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Development of a three-dimentional digital image correlation for displacement and strain measurement of seeded endothelial cells.

INