Nanostructures in the Ca aluminate-Ca phosphate system

Six different mechanisms have been found to be involved during the hydration and curing of chemically bonded bioceramics in the Ca aluminate-Ca phosphate (CAPH) system [12]. The biomaterials will be in contact with different tissue— enamel, dentine, hard bone tissue, and soft tissue—as well as other biomaterial contact surfaces. These six mechanisms affect the integration differently depending on (1) what type of tissue the biomaterial is in contact with, (2) in what state (unhydrated or hydrated) the biomaterial is introduced, and (3) what type of application is aimed for (cementation, dental fillings, endodontic fillings, sealants, coatings, or augmentation products). The actual contact zone developed depends on a combination of the discussed mechanisms and the surrounding tissue. The latter varies from a cellular-free high content apatite tissue in the case of a dental enamel, via dentine to a bone structure with cellular and body liquid contact. Both a pure nanostructural, mechanically controlled integration, and a chemically induced integration seem plausible.

Table 1.6 presents a summary of the six mechanisms involved in the integration of CAPH materials toward tissues and implant surfaces.

Figs. 1.2-1.6 show the nanostructure of phases and porosity formed. Fig. 1.2 illustrates the typical nanosized microstructure of the hydrated material with nanosized porosity between precipitated nanosized hydrates [12].

Fig. 1.3 illustrates the integration between the biomaterial and the tissue, in this case dentine. Even in high magnification a complete integration without gaps seems possible [12].

Table 1.6 Chemical reactions of the CaO-Al2O3-P2O5-H2O (CAPH) system in contact with different environments [11]

Reaction mechanism

Description

Comments

Mechanism 1.

Main reaction

  • 3 (CaO AI2O3) + I2H2O ^ 3Ca2 + + 6Al3 + +
  • 4(OH)- ^ 3Ca2 + + 6Al(OH)-

^ Саз [Al(OH)4]2 (OH)4 (katoite) + 4Al(OH)3 (gibbsite)

Katoite and Gibbsite are formed as the main nanosized hydrates

Mechanism 2.

Complementary reaction with phosphate-containing solution

5Ca2 + + 3PO4-+ OH-^ Ca5(PO4 )3OH

Additional phase formed: nanosized apatite

Mechanism 3.

Contact zone reaction with body liquid in presence of the basic Ca phase

HPO4- + OH - ^ PO3- + H2O

Thereafter the apatite formation reaction occurs as mechanism 2,

5Ca2 + + 3PO4-+ OH - ^ Ca5(PO4)3OH

Nanosized apatite formation in the contact zone in presence of body liquid

Mechanism 4.

Transformation reaction of the originally formed phase Katoite.

Саз • (Al(OH)4)2 • (OH)4 ^ 2Ca2 + + hpo2- + 2H2P0- ^

Ca5 • (PO4)3 • (OH) + 2Al(OH)3 + 5H2O

Nanocrystals of apatite and

Gibbsite formed in the biomaterial contact zone toward tissue

Mechanism 5.

Biologically induced integration and ingrowth

Bone ingrowth toward the biomaterial contact area allows the new bone structure to come into integrated contact with the biomaterial.

New bone formation at the contact zone

Mechanism 6.

Mass increase reaction due to presence of unhydrated Ca

3CaO AI2O3 + I2H2O ^ 3CaO AI2O3 6H2O + 2AI2O3 ЗН2О

Mass increase and point welding

Nanostructure of Ca aluminate hydrates. The pore channels are estimated to be 1-2 nanometers and the hydrates in the interval 10-40 nm. (white bar 10 nm)

Figure 1.2 Nanostructure of Ca aluminate hydrates. The pore channels are estimated to be 1-2 nanometers and the hydrates in the interval 10-40 nm. (white bar 10 nm).

Nanostructural integration of CAPH material with dentine (gray particles in the biomaterial are glass particles)

Figure 1.3 Nanostructural integration of CAPH material with dentine (gray particles in the biomaterial are glass particles).

Tissue integration toward the nanostructured biomaterial, a hydrated Ca aluminate-based material with nanocrystals in the range 10-40 nm (black bar=500 nm)

Figure 1.4 Tissue integration toward the nanostructured biomaterial, a hydrated Ca aluminate-based material with nanocrystals in the range 10-40 nm (black bar=500 nm).

HRTEM of a precipitated hydroxyapatite crystal approximately 30 nm in size in the Ca aluminate-phosphate system

Figure 1.5 HRTEM of a precipitated hydroxyapatite crystal approximately 30 nm in size in the Ca aluminate-phosphate system.

Four dental crowns cemented by Ceramir C&B, a Ca aluminate-based chemically bonded bioceramic

Figure 1.6 Four dental crowns cemented by Ceramir C&B, a Ca aluminate-based chemically bonded bioceramic.

When apatite is formed at the interface according to any of the reaction mechanisms 2-4 above, at the periphery of the bulk biomaterial, the biological integration may start. Bone ingrowth toward the apatite allows the new bone structure to come in integrated contact with the biomaterial. The transition from tissue to the biomaterial is smooth and intricate [12]. For an experimental Ca aluminate-based system the ingrowth is shown in Fig. 1.4.

Fig. 1.5 shows how a nanocrystal of apatite is formed in the contact to tissue [12].

 
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