1450ºC) </li></ul><ul><li>Can be cast into intricate shapes (in glass form) </li></ul><ul><li>Can be ground into powders, sized, and used for packing material, etc. </li></ul>3. Bioactive Ceramics
  31. 31. Glass <ul><li>Structure is isotropic, so the properties are uniform in all directions </li></ul><ul><li>Brittle </li></ul><ul><ul><li>No planes of atoms to slip past each other </li></ul></ul><ul><ul><li>No way to relieve stress </li></ul></ul><ul><ul><li>Often more brittle than (crystalline) ceramics </li></ul></ul><ul><li>Typically good electrical and thermal insulators </li></ul><ul><li>Transparent (amorphous) </li></ul><ul><li>A supercooled liquid or a solid? </li></ul><ul><ul><li>Viscosity of water at room temp is ~ 10 -3 Poise </li></ul></ul><ul><ul><li>Viscosity of a typical glass at room temp >> 10 16 P </li></ul></ul>
  32. 32. Glass Processing <ul><li>Completely melting ingredients to a homogeneous liquid and cooling to a homogeneous material. </li></ul><ul><li>Glasses are most commonly made by rapidly quenching a melt </li></ul><ul><ul><li>Elements making up the glass material are unable to move into positions that allow them to become crystalline </li></ul></ul><ul><ul><li>Result is a glass structure – amorphous </li></ul></ul>melt
  33. 33. Glass Structure <ul><li>Glass-forming oxides </li></ul><ul><ul><li>e.g., SiO 2 ; B 2 O 3 ; P 2 O 5 ; GeO 2 </li></ul></ul><ul><ul><li>glass-forming network: often the major component </li></ul></ul><ul><li>Glass-modifying oxides </li></ul><ul><ul><li>e.g., Na 2 O; CaO; Al 2 O 3 ; TiO 2 </li></ul></ul><ul><ul><li>modify glass network: add positive ions to the structure and break up network </li></ul></ul><ul><ul><li>minor to major component: alter glass properties (e.g. softening pt) </li></ul></ul><ul><li>Even when molten, chains not free to move, very viscous </li></ul>
  34. 34. Types of Glass <ul><li>Silicate glass (fused silica) </li></ul><ul><ul><li>SiO 2 </li></ul></ul><ul><ul><li>Each silicon is covalently bonded to 4 oxygen atoms </li></ul></ul><ul><li>Soda-lime glass </li></ul><ul><ul><li>70 wt% SiO 2 ; 15 wt% Na 2 O; 10 wt% CaO </li></ul></ul><ul><ul><li>Window glass, bottles, etc. </li></ul></ul><ul><li>Borosilicate glass: </li></ul><ul><ul><li>Some SiO 2 replaced by B 2 O 3 </li></ul></ul><ul><ul><li>80 wt% SiO 2 ; 15 wt% B 2 O 3 ; 5 wt% Na 2 O </li></ul></ul><ul><ul><li>Pyrex glass; cooking and chemical glass ware </li></ul></ul>
  35. 35. Glass-Ceramics <ul><li>Composite structure consisting of a matrix of glass in which fine crystals have formed </li></ul><ul><ul><li>Crystals can commonly be very fine (avg. size < 500 nm) </li></ul></ul><ul><li>Glass-ceramics are 50 to 99% crystalline </li></ul><ul><li>The result is a mixture of glass-like and crystalline regions that: </li></ul><ul><ul><li>Prevents thermal shock </li></ul></ul><ul><ul><li>Lowers porosity </li></ul></ul><ul><ul><li>Increases strength </li></ul></ul>
  36. 36. Glass-Ceramic Processing <ul><li>Glass with nucleating agent like TiO 2 is formed into the desired shape </li></ul><ul><li>Nucleating agents aide in the formation of the crystals </li></ul><ul><ul><li>Barely soluble in the glass </li></ul></ul><ul><ul><li>Remain in solution at high temperatures </li></ul></ul><ul><ul><li>Precipitate out at low temperature </li></ul></ul><ul><ul><li>Act as nuclei for crystal growth at elevated temperatures </li></ul></ul><ul><li>Conversion takes part in two phases </li></ul><ul><ul><li>First glass is seeded with nuclei </li></ul></ul><ul><ul><ul><li>The formed material may be lowered to the nucleating temperature after forming OR </li></ul></ul></ul><ul><ul><ul><li>It may be lowered to room temperature, then reheated to the nucleating temperature. </li></ul></ul></ul><ul><ul><li>Second crystals grow around the nuclei </li></ul></ul><ul><ul><ul><li>Following nucleation the temperature is then raised to the crystal growth temperature </li></ul></ul></ul>
