Cortronik GmbH

Rostock, Germany

Cortronik GmbH

Rostock, Germany
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Rahim M.,Ruhr University Bochum | Frenzel J.,Ruhr University Bochum | Frotscher M.,Ruhr University Bochum | Frotscher M.,Cortronik GmbH | And 5 more authors.
Acta Materialia | Year: 2013

In the present work we show how different oxygen (O) and carbon (C) levels affect fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation fatigue (up to cycle numbers >108) and scanning electron microscopy (for investigating microstructural details of crack initiation and growth) to study fatigue behavior. High cycle fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of structural fatigue of pseudoelastic NiTi shape memory alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Khatibi G.,Vienna University of Technology | Lederer M.,Vienna University of Technology | Betzwar Kotas A.,Vienna University of Technology | Frotscher M.,Cortronik GmbH | And 2 more authors.
International Journal of Fatigue | Year: 2015

Abstract Thin-walled tubes made of the biocompatible Cobalt-Chromium alloy L605 are being used as the starting material for manufacturing of stents with fine struts of about 100 μm due to their excellent mechanical properties. The required long-term reliability of these medical implants is endurance of 4·108 loading cycles without failure. Knowledge of the high-cycle fatigue properties of the tubing material used for the stent fabrication is a prerequisite for optimization of these high quality products during the stages of design and fabrication. In the present study, the high-cycle fatigue behavior of thin-walled tubes of CoCr L605 was studied by using an ultrasonic resonance fatigue testing system. Laser cutting technique was used to introduce holes in the midsection of the tubes followed by electro-polishing and passive coating processes. Standard stent manufacturing processes were applied for preparation of the tube samples, ensuring representative microstructures and mechanical properties to final stent products. Vibrometry measurements and finite element simulations were conducted to analyze the overall modal response of the tubes during the vibrational loading and to calculate the stress-strain distribution in the periphery of the holes. A lifetime prediction model covering the low- and high-cycle fatigue regime is proposed, based on the experimentally obtained high-cycle fatigue curves in combination with the literature data and finite element modeling. © 2015 Elsevier Ltd.

Kassab E.,Ruhr University Bochum | Neelakantan L.,Ruhr University Bochum | Frotscher M.,Indian Institute of Technology Madras | Frotscher M.,Cortronik GmbH | And 6 more authors.
Materials and Corrosion | Year: 2014

The goal of this study is to compare the corrosion behaviour of selected ternary nickel titanium (NiTi)-based alloys (Ni45Ti 50Cu5, Ni47Ti50Fe3 and Ni39Ti50Pd11) with a binary, pseudoelastic Ni50.7Ti49.3 alloy. We examine the influence of the ternary elements on the corrosion behaviour using standard electrochemical techniques. All measurements were done in a physiological solution (0.9% NaCl) simulating a body temperature of 37 ± 1 °C. The influence of Cu and Pd addition on the surface oxide film was characterised by X-ray photoelectron spectroscopy (XPS). The results revealed that, the localised corrosion resistance of these ternary alloys is lower than the binary NiTi alloy. By comparing the different NiTi-based alloys, the following relation has been proposed for their localised corrosion resistances: NiTiCu < NiTiFe < NiTiPd < NiTi. Depth profiling by XPS showed that the surface oxide film on all the investigated NiTi-based alloys is mainly of TiO2, however, the NiTiPd and NiTiCu alloys showed metallic ternary element distributed within TiO2 layer. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Neelakantan L.,Ruhr University Bochum | Neelakantan L.,Indian Institute of Technology Madras | Zglinski J.K.,Ruhr University Bochum | Frotscher M.,Ruhr University Bochum | And 2 more authors.
Review of Scientific Instruments | Year: 2013

The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of fatigue on corrosion and vice versa. This can be achieved by performing ex situ andor in situ measurements. The versatility of the combined electrochemicalmechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9 NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-fatigue on the localized corrosion resistance, or the influence of hydrogen on fatigue life. Ex situ experiments showed that a pre-fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the fatigue failure. © 2013 American Institute of Physics.

Rahim M.,Ruhr University Bochum | Frenzel J.,Ruhr University Bochum | Frotscher M.,Ruhr University Bochum | Frotscher M.,Cortronik GmbH | And 3 more authors.
Materialwissenschaft und Werkstofftechnik | Year: 2013

In the present work, a computer-controlled test rig for simultaneous fatigue testing of several pseudoelastic NiTi wires through bending rotation is described. Bending rotation fatigue (BRF) testing represents a displacement-controlled experiment where a straight wire is bent into a semi-circle und forced to rotate around its axis. Thus, each point on the wire surface is subjected to alternating tension and compression. A test rig, which allows to control loading amplitudes, rotation frequencies and temperatures is described. We report preliminary results of an experimental program, which aims for a better understanding of fatigue lives, crack initiation, and crack growth in pseudoelastic NiTi wires. It was found that a good surface quality is of utmost importance to avoid early crack initiation. Wöhler curves of pseudoelastic NiTi wires typically show two different regimes depending on the maximum imposed surface strain during bending rotation fatigue testing. Larger strain amplitudes, which are associated with macroscopic formation of stress-induced martensite, result in relatively low fatigue lives (LCF regime). In contrast, cycle numbers exceeding 107were obtained for strain amplitudes where no large scale stress-induced formation of martensite occurred (HCF regime). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kowalski W.,University of Rostock | Dammer M.,Cortronik GmbH | Bakczewitz F.,Cortronik GmbH | Schmitz K.-P.,University of Rostock | And 2 more authors.
Journal of the Mechanical Behavior of Biomedical Materials | Year: 2015

Drug eluting stents (DES) consist of platform, coating and drug. The platform often is a balloon-expandable bare metal stent made of the CoCr alloy L-605 or stainless steel 316. L. The function of the coating, typically a permanent polymer, is to hold and release the drug, which should improve therapeutic outcome. Before implantation, DES are compressed (crimped) to allow implantation in the human body. During implantation, DES are expanded by balloon inflation. Crimping, as well as expansion, causes high stresses and high strains locally in the DES struts, as well as in the polymer coating. These stresses and strains are important design criteria of DES. Usually, they are calculated numerically by finite element analysis (FEA), but experimental results for validation are hardly available. In this work, the X-ray diffraction (XRD) sin2ψ-technique is applied to in-situ determination of stress conditions of bare metal L-605 stents, and Poly-(L-lactide) (PLLA) coated stents. This provides a realistic characterization of the near-surface stress state and a validation option of the numerical FEA.XRD-results from terminal stent struts of the bare metal stent show an increasing compressive load stress in tangential direction with increasing stent expansion. These findings correlate with numerical FEA results. The PLLA-coating also bears increasing compressive load stress during expansion. © 2015 Elsevier Ltd.

Kowalski W.,University of Rostock | Dammer M.,Cortronik GmbH | Bakczewitz F.,Cortronik GmbH | Kessler O.,University of Rostock
Materials Science Forum | Year: 2014

Stents are medical implants, which are applied to keep cavities in the human body open, e.g. blood vessels. Typically they consist of tube-like grids of suitable metal alloys. Typical dimensions depend on their applications: outer diameters in the mm-range and grid bar thickness in the 100 μm range. Before implantation, stents are compressed (crimped) to allow implantation in the human body. During implantation, stents are expanded, usually by balloon catheters. Crimping as well as expansion causes high strains and high stresses locally in the grid bars. These strains and stresses are important design criteria of stents. Usually, they are calculated numerically by Finite Element Analysis (FEA). The XRD-sin2ψ-technique is applied for in-situ-determination of stress conditions during crimping and expansion of stents of the CoCr-alloy L-605. This provides a realistic characterization of the near-surface stress state and an evaluation of the numerical FEA results. XRD-results show an increasing compressive load stress in circumferential direction with increasing stent expansion. These findings correlate with the numerical FEA results. Further residual stresses after removing the expansion device have been measured. © (2014) Trans Tech Publications, Switzerland.

Quosdorf D.,University of Rostock | Brede M.,University of Rostock | Leder A.,University of Rostock | Lootz D.,Cortronik GmbH | And 2 more authors.
Technisches Messen | Year: 2011

This article de-scribes a Micro-Particle-Image-Velocimetry measuring setup (Micro-PIV), which allows to measure the three-dimensional flow field inside a transparent 1: 1 model of an original co- ronary stent. The treatment of the coronary heart disease is of utmost importance due to it's high prevalence. Often coronary stents are implanted which lead to adverse effects originating from the altered blood flow through the vessel treated. © Oldenbourg Wissenschaftsverlag.

The invention describes a stent (1) formed of at least two meander-shaped segments (3a and 3b), which has a supporting structure (8) for an active ingredient in areas of abated flow.

The invention relates to a device for coating a stent, comprising a holder for the stent, a spraying unit comprising a spray mandrel and an air nozzle. The spray mandrel, the air nozzle and the holder are designed and disposed relative to each other such that the spray mandrel projects from one side into the stent during coating and the air nozzle projects into the stent from the opposing side. The invention further provides a method for coating a stent which employs the device according to the invention. Finally, the invention relates to stents that can be obtained according to the method.

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