Surface functionalization of nitinol utilizing ultrashort laser pulses

Ultrashort pulse (USP) lasers are an established technology to manufacture nitinol medical devices. USP lasers offer a great variety of processing parameters which can be utilized for fine cutting, laser lathe and ablation of nitinol. Usually, several hundreds of kHz are used to penetrate the material with pulses of fixed energy and frequency. New USP laser sources are offering a so-called burst mode which can be used to precisely control the energy deposition into the material by adjusting the temporal pulse distribution. Depending on the applied process parameters, this leads to a cut through or to ablation of some material and a modification of the irradiated surface. Previous work has shown that the bulk material is not affected by such laser light whereas the laser-matter interface is changed. The irradiated surfaces are dominated by laser-induced periodic surface structures (LIPSS) or cone-like protrusions (CLPs). These modified surfaces allow e.g. different roughness, wetting, corrosion, bioactivity and ultimately tribological properties. The effect of such femtosecond laser-generated structures has already been shown for stainless steel and titanium. Complex medical devices might benefit from locally adjusted surface properties e.g. reduced frictional force between tissue and device or improved adhesion due to an increased surface area through micro structures.

In this work, we report on recent findings on different nitinol surfaces with LIPSS and CLPs in comparison to an electropolished nitinol surface. To analyze specimen metallographic cross sectioning, laser scanning and scanning electron microscopy, focused-ion beam, corrosion testing and friction force testing are performed.