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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)

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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


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              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



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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

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               Digression
Manufacturing of artificial diamonds under
pressure
   http://radio.weblogs.com/0105910/2002/12/30
    .html
   http://www.lifegem.com




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     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



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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




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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




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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




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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




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    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

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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
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