  37. 37. Bioactivity <ul><li>Bioactivity is very sensitive to composition </li></ul><ul><ul><li>Both in glasses and glass-ceramics </li></ul></ul><ul><ul><li>Less than 60 mol% SiO 2 </li></ul></ul><ul><ul><li>High Na 2 O and CaO content </li></ul></ul><ul><ul><li>High CaO/P 2 O 5 ratio, minimum 5:1 </li></ul></ul><ul><li>Composition makes the surface highly reactive when it is exposed to an aqueous environment </li></ul>Bioactive glass implants (45S5) and matching drill bits used to replace the roots of extracted teeth
  38. 38. Bioactive Ceramic Interfacial Reactions <ul><li>When the bioactive glass is immersed in body fluids sodium ions leach from the surface and are replaced by H + through an ion exchange reaction. </li></ul><ul><ul><li>This produces a silica rich layer </li></ul></ul><ul><li>An amorphous calcium-phosphate layer is formed on the silica rich layer due to migration of the calcium and phosphate ions from the bulk of glass. </li></ul><ul><ul><li>Biological moieties such as blood proteins, growth factors and collagen are incorporated into the layer. </li></ul></ul><ul><li>The amorphous layer crystallizes into carbonate hydroxyapatite (equivalent to natural bone mineral). </li></ul><ul><li>Body’s tissues are able to attach directly to the crystallized layer </li></ul><ul><li>Layer grows to be approximately 100-150  m in depth. </li></ul><ul><li>Occurs within 12-24 hr </li></ul><ul><li>Cells arrive within 24 to 72 hr and encounter a bonelike surface, complete with organic components </li></ul>
  39. 39. Bioactive Applications Glass and Glass-Ceramic <ul><li>A/W Solid Glass-Ceramics (Cerabone®) </li></ul><ul><ul><li>Vertebral prostheses (for spinal fractures) </li></ul></ul><ul><ul><li>Vertebral spacers (for lumbar instability) </li></ul></ul><ul><ul><li>Iliac crest prostheses (restoration after bone graft removal) </li></ul></ul><ul><li>Solid Bioglass® </li></ul><ul><ul><li>Douek cochlear implants (100% effective after 10 years vs. 72% failure for metallic and polymeric implants of same type) </li></ul></ul><ul><li>Particulate Bioglass® </li></ul><ul><ul><li>PerioGlas® – for treatment of periodontal disease </li></ul></ul><ul><ul><li>NovaBone – bone grafting material for orthopedics & maxillofacial repair </li></ul></ul>Glass-Ceramic implants for spinal repair Glass-Ceramic cochlear implants
  40. 40. 4. Resorbable Ceramics <ul><li>Degrade upon implantation in the host </li></ul><ul><li>Rate of degradation varies from material to material </li></ul><ul><ul><li>rate needs to be equal to rate of tissue generation at specific site of application </li></ul></ul><ul><li>Almost all bioresorbable ceramics (except Biocoral and Plaster of Paris – calcium sulfate dihydrate) are variations of calcium phosphate </li></ul><ul><li>Uses of biodegradable bioceramics: </li></ul><ul><ul><li>Drug-delivery devices </li></ul></ul><ul><ul><li>Repair material for bone damaged by trauma or disease </li></ul></ul><ul><ul><li>Space filling material for areas of bone loss </li></ul></ul><ul><ul><li>Material for repair and fusion of spinal and lumbosacral vertebrae </li></ul></ul><ul><ul><li>Repair material for herniated disks </li></ul></ul><ul><ul><li>Repair material for maxillofacial and dental defects </li></ul></ul><ul><ul><li>Ocular implants </li></ul></ul>
