Lakshman Neelakantan

Bio: Lakshman Neelakantan is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Corrosion & Materials science. The author has an hindex of 14, co-authored 40 publications receiving 496 citations. Previous affiliations of Lakshman Neelakantan include Ruhr University Bochum & Max Planck Society.

Processing and characterization of braided NiTi microstents for medical applications

Abstract: In the present exploratory study we investigate the processing of NiTi microstents, which were produced by braiding. We give a short description of the braiding of microstents using 0.1 mm pseudoelastic NiTi wires. We compare our braided microstents with microstents which were produced, using the traditional tube making and laser cutting technology. We outline why braiding may avoid some of the inherent problems related with the traditional procedure and highlight its cost effectiveness. We report on the thermal, chemical and mechanical properties of braided microstents. Most importantly, we provide information on the elementary damage processes which characterize displacement controlled fatigue loading up to 30·106 cycles. The results presented in this work provide information on a research field, which links the engineering manufacturing field braiding (as a potential new processing technology for NiTi stents) with state-of-the-art materials science techniques for the characterization of pseudoelastic NiTi shape memory alloys. In der vorliegenden Arbeit wird die Herstellung von NiTi Stents mittels Flechttechnologie untersucht. Der Flechtprozesses von NiTi Mikrostents, hergestellt aus pseudoelastischen Drahten mit 0,1 mm Durchmesser, wird kurz dargestellt. Die Geflechte werden mit Stents, welche aus Rohren durch den traditionellen Laserschneidprozess hergestellt wurden, verglichen. Es wird diskutiert, warum Flechten einige der inharenten Probleme der traditionellen Prozessroute vermeiden konnte. Daruber hinaus wird die Wirtschaftlichkeit dieses Verfahrens aufgezeigt. Die thermischen, chemischen und mechanischen Eigenschaften von geflochtenen Mikrostents werden charakterisiert. Dabei werden auch die elementaren Schadigungsprozesse, die bei einer verschiebungskontrollierten Ermudungsbelastung bis zu 30·106 Zyklen auftreten, untersucht. Die Ergebnisse dieser Arbeit verbindet das Ingenieurgebiet des Flechtens (als ein potenziell neues Fertigungsverfahren fur NiTi Stents) mit modernsten Methoden der Werkstoffwissenschaft zur Charakterisierung von pseudoelastischen NiTi Formgedachtnislegierungen.

Initiation of Stress Corrosion Cracking in Cold-Drawn Prestressing Steel in Hardened Cement Mortar Exposed to Chlorides

Abstract: Cold-drawn, high-strength, prestressing (PS) steel strands are widely used in pretensioned concrete (PTC) structures. This paper discusses the stress corrosion cracking (SCC) of PS steel embedded in cement mortar and gradually exposed to chlorides. Various stages of the passive to active (P-to-A) transition, which marks the onset of SCC, were investigated using the electrochemical impedance spectroscopy technique. The key mechanisms were identified and confirmed using scanning electron microscopy/energy dispersive x-ray analysis, x-ray diffarction, and confocal Raman spectroscopy. It was found that the passive film on unstressed PS steel has better electrochemical characteristics than that on conventional steel rebars. However, the residual tensile stress at the surface of PS steels can assist passive film cracking after chloride attack—contrary to the pitting corrosion without cracking of passive film in conventional steels. Further, tests indicated that the concentration of chlorides required to crack the passive film in PS steels can reduce by about 50% when prestressed—as in field structures. Chemical composition, stress state, and microstructural features at the PS steel surface were identified as possible factors influencing the initiation of SCC in PTC structures.

Note: Design and fabrication of a simple versatile microelectrochemical cell and its accessories.

Abstract: A microelectrochemical cell housed in an optical microscope and custom-made accessories have been designed and fabricated, which allows performing spatially resolved corrosion measurements. The cell assembly was designed to directly integrate the reference electrode close to the capillary tip to avoid air bubbles. A hard disk along with an old optical microscope was re-engineered into a microgrinder, which made the vertical grinding of glass capillary tips very easy. A stepper motor was customized into a microsyringe pump to dispense a controlled volume of electrolyte through the capillary. A force sensitive resistor was used to achieve constant wetting area. The functionality of the developed instrument is demonstrated by studying μ-electrochemical behavior of worn surface on AA2014-T6 alloy.

