Redes inmunológicas artificiales (redes idiotípicas)

(deoisze G eoruiou

2.4.1 Crystallographic structure

Crystallographic structures o f crystals can be categorized depending on the type of symmetry inherent to them. These structures can therefore be categorized according to their space group. Hydroxyapatite, in particular calcium hydroxyapatite, has a definite crystallographic structure and composition, and has an intrinsic space group (Pbg/m) which is based on a repeating unit cell (Figure 9). The unit cell o f hydroxyapatite consists o f a six fold c-axis perpendicular to three equivalent a-axes (ai,a2,a3) at angles 120"C to each other.

The groups that make up the closely packed hexagonal unit cell are Ca^^, P0 4^‘, and OH groups.31,4 7,5 3,59-62

Cal

OH

Call

PO.

D evelopm ent o f G lass R einforced H ydroxyapatite for Hard Tissue S urgery G eorge G eorgiou

The exact atomic positions o f these atoms can be located by x,y,z coordinates within the unit cell. From the ten calcium atoms in the structure, two occupy a position z = 0 and two at z = 0.5, hence these four calcium atoms belong to the Ca(I) subset. The other six form a triangular arrangement perpendicular to the c-axis, a set o f three located at z = 0.25 and another set o f three at z = 0.75, with each arrangement surrounding an OH group in the same plane, one OH located at z = 0.25 and the other at z = 0.75.^^'"^^'^^'^^'^^

The six P0 4^' groups belong to a tetrahedral symmetry and are responsible for the stability o f the apatite structure. These groups occupy the positions z = 0.25 and z = 0.75. The oxygens bonded to the phosphorous atoms in the phosphate groups are also labelled, these are denoted by Oi, On and two Om. The atomic arrangements that the atoms adopt in hydroxyapatite are similar to those seen in F-Apatite, Caio(P0 4)3F2, and Cl-Apatite, Ca,o(P0 4)3Cl2, but the OH atomic positions in this instance are described for F' and Cl" ions that substitute for the OH groups.

Changes in the properties o f Ca-hydroxyapatite can occur as a result o f ionic substitutions o f Ca^^, P0 4^" and OH' groups within the apatite structure. The changes in properties

include lattice parameters, morphology and solubility, however, they can take place without significantly altering the hexagonal symmetry. It has, though, been reported that the symmetry changes from hexagonal to monoclinic occur upon CT substitution for OH'and this is in fact a loss in symmetry which is thought to reflect the alternating positions o f Cl" atoms and an expansion o f the cell in the b-axis.^^

D evelopm ent o f G lass R einforced H ydroxyapatite for H ard Tissue S urgery G eorge G eorgiou

The solubility properties o f apatites can give an indication o f the crystallinity, stability and crystal size o f a particular apatite. Fluoroapatite for instance, exhibits a higher degree o f the aforementioned properties, and consequently are less soluble, compared to fluoride free apatites and biological apatites. This can be attributed to a contraction in the a-axis without any change in the c-axis, and arises from substitution o f F' for

The effect on lattice dimensions and hence the crystal properties within Ca-HA by type A and type B carbonate substitution as well as coupled substitutions such as COg^ for P0 4^‘ and Na^ for Ca^^ within Ca-hydroxyapatite have already been previously mentioned in the carbonate apatite section.

Differences in lattice parameters can also be observed for Ca-hydroxyapatite that undergoes cationic substitution. Ions such as strontium (Sr^"^), magnesium (Mg^"^), barium (Ba^^), lead (Pb^^), etc. substitute for Ca^^ in the crystal lattice. The extent to which these changes take place reflect the size and amount o f the substituting ions. Crystallinity, thermal stability and dissolution properties would consequently alter as cationic substitutions take place, for instance the extent o f dissolution would increase for apatites with the substitution o f Sr^^ or

Mg^"^ for Furthermore, the presence two substituents such as Mg^^ and

CO]^ would have synergistic effects on crystallinity and dissolution properties o f synthetic apatites. Alternatively, antagonistic effects can also be seen with the presence o f two substituents such as magnesium and fluoride or carbonate and fluoride, with the fluoride effect being the greatest.^^

D evelopm ent of'G lass R einforced Hydroxyapatite for H ard Tissue Sui'gery G eorge Georgiou

The type o f substitutions that have been metioned are important in understanding how the change in properties o f the apatite relate to the way HA behaves as a biomaterial, in particular the manner that HA interacts with bone mineral o f the bone-biomaterial interface. HA can be used for clinical applications such as bone repair, augmentation, substitution and coatings o f metals used as dental and orthopaedic implants.

2.4.2 Hydroxyapatite preparation

The preparation o f HA can be carried out via a number o f methods, which include hydrothermal conversions, solid state reactions, precipitation and hydrolysis.^^ However, under aqeuous conditions such as precipitation or hydrolysis the apatite formed is usually calcium deficient giving a lower Ca/P molar ratio compared to the stoichiometric value o f pure HA o f 1.67. In contrast, precipitation can also result in a higher Ca/P molar ratio when the reaction is carried out under very basic conditions, and will produce HA containing carbonate.

2.4.2.1 Precipitation method

1 0Ca(OH)2 + 3H3(P0 4 ) 2 Caio(P0 4)6(OH)2

The reaction described above is, commercially, the most commonly used precipitation m e th o d .T h e reaction which is based on the Rathje method involves a stirring suspension o f calcium hydroxide, Ca(0 H)2 in water into which phosphoric acid, H 3 P O 4 , is added

D evelopm ent o f G lass R einforced H ydrosyapatiie for Hard Tissue S urgery G eo rg e G eorgiou

1 0Ca(NO3)2 + 6(NH4)2HP0 4 + 2NH4OH -> Ca,o(P0 4)6(OH)2

The alternative precipitation route above which is based on the Hayek and Newesley method is sensitive to the concentrations o f the reactants and pH o f the reaction, which consequently has an effect on the product formed upon sintering o f the precipitated apatite.^^ In order to avoid the formation o f octacalcium phosphate and subsequent conversion to P-TCP, the pH o f the reaction needs to be controlled. This can be done by the addition o f ammonium hydroxide ( N H 4 O H ) to both solutions o f calcium nitrate, Ca(N0 3)2,

and ammonium phosphate (NH4)2HP0 4 to give a significantly high pH level, typically between 1 0-1 2,

2.4.2.2 Hydrolysis method

Acid calcium phosphates can be used as reagents for the preparation o f apatite. The

hydrolysis o f acid calcium phosphates in ammonium, sodium or potassium hydroxide,

carbonate, fluoride or chloride solutions is an alternative method for the preparation o f apatite. The calcium reagents can involve dicalcium phosphate dihydrate, DCPD, CaHP0 4.2H2 0; octacalcium phosphate, OCP, Ca8H2(P0 4)6.5H2 0; or monetite, dicalcium

phosphate anhydrous, DCP, CaHP0 4.^^

2NH4OH