Ultrashort Laser Texturing of Superelastic NiTi: Effect of Laser Power and Scanning Speed on Surface Morphology, Composition and Wettability

Abstract

Among metallic biomaterials, near-equiatomic NiTi is one of the most promising intermetallic system applicable for biomedical applications, thanks to its high biocompatibility and unique superelasticity (or pseudoelasticity), offering a complete recoverable strain up to 8%. In the prospective uses for bio-devices, the surface processing of NiTi medical components plays a fundamental role for guaranteeing both a Ti oxide passivizing layer for avoiding Ni ion release into the human body and surface morphology for controlling the cell proliferation. Mechanical polishing, thermal, chemical or electro-chemical treatments are typically performed for surface modifications. Recently, laser texturing has been successfully applied for different materials, included NiTi shape memory alloys, and also for tuning the surface properties, such as wettability. In the present work, ultrashort laser surface modification was carried out, through the use of a femtosecond laser, for the surface texturing of commercial superelastic NiTi plates. The main goal is to investigate the correlation among morphology, chemical composition and wettability with the principal process parameters, such as average power and scanning velocity in high power ultrashort laser texturing. Laser patterned surfaces were characterized by means of scanning electron microscopy, 3D-profilometry, XPS analysis and wetting measurements. After the laser treatments, both surface morphology and Ni/Ti ratio were largely modified from the initial surface, depending on the adopted process parameters. The wettability of the laser textured surfaces can be also varied with respect to the initial surface, due to the roughness values and grooves induced by the laser beam scans. The laser texturing process induced a combination between micro and nano structures, depending on the input energy. In details, the surfaces were tuned to lower roughness values (from 0.4 μm to 0.3 μm) with a laser power of 1 W, while it was increased up to 0.65 μm with a laser power of 13 W. The laser surface modification promoted a change of the contact angle from 70° of the untreated condition up to 135° to the surface laser treated with a power of 13 W.

Enhancement of Surface Properties Using Ultrashort-Pulsed-Laser Texturing: A Review

“…Table 1 summarizes some compelling findings from recent research that highlight the importance of texture design and LST for surface wettability. Biff et al [30] explored the wettability characteristics of superelastic NiTi, a key material in biomedical applications, using an fs laser to texture the material surface. The experiments were conducted in ambient air at different laser powers (1 and 13 W) and scanning speeds (2 and 4 m/s).…”

“…The CAs of the laser-textured surfaces depended on the irradiation energy; at the lowest energy, the CA was not substantially altered from that of the untreated surface (~70 • ). As the energy increased, the CA rose to 131 • , indicating the reduced wettability of the laser-textured surface [30].…”

“…The shock waves resulting from bubble expansion induce structural transformations on the surface. Increasing the laser power or energy increases the size and pressure of the bubbles, consequently expanding the depth and width of shock-induced ruptures and enlarging the surface pores [30,40,41].…”

“…The laser power intensity of ablation-based LST shapes the geometry (depth and size) of the texture features [40]. Chen et al and Biff et al [30] demonstrated that besides increasing the surface roughness, a high laser power converts hydrophilic Ni-based superalloy (C-263) and NiTi surfaces to hydrophobic surfaces. The laser scanning speed, which defines the scanning rate of the laser beam over the substrate, influences both the heat input and cooling rate of the processed material.…”