Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 62

converted coral increased its mechanical properties. This nanocoating was reported to
increase all of the mechanical properties. Coral, in addition to the macropores, also con-
tains meso- and nanopores ranging between 5 and 50 nm within a fibrous structure that
constitutes the interpore area between the large macropores.
The method involves a unique patented two-step conversion procedure. In the first
stage, corals possessing a suitable macropore structure are converted to pure HAp, with
complete replacement of calcium carbonate by phosphatic material throughout the speci-
men. The HAp can be prepared from coral by a hydrothermal reactor. The reaction can be
carried out at 250°C with an ammonium monohydrogen phosphate solution for a prede-
termined period of time. Additional stage involves the use of sol-gel derived HAp with
the converted coralline apatite.
In general, the alkoxide method used involves the formulation of a homogeneous solu-
tion containing all of the component metals in the correct stoichiometry. Mixtures of metal
alkoxides and/or metal alkanoates in organic solvents that have been stabilized against
precipitation by chemical additives (amines, glycols, acetylacetone, etc.) have proven the
most successful.
All synthetic procedures for the preparation of sols were carried out in a dry nitrogen
atmosphere due to the hygroscopic nature of the starting alkoxide precursors. The calcium
alkoxide precursor solution was prepared by dispersing calcium diethoxide (Ca(OEt) 2 ) in
absolute ethanol (Et(OH)). Phosphorous precursor solution was then prepared by diluting
triethyl phosphite (P(OEt) 3 ) in absolute Et(OH) (Ben-Nissan et al. 2001).
After complete dissolution of the calcium diethoxide, the phosphorous precursor solu-
tion can be added to the calcium precursor solution. The clear solutions were aged at ambi-
ent temperature before being used. Coatings were formed using these solutions followed
by subsequent heat treatment for further densification.
Detailed analyses using SEM revealed that the coating step effectively obliterates the sur-
face meso- and nanopore systems while leaving the macropore system intact (Figure 2.11).
Application of nanocoating had two major effects; first, the improvement of the mechan-
ical properties compared to traditional coral materials, and second, it enhanced the bio-
activity of the material. Mechanical testing indicated that all properties have increased
including compression, biaxial, flexural strengths, and the elastic modulus (Ben-Nissan

300 nm Mag = 40.34 K X EHT = 1.00 kV Noise Reduction = Line Int. Diagonal A = SE2 Date: 4 Aug 2003

WD = 6 mm Extractor I = 220.40 µA Gun Vacuum = 7.51e-0.10 mBar Time: 14:15:59

FIGURE 2.11
SEM reveals that the coating step effectively obliterates the surface meso- and nanopore systems, whilst leaving
the macropore system intact.