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cellTL2007 - Tribol Lett(2007 27:233238 DOI...

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ORIGINAL PAPER Macroscopic Friction Coefficient Measurements on Living Endothelial Cells Alison C. Dunn Æ Toral D. Zaveri Æ Benjamin G. Keselowsky Æ W. Gregory Sawyer Received: 14 December 2006 / Accepted: 30 April 2007 / Published online: 31 May 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Arterial stent deployment by balloon or self- expandable structure introduces shear forces and radial forces that can damage or remove the endothelial cell layer. These factors can subsequently cause failure by restenosis or endothelial leaks. These conditions can be exacerbated by pulsatile blood flow and arterial asymmetry, which can cause migration or displacement. In mechanical or finite- element models which attempt to explain this motion, friction between the stent materials and endothelial cells is eclipsed by pressure, or assumptions that cells are moved along with the stent. During device deployment or migra- tion, some relative motion between stent materials and endothelial cells occurs. This study aims to quantify fric- tion between a polished glass pin and a single layer of arterial endothelial cells, and include observations of cell damage in an attempt to better understand the biological response to tribological stresses. Measured friction coeffi- cient values were on the order of l = 0.03–0.06. Keywords Biotribology Á Stent Á Cells Introduction Vascular tribological research often focuses on the mate- rials and mechanics associated with arterial stents and catheters [ 1 ]. Coronary artery stents are scaffold-like medical devices that are inserted into a damaged artery and opened in an effort to stabilize the vessel diameter, and blood flow. Biocompatible materials that have been used to create these scaffolds are the following: stainless steel, cobalt–chromium–nickel–molybdenum–iron alloys, tanta- lum, nitinol, polyethylene, polyurethane, and ePTFE coated nitinol to name a few [ 2 ]. Figure 1 illustrates an artery with a stent during expansion into contact with the endothelial cell layer (greatly exaggerated) that makes up the inner lining of a healthy artery. This research reports on the measurements of the friction forces between a smooth inert surface and a confluent layer of bovine aortic endothelial cells (BAEC). There are a number of biotri- bological interactions of importance in stent design, insertion, and fixation. We are unaware of any direct contact tribology experiments on endothelial cells. Nano- tribology techniques such as atomic force microscopy (AFM) have been used to assess mechanical properties, but have not as, yet identified friction coefficient values [ 3 ]. At the macroscale, synthetic arteries have been constructed to assess the forces necessary to prevent retrograde dis- placement; the predicted forces for typical geometries are in the range of 2.05 ± 0.02 N [ 4 ].
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