Nanoporous metal surfaces produced by plasma immersion ion implantation

Development of novel materials and structures for drug delivery systems is currently a very active field of research. For bare metal stents the in-stent restenosis was a serious problem for about 25 - 35% of the patients and this spurred the medical device companies to come up with a solution. Recently the drug-eluting stents were designed to deliver a drug locally from a surface layer to reduce restenosis.
High-fluence ion implantation of noble gas ions into metals can be used to create porous layers on metal surface. These void structures may show unique characteristics which offer potential for medical applications such as metal-based drug-eluting stents. This application requires interconnected pores with the dimension in the range on the nano- to microscale. Systematic investigations of the influence of implantation parameters on the surface morphology and cavity characteristics (e.g. size, distribution, and degree of interconnection) have to date been rather limited.
The aim of the present work is to study the formation of nanostructures on stainless steel surfaces by means of plasma immersion ion implantation (PIII) using different gases (helium or argon).
Argon and/or helium ion implantation was performed at following parameters: ion energies ranging from 5 to 35 keV, ion fluence of more than 10e18 cm -2, substrate temperature in the range 50 – 400°C. The surface topography of the modified steel has been analyzed by scanning electron microscopy and atomic force microscopy. Focused ion beam (FIB) tool was used to prepare sections perpendicular to implanted surface (i.e. transverse section). The phase and element compositions have been examined by grazing incidence X-ray diffraction analysis (GIXRD), X-ray photoelectron spectroscopy, and elastic recoil detection analysis.
Varying the ion energy, fluences, and substrate temperature has been found to produce either void or sponge like structures on the nano- (~10 nm) to micro-scale (~1 µm). Argon PIII treatment at elevated temperatures leads to spongy structure formation. Helium implantation results in a surface roughening and creation of voids in high concentration with size in the range 300 – 500 nm as well as nano-scale cavities (5-50 nm). Apart from the austenite iron peaks, the GIXRD patterns of the implanted samples display weak peaks of ferrite (bcc iron) as well as oxide phases.