## 4 Discussion  
The LAB process is a facile process to produce an area-specific CaP coating on a Co-Cr alloy substrate, as reported for several other substrates [14; 18; 19]. Surface and cross-sectional analyses (Figures 2, 6 and 7) confirmed that the LAB-processed substrate induced CaP precipitation within only 5 min of surface irradiation and was coated with a micron-thick CaP layer within 30 min. In conventional biomimetic processes, a micron-thick CaP layer is formed only after a surface-modified substrate is immersed in a supersaturated CaP solution for hours to days [11; 12; 13]. Thus, the present LAB process, which can be accomplished with a single-step (one-pot) procedure within 30 min, is a simple and more practical process than conventional biomimetic processes. Note that the CP solution remained transparent without inducing homogeneous CaP precipitation during the LAB process up to 30 min. Therefore, CaP was precipitated heterogeneously at the laser-irradiated substrate surface without the sedimentation of homogeneously precipitated CaP particles. The use of a liquid CaP source (CP solution) makes the LAB process different from other laser-assisted CaP coating techniques, such as  
Figure 9: (**a**) HAADF image, (**b**) STEM-EDX elemental (Ca, P, O, Co, Cr and Mo) maps and (**c**) STEM-EDX merged elemental (Ca + Cr) map of the ultra-thin cross-section of the LAB-processed (30 min) Co–Cr alloy substrate (interface with the CaP layer). (**d**) STEM-EDX spectra obtained from each region in (**c**). The Cu peaks in (**d**) are derived from the grid that was used to mount the cross-sectional ultra-thin sample. The C peak in (**d**) is derived from the oil-based ink (soft protective cover) that was applied to the substrate surface prior to the FIB processing. pulsed laser deposition [9] and laser-assisted powder fusion [10], both of which use solid CaP sources. A comparison with a non-irradiated surface of the LAB-processed (30 min) substrate revealed that the CaP coating was confined to the laser-irradiated surface; the non-irradiated surface was not affected, even in the same substrate (Figure 2c), which indicates the essential role of laser irradiation to induce CaP precipitation on the Co-Cr alloy substrate. A trace amount of CaP could possibly precipitate on the non-irradiated surface [21], although this was not detected under the tested analytical conditions. The role of laser irradiation in the LAB process for the Co-Cr alloy substrate is considered to be similar to that reported for titanium metal, i.e., heating and surface modification (oxidative deformation) [19]. The heating effect due to laser irradiation was confirmed in the control experiment without the use of a temperature-controlled water bath; the temperature of the CP solution increased with the irradiation time for the Co-Cr alloy substrate (Figure 4). The CP solution does not readily absorb light at a wavelength of 355 nm [14]; therefore, the temperature increase observed in the CP solution was due to laser light absorption and light-to-heat energy conversion at the Co-Cr alloy surface. The heating of the solution has an accelerating effect on CaP precipitation by an increase in the mass transfer rate and the degree of supersaturation of the CP solution (solubility of apatite and OCP decreases with an increase in the medium temperature [22; 23; 24; 25]). Besides the heating effect, laser irradiation has a surface-modifying effect on the Co-Cr alloy substrate. As shown in Figures 2a and 3, the LAB process caused micro-roughening and oxidation of the substrate surface. This is caused by laser-induced thermal reactions such as melting, vaporization, ionization and oxidation. The Co-Cr alloy substrate showed a similar micro-roughening on its surface under laser irradiation in ultrapure water (Figure 5a). Cross-sectional STEM-EDX analyses (Figure 9) indicated the surface segregation of Co-Cr alloy on the LAB-processed substrate; a Cr-rich layer deficient in Co was identified above the Co-rich alloy layer deficient in Cr on the substrate.