- Alumina - Zirconia - Carbon - Hydroxyapatite - glasses (vetroceramics, bioglasses)

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: [Ca 10 (PO 4 ) 6 ](OH) 2 Apatites are a family of compounds, close in structure and chemical formula to the mother compound, the mineral fluoroapatite (FA): [Ca 10 (PO 4 ) 6 ]F 2

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 Ca 2+ ions, of which 6 within the cell, and 8 shared with as many adjacent cells (8/2). Total = 10 Ca 2+ ions 10 PO 4 3- ions, of which 2 within the cell, and 8 peripheral, shared with as many adjacent cells. Total = 6 PO 4 3- 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

Precipitation from aqueous solutions 5 Ca(OH) H 3 PO 4 --> Ca 5 (PO 4 ) 3 OH + 9 H 2 O

Solid state : intimate mixtures of powders are heated at the needed temperature, to yield the required compounds: 12 CaHPO CaCO 3  2 Ca 10 (PO 4 ) 6 (OH) CO H 2 O

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 MPa, sometimes with liquid ligands, then sintered in air at °C for some hours, with a temperature gradient of 100°C/h. Excellent mechanical properties

Synthesis of porous HA

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)]

- Alumina - Zirconia - Carbon - Hydroxyapatite - glasses (vetroceramics, bioglasses)

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

Vetroceramic materials

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 T 1, 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.

BIOGLASSES

Typical composition (weight): 45% SiO 2, 25.5 % CaO, 24.5 Na 2 O, 6% P 2 O 5. i.e.: SiO 2 content < 60% mol High content in Na 2 O and CaO High ratio CaO/P 2 O 5

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

The end