Sachiko Hiromoto

Effect of chloride ion on the anodic polarization behavior of the Zr65Al7.5Ni10Cu17.5 amorphous alloy in phosphate buffered solution

The corrosion behavior of the Zr65Al7.5Ni10Cu17.5 amorphous alloy was investigated in a phosphate buffered saline solution and in phosphate–citric acid buffered solutions with various chloride-ion concentrations from 0 to 1.0 M, to examine the performance of the alloy as a biomaterial. The surface condition of the non-polarized and polarized alloy in the passive region was characterized using X-ray photoelectron spectroscopy. The susceptibility of this alloy to pitting corrosion was small at a small amount of chloride ion, but it increased with the chloride-ion concentration. Phosphate ion adsorbed on zirconium oxide in the surface raised the susceptibility to the pitting corrosion. On the other hand, phosphate ion is hardly adsorbed by aluminum oxide, so aluminum oxide in the surface diminished the susceptibility to the pitting corrosion.

In vitro and in vivo biocompatibility and corrosion behaviour of a bioabsorbable magnesium alloy coated with octacalcium phosphate and hydroxyapatite.

Octacalcium phosphate (OCP) and hydroxyapatite (HAp) coatings were formed on Mg-3 mass% Al-1 mass% Zn (AZ31) magnesium alloy by a single-step chemical solution deposition method. Chemically polished AZ31 (Cpol-AZ31) and HAp- and OCP-coated AZ31 (HAp- and OCP-AZ31) were immersed in a medium for 52 weeks or implanted in transgenic mice for 16 weeks to examine the long-term corrosion behaviour and in situ inflammation behaviour. In the medium, Mg-ion release was restricted for the initial several days and the corrosion rate thereafter was suppressed by approximately one-half with the HAp and OCP coatings. HAp-AZ31 showed a ∼20% lower corrosion rate than OCP-AZ31. Tissues of the transgenic mouse emit fluorescence in proportion to the degree of inflammation in situ. The luminescence intensity level was too low to be a problem regardless of the coatings. A thinner fibrous tissue layer was formed around OCP- and HAp-AZ31 than around Cpol-AZ31, indicating that the HAp and OCP coatings suppressed corrosion and foreign-body reaction in vivo. Visible pits were formed in filiform and round shapes in vitro and in vivo, respectively. Corrosion was observed underneath the coatings, and almost uniform corrosion took place in vitro, while local corrosion was predominant in vivo. These differences in corrosion morphology are attributed to the adhesion of tissues and the lower diffusivity on the surface in vivo than that in vitro. Dissolution behaviour of OCP crystals in vivo was different from that in vitro. It was demonstrated that the HAp and OCP coatings developed have great potential for a biocompatible and corrosion protection coating.

Effects of surface finishing and dissolved oxygen on the polarization behavior of Zr65Al7.5Ni10Cu17.5 amorphous alloy in phosphate buffered solution

Abstract The present study evaluates the effects of surface finishing and dissolved oxygen in phosphate buffered saline solution on the corrosion behavior of Zr65Al7.5Ni10Cu17.5 amorphous alloy for the application of the alloy to biomaterials. Non-polished and polished alloys were polarized in deaerated electrolyte, and the polished alloy was also polarized with various concentrations of dissolved oxygen. The surface of the alloy was characterized using X-ray photoelectron spectroscopy. The corrosion resistance of the alloy decreased with metallic nickel and hydroxide ion in surface oxide after polishing. The pitting corrosion resistance also decreased with the hydroxide ion after polishing. On the other hand, the pitting-corrosion resistance increased with aerating 4% O2 because of the decrease of hydroxide ion and adsorbed phosphate ion in the surface oxide. However, oxygen above 4% O2 prevented dissolution of nickel, causing the decrease of the pitting-corrosion resistance. When the pH was 2.2, the dissolved oxygen did not influence the pitting-corrosion resistance because of the increased solubility of the surface oxide.

Effect of pH on the polarization behavior of Zr65Al7.5Ni10Cu17.5 amorphous alloy in a phosphate-buffered solution

Abstract For the biomedical use of amorphous alloys, the effect of pH on the polarization behavior of Zr 65 Al 7.5 Ni 10 Cu 17.5 amorphous alloy was evaluated in phosphate-buffered solutions with and without chloride ion. In the solution without chloride ion, the corrosion resistance at open-circuit potential decreased with a decrease in pH. The open-circuit potential was ennobled with a decrease in pH by the acceleration of a cathodic reaction. However, the open-circuit potential was not changed by chloride ion, and the pitting potential was not changed with the decrease in pH. Then, the open-circuit potential was always less noble than the pitting potential. Therefore, the pitting-corrosion resistance of the zirconium-based amorphous alloy decreases with a decrease in pH due to the shift of the open-circuit potential. However, pitting corrosion does not occur spontaneously with a decrease in pH.

Development of electrolytic cell with cell-culture for metallic biomaterials

Abstract The interface reactions between materials and tissues must be understood to improve the corrosion resistance in vivo and biocompatibility of the materials. Then, we have developed a new electrolytic cell by which electrochemical measurement can be performed during culturing cells on specimen. Open-circuit potential ( E open ) and AC impedance of commercially pure titanium at E open were measured in a cell-culture medium with and without culturing murine fibroblasts L929 and in the medium without proteins. The biomolecule adsorption layer containing proteins and cells was formed on titanium oxide film and worked as a capacitor by preventing the diffusion of molecules. The cell enhanced this prevention.

Electrochemical properties of an interface between titanium and fibroblasts L929

Various electrochemical measurements were performed on titanium with and without culturing murine fibroblasts L929 to characterize the effects of cells on interface electrochemical properties between titanium and cells. Open-circuit potential of titanium decreased with L929 cells, which was caused by the shift of equilibrium potential between cathodic and anodic reactions indicated by the decrease in cathodic current density with L929 cells. In cathodic potential step test, the decrease in current density following to the peak current density was delayed with L929 cells, indicating that diffusivity of molecules and ions decreased with the cells. In addition, alternating current impedance measurement and data approximation to the electrical equivalent circuit model revealed that the circuit element for diffusion resistance of biomolecule adsorption layer increased with L929 cells. Consequently, the effect of cells on the interface property is the retardation of diffusion through the biomolecule adsorption layer due to the increase in biomolecule density with extracellular matrix consisting with proteins and glycosaminoglycans generated by the cells.

Re-passivation current of amorphous Zr65Al7.5Ni10Cu17.5 alloy in a Hanks’ balanced solution

Abstract To evaluate the repassivation behavior of amorphous phase with chemical and structural homogeneity, time transient of re-passivation current density was measured on amorphous Zr 65 Al 7.5 Ni 10 Cu 17.5 alloy and crystalline pure Zr with a momentary fracture of a ribbon shaped specimen in an artificial body fluid. Current density abruptly increased to a peak, J peak , and exponentially decreased to the constant value, J ∞ , J peak and J ∞ on the amorphous specimen were smaller than those on the crystalline specimen, indicating that the amorphous phase would show the smaller dissolution rate from bare metal surface and from a re-passive film than the crystalline specimen, respectively. On the other hand, decrease rate of the current density on the amorphous specimen was smaller than that on the crystalline specimen, indicating the regeneration of passive film on the amorphous specimen was delayed probably because the amount of metal ions dissolved at the initial stage, i.e. the source of re-passive film, was smaller on the amorphous specimen. Consequently, the total charge for the re-passivation of the amorphous specimen was smaller than that of the crystalline specimen, supposing that the amount of dissolved metal ions during re-passivation was smaller on the amorphous specimen than the crystalline specimen.

Electrochemical properties of 316L stainless steel with culturing L929 fibroblasts

Potentiodynamic polarization and impedance tests were carried out on 316L stainless steel with culturing murine fibroblast L929 cells to elucidate the corrosion behaviour of 316L steel with L929 cells and to understand the electrochemical interface between 316L steel and cells, respectively. Potential step test was carried out on 316L steel with type I collagen coating and culturing L929 cells to compare the effects of collagen and L929 cells. The open-circuit potential of 316L steel slightly shifted in a negative manner and passive current density increased with cells, indicating a decrease in the protective ability of passive oxide film. The pitting potential decreased with cells, indicating a decrease in the pitting corrosion resistance. In addition, a decrease in diffusivity at the interface was indicated from the decrease in the cathodic current density and the increase in the diffusion resistance parameter in the impedance test. The anodic peak current in the potential step test decreased with cells and collagen. Consequently, the corrosion resistance of 316L steel decreases with L929 cells. In addition, collagen coating would provide an environment for anodic reaction similar to that with culturing cells.

Bioinspired adhesive polymer coatings for efficient and versatile corrosion resistance

The anticorrosion ability of ultrathin coatings with bio-inspired organic polymers is demonstrated. We prepared a series of catechol-containing poly(alkyl methacrylate)s by free radical polymerization. These copolymers were spin-coated on various corrosion susceptible metal/alloy substrates of magnesium, aluminum, copper and iron without any harsh pretreatment. Several key factors like molecular structure, composition ratio and processing conditions were wisely tailored to afford a transparent, firm and sub-micron polymer coating on those substrates. Corrosion resistance of the polymer-coated substrates was thoroughly investigated by immersion tests in salt-water and acidic solutions, polarization tests, and visual inspection. Formation of an anomalous dense layer of ca. 5 nm thickness adjacent to the metal surface and a remarkable effect of thermal treatment were clearly observed by neutron reflectivity measurements, leading to a highly protective ability against foreign molecules e.g. water or corrosive ions. A key molecular design for anticorrosive polymer coating was revealed to be a combination of strong and versatile binding ability of catechol units and defect free polymer layers formed on the metal substrates in the presence of hydrophobic alkyl chains.

In vivo evaluation of Zr-based bulk metallic glass alloy intramedullary nails in rat femora

Zr-based bulk metallic glasses (BMG) show high corrosion resistance in vitro and higher strength and lower Young’s modulus than crystalline alloys with the similar composition. This study aimed to perform an in vivo evaluation of Zr65Al7.5Ni10Cu17.5 BMG. Osteotomy of the femur was done in rats and stabilized with intramedullary nails made of Zr65Al7.5Ni10Cu17.5 BMG, Ti–6Al–4V alloy, or 316L stainless steel. Systemic and local effects of each type of nail were evaluated by measuring the levels of Cu and Ni in the blood and the surrounding soft tissue. Changes of the surface of each nail were examined by scanning electron microscopy (SEM). Healing of the osteotomy was evaluated by peripheral quantitative computed tomography and mechanical testing. No increase of Cu and Ni levels was recognized. Surface of the BMG showed no noticeable change, while Ti–6Al–4V alloy showed Ca and P deposition and 316L stainless steel showed surface irregularities and pitting by SEM observation. The stress strain index, maximum torque, torsional stiffness, and energy absorption values were larger for the BMG than those for Ti–6Al–4V alloy, although there was no significant difference. The Zr-based BMG can promote osteotomy healing as fast as Ti–6Al–4V alloy, with the possible advantage of the Zr-based BMG that bone bonding is less likely, allowing easier nail removal compared with Ti–6Al–4V alloy. The Zr-based BMG is promising for the use in osteosynthetic devices that are eventually removed.