Ramakrishna Venugopalan

Evaluation of restorative and implant alloys galvanically coupled to titanium.

OBJECTIVES As the success of implants leads to their increasing use in restorative dentistry, attention should be devoted to the galvanic combination of restorative materials with titanium. This paper used continuous corrosion potential monitoring in conjunction with zero-resistance ammetry to obtain galvanic corrosion properties of restorative and implant materials coupled with titanium (ASTM F67-Grade II). METHODS Direct coupling or galvanic experiments were conducted on eight restorative and implant materials coupled to titanium. Deaerated artificial saliva solution in a specifically designed corrosion cell simulated an oral crevice situation. Open circuit potentials (Eo.c.) of each material in the couple, coupled corrosion potentials (Ecouple corr), coupled corrosion current density (lcouple corr) and the resultant charge transfer were monitored. The results were analyzed using single factor ANOVA and Duncans multiple range tests. RESULTS Noble restorative (Au-, Ag-, and Pd-based) alloys coupled to titanium were found to be least susceptible to galvanic corrosion. Co-Cr-Mo, Ni-Cr and Fe-based alloys coupled to tatanium were found to be moderately susceptible to galvanic corrosion due to mechanical-electrochemical interaction. Ni-Cr-Be alloy coupled to titanium was found to be highly susceptible to galvanic corrosion. The in vitro test results for the titanium/Disperalloy combination does not concur with the published clinical performance of this combination, and thus warrants further investigation. SIGNIFICANCE From the data obtained in this study and current literature profiles, acceptable restorative couples were developed for use as clinical guidelines in restorative dentistry.

Comparative MRI Compatibility of 316L Stainless Steel Alloy and Nickel–Titanium Alloy Stents

The initial success of coronary stenting is leading to a proliferation in peripheral stenting. A significant portion of the stents used in a clinical setting are made of 316 low carbon stainless steel (SS). Other alloys that have been used for stent manufacture include tantalum, MP35N, and nickel-titanium (NiTi). The ferromagnetic properties of SS cause the production of artifacts in magnetic resonance imaging (MRI). The NiTi alloys, in addition to being known for their shape memory or superelastic properties, have been shown to exhibit reduced interference in MRI. Thus, the objective of this study was to determine the comparative MRI compatibility of SS and NiTi stents. Both gradient echo and spin-echo images were obtained at 1.5 and 4.1 T field strengths. The imaging of stents of identical geometry but differing compositions permitted the quantification of artifacts produced due to device composition by normalizing the radio frequency shielding effects. These images were analyzed for magnitude and spatial extent of signal loss within the lumen and outside the stent. B1 mapping was used to quantify the attenuation throughout the image. The SS stent caused significant signal loss and did not allow for visibility of the lumen. However, the NiTi stent caused only minor artifacting and even allowed for visualization of the signal from within the lumen. In addition, adjustments to the flip angle of standard imaging protocols were shown to improve the quality of signal from within the lumen.

Structural and mechanical properties of nanostructured metalloceramic coatings on cobalt chrome alloys

A functionally graded nanocrystalline metalloceramic coating on cobalt–chrome alloys was investigated with thin-film x-ray diffraction (XRD), nanoindentation, and scratch adhesion testing. The gradual transition in bonding from metallic to predominantly covalent along with a nanocrystalline grain structure provides a unique material system with excellent strength, toughness, and adhesion properties. XRD analysis of the (CrTiN) coating suggests a cubic sodium chloride phase structure with a=4.2169±0.0035 A. Nanoindentation measurements of the coating result in a hardness of 27 GPa and Young’s modulus of 320 GPa. The graded metallic/covalent nature of the coating with high plasticity also results in excellent film/substrate adhesion as shown by an average critical force of 44±5 N in scratch testing.

Middle Infrared, Quantum Cascade Laser Optoelectronic Absorption System for Monitoring Glucose in Serum:

Advances in middle infrared technology are leading researchers beyond the Fourier transform infrared spectrometer and to the quantum cascade laser. While most research focuses on gas-phase detection, recent research explores its use for condensed-phase matter studies. This work investigates its use for monitoring biologically relevant samples of glucose in serum. Samples with physiological glucose concentrations were monitored with a laser at 1036 cm−1. A 0.992 R2 linearity value was observed. In addition, using another laser at 1194 cm−1 as a measure of the background spectroscopic characteristics, a linearity of 0.998 R2 was observed. The average predictive standard errors of the mean (SEM) were 32.5 and 24.7 mg/dL, respectively, for each method. Quantum cascade lasers could be used to develop middle infrared devices for uses beyond the confines of the laboratory.

