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Effects of shear stress and tensile strain on cell monolayers

 

Introduction

Endothelial cells, which line the arteries, are subject to shear stress due to the flow of blood over them, and tensile hoop strain due to the cyclic stretching and contraction of the artery walls. It is known that certain combinations of shear stress and tensile strain can increase the likelihood of atherosclerosis. In order to understand the causes of this disease, it is useful to study the response of cells to mechanical loading outside the body, in the laboratory. For this project, a unique bioreactor has been designed and built to expose cells simultaneously to shear stress and tensile strain. The bioreactor allows engineers to control both types of loading precisely, replicating mechanical conditions occurring in healthy and diseased arteries so that the effects on cells can be investigated.

People

Prof. Peter McHugh, Dr. Liam Breen, Dr Bruce Murphy

In collaboration with

Dr. Brendan McCormack, Dr. Gordon Muir, Institute of Technology Sligo

Project

The bioreactor design combines a cone-and-plate rheometer with flexible silicone substrates on which cells are seeded and grown. In a standard cone-and-plate rheometer, cells are seeded simply on the plate below the cone, which rotates in a bath of media. However, in this system, cells are grown within a 14x14 mm ² region on flexible substrates, each of which is centered in a 16x16 mm ² square well in the plate. A system of stepper motors and pulleys applies any desired time-varying stretch to the substrates (and cells) while the cone rotates, driving the rotational flow of media to apply the desired shear stress.

 
A schematic diagram of the bioreactor design.

Computational fluid dynamics (CFD) models were used to calibrate the wall shear stress generated within the bioreactor, yielding a master curve correlating cone speed to wall shear stress. Cellular substrate strains were validated by video extensometry.

Endothelial cells have been placed in the bioreactor for up to 24 hours without significant cell death. In a mechanical test, a low steady wall shear stress of 14 dyn/cm ² and cyclical strain of 0%–12% at frequency of 1 Hz were applied simultaneously for three hours. The cells were observed to elongate and align perpendicular to the stretch and parallel to the flow (below), which is a characteristic response to this applied mechanical environment.


Images of endothelial cells after exposure to a low steady wall shear stress (WSS) and cyclical tensile hoop strain (THS) in the bioreactor.

Publications

Breen LT, McHugh PE, McCormarck BA, Muir G, Quinlan NJ, Heraty KB, Murphy BP (2006) Development of a novel bioreactor to apply shear stress and tensile strain simultaneously to cell monolayers, Review of Scientific Instruments 77:104301

Acknowledgements

This work was supported by HEA-PRTLI3 and by an IRCSET Embark Scholarship to Liam Breen.



    

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