Surface Modification of Various Metals for Biomedical Applications

Abstract

Metallic biomaterials including cobalt-chrome (Co-Cr) alloys, stainless steel and titanium have been extensively used for orthopedic implant and vascular stent application because of their superior mechanical properties and chemical stability. However, their relatively low biocompatibility and bioactivity are their major limitation of wider applications. The biocompatibility of material has close relationship with its surface properties, such as composition, surface roughness, and structure. Therefore, there have been many studies focusing on the modification of surface roughness and chemistry to improve their bioactivity and biocompatibility. Recently, a variety of new surface roughening technologies have emerged, which attempt to precisely design and modulate at the nanoporous surface features and properties of materials. Because of high surface area, nanoporous materials have attracted great interest for their wide applications in tissue engineering, catalysis, sensor, fuel cells, actuators, and so on. Especially, metallic biomaterials with engineered nanoporous structure on its surface possess the unique capacity of directly affecting the molecular and cellular events that ultimately determine the overall biological response to implanted materials, such as protein adsorption, cell adhesion and proliferation, among others. As a result of this exceptional ability, various nanotechnology-based techniques have been developed to generate nanoporous surface features on existing metallic biomaterials. However, so far, only a few papers have reported on creating a simple rough surface, and to the best of our knowledge, no attempts have been made to create nanoporous surfaces with a well-defined structure, which would open now avenues for designing metallic biomaterials with advanced functions. In the first chapter of this study, we investigated selective plasma etching with Ta ions on various metal substrates with the aim to figure the mechanism of nano-size surface pattern formation out and identify processes under various conditions. We have shown that sputtering with extremely high negative substrate bias triggers and drives the self-organized long-range order surface nano-sized patterns. The dimension and shape of nano-pattern structures were varied by changing applied negative substrate bias voltage and processing time. Our results provide the relevant parameters to achieve well-controlled nano-sized surface pattern formation, which would possibly applicable for various industrial fields. In the recent studies, the surface roughness of orthopedic implants significantly affects the rate of osseointegration and biomechanical fixation. The macro- and micro-roughness of the implant surface maximizes the interlocking effect between surrounding the mineralized bone and the surface of the implant. For the surface roughening of orthopedic implant, the most widely used commercial technique is sandblasting and acid etching (SLA) treatment. SLA-treated implants greatly improve osteoblast adhesion, migration, signaling, proliferation and differentiation in vitro, and bone formation in vivo due to the macro-roughness and micro-texture. Despite the relative success of SLA implants, osseointegration remains a clinical problem in the orthopedic application where insufficient bone mass is common, as well as in patients with systemic conditions such as diabetes and metabolic bone diseases. For the stent application, Cobalt-chrome alloy (Co-Cr) bare metal stent has been developed in the treatment of artery disease because of its excellent mechanical properties and chemical stability. However, Co-Cr bare metal stent implantation is associated with an excessive proliferation of vascular smooth muscle cells(SMCs) inside vessel wall after stent insert surgery due to the poor biocompatibility of stent materials. The aim of second and third chapter was to fabricate nano-roughened surface on the SLA-treated titanium and bare Co-Cr substrate by using tantalum ion-induced selective plasma etching process. Tantalum has displayed an exceptional biocompatibility and safety record in orthopedic and vascular stent application. Specially, the nano-roughness onto the SLA was significantly improved the level of in vitro cellular responses new bone regeneration rate. And a highly nanoporous Ta-incorporated Co-Cr surfaces were remarkably improved endothelial cell adhesion behavior, which was attributed to the nano-topographic feature and large surface area of the nano-patterned surface. These results show that the SPE method is very useful to create nano-patterned surface feature onto various metal substrates, and the potential for increasing biological properties of any substrates by mimicking the scale of natural tissues.