Surface topography, corrosion and microhardness of nitrogen-diffusion-hardened titanium alloy

Mechanical-electrochemical interactions accelerate corrosion in mixed-metal modular hip prostheses. These interactions can be reduced by improving the modular component machining tolerances or by improving the resistance of the components to scratch or fretting damage. Wrought cobalt-alloy (CoCrMo) is known to have better tribological properties compared to the titanium alloy (Ti64). Thus, improving the tribological properties of this mixed-metal interface should center around improving the tribological properties of the Ti64 alloy. This study used scanning probe microscopy (contact, tapping and phase contrast mode), scanning electron microscopy, corrosion testing, and microhardness testing to determine the effect of a nitrogen-diffusion hardening process on the surface morphology, electrochemistry and surface hardness of the Ti64 alloy. The nitrogen-diffusion-hardened titanium alloy samples (N-Ti64) had a more pronounced grain structure, more nodular surface, and significantly (P<0.01) higher mean roughness values than the control-Ti64 samples. The N-Ti64 samples also exhibited at least equivalent corrosion behavior and a definite increase in surface hardness compared to the control Ti64 samples. The equivalent corrosion behavior and improved surface hardness indicate the potential for N-Ti64 samples to resist similar and mixed-metal scratch and fretting damage. The use of N-Ti64 as opposed to control-Ti64 may therefore reduce the occurrence of mechanical-electrochemical degradation in mixed-metal modular total hip prostheses.

Using two discrete frequencies within the middle infrared to quantitatively determine glucose in serum

Tight glucose monitoring is essential for the reduction of diabetic complications. This research investigated the changes of absorption spectra observed in serum at three prominent glucose absorption peaks in the middle infrared using a demountable liquid, transmission cell. Two frequencies of light were used to determine the glucose absorption: one at 1193 cm(-1 ) to determine the background water absorption and the other at one of the characteristic peaks (1035, 1080, and 1109 cm(-1)). The peak at 1035 cm(-1) was best for quantitative determination with a standard of error of 20.6 mg/dl (1.1 mmol/L). While interference from other serum constituents could cause problems, urea and albumin-two constituents known to have close absorption peaks-were determined to have no effect on the ability to determine the glucose levels at 1035 cm( -1).

Structure and Mechanical Properties of Functionally-Graded Nanostructured Metalloceramic Coatings

A functionally graded nanocrystalline metalloceramic coating on cobalt-chrome alloy was investigated using thin film x-ray diffraction (XRD), cross-sectional transmission electron microscopy (TEM), nanoindentation, and scratch adhesion testing. The gradual transition in bonding from metallic (Cr/CrTi) near the interface to predominantly covalent (CrTiN) near the surface provides a combination of high toughness and high surface hardness. XRD analysis of the (CrTiN) coating suggests a cubic sodium chloride type phase structure with lattice parameter a = 4.2169±0.0035 A. The surface layer structure is described as a tertiary Ti-N-Cr disordered solid solution that is predominantly cubic TiN, but with some Cr atoms substituted for Ti. TEM shows a transition from equiaxed 20-40 nm-sized grains at the surface to larger, elongated columnar grains below the surface. Nanoindentation measurements of the coating result in a hardness of 27 GPa and Youngs modulus of 320 GPa. In addition, the high plasticity of 55% observed for this coating represents an increase in toughness over other ceramic coatings having similar hardness. The unique, functionally graded, smooth nanocrystalline metalloceramic coating structure provides an opportunity to reduce wear and increase longevity of total hip joint replacements.

Magnetic resonance phase velocity mapping through NiTi stents in a flow phantom model.

To assess constant and pulsatile flow velocity within the lumen of a peripheral NiTi stent using phase velocity mapping for comparison with independent assessments of flow velocity in a phantom model.

Effect of a Flow-Streamlining Implant at the Distal Anastomosis of a Coronary Artery Bypass Graft

AbstractIntimal thickening in the coronary artery bypass graft (CABG) distal anastomosis has been implicated as the major cause of restenosis and long-term graft failure. Several studies point to the interplay between nonuniform hemodynamics including disturbed flows and recirculation zones, wall shear stress, and long particle residence time as possible etiologies. The hemodynamic features of two anatomic models of saphenous-vein CABGs were studied and compared. One simulated an anastomosis with both diameter and compliance mismatch and a curvature at the connection, analogous to the geometry observed in a conventional cardiothoracic procedure. The other, simulated an anastomosis with a flow stabilizing anastomotic implant connector which improves current cardiothoracic procedures by eliminating the distal vein bulging and curvature. Physiologic flow conditions were imposed on both models and qualitative analysis of the flow was performed with dye injection and a digital camera. Quantitative analysis was performed with laser Doppler velocimetry. Results showed that the presence of the bulge at the veno-arterial junction, contributed to the formation of accentuated secondary structures (helices), which progress into the flow divider and significantly affect radial velocity components at the host vessel up to four diameters downstream of the junction. The model with the implant, achieved more hemodynamically efficient conditions on the host vessel with higher mean and maximum axial velocities and lower radial velocities than the conventional model. The presence of the sinus may also affect the magnitude and shape of the shear stress at locations where intimal thickening occurs. Thus, the presence of the implant creates a more streamlined environment with more primary and less secondary flow components which may then inhibit the development of intimal thickening, restenosis, and ultimate failure of the saphenous vein graft.

Evaluation of in-stent stenosis by magnetic resonance phase-velocity mapping in nickel-titanium stents.

To evaluate different grades of in‐stent stenosis in a nickel‐titanium stent with MRI.