Hydroxyapatite dehydroxylation

Non-thermal deposition methods are defined as those ones carried out at temperatures much below the incongruent melting point of hydroxyapatite, in particular at or near ambient temperature. However, frequently post-depositional heat treatment must be applied to either crystallise amorphous calcium phosphate (ACP), transform (dehydrated/dehydroxylated) precursor phases such as octacal-cium phosphate (OCP) to hydroxyapatite, and/or to remove organic compounds used in coating preparation, for example during sol-gel, dip coating, electrochemical and EPD. 

The extremely high temperature in a plasma jet leads, even during the very short residence time (hundreds of microseconds to few milliseconds, depending on particle density and size) of the hydroxyapatite particles, to dehydroxylation and finally thermal decomposition by incongruent melting. This thermal decomposition of hydroxyapatite in the hot plasma jet occurs in four consecutive steps as shown in Table 6.7. 

The existence of OHAp as products of partial dehydroxylation of hydroxyapatite was postulated earlier (Bredig, Franck and Fiildner, 1933 Trombe and Montel, 1978) but the existence of OAp was subject to controversy for many years until it has now been well established thanks to modern analytical techniques (e.g., Gross and Pluduma, 2012 see Chapter 6.2.1.4). Liao etal. (1999) reported that (i) a small amount of OH- ions is always present in the structure of OAp and (ii) even a loss of 75% of the chemically bound water maintains the apatite channel structure. The limiting composition is then Ca10(PO4)6(OH)0 5O0 75 that corresponds to a water loss of 1.34% (Trombe and Montel, 1971). Hence, partially dehydroxylated hydroxyapatite (designated OHAp) could be described either as a non-stoichiometric... 

The thermal dehydroxylation during plasma spraying of hydroxyapatite towards oxyhydroxyapatite and oxyapatite can be followed by Raman as well as infrared spectroscopy (Blakeslee and Condrate, 1971 Fowler, 1974 Weinlaender et al., 1992 Park, Condrate and Lee, 1998 Hartmann et al., 2001 Park et al., 2002 Fleet... 

Information on the degree of disorder introduced by thermal treatment of hydroxyapatite during plasma spraying could be obtained by NMR spectroscopy. The positions and shift of 1H-MAS and 31P-MAS NMR peaks are indicative of the environment of the P043- tetrahedra in calcium orthophosphates and allow distinguishing between dehydroxylation (oxyhydroxyapatite) and decomposition (tricalcium phosphate, tetracalcium phosphate) products of hydroxyapatite. 

Quantitative evaluation of the 2D-HETCOR spectra are presented in Table 7.3. As expected incubation in r-SBF causes over time dissolution of phases characterised by the distorted states 1 and 2 associated with oxyhydroxyapatite, that is partially dehydroxylated SRO-structured hydroxyapatite as well as dissolution of the thermal decomposition products TTCP and TCP, but to a lesser extent. Concurrently the relative proportion of crystalline well-ordered hydroxyapatite increases from 46 mass% in an as-sprayed coating to 74 mass% in a coating incubated under physiological conditions for 12 weeks. 

---