Biomedical subjects

R Venugopalan

Publications and source records attributed to R Venugopalan.

At least 19 recordsLinked to original sources

PubMed2003

Radiopaque markers in endovascular stents--benefit and potential hazards.

AIM: The primary objective of this study was to evaluate the impact of radiopaque markers on stent visibility. The secondary objective was to investigate the impact of such radiopaque markers on localized corrosion at the stent corpus-marker interfaces. MATERIAL AND METHODS: The radiopacity of the following stents was evaluated at different fluoroscopy modes (spotfilm, continuous fluoroscopy, 15 p/s, 7.5 p/s, 3 p/s): Memotherm Flexx (FX), Memotherm Luminexx (LX)(Bard), SMART (SM), SMARTeR (SMR)(Cordis). Four readers evaluated a total of 3200 images (160 per stent per fluoro mode) according to the following radiopacity score (RS): 0 = no stent visible, 1 = poor, 2 = acceptable, 3 = good, and 4 = very good stent visibility. LX and SMR stents (n = 5/group) were subjected to potentiodynamic polarization testing in de-aerated Hanks salt solution at 37 degrees C using a potentiostat. Palmaz-Schatz stents (n = 3) were used as the control group for comparative evaluation. The corrosion current density (I corr ) and the breakdown value (E bd ) were compared parameters of interest. RESULTS: At the spotfilm mode the LX, the SM and the SMR were rated well to very well visible in 99 %, 96 %, 96 %, and the FX only in 64 %. At 7.5 p/s (standard fluoro mode) the LX was considered to be well to very well visible in 77 %, followed by the SMR in 12.5 %, the SM in 1 %, and the FX in 0 %. The SMR stents exhibited the highest E bd values (802 +/- 112 mV vs SCE), while the LX stents exhibited the lowest E bd values (155 +/- 38 mV vs SCE). Also, the average E bd values for the SMR stents were better than those exhibited by the PS stents (503 +/- 107 mV vs SCE). CONCLUSION: Stent radiopacity can be increased significantly with the help of radiopaque markers (p < 0,0001 LX vs. FX). However, the surface condition of the stent corpus and the mode of attachment of the marker onto the corpus may have a significant impact on the uniformity of the final corrosion behavior. It is not feasible to determine the clinical impact of this localized corrosion behavior from this in vitro study. Further experimentation is recommended to ascertain the same.

Coated Materials, Biocompatible↗

PubMed2001

Coherent gluon production in very-high-energy heavy-ion collisions.

The early stages of a relativistic heavy-ion collision are examined in the framework of an effective classical SU(3) Yang-Mills theory in the transverse plane. We compute the initial energy and number distributions, per unit rapidity, at midrapidity, of gluons produced in high-energy heavy-ion collisions. We discuss the phenomenological implications of our results in light of the recent Relativistic Heavy-Ion Collider data.

Journal Article↗

PubMed2001

Initial gluon multiplicity in heavy-ion collisions.

The initial gluon multiplicity per unit area per unit rapidity, dN/L2/d eta, in high energy nuclear collisions, is equal to f(N)(g(2)mu L) (g(2)mu)(2)/g(2), with mu(2) proportional to the gluon density per unit area of the colliding nuclei. For an SU(2) gauge theory, we compute f(N)(g(2)mu L) = 0.14 +/- 0.01 for a wide range in g(2)mu L. Extrapolating to SU(3), we predict dN/L2/d eta for values of g(2)mu L relevant to the Relativistic Heavy Ion Collider and the Large Hadron Collider. We compute the initial gluon transverse momentum distribution, dN/L2/d(2)k( perpendicular), and show it to be well behaved at low k( perpendicular).

Journal Article↗

PubMed2000

Initial energy density of gluons produced in very-high-energy nuclear collisions

In very-high-energy nuclear collisions, the initial energy of produced gluons per unit area per unit rapidity, (dE/L2)/deta, is equal to f(g(2)&mgr;L) (g(2)&mgr;)(3)/g(2), where &mgr;(2) is proportional to the gluon density per unit area of the colliding nuclei. For an SU(2) gauge theory, a nonperturbative computation of f(g(2)&mgr;L) shows that it varies rapidly for small g(2)&mgr;L but varies only by approximately 25%, from 0.208+/-0.004 to 0.257+/-0. 005, for a wide range 35.36- 296.98 in g(2)&mgr;L. This includes the range relevant for collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Extrapolating to SU(3), we estimate dE/deta for Au-Au collisions in the central region at RHIC and LHC.

