Fabrication And Biocompatibility Of Ultrafast Laser Processed Micro/Nano Structures On Titanium Functional Surfaces

Titanium is widely used to fabricate medical implants owing to its excellent mechanical properties and good biocompatibility.To improve the stability,antibacterial resistance,and abrasion resistance of titanium implants in organisms,the surface must be modified.Although traditional processing technologies can already obtain high efficiency and proper quality on implants by changing their surface morphology and composition,it is difficult to control the modified area and is prone to pollution.Ultrafast laser processing has extremely high peak power and short pulse width.It can not only actively control the ablated area,directly process micro/nano multi-level structure,but also afford oxide layers,promoting cell adhesion.However,the processing efficiency is low and fewer types of structures are realized.As a result,adjusting and optimizing the process to meet the growing demand for diverse microtextures and high processing efficiency on titanium is urgently needed.Therefore,this study investigated the direct writing of micro-protrusion and micro-groove on titanium by femtosecond and picosecond lasers,analyzed the formation mechanism of surface microtextures under different laser processing parameters,and compared the differences in surface morphology,surface profile,surface wettability,and biocompatibility of micro-groove and micro-protrusion fabricated by two pulse width lasers.This research could provide a new method for implant modification,expand the types of titanium surface microtextures,and serve as a reference for the expansion of implant surface functions.Herein,the femtosecond laser processing mechanism and technology of different titanium surface microstructures were studied.The ablation threshold of femtosecond laser-ablated titanium was numerically calculated to obtain 1.01 J·cm-2.Combined with the photothermal effect of laser and Marangoni convection,the formation mechanism of defocusing micro-groove and micro-protrusion was revealed.The influence of different laser processing parameters on surface morphology and profile of the micro-groove and micro-protrusion structures were studied.The surface morphology evolution of structures under different processing tracks was analyzed.And it was found that better surface quality of line array structure could be obtained only when the processing tarck was along the laser polarization direction.And the surface oxygen content of the micro-groove line array could reach 20.22%.Then,in order to improve processing efficiency and realize curved surface processing,picosecond laser was used to process micro-groove and micro-protrusion structures on titanium surface.Meanwhile,the processing mechanism and technology were studied.Based on the numerical calculation method,when the pulse number of 100 and 500 were used in the single point ablation experiment of titanium,yielding ablation thresholds of 3.23 J·cm-2 and 2.36 J·cm-2,respectively.The formation mechanism of micro-groove and micro-protrusion line structures was analyzed by combining the convection and thermal influence effects.The effects of laser processing parameters on the morphology and size of the microstructures was analyzed and found that the size of the structures ablated using picosecond laser was mainly affected by the laser influence,while their shape was influenced by the spot overlapping rate.Moreover,by analyzing the surface morphology of the microstructures under different processing tracks,the quality of micro-groove and micro-protrusion only processed in the direction along to the laser polarization were better.And the oxygen content of line array surface could reach 38.32%.Finally,the surface wettability and biocompatibility of two ultrafast laser-machined microtextures were investigated.The water contact angle measurement results showed that after modifying using the femtosecond and picosecond lasers,the contact angle of titanium surface decreased from the 40.25° to 9.88° and 0°,respectively.The contact angle of modified samples stored in air,vacuum and the 0.9%NaCl solution for 3 and 90 days was compared.It was found that the picosecond laser-ablated arrays stored in 0.9%NaCl solution could maintain good 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 results of osteoblast adhesion,proliferation and differentiation experiments showed that the micro-protrusion or micro-groove arrays processed using the femtosecond laser were conducive to cell adhesion and arrangement,while those prepared using the picosecond laser facilitated cell spreading and migration.Among them,the femtosecond laser-ablated micro-groove line arrays and picosecond laser prepared micro-protrusion line arrays had better osteogenic performance,which was mainly due to their large ablation area and high surface oxygen content.Moreover,the picosecond laser-ablated micro-protrusion could be potenialy applied to the surface modification of complex-shaped implants.