Porous metal surfaces produced by ion implantation

Development of novel materials and structures for drug delivery systems is currently a very active field of research. Plasma immersion ion implantation (PIII) using helium or argon plasmas has been employed for porous layers creation on metal surfaces. These void structures may show unique characteristics which offer potential for medical applications such as metal-based drug-eluting stents. The paper addresses the influence of implantation parameters on surface morphology, cavity characteristics and mechanical properties of stainless steel stents. Argon and/or helium PIII processing of stainless steel samples has been performed at ion energies ranging from 5 to 35 keV, ion fluence of more than 1e17 cm^-2, and substrate temperature in the range 50 – 400 C. Scanning electron microscopy, grazing incidence X-ray diffraction analysis, X-ray photoelectron spectroscopy, and elastic recoil detection analysis have been employed for sample characterization. Argon PIII treatment at elevated temperatures of 200 – 350 C leads to spongy structure formation of a size of 1-2 micron. 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). So, varying the ion species (helium or argon), ion energy and fluence, and substrate temperature has been found to produce either void or sponge like structures at the nano- (~10 nm) to micro-scale (~1 micron). The best mechanical properties have been obtained in the stainless steel samples implanted at elevated temperature (higher than 250 C). Surface flaking and cracks formation have been greatly reduced by subsequent post-implantation annealing at temperatures of 600 – 800 C.