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Most used bioactive ceramics in the medical field.
Depending on which part of bone is replaced, either the dense or the porous form is used.
Hydroxyapatite (HA) is a double salt made of tricalcium phosphate and calcium hydroxide:
[Ca10(PO4)6](OH)2
Apatites are a family of compounds, close in structure and chemical formula to the mother compound, the mineral fluoroapatite (FA):
[Ca10(PO4)6]F2
Ca may be replaced by other divalent metals (Ba++, Sr++, Pb++, etc), P by other elements (As, V, Cr), fluorine by other halogens or monovalent anions (Cl-, OH-, Br-, I-, etc).
Only phospho-calcium apatites are of biomedical interest.
Besides, apatites may show a vast number of isomorphic substitutions, either at the cations and the anions (solid solutions).
Both biological and synthetic HA’s crystallize in the hexagonal system.
The crystal structure shows tetrahedra of Ca phosphate around pillars of hydroxyls, hosted in cavities, which can also host other ions, e.g. fluoride ions in FA.
The unit cell can be interpreted as the superposition of two structures, with a symmetry plane in the middle. In each cell there are:
14 Ca2+ ions, of which 6 within the cell, and 8 shared with as many adjacent cells (8/2). Total = 10 Ca2+ ions
10 PO43- ions, of which 2 within the cell, and 8 peripheral, shared
with as many adjacent cells. Total = 6 PO43- ions
8 OH– ions, all belonging to the unit cell for ¼, being located at vertexes. Total 8/4 = 2 OH- ions
Three ways to prepare HA powders are possible:
• Through solution chemistry
• Through solid state reactions
• Through hydrothermal treatments
Solid state:
intimate mixtures of powders are heated at the needed temperature, to yield the required compounds:
12 CaHPO4 + 8 CaCO3 2
Ca10(PO4)6(OH)2 + 8 CO2 + 4 H2O
Hydrothermal treatments:
Precipitation of HA from aqueous solutions at elevated temperatures and pressures, which yields purer and larger crystals (hexagonal prisms 0,1 mm in length).
Autoclave needed
Dense hydroxyapatite :
From sintering of powders: compression at 60-80 MPa, sometimes with liquid ligands, then sintered in air at 950-1300°C for some hours, with a temperature gradient of 100°C/h.
Excellent mechanical properties
Mechanical Properties
Depend on microporosity and synthesis procedures
Increase with sintering temperature of:
• density,
• grain size
• toughness
• resistance to compression, torsion and bending
• elastic modulus
Toughness may decrease if Ca phosphate is formed at high temperature
Sintered HA shows features higher than cortical bone, enamel and dentine, but resistance to wear is low.
Applications where substantial mechanical stress are absent:
Coatings of metallic prostheses
Small bone substitutes
Surface Chemistry (Bioactivity)
Depends much on pH. Usually biological fluids are acidic:
partial surface dissolution and formation of a hydrated layer, then
dissolution/precipitation processes yield a layer of hydroxo-carbonato-apatite on top of Hap
[Ca3(PO4)2]3 Ca[0,4 (OH)2 0,6 (CO3)]
Glasses: amorphous materials arising from the progressive stiffening of a liquid, not crystallized
upon cooling
With crystalline solids the solid-liquid transition takes place abruptly at a definite temperature (melting temperature).
With an amorphous solid (glass), the transition to the solid state takes place by continuous increase in viscosity upon cooling, till complete stiffness
The glass is characterized by a highly disordered spatial organization, similar to that of the liquid it comes from
Similarly, upon heating the glass does not melt at a given temperature but just decreases its viscosity becoming eventually fluid
A liquid becomes a glass when its crystallization rate is small with respect
to the cooling rate.
Because the vitreous state is a non-equilibrium one, a glass is
thermodynamically unstable, and tends to evolve towards more stable
situations, i.e. to crystallize.
Devetrification, i.e. phenomena of nucleation and growth of crystalline species, takes place when the glass is kept at a temperature at which crystallization rate is substantial
a vetroceramic material
The transition from the liquid-like state to the crystalline one is a local phenomenon taking place by the growth of nuclei made of few hundreds of atoms.
vetroceramics are made by:
- casting and cooling of a glass form (step I)
- heating up to T1 (step II)
- successive thermal treatment at a temperature T1, at which nuclei acting as crystallization centers are formed (nucleation).
-Heating up to T2 (step III)
- the sample is kept at T2 to allow the growth of the crystalline phases
-The sample is cooled (step IV)
Crystalline phases generated within the amorphous matrix amy be evidenced by an acid attack (the amorphous phase is more soluble in acids) .
Usually crystallization takes place at temperatures (T2!) at which flow is negligible, and no change in the shape of the sample occur.
Typical composition (weight):
45% SiO2, 25.5 % CaO, 24.5 Na2O, 6% P2O5.
i.e.:
SiO2 content < 60% mol
High content in Na2O and CaO
High ratio CaO/P2O5
Bioactivity of such materials takes place through a time-dependent modification of the surface.
Either in contact with SBF (in vitro) or biological fluids (in vivo).
The in vivo outcome is the complete integration with tissues either calcified (osteointegration) or non calcified.
Three component phase diagrams, showing the ultimate formation of Hap from bioglasses of different composition