*4.4. Features of the SPT for HA Film Coating*

A HA film coating on titanium was achieved by combining a simple chemical treatment (5 M NaOH treatment) with SPT in a calcium phosphate solution. One of the striking features of this proposed method was that the rate of HA film formation during SPT was faster than that in most wet processes. For example, Kim et al. reported that a bone-like apatite layer formed on the 5 M NaOH treated titanium substrate when the substrate had been soaked in simulated body fluid (SBF) for more than one day [5]. In contrast, a uniform HA film was observed to form on the 5 M NaOH treated titanium by SPT for only 30 min (Figure 3b). As stated before, the thermal energy produced during SPT increased the degree of solution supersaturation with respect to HA, and thus greatly increased the growth rate of the HA crystals.

The substrate heating method in liquid with the application of a large current can be applied to electrically conducting materials such as metals and alloys. For SPT, the solution in contact with titanium is heated with the thermal energy generated by plasma, suggesting that this HA coating method can be applied to materials without electrical conductivity such as ceramics.
