Titanium is widely used as an implant material owing to its excellent mechanical properties and good biocompatibility. It is often used in the manufacturing of artificial joints, bone plates, dental implants, etc. To improve the stability, antibacterial resistance, and abrasion resistance of titanium implants in organisms, their surface must be modified. An ultrafast laser can actively control the surface processing area and afford oxide layers, promoting cell adhesion. Currently, some researchers have realized many functions of titanium. However, the processes of such functions are complex and fewer types of structures are realized. Therefore, this study investigates the direct writing of microprotrusion and microgroove on titanium by modifying the processing parameters of femtosecond and picosecond lasers, systematically analyzes the difference in the microtexture, and explores a post-treatment method for regulating wettability. Finally, cell adhesion and proliferation experiments are performed to evaluate the biological properties of different microtextured surfaces.

Titanium samples with a size of 10 mm were mechanically ground and polished. The samples were cleaned two times using ethanol for 10 min each time and then dried in air. Herein, both femtosecond laser (Spectra Physics Spitfire Ace; pulse width: 35 fs, wavelength: 800 nm, and repetition frequency: 1 kHz) and picosecond laser (BGL-1064-50B; pulse width: 15 ps, wavelength: 1064 nm, and repetition frequency: 10-1000 kHz) were used to ablate the titanium surface. For a comparison of hydrophilicity stability, the samples were separately stored in air, vacuum, and a 0.9% NaCl solution. The modified samples were immersed in a 1% fluoroalkylsilane solution (in ethanol) for one day to reduce the surface energy and then dried naturally. The ablated samples were loaded with rat bone marrow mesenchymal stem cells (rBMSCs) in a 24-well plate and cultured for 48 h. After immersing the cells with a 4% paraformaldehyde solution, dehydrating the cells with ethanol, and drying naturally, the samples were sprayed with gold to observe their morphology. The loaded samples were cultured for one, three, and five days and then mixed with a CCK-8 solution to measure their absorbance. The surface morphology and elemental content of the samples were characterized using scanning electron microscopy (JSM-6610LV) and energy dispersive spectroscopy (EDS). The surface profiles of the samples were observed using a VK-X200 confocal laser microscope. The surface wettability was evaluated using a contact angle measurement device (SDC-200S). The absorbance at 450 nm was measured using a M200 PRO NanoQuant microplate reader.

Herein, both femtosecond and picosecond lasers were used to prepare microprotrusions and microgrooves on titanium surfaces. When the spot diameter of the femtosecond laser increased and the laser influence decreased, the microgroove width gradually increased while the depth decreased and both the width and height of the microprotrusion increased. As the overlapping rate decreased, the microgroove approached a U shape and the top of the microprotrusion became sharp (Fig. 3). The size of the microprotrusion ablated by the picosecond laser was considerably larger than that of the microgroove. When the laser influence or overlapping rate increased, the width and height of the microprotrusion increased. The EDS results revealed that the oxygen content in the picosecond laser-ablated surface was higher than that in the femtosecond laser-ablated surface (Fig. 5). After modifying using the femtosecond and picosecond lasers, the contact angle of the titanium surfaces reduced from 40.25° to 9.88° and 0°, respectively. When the samples were stored in vacuum and the 0.9% NaCl solution, the picosecond laser-ablated arrays could maintain good superhydrophilicity. The silanization could reduce the surface energy of the sample without laser modification, femtosecond laser-ablated sample, and picosecond laser-ablated sample, yielding contact angles of 113.63°, 152.80°, and 146.38°, respectively (Fig. 6). The cells were mostly adhered along the top and edge strips of the microprotrusion and inside the microgroove processed using the femtosecond laser (Fig. 7). The cell proliferation results were consistent with the cell adhesion results (Fig. 8). Although the number of cells adhering to the picosecond laser-ablated surface was relatively small to the femtosecond laser-ablated surface, the picosecond laser-ablated surface could still afford more pseudopodia and then improved the cells spread on the top of the microprotrusion and the edge of the microgroove (Fig. 9).

Herein, femtosecond and picosecond lasers were used to prepare a conventional microgroove and a special microprotrusion structure on titanium. The size of the structures ablated using both the lasers was mainly affected by the laser influence, while their shape was influenced by the overlapping rate. The oxygen contents in the femtosecond laser- and picosecond laser-ablated surfaces could reach 20.22% and 38.32%, respectively. Because the surface wettability was mainly affected by different microtexture morphologies, the contact angle of the titanium surface after femtosecond laser ablation decreased from the 40.25° to 9.88°, while that of the picosecond laser-ablated surface reached 0°. When the samples were stored in vacuum or a 0.9% NaCl solution for three days, the picosecond laser-ablated surface could maintain stable superhydrophilicity. Combined with silanization, the femtosecond laser-ablated surface became superhydrophobic at a contact angle of 152.80°, while the contact angle of the picosecond laser-ablated surface was 146.38°. Furthermore, the microprotrusion or microgroove arrays processed using the femtosecond laser were conducive to cell adhesion and arrangement, while those prepared using the picosecond laser promoted the growth of the pseudopodia of cells, thereby facilitating cell spreading and migration. The cell proliferation results were consistent with the cell adhesion results, showing that femtosecond laser processing could likely promote osteogenic differentiation. The combination of ultrafast laser-based micro/nano processing and hydrophilic/hydrophobic surface preparation technology can enhance the surface activity of titanium implants.