## 3 Results and Discussion
### Surface Morphology and Roughness Analysis  
In a low-magnification SEM image (100\(\times\)) in Figure 2a, microtexturing is evident on the surface of the Ti64 specimens, where unmelted Ti64 metal powder particles were sintered to the surface of the structure during manufacturing. In Figure 2d, heat treatment caused some of the unmelted and partially melted Ti64 metal powder particles to become further fused and bonded to the surface. Moderately magnified images (1000\(\times\)) (Figure 2b,e) further revealed a distinct difference between the two surfaces: when the 3D-printed Ti64 specimen was annealed at 820 \({}^{\circ}\)C, heat treatment led to the formation of large-scale aggregations of Ti64 metal powder particles (Figure 2e) compared to the surface of control specimens. In higher-magnification images (10\({}^{4}\times\)) in Figure 2c,f, nanotexturing is evident on the surface of the heat-treated Ti64 specimens, which revealed a difference at the nanometer scale, namely, a closely spaced lattice layer formed on heat-treated specimens, which resulted in nano-elevation of the surface roughness. Figure 1: Optical microscope image and the particle size distribution of Ti64 powder. The mean particle size of the Ti64 powders determined using ImageJ was 31.42 \(\pm\) 9.27 \(\upmu\)m (\(n\) = 116). Figure 2: SEM micrographs of untreated and heat-treated Ti64 specimens at different magnifications. (**a**,**d**) 100\(\times\); (**b**,**e**) 1000\(\times\); (**c**,**f**) 10\({}^{4}\times\). As can be seen in Figure 2, the untreated 3D-printed Ti64 specimen primarily consists of microscale features (unmelted and partially melted Ti64 metal powder particles); however, the surface of a heat-treated specimen had a hierarchical structure consisting of microscale features (partially melted Ti64 spheroids) and nanoscale features (closely spaced nano-lattices). It was suggested that roughness in the micrometer range provides better biomechanical interlocking, while roughness in the nanometer range provides more adhesion sites for extracellular matrix (ECM) proteins [35]. The present experimental evidence possibly support why heat-treated SLM specimens are thought to promote implant stabilization and osseointegration. Figure 3a-e,g-k respectively show 3D surface topographies of untreated and heat-treated Ti64 specimens, where the surface height was measured using a laser confocal 3D profiler. To provide an easily interpreted encoding of surface heights, Parula mapping of height to color varying from blue to yellow was used in the current figure. Five interrogation areas with a FOV of 146.52 \(\times\) 146.52 \(\upmu\)m were scanned on each Ti64 specimen, as illustrated in Figure 3f,l. In the case of the untreated 3D-printed Ti64 specimen, yellow areas represent locations of unmelted Ti64 metal powder particles sintered to the surface, where the diameter of the yellow areas was of the same order as that of Ti64 powder, as shown in Figure 3a-e. In contrast, the formation of large-scale aggregations of Ti64 metal powder particles can be seen on the heat-treated surface, as shown in the large yellow areas in Figure 3i,k. As can be seen in Figure 3, the 3D surface topography depicted the same microscale features as did the SEM images. Figure 3: 3D surface topographies of Ti64 specimens measured using a laser confocal 3D profiler. (**a–e**)/(**g–k**) Surface heights of untreated/heat-treated Ti64 specimens; (**f,l**) untreated and heat-treated Ti64 specimens, where five interrogation areas equally spaced on each Ti64 specimen were selected for surface height measurements. In this study, a Gaussian high-pass filter with a cut-off value of 0.1 mm was used to obtained roughness and waviness features. Figure 4a-f respectively show the real surfaces, the roughness features, and the waviness features of untreated/heat-treated Ti64 specimens, where the real surface data were directly obtained from the position (a) and position (i) (see Figure 3).