Physical and bio cellular response of laser induced periodic surface structures formed using a femtosecond laser on a platinum alloy

Abstract

Minimally invasive implantable medical devices represent a major advance in the treatment of obstructive coronary artery disease with stent implantation becoming the standard coronary angioplasty procedure. Such devices interact with tissues at their surfaces and therefore surface quality can alter the delivery and performance of the device and can affect factors such as the adhesion of drug eluting coatings, biocompatibility and friction. Laser Induced Periodic Surface Structures (LIPSS) are ripples created on the surface of a material after ultra-short laser material interaction. Applications include improved drug coating adhesion, increased surface area and surface energy of devices, altered hydrophilic/hydrophobic performance on a surface, and modified cell growth.

This study investigates the evolution of LIPSS generated on biomedical alloy materials and includes understanding the formation of nanostructures and also the biological response of cells. The work focuses on three questions: • How does material reflectivity affect the material response and specifically the incubation coefficient? • How do nanostructures develop with increasing number of laser pulses? • What is the biological response on cardiac stents?

LIPSS were generated by applying femtosecond pulses, with a 500 fs pulse duration at a repetition rate of 100 kHz, to smooth polished 316LSS and Pt:SS (roughness Ra value of 2.9 ± 0.2 nm and 1.5 ± 0.2 nm respectively) cardiac stent surfaces. Exposing the sample to laser radiation slightly above the damage threshold fluence allows the formation of LIPSS structures. In order to create different periodic topographies, experiments were performed at two different laser wavelengths, 515 nm and 1030 nm. The LIPSS period and depth increased with increasing laser wavelength from a period of 388.7 ± 9.0 nm and depth of 86.1± 1.7 nm at a wavelength of 515nm to 740.0 ± 13.9 nm and 177± 5.2 nm at a wavelength of 1030 nm.

The role of reflectivity in multiple pulse incubation was investigated on the surface of Pt:SS, 316LSS and bulk Au. By using fluences below the damage threshold of the material, reflectivity measurements were performed on samples that were modified using multiple pulses. The study found that the incubation coefficient significantly increases with successive pulses when reflectivity is taken into account. In the case of Pt:SS, an incubation parameter was estimated to be 0.75 ± 0.02 and 0.79 ± 0.03 but when reflectivity was incorporated, increased to 0.90 ± 0.02 and 1.02 ± 0.03, at 515 nm and 1030 nm, respectively. This result highlights the important role of reflectivity in incubation and provides new insights into the laser material interaction of alloys.

The topography of LIPSS was characterised using high-resolution techniques such as Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Focused Ion Beam (FIB), X-Ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM). The initial exposure to femtosecond pulses induces nanostructures such as nanopillars, nanoparticles and bridges, on the material surface. As the number of pulses increases, LIPSS form alongside these nanostructures and it is believed that these nanostructures contribute to the onset of LIPSS formation. It was also found using FIB that there is no change in the grain structure after LIPSS formation. An examination of the crystal structure of Pt:SS using TEM revealed no change in the crystallinity of the material with the formation of LIPSS, a useful property in medical device applications.

The interaction of cells with a biomaterial is affected by the surface charge, chemistry, nano-topography and curvature of the surface. Cells attach to the surface through adhesion proteins, such as collagen, fibronectin, vitronectin and laminin. An in vitro biological study examined the response of monocytes (cells that circulate in the blood), fibroblasts (cells located in connective tissue) and endothelial cells (cells lining the interior surface of blood vessels) to LIPSS on Pt:SS and 316LSS coronary stents with surface roughness Ra values ranging from 29 nm to 50 nm. Varying bio-responses were observed for different cell lines and roughness values. Monocytes show a high affinity to bare un-textured surfaces but failed to firmly attach onto textured surfaces. Fibroblast cells did not adhere to un-textured surfaces but formed a monolayer on LIPSS surfaces with roughness values of approximately 50 nm indicating that the LIPSS surface supports adhesion after 24 hours. Endothelial cells did not attach to either un-textured or textured surfaces. However, it is likely that endothelial cells could attach onto LIPSS surfaces that have a roughness value outside the parameters used in this study.

A deeper understanding of LIPSS formation and biological interaction of various biomaterials is useful in the development of new technologies. Laser-structured surfaces with various topographies can offer new bio-functionalities in the area of medical implants both in coronary applications and other surface-contacting devices such as orthopaedic joint replacements.