Lectire 8 - Ceramics and Other Materials

What are Ceramics, Glasses and Glass Ceramics?

• Definition: an inorganic, nonmetallic solid
  prepared from powdered materials and
  fabricated into products through the application
  of heat

  • Primarily ionic and covalent bonds

• Categories of these materials

  • crystalline (bio-resorbable or inert) ceramics

  • glass, amorphous,

  • glass-ceramics, starts as a glass and ends up as
    a polycrystalline ceramics possibly with a
    residue glassy matrix.

• General properties:

  • Ionic bond =>Difficult to shear, very low ductility,
    high compressive strength, low tensile strength;

  • Low thermal and electrical conductivity

  • Refractory and high Tm

  • High hardness  dental materials

  •  Aesthetically pleasing appearance 

Variation in Slip Between Metal and Ceramics

• In ceramics, dislocation glide must occur over 2 atomic
positions due to the electroneutrality requirement
• Less slip in ceramics
=> More brittle fracture

Nearly Inert Bioceramics

• Alumina (Al2O3)
• Sapphire or ruby
• Single crystal or polycrystalline
• Excellent corrosion resistance and biocompatibility (very thin fibrous layer)

• High strength → structural support such as bone plates, bone screws

• Small grain size and narrow distribution → high hardness and
low surface roughness → low friction and wear → joint replacement.

Joint Replacement Applications of Alumina

Alumina socket and ball, coefficient of friction decreases with time and approaches the value of a normal joint  wear 10 times slower than metal-PE surfaces

Biodegradable or Resorbable Ceramics:

What causes the biodegradability or resorption?

• Resorption or biodegradation is caused by
– Physiochemical dissolution, depending on the solubility of the
material and local pH
– Physical disintegration into small particles as a result of
preferential chemical attack of grain boundaries
– Biological factors, such as phagocytosis, which also causes a
decrease in local pH

Calcium Phosphate

• The mineral phase of bone and teeth is mainly calcium and
phosphate ions.

bIODEGRADABLE CERAMIC

• Solubility and hydrolysis decrease with increasing Ca/P ratio.
• Ca/P ratio  1 is not suitable for biological implantation.

Hydroxyapatite (HA)

• Ca₁₀(PO4)₆(OH)₂,

• Hard tissue contain 60% HA (mostly carbonate HA), 25% water and 15% organic materials. HA can be converted from coral or animal bone

• Manufacturing:
  – Ca(NO₃)₂ + NaH₂PO₄ → precipitate of HA → drying and filtering → furnace 1150 degrees C → grounding →sieving → pressing in a die → sintering

• Elastic modulus (40-117 GPa),
  – Enamel: 74GPa
  – Dentin: 21GPa
  – Compact bone: 12-18 GPa

• Hexagonal rhombic crystal
  – Substitute of OH- by F-, chemical stability
  – Defects and impurities can be characterized by X-ray diffraction (crystalline phase), FTIR (chemical groups)

X-Ray Diffraction of HA

Typical FT-IR Spectrum of HA

Factors Influence Degradation Rate of Calcium Phosphate

– Rate of degradation increases as
• Chemical susceptibility to dissolution increases
• Surface area increases
• Crystallinity decreases
• Crystal perfection decreases
• Grain size decreases
• F- substitution decreases

Clinical Application of Calcium Phosphate

• Advantages: bioactive and osteoconductive
• Bioactive bonding mechanism
– differentiated osteoblasts produce a cellular
bone matrix of 3-5 micrometer layer at the surface →
0.05 to 0.2 micrometer → normal bone attached
through a thin epitaxial bonding layer to the bulk
implant
• As dense form:
– small unloaded implants such as in the middle
ear implant,
• As porous form
– granules for filling bony defects in orthopedic
and dental surgery
• As coatings
– with reinforcing metal posts as in dental
materials,
• As fillers in composites

Bioactive Glasses and Glass-ceramics

• Specific composition highly reactive surface in aqueous medium
– SiO2 greater than or equal to 60%,
– high Na2O and CaO,
– high CaO/P2O5
• The surface forms a biologically active carbonated HA layer that
provides the bonding interface with tissue. The interfacial strength 
the bulk strength of both the implant and tissue
• Clinical applications
– 45S5, SiO2 45%, Ca/P=5:1
– Ceravital®, middle ear surgery to replace ossicles damaged by chronic infection
– Periodontal defect repair, maintenance of the alveolar ridge for denture wearers
– Toothpaste ingredient against sensitivity

Compositional Dependence (w%) of Bone and Soft Tissue Bonding

The level of bioactivity of a specific material can be related to the time for more than
50% of the interface to be bonded, (t0.5bb) e.g.: (Index of Bioactivity) IB = (100/t0.5bb).

Sequence of Interfacial Reactions Involved in Forming a Bond Between Tissue and Bioactive Glass

Fill out this chart

Crystalline Carbon Materials

• Crystalline: diamond, graphite, and fullerene

Quasicrystalline Carbon Materials

• Quasicrystalline,
– glassy carbon, extremely inert,
used as electrodes in
electrochemistry or prosthetics
– pyrolytic carbon as implant
surface coating, has high
mechanical strength than glassy
and graphite carbon, excellent
tissue and blood compatibility,
used in heart valves and finger
joint implants

Composite Materials

• Consisting of 2 or more chemically distinct parts in the macro-scale, having a distinct interfaces separating them
• Fiber or particulate composites usually consists of one or more
discontinuous phases (usually stronger, called reinforcing materials)
embedded in a continuous phase (called matrix)
• The property of the composite material depends on
– Properties of each constituent
– Shape of the heterogeneities
– Volume fraction
– Interface
• Natural tissue such as bone and tendon or vessel are composites.

HAPEX^TM

• Composite of hydroxyapatite in a polyethylene matrix
• Stiffness similar to cortical bone
• High toughness
• Bone bonding in vivo
• Orbital implant and middle ear implants