On the corrosion behavior of Al2Cu by local electrochemical impedance spectroscopy using droplet cell microscopy

Abstract: In this short communication, we study the electrochemical behavior of an individual Al2Cu intermetallic phase (coexisting with α-Al) in 0.1 M NaCl solution (with and without inhibitor) using droplet cell microscopy (DCM). The influence of CeCl3 (250 ppm) addition to the electrolyte is studied through potentiodynamic polarization (DC method). The results suggest that the addition of CeCl3 retards the cathodic reaction rate, thereby decreasing the corrosion rate. This study also highlights on the stability of the system, which is essential to perform local electrochemical impedance spectroscopy (LEIS) using DCM. Fabrication of a proper silicone rubber gasket at the flat capillary tip is important to achieve reproducibility, in order to use DCM in contact mode (as confined droplet). The stability and linearity of the system is proved by sweeping frequencies in both directions and by performing Kramers-Kronig transformation (KKT), respectively. LEIS reveals a charge transfer resistance of 2.83 kΩ cm2 as an effect of inhibitor addition, whereas the charge transfer resistance is 1.9 kΩ cm2 in 0.1 M NaCl solution.

Electropolishing of a Nickel–Titanium–Copper Shape Memory Alloy in Methanolic Sulfuric Acid

Abstract: Electropolishing of NiTiCu shape memory alloy was performed in methanolic sulfuric acid as a surface processing method. The current investigation, using a rotating disk electrode, aims at identifying appropriate electropolishing conditions for this alloy based on understanding the underlying mechanism. The influence of temperature, rotation rate, and sulfuric acid concentration 3 or 5 M H2SO4 was quantified. Electropolishing in methanolic 5 M H2SO4 is solely mass-transport-controlled, while a mixed control is found for the methanolic 3 M H2SO4. This behavior is different from that of the binary NiTi alloy. From the electrochemical perspective, a methanolic 5 M H2SO4 solution is more suitable for electropolishing NiTiCu. The activation energy for electrodissolution is 21.5 kJ mol 1

A Review on TiO2 Nanotubes: Influence of Anodization Parameters, Formation Mechanism, Properties, Corrosion Behavior, and Biomedical Applications

Abstract: In this article, influence of anodization parameters on the formation of tubes, tube dimensions, formation mechanism, properties of TiO2 nanotubes (TNT), and their applications in biomedical field are reviewed. The fabrication of TNT of a different shape such as pore size, length, and wall thickness by varying anodization parameters including electrolytes, pH, voltage, electrolyte bath temperature, and current density is examined and discussed. The crystallographic nature of the nanotube obtained by various methods has also been discussed. Finally, the article concludes by examining the key properties including the corrosion aspect and various applications in biomedical field in depth.

Electrocatalytic reduction of nitrate - a step towards a sustainable nitrogen cycle.

Abstract: Nitrate enrichment, which is mainly caused by the over-utilization of fertilisers and industrial sewage discharge, is a major global engineering challenge because of its negative influence on the environment and human health. To solve this serious problem, many technologies, such as the activated sludge method, reverse osmosis, ion exchange, adsorption, and electrodialysis, have been developed to reduce the nitrate levels in water bodies. However, the applications of these traditional techniques are limited by several drawbacks, such as a long sludge retention time, slow kinetics, and undesirable by-products. From an environmental perspective, the most promising nitrate reduction technology is enabled to convert nitrate into benign N2, and features low cost, high efficiency, and environmental friendliness. Recently, electrocatalytic nitrate reduction has been proven by satisfactory research achievements to be one of the most promising methods among these technologies. This review provides a comprehensive account of nitrate reduction using electrocatalysis methods. The fundamentals of electrocatalytic nitrate reduction, including the reaction mechanisms, reactor design principles, product detection methods, and performance evaluation methods, have been systematically summarised. A detailed introduction to electrocatalytic nitrate reduction on transition metals, especially noble metals and alloys, Cu-based electrocatalysts, and Fe-based electrocatalysts is provided, as they are essential for the accurate reporting of experimental results. The current challenges and potential opportunities in this field, including the innovation of material design systems, value-added product yields, and challenges for products beyond N2 and large-scale sewage treatment, are highlighted.

Impact of surface topography and coating on osteogenesis and bacterial attachment on titanium implants.

Abstract: Titanium (Ti) plays a predominant role as the material of choice in orthopaedic and dental implants. Despite the majority of Ti implants having long-term success, premature failure due to unsuccessful osseointegration leading to aseptic loosening is still too common. Recently, surface topography modification and biological/non-biological coatings have been integrated into orthopaedic/dental implants in order to mimic the surrounding biological environment as well as reduce the inflammation/infection that may occur. In this review, we summarize the impact of various Ti coatings on cell behaviour both in vivo and in vitro. First, we focus on the Ti surface properties and their effects on osteogenesis and then on bacterial adhesion and viability. We conclude from the current literature that surface modification of Ti implants can be generated that offer both osteoinductive and antimicrobial properties.