In recent decades, the global market demand for dental and orthopedic implants has grown steadily owing to an aging population, an increase in bad habits, and frequent accidents. Titanium alloys have a low specific gravity, high corrosion resistance, better compressive strength, and fracture toughness than other implant materials and are currently widely used in biomedical implant engineering. However, owing to the biological inertia of titanium alloys, long-term implantation may induce inflammation around the implant and affect its lifespan. To realize a tight connection between the implant and the surrounding tissue of the human body and form a stable osseointegration interface, the modification of the implant surface is very important. Among these surface properties, wettability mainly affects the type, quantity, and conformation of proteins deposited on the implant surface, and subsequently affects the cell response at the material interface. With deeper research on the surface modification of laser-irradiated materials, the coupling relationship between various surface properties after modification becomes complicated when considering the functional effect of hydrophilicity alone. However, the differences in the surface morphology make the relationship between wettability and cell biological behavior inconclusive. Therefore, it is necessary to systematically evaluate the wettability of surfaces with different structural features after femtosecond-laser texturing. The scanning path strategy and structural changes caused by laser parameters proposed in this study provide ideas for the design and construction of biomedical implant surface structures.

Ti6Al4V sheets with a thickness of 0.5 mm are prepared for the experimental study. Before processing, the samples are polished using a abrasive wheel, followed by ultrasonic cleaning in deionized water, acetone, and absolute ethanol for 15 min. Surface texturing is performed using a ytterbium fiber laser with a central wavelength of 1030 nm and a pulse duration of 300 fs. Sixteen samples are fabricated using two scanning path strategies to determine the effects of laser parameters on the surface structure and wettability. A scanning electron microscope (SEM) is used to observe the structural morphology of the processed sample surfaces. The cross-sectional morphology and surface roughness S_a of the samples are characterized using a confocal microscope. To evaluate the wetting properties, the static contact angles of the sample surfaces are measured using a video optical contact angle meter. The X-ray photoelectron spectroscopy (XPS) is used to detect the chemical compositions of the sample surfaces before and after femtosecond laser texturing.

In this study, the surface of titanium alloy is textured by changing the laser energy density and number of scans. With increasing laser energy density, the top of the microstructure gradually bulges, and the laser induced periodic surface structure (LIPSS) bifurcates and produces a large number of surface nanoparticles. When the number of scans increases, a thick recast layer appears on the edge of the microstructure, whereas the LIPSS structure does not disappear because of the low energy density (Fig. 3). The surface topography becomes more undulating as the laser energy density increases, which is consistent with the SEM image results (Fig. 4). The surface roughness S_a and microstructure height do not increase linearly with the energy density but tend to saturate (Fig. 5). Increasing the number of scans results in slight changes in the overall surface topography (Fig. 6). With an increase in the number of scans, the surface roughness and microstructure height exhibit continuously increasing trends (Fig. 7). After femtosecond laser texturing, the hydrophilicity of all surfaces improves, and the contact angle decreases with increasing energy density. The effect of the number of scans on the surface wettability is not significant at low energy density values, and the contact angle decreases slightly compared to that of the untextured samples. However, when both the laser energy density and number of scans are increased, the wetting behavior of the material surface changes drastically, and a superhydrophilic surface that is rapidly wetted by the droplet within 3 s is observed (Fig. 8). The XPS results show that the femtosecond-laser texturing changes the chemical composition of the titanium alloy surface. The carbon content decreases and the oxygen content increases significantly (Fig. 9). The further analysis of the Ti2p fine spectra shows that an oxidation reaction occurs when the femtosecond laser textures the surface of the titanium alloy, and a high laser energy density or number of scans promotes the conversion of elemental titanium to high-valence titanium oxide (Fig. 10).

Femtosecond laser irradiation of a Ti6Al4V surface can be used to effectively construct designed structures with different morphological and dimensional characteristics. Although both the laser energy density and number of scans contribute to an increase in the surface roughness of titanium alloys, there is a fundamental difference in terms of ablation. The energy density directly affects the degree of laser ablation on the surface, and the surface roughness is saturated by the plasma shielding at high energy density. The number of scans determines the height of the microstructure and the depth of the grooves; the higher the number of scans, the greater the surface roughness. Appropriate energy density values and numbers of scans help maintain a titanium alloy surface with regular microstructural features. Moreover, femtosecond laser processing significantly improves the hydrophilicity of the titanium alloy surface. A superhydrophilic surface with rapid droplet spreading is obtained at the laser energy density of 2.31 J/cm2 and the number of scans of 50, which may be attributed to the porous morphology with high roughness created by the increase in the laser energy density and number of scans. However, further research shows that the surface hydrophilicity of titanium alloys is not entirely dependent on the surface structural dimensions, and that the surface chemical composition changes caused by laser ablation also mediate the surface wetting process.