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Ceramics (keramikos-burnt stuff in Greek) Ceramics are refractory polycrystalline compounds Refractory: difficult to fuse, corrode, or draw out, capable of enduring high temperature Usually inorganic Highly inert; thus, biocompatible. 1 Ceramics (keramikos-burnt stuff in Greek) Hard and brittle High compressive strength Generally good electric and thermal insulators Good aesthetic appearance 2 Ceramics Usually compound between metallic and non-metallic elements Always composed of more than one element Bonds are partially or totally ionic, and can have combination of ionic and covalent 3 Ceramics Majority has ionic (in salt compounds) OR metallic and nonmetallic elements (as in oxides Al2O3, MgO, SiO2) Applications: heart valves orthopaedic implants dental applications (see Handout #9 for further applications) 4 Remember from earlier courses! Stability of ceramics depend on two factors: Coordination number: maximum number of ions adjacent to another ion without overlap in electron orbitals (see Handout #2) Electronegativity: how willing atoms are to accept electrons 5 Ceramics Ceramic Structure: AmXn A: metal, +ve X: nonmetal, CsCl NaCl ZnS -ve 6 Ceramics: Physical Properties Hard (on the Moh’s scale: Diamond = 10, talc = 1, alumina = 9) High melting temperature Difficult to shear plastically due to ionic bond (Which material can shear easily?) Non-ductile, brittle. Thus, sensitive to notches and cracks (draw a brittle and a ductile stress strain curve, heart valve) 7 Inert Ceramics: Aluminum Oxides (Alumina) Natural alumina is known as sapphire or ruby (Al2O3) 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 8 Inert Ceramics: Alumina History: since early seventies more than 2.5 million femoral heads implanted worldwide. alumina-on-alumina implants have been FDA monitored over 3000 implants have been successfully implemented since 1987 9 Inert Ceramics: Aluminum Oxides (Alumina) The smaller the grain size and porosity the higher the strength Hardness = 9 on the Moh’s scale High hardness: low friction Good for which Joint replacement, low wear application? despite its corrosion resistance brittleness Friction: surface finish less than 0.02 µm Wear: no generation of wear particles; thus, biocompatible 10 Inert Ceramics: Aluminum Oxides (Alumina) Fabrication: single crystal form powdered form Single crystal: prepare a seed crystal at high temperature and feed fine alumina powders to grow on the seed crystal 11 Inert Ceramics: Aluminum Oxides (Alumina) Powdered form: Pressing and sintering fine alumina powders at about 1600 C MgO added (<0.5%) to limit grain growth very small grain size (<7 µm), 1.4 µm for medical grade alumina narrow grain size Advantage of small grain size? inhibiting crack growth under fatigue 12 Digression Manufacturing of artificial diamonds under pressure http://radio.weblogs.com/0105910/2002/12/30 .html http://www.lifegem.com 13 Inert Ceramics: Alumina Inertness: advantage: biocompatible disadvantage: nonadherent fibrous membrane at the interface. interfacial failure can occur, leading to implant loosening 14 Inert Ceramics: Alumina Mechanical behaviour simulated physiological environments predict a 50 year 99.9% survival probability at 112 MPa Wear on total hip: wear on alumina / UHMWPE is much less then metal/UHMWPE wear on alumina / alumina near zero 15 Inert Ceramics: Zirconia, ZrO2 zirconium; named from the Arabic, zargun = gold color Fabrication: Obtained from the mineral zircon Addition of MgO, CaO, CeO, or Y2O3 stabilize tetragonal crystal structure (e.g. 97 mol%ZrO2 and 3 mol%Y2O3) Usually hot-pressed or hot isostatically pressed 16 Inert Ceramics: Zirconia, ZrO2 Applications: orthopaedics: femoral head, artificial knee, bone screws and plates, favored over UHMWPE due to superior wear resistance dental: crowns and bridges made up about 25% of the total number of operations per year in Europe 8% of the hip implant procedures in USA. over 400,000 zirconia hip joint femoral heads have been implanted since 1985 until 2001 17 Inert Ceramics: Zirconia, ZrO2 Compared to alumina, partially stabilised zirconia (PSZ) higher flexural strength fracture toughness better reliability lower Young's modulus ability to be polished to a superior surface finish lower hardness 18 Inert Ceramics: Zirconia, ZrO2 Soluble in water: phase transformation from tetragonal to monoclinic phase… Success in hip simulator? Depends on the medium: Water: not good (weight loss of 28mg after 6000 cycles) Saline: so so Bovine serum: good (weight loss of 0.7mg after 20mill cycles) The water lubrication resulted in a wear about 10,000 times greater than with serum lubrication. 