[BIC-announce] Seminar in Biomedical Engineering - Wednesday September 26th - 1 pm- Room 333

Tue Sep 25 09:22:03 EDT 2012

Dear all, 

Our  next Biomedical Engineering Dpt seminar is this coming wedneday 

Wednesday - September 26th,  at 1 pm 

Location: Room  333 Lyman Duff Building (Biomedical Engineering Dpt, 3775 
University Street).

Speaker: Dr. Rosaire Mongrain PhD, Mechanical Engineering, McGill University, Montreal, Canada

Title : Development of a new biodegradable stent using nano-structured metallic amalgamates

Authors: R. Mongrain*, Bandar Al-Mangour**, Stephen Yue**, O. Bertrand***
* Mechanical Engineering, McGill University, Montreal, Canada
**Mining and Metallurgy, McGill University, Montreal, Canada
** Interventional Cardiology, Hopital Laval, Laval University, Quebec, Canada

Abstract:
There is mounting evidence that relates stent fracture (SF) to in-stent restenosis (ISR) and
thrombosis and consequently stent functional failure. These events remain mostly clinically silent
because of the limited visibility of imaging modalities [1]. In fact, clinically the incidence of stent
fracture is reported in 1-2.6 % of patients. Higher rates (up to 29%) are reported based of pathologic
investigations [1]. However, only the severe SF (multiple strut fractures with gap in the stent body)
were associated with adverse pathological findings. Interestingly, the severity of SF is correlated
with implant duration and the severe SF had the longest implant duration. This suggests a fatigue
phenomenon related with the dynamic loading of the stent.
A recent standard was proposed to assess stent design under cyclic loading until fracture [2]. In that
context, the ideal stent material should have a high density (for radio-opacity), a high elastic
modulus (to minimize recoil), low yield stress (to facilitate expansion), high tensile strength (for
radial strength and thinner struts), high ductility (to withstand deformation during expansion) [3].
For example, the L605 Cobalt-Chromium alloy compared to the 316L Stainless-Steel alloy has
higher density, elastic modulus, yield strength and ductility which support its choice as a stent
material. Stent fatigue resistance is related to material strength but also to its microstructure because
of the small relative size of the stent struts. In general, small grain sizes favor fatigue resistance. In
principle, achieving smaller grain dimensions of current stent materials should improve their fatigue
resistance. Even the best alloys show limitations with respect to stent fractures and corrosion. In this
work, we propose a new method to improve the mechanical and corrosion properties of these
materials.
We propose using cold gas-dynamic spraying (CGDS) process to generate improved materials with
smaller grain sizes. The cold spray process essentially used the energy stored in a high pressure gas
to propel ultra fine powder (nano-powder) particles at supersonic velocities (300-1500 m/s). The
compressed gas is preheated and exits through a nozzle at high velocity. The compressed gas is also
fed to a powder feeder which introduces the ultrafine powder in the gas stream jet. The nanostructured
powder impacts with a substrate, the particles deform and adhere to form a coating on the
substrate. The particles remain relatively cold and retain their submicron to micron range
dimensions. No melting is observed and interestingly particles flow and mix under very high strain
rates generating complex microstructures. Therefore, unwanted effects of high temperatures, such as
oxidation, grain growth and thermal stresses, are absent. As mentioned above, this study specifically
addresses the development and characterization of 316L and L605 coatings produced by the CGDS
process for improved mechanical and corrosion behavior for stent manufacturing. Scanning electron
microscopy and electron backscatter diffraction were used to investigate the microstructural changes of
these coatings before and after annealing. The effect of gas type on the microstructure of 316L coatings
and the role of post-heat treatment in the microstructure and properties are also studied. Of particular
interest are grain refinement, heat treatment, mechanical properties and corrosion behavior of the cold
sprayed material. Most recent thermo mechanical treatment results provide a material with higher
strength and ductility comparable to cobalt-chromium alloys [4]. Methods of stent fabrication with the
new material are presented.

[1] Nakazawa G. et al., J Am Coll Cardiol, 54(21) : 1924-1931, 2009
[2] Gong XY et al., Fatigue to fracture : An informative, fast and reliable approach for assessing medical implant
durability, A review of ASTM F04.30.06 Fatigue to fracture and beyond working group activities, JAI-ASTM, 2009
[3] Poncin P., Materials & Processes for Medical Devices Conference, ASM International, 8-10 September 2003
[4] Al-Mangour B., The use of cold sprayed alloys for metallic stents, Master of Engineering, Department of Mining
and Materials Engineering, McGill University, Montreal, February 2012

A list of upcoming seminars can be found at : http://www.mcgill.ca/bme/news/seminars

See you there

Christophe Grova 

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Christophe Grova, PhD 
Assistant Professor
Biomedical Engineering Dpt 
Neurology and Neurosurgery Dpt 

Montreal Neurological Institute
Centre de Recherches en Mathématiques

Biomedical Engineering Department - Room 304
McGill University
3775 University Street, Montreal, Quebec, Canada, H3A 2B4
email : christophe.grova at mcgill.ca
tel : (514) 398 2516
fax : (514) 398 7461

web: 
http://www.mni.mcgill.ca/research/gotman/members/christophe.html
http://www.bmed.mcgill.ca/
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