Laser surface texturing of stent materials for cardiovascular applications

Cardiovascular stents are widely used biomedical devices for cardiovascular diseases. During the process of three generations of stents (bare metal stents, drug-eluting stents, and biodegradable stents), changing materials or loading novel drugs involved long-term research on the many problems introduced by them, such as biocompatibility, strength, manufacturing, availability of the materials, drug loading and releasing, time-effects of the drug. Laser surface texturing is a fast, contactless, tailorable, precise surface engineering technique that can fabricate nano-to-micron features on the surface, which is a highly appropriate and universal solution for high-value biomedical devices, such as cardiovascular stents. Laser surface texturing for cardiovascular stents and related materials is one of the frontier topics and has attracted a growing number of research. However, the application of cheaper nanosecond lasers for cardiovascular stents still lacks comprehensive and systematic analysis. This project used nanosecond laser surface texturing as a cheaper and more convenient method to improve the stent performance for industrial applications.

This research was designed in four steps. The basic analysis first analysed the influence of laser processing parameters (fluence, pulse width, scanning speed, frequency and scanning pitch) on the surface topography and roughness through scanning electron microscopy (SEM) and white light profilometry. The function of each laser parameter was revealed for more controlled texturing on stainless steel through a nanosecond laser. Secondly, the fabrication of four typical types of textures (smooth/rough grooved textures and smooth/rough near-isotropy textures) and two novel textures (porous and feather-like crystal-covered) through one-time simple nanosecond laser direct scanning aimed at a controlled texturing process. The relationship between wettability, surface topography and surface chemistry was revealed with SEM, white light profilometer, X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA) measurements. These analyses aimed to support the mechanisms of the controlled laser surface texturing process through nanosecond laser on stainless steel. Then, a static blood contact test was used to characterise the hemocompatibility among positive/negative controlled samples, samples without laser texturing and laser-textured samples aiming to reduce clot formation. Lastly, mechanical tests, including tensile, bending, and tensile fatigue tests, tested samples mainly for bending and long-term performance.

The results of this project confirmed the success of controlled texturing through nanosecond laser, the increasing hemocompatibility, bending flexibility and modulus of resilience. The theory of accumulation effects from single-point ablation to single-line ablation and multiple-line ablation was built based on the fabrication of six types of textures and the effects of each laser parameter on topography and textures. This comprehensive and systematic analysis could rarely be found in other research but was important for laser surface texturing on metal. Analysis of the water contact angle revealed the dominant influence of surface topography over surface chemistry. WCA decreased with increased groove width/depth ratio (for grooved textures) and increased surface roughness (for near-isotropy textures). More interesting results were found from the blood contact tests, with improved hemocompatibility observed on rough textures. Still, the key factor in controlling hemocompatibility was not only the height roughness (Sa/Ra), but also the size of and the distance between surface features. Lastly, the three-point bending test of the textured samples showed an improved modulus of resilience and flexibility, and tensile fatigue test results revealed reduced cyclic softening, which is preferable for cardiovascular stent applications. However, the reduced fatigue life is also noticeable from the laser-textured surface.

This research has revealed the possibility of fabricating different types of textures through nanosecond laser with tuneable morphology, surface chemistry, and wettability. Most importantly, the results indicated increased hemocompatibility and comparable mechanical properties. These results provided a reference for future studies on the laser surface texturing process with nanosecond laser on cardiovascular stent applications to simplify the processing and reduce the cost.