19 Inert Ceramics: Zirconia, ZrO2 Bad publicity: so called “TH-zirconia” implants in 1998 a small (nine batches) but catastrophic experience of zirconia ball fractures in patients unique to one ceramic company (St. Gobain Desmarquest) and one manufacturing process since the end of 2000, 317 head breakages have been reported in these implants as of august 2001, the company in question has no longer been making implants for medical applications 20 Inert Ceramics: Zirconia, ZrO2 New generation: TZP/alumina composite 80% TZP and 20% Al2O3, TZP is 90 mol% ZrO2-6 Mol%Y2O3-4 mol%Nb2O5 composition 70 vol% TZP (stabilized with 10mol%CeO2), and 30 vol% Al2O3 and 0.05mol% TiO2 21 Biodegradable Ceramics: Calcium Phosphates Degrade on implantation to the host Desirable to degrade at the rate at which the host tissue regenerates Almost all bioresorbable ceramics are variations of calcium phosphate 22 Biodegradable Ceramics: Calcium Phosphate Uses drug-delivery repair material for bone damaged trauma or disease void filling after resection of bone tumours repair and fusion of vertebrae repair of herniated disks repair of maxillofacial and dental defects ocular implants 23 Biodegradable Ceramics: Calcium Phosphate Degradation caused by: physiologic dissolution (depends on environment pH, type of calcium phosphate) physical disintegration (grain boundary attack) biological factors (phagocytosis etc.) Form of calcium phosphate depends on Ca:P ratio 5:3 hydroxyapatite, Ca10(PO4)6(OH)2 3:2 -tricalcium phosphate Ca3(PO4)2 24 Biodegradable Ceramics: Calcium Phosphate Most stable form is crystalline hydroxyapatite crystallizes into hexagonal rhombic prism closely resembles the mineral phase of bone and teeth Excellent biocompatibility High elastic modulus (40-117 GPa) Moduli of mineralized tissues: Enamel: 74 GPa Dentin: 21 GPa Bone: 12-18 GPa 25 Biodegradable Ceramics: Calcium Phosphate Structure resembles bone mineral; thus used for bone replacement Coating of metal implants to promote bone ingrowth Different forms exist depending on Ca/P ratio, presence of water, impurities and temperature 26 Biodegradable Ceramics: Calcium Phosphate Wet environment and low temperature: hydroxyapatite Dry environment and higher temperature: -whitlockite Both forms are tissue compatible 27 Biodegradable Ceramics: Calcium Phosphate Manufacturing involves precipitation of hydroxyapatite crystals in aqueous solutions of Ca(NO3)2 and NaH2PO4. Precipitates filtered, dried Kept at 900 C for 3h to promote crystallization Pressed into final form Sintered at about 1100 C for 3 hours 28 Bioactive Ceramics: Glass Ceramics Bioactive: capable of direct chemical bonding with the host biological tissue Glass: an inorganic melt cooled to solid form without crystallization an amorphous solid possesses short range atomic order BRITTLE! Glass-ceramic is a polycrystalline solid prepared by controlled crystallization of glass LESS BRITTLE 29 Bioactive Ceramics: Glass Ceramics Controlled crystallization is maintained by precipitating Cu, Ag, and Au by UV light irradiation These precipitates help to nucleate and crystallize the glass into a fine grained ceramic 30 Bioactive Ceramics: Glass Ceramics 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 31 Bioactive Ceramics: Glass ceramics SiO2 B C A D CaO Na2O A: Bonding within 30 days B: Nonbonding, reactivity too low C: Nonbonding, reactivity too high D: Bonding 32 Ceramics: Classification based on tissue attachment Type 1: Dense, inert, nonporous ceramics. Attach bone by tissue growth into surface irregularities OR by press fitting (termed as morphological fixation) 33 Ceramics: Classification based on tissue attachment Type 2: Porous inert ceramics attach by bone ingrowth into pores resulting in mechanical attachment of bone to material (termed as biological fixation) 34 Ceramics: Classification based on tissue attachment Type 3: Dense, nonporous surface-reactive ceramics attach directly by chemical bonding with bone (termed as bioactive fixation) 35 Ceramics: Classification based on tissue attachment Type 4: Dense, nonporous (or porous) resorbable ceramics which are slowly resorbed and replaced by bone 36
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