Nanostructured DLC-Ag Composites for Biomedical Applications
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NANOSTRUCTURED DLC-Ag COMPOSITES FOR BIOMEDICAL APPLICATIONS R.J. Narayan, H. Wang, A. Tiwari North Carolina State University, Raleigh, NC ABSTRACT We have synthesized novel diamondlike carbon coatings with silver nanoparticles embedded into the DLC film. The size of silver nanoparticles that are confined into layered structures has been varied from 5 nm to 50 nm using an ingenious pulsed laser deposition technique. The size of nanoparticles was found to be remarkably uniform within 15%. We have characterized these samples using high-resolution cross-section TEM and STEM-Z contrast techniques, electron energy loss spectroscopy (EELS), Raman, nanohardness, adhesion, and biocompatibility measurements. In the STEM-Z, where the contrast is proportional to atomic number2, we have obtained the details of the atomic structure of silver particles. We have correlated the microstructure with hardness and adhesion properties. The EELS was used in conjunction with STEM-Z to obtain sp3/sp2 bonding ratio. This ratio was compared with Raman result to provide an average bulk value. The role of silver nanoparticles is surmised to provide a reservoir of electrons for antimicrobial activity on the surface, as revealed by our biocompatibility tests. INTRODUCTION The introduction of implantable medical devices into the body has been shown to greatly increase the risk of infection. The number of bacteria required to cause an infection reduced by the presence of a biomaterial; in addition, the persistence of bacteria is enhanced. Bioengineering of hybrid implant materials in order to achieve optimal performance and to prevent infection and inflammatory reactions is a field undergoing rapid development. The sustained delivery of antimicrobial drugs into the local micro-environment of implants systemic side-effects and exceeds usual systemic concentrations by several orders of magnitude. Silver antimicrobial coatings have been designed to slowly deliver antimicrobial drugs to reduce implant infections. DLC describes hydrogen-free hard carbon solids with atomic number densities> 0.19 g-atom/cm3.1 It is a cross-linked, non-crystalline network of sp2- and sp3- hybridized carbon. Film densities of these films have been reported to be as high as 3.1 g/cm3, and film friction and wear coefficient are among the lowest recorded to date. The mechanical properties and electronic properties can be tailored by the sp3/ sp2 ratio. Films demonstrate optical gap up to 3 eV, transparency to light from deep UV through visible to far infrared, high refractive index, and wide resistance to radiation. Finally, these films exhibit excellent thermal conductivity, and extremely low thermal expansion. However, these films have internal stresses leading to poor adhesion and peel off. In this paper, we have designed a new architecture where silver,a compliant material, is incorporated as nanoparticles inside the DLC thin film matrix. It is based upon the theory that a more compliant entity in the hard carbon film may accommodate the large compressive stress a
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