Journal Article↗

PubMed2000

The effect of nitrogen diffusion hardening on the surface chemistry and scratch resistance of Ti-6A1-4V alloy.

Modular, head-stem, mixed-metal connections are susceptible to mechanically mediated electrochemical interactions. Any attempt to improve the performance of these connections should center around increasing their resistance to mechanical damage, particularly the titanium alloy (Ti64). This study investigated the effect of a nitrogen-diffusion-hardening process on Ti64, with specific reference to changes in composition, chemistry, electrochemistry and its ability to resist and/or repassivate scratch damage. The nitrogen-diffusion-hardened Ti64 alloy had TiN and TiNO complexes at the immediate surface and sub-surface layers. The diffusion-hardened samples also had a deeper penetration of oxygen compared to regular Ti64 alloy samples. The electrochemical impedance spectroscopy data corroborated the increased thickness of the barrier oxide on the diffusion-hardened samples. The nitrogen-diffusion-hardened samples were more resistant to scratch damage and repaired/repassivated faster after such damage. The results suggest that the nitrogen-diffusion-hardened titanium alloy should exhibit increased resistance to mechanical-electrochemical interactions in mixed-metal modular interfaces in total hip prostheses.

Alloys↗

PubMed1999

Corrosion testing of stents: a novel fixture to hold entire device in deployed form and finish.

Stainless steel and nickel-titanium alloy stents for cardiac, biliary, and prostatic clinical applications may be susceptible to loss of mechanical integrity/failure due to their small size and unique geometry. Because their corrosion behavior is a critical aspect of their biocompatibility, it is necessary to test such devices for corrosion behavior using a final deployed form and finish condition. Welding wire leads or holding with metallic clips can result in surface variability or damage to the stents. A novel fixture to hold stents for corrosion testing was designed and evaluated in this study. This fixture design minimally influenced the properties of stents, and also facilitated easy removal post-testing for other types of characterization.

Alloys↗

PubMed1999

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.

Alloys↗

PubMed1998

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 Duncan's 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.

Analysis of Variance↗

PubMed1998

The effect of water contamination on dimensional change and corrosion properties of a gallium alloy.

OBJECTIVE: This aim of this study was to determine the effect of water contamination on the dimensional change over time and the corrosion properties of a gallium-based alloy (Galloy, SDI), compared to an amalgam (Tytin, Kerr). METHODS: Normal and water-contaminated (10 microL of dH2O during condensation) Galloy and Tytin samples were fabricated using a split metal mold and mechanical condenser. The normal samples were tested for dimensional change under dry and wet (immersed in dH2O) conditions. The water-contaminated samples were tested for dimensional change under only dry conditions. Linear measurements were made over a 7 day period using a stage-equipped dial micrometer. The corrosion potential (Ecorr), the corrosion current density (lcorr), the passive current density (lp), the protective scale formation current density (lps) and potential (Eps), and the breakdown potential (Ebd) were obtained from the polarization curves generated in oxygenated saline solution. Single factor ANOVA and Duncan's multiple range tests were used to determine differences between the various samples. RESULTS: The water-contaminated Galloy samples contracted to a minimum of -3.42 +/- 1.73 microns cm-1 in 5 h followed by expansion to a maximum of 22.56 +/- 3.90 microns cm-1 at the end of 7 days. Normal Galloy and Tytin samples and water-contaminated Tytin samples exhibited rapid contraction during the first 24 h, followed by minimal change over the next 7 days. The Ecorr of normal and water-contaminated Galloy samples was more active than that of the normal and water-contaminated Tytin samples. The lcorr of the normal and water-contaminated Galloy samples was three orders of magnitude greater than the lcorr of normal and water-contaminated Tytin samples. The Eps for the water-contaminated Galloy and Tytin samples was more noble than for the normal Galloy and Tytin samples. The Ebd for the Tytin and water-contaminated Tytin samples was greater than the corresponding values for the Galloy and water-contaminated Galloy samples. SIGNIFICANCE: The gallium-based alloy exhibited expansion if contaminated with water during the condensing and setting process. Post-setting exposure to water did not result in expansion of the gallium-based alloy. The alloy also exhibited a greater susceptibility to corrosion than the amalgam. Due to the possibility of delayed expansion, this material should be used cautiously, particularly in applications involving weakened tooth structure.

Analysis of Variance↗

PubMed1996