  41. 41. Calcium Phosphate <ul><li>Calcium phosphate compounds are abundant in nature and in living systems. </li></ul><ul><li>Biologic apatites which constitute the principal inorganic phase in normal calcified tissues (e.g., enamel, dentin, bone) are carbonate hydroxyapatite, CHA. </li></ul><ul><li>In some pathological calcifications (e.g., urinary stones, dental tartar, calcified soft tissues – heart, lung, joint cartilage) </li></ul><ul><li>Form of calcium phosphate depends on Ca:P ratio </li></ul><ul><ul><li>Most stable form is crystalline hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ] </li></ul></ul><ul><ul><ul><li>Ideal Ca:P ratio of 10:6 </li></ul></ul></ul><ul><ul><ul><li>Crystallizes into hexagonal rhombic prisms </li></ul></ul></ul><ul><ul><ul><li>This apatite form of calcium phosphate is closely related to the mineral phase of bone and teeth </li></ul></ul></ul><ul><ul><ul><li>Very low bulk solubility; can be used as a structural biomaterial </li></ul></ul></ul>
  42. 42.  -Tricalcium Phosphate (TCP) <ul><li>Another widely used form is β -tricalcium phosphate [ β -Ca 3 (PO 4 ) 2 ] </li></ul><ul><ul><li>In aqueous environment surface reacts to form HA </li></ul></ul><ul><ul><li>4Ca 3 (PO 4 ) 2 + 2H 2 O -> Ca 10 (PO 4 ) 6 (OH) 2 + 2Ca 2+ + 2HPO 4 2- </li></ul></ul><ul><li>Often porous (partially sintered powders) </li></ul>Tricalcium phosphate thin film (Osteologic) used in orthopedic applications
  43. 43. Stability <ul><li>Resorption caused by 3 factors </li></ul><ul><ul><li>Physiologic dissolution (depends on environment pH, type of CaP) </li></ul></ul><ul><ul><li>Physical disintegration into small particles as a result of preferential chemical attack of grain boundaries (enhanced by porosity) </li></ul></ul><ul><ul><li>Biological factors, such as phagocytosis, which causes a decrease in local pH concentration </li></ul></ul><ul><li>Apatite forms are the most stable </li></ul><ul><li>high rate of dissolution  low rate of dissolution </li></ul><ul><li>TTCP > α -TCP > β -TCP > HA > Fluorapatite </li></ul><ul><li>Substitution of F- for OH- in HA greatly increases the chemical stability </li></ul><ul><ul><li>Get fluorapatite [Ca 10 (PO 4 ) 6 (F) 2 ] </li></ul></ul><ul><ul><li>Found in dental enamel </li></ul></ul><ul><ul><li>Principle is used in dental fluoride treatments (~ 1 in 100 OH- replaced) </li></ul></ul>
  44. 44. Mechanical Properties <ul><li>Dense HA (properties are similar to enamel – stiffer and stronger than bone) </li></ul><ul><ul><li>Elastic modulus = 40 – 115 GPa </li></ul></ul><ul><ul><li>Compressive Strength = 290 MPa </li></ul></ul><ul><ul><li>Flexure Strength = 140 MPa </li></ul></ul><ul><li>Porous HA </li></ul><ul><ul><li>not suitable for high load bearing applications </li></ul></ul><ul><li>TCP </li></ul><ul><ul><li>Generally poor (more of a packing material) </li></ul></ul>
  45. 45. Summary <ul><li>4 groups of ceramic for biomedical applications </li></ul><ul><ul><li>Nonporous, nearly inert – structural components </li></ul></ul><ul><ul><li>Porous, inert – non-load bearing, coatings, fillers </li></ul></ul><ul><ul><li>Nonporous, bioactive – coatings, dental applications, strong attachment to bone </li></ul></ul><ul><ul><li>Resorbable – fillers, spinal/defect repair, drug delivery </li></ul></ul><ul><li>Function greatly affected by </li></ul><ul><ul><li>Composition – bioactivity </li></ul></ul><ul><ul><li>Structure (crystal and grains) – mechanical properties </li></ul></ul><ul><ul><li>Processing – mechanical properties </li></ul></ul><ul><ul><li>Porosity – reactivity, degradation </li></ul></ul><ul><ul><li>In vivo environment – reactions with tissue/fluids </li></ul></ul>