Chun Ming Shih

Effect of surface oxide properties on corrosion resistance of 316L stainless steel for biomedical applications

Surface passivation is a promising technique for improving the corrosion resistance both in vitro and in vivo as well as the biocompatibility of 316L stainless steel. In this work, we studied the effect of different passivative processes on the in vitro corrosion resistance of 316L stainless steel wire. Characterization techniques such as anodic polarization test, scanning electron microscopy, auger electron spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were employed to co-relate the corrosion behavior to various surface characteristics and surface treatments. Results showed that all of these surface treatments did not improve the corrosion resistance of the alloy satisfactorily except amorphous oxidation. This improvement is attributed to the removal of plastically deformed native air-formed oxide layer and the replacement of a newly grown, more uniform and compact one which is composed of nano-scale oxide particles with higher oxygen and chromium concentrations. The properties of surface oxide layer, rather than its thickness, seem to be the predominant factor to explain the improvement of in vitro corrosion resistance.

Growth inhibition of cultured smooth muscle cells by corrosion products of 316 L stainless steel wire

The potential cytotoxicity on vascular smooth muscle cells of corrosion products from 316 L stainless steel, one of most popular biomaterials of intravascular stents, has not been highlighted. In this investigation, 316 L stainless steel wires were corroded in Dulbeccos modified eagles medium with applied constant electrochemical breakdown voltage, and the supernatant and precipitates of corrosion products were prepared as culture media. The effects of different concentrations of corrosion products on the growth of rat aortic smooth muscle cells were conducted with the [3H]-thymidine uptake test and cell cycle sorter. Both the supernatant and precipitates of corrosion products were toxic to the primary culture of smooth muscle cells. The growth inhibition was correlated well with the increased nickel ions in the corrosion products when nickel concentration was above 11.7 ppm. The corrosion products also changed cell morphology and induced cell necrosis. The cell growth inhibition occurred at the G0/G1 to S transition phase. Similar to our recent study of nitinol stent wire, the present investigation also demonstrated the cytotoxicity of corrosion products of 316 L stainless steel stent wire on smooth muscle cells, which might affect the poststenting vascular response.

Characterization of the thrombogenic potential of surface oxides on stainless steel for implant purposes

Abstract Marketed stents are manufactured from various metals and passivated with different degrees of surface oxidation. The functional surface oxides on the degree of antithrombotic potential were explored through a canine femoral extracorporeal circuit model. Related properties of these oxide films were studied by open-circuit potential, current density detected at open-circuit potential, the electrochemical impedance spectroscopy, transmission electron microscopy, Auger spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. Experimental evidences showed that blood clot weight after a 30-min follow-up was significantly lower for the stainless steel wire passivated with amorphous oxide (AO) compared to the wire passivated with polycrystalline oxide (PO) or commercial as-received wire coils (AS). Surface characterizations showed that a stable negative current density at open-circuit potential and a significant lower potential were found for the wire surface passivated with AO than for the surface passivated with PO. Time constant of AO is about 25 times larger than that of polycrystalline oxide. Significant difference in oxide grain sizes was found between PO and AO. Surface chemistries revealed by the AES and XPS spectra indicated the presence of a Cr- and oxygen-rich surface oxide for AO, and a Fe-rich and oxygen-lean surface oxide for PO. These remarkable characteristics of AO surface film might have a potential to provide for excellent antithrombotic characteristics for the 316L stainless steel stents.

Atomic layer deposition-based functionalization of materials for medical and environmental health applications

Nanoporous alumina membranes exhibit high pore densities, well-controlled and uniform pore sizes, as well as straight pores. Owing to these unusual properties, nanoporous alumina membranes are currently being considered for use in implantable sensor membranes and water purification membranes. Atomic layer deposition is a thin-film growth process that may be used to modify the pore size in a nanoporous alumina membrane while retaining a narrow pore distribution. In addition, films deposited by means of atomic layer deposition may impart improved biological functionality to nanoporous alumina membranes. In this study, zinc oxide coatings and platinum coatings were deposited on nanoporous alumina membranes by means of atomic layer deposition. PEGylated nanoporous alumina membranes were prepared by self-assembly of 1-mercaptoundec-11-yl hexa(ethylene glycol) on platinum-coated nanoporous alumina membranes. The pores of the PEGylated nanoporous alumina membranes remained free of fouling after exposure to human platelet-rich plasma; protein adsorption, fibrin networks and platelet aggregation were not observed on the coated membrane surface. Zinc oxide-coated nanoporous alumina membranes demonstrated activity against two waterborne pathogens, Escherichia coli and Staphylococcus aureus. The results of this work indicate that nanoporous alumina membranes may be modified using atomic layer deposition for use in a variety of medical and environmental health applications.

Atomic layer deposition of titanium dioxide on cellulose acetate for enhanced hemostasis

TiO₂ films may be used to alter the wettability and hemocompatibility of cellulose materials. In this study, pure and stoichiometric TiO₂ films were grown using atomic layer deposition on both silicon and cellulose substrates. The films were grown with uniform thicknesses and with a growth rate in agreement with literature results. The TiO₂ films were shown to profoundly alter the water contact angle values of cellulose in a manner dependent upon processing characteristics. Higher amounts of protein adsorption indicated by blurry areas on images generated by scanning electron microscopy were noted on TiO₂ -coated cellulose acetate than on uncoated cellulose acetate. These results suggest that atomic layer deposition is an appropriate method for improving the biological properties of hemostatic agents and other blood-contacting biomaterials.

Electrochemical Behavior of MP35N Implant Alloy in Simulated Physiological Media

For years, MP35N alloy has been regarded as a very high corrosion-resistant material and has been widely used in various medical applications such as orthopedic implants, lead wire for pacemakers, and cardiovascular stents. Previous researchers of this alloy revealed corrosion pits or localized oxidation on the surface of this material after exposure to Ringers solution. Therefore, a detailed examination of this multiphase alloy was designed to understand its electrochemical properties and to explore its corrosion behavior. Corrosion resistance of MP35N was investigated by open-circuit potential, anodic polarization, potentiostatic control, electrochemical impedance spectroscopy, and linear voltammetry techniques. Compositional studies of oxide film were examined by electron spectroscopy for chemical analysis. Surface morphologies of MP35N after electrochemical studies were characterized by scanning electron spectroscopy. Elemental distribution and chemical composition was identified by energy dispersive spectrum. Results indicate that chemical composition of the oxide film after passivation plays an important role in the determination of corrosion resistance of MP35N in Ringers solution. A high concentration of hydroxyl and hydrate groups inside the passivated oxide film is beneficial to the corrosion resistance of this multiphase alloy.

Electrochemical and SEM Characterization of Gold-Coated Stents In Vitro

The inert properties of gold make it a useful material for implants. Gold was applied as a coating on 316L stainless steel stents to improve the radiopacity and to provide better thrombogenic resistance and a lower degree of restenosis after deployment. Reports of clinical trials using gold-coated stents showed controversial results. A detailed examination of gold-coated stents was designed to understand the influence of surface conditions on the electrochemical properties in vitro. Corrosion resistance of gold-coated stents was investigated by cyclic polarization, and potentiostatic control. Surface morphologies of gold-coated stents in as-received condition, postinflated condition, and after cyclic polarization were characterized by scanning electron spectroscopy (SES). X-ray mapping identified elemental distribution. Results found severe defects in the gold-coating in the as-received condition, cracks and wrinkles on inflated gold-coated stents, and severe corrosion after cyclic polarization. Leaching of metallic ions in Ringers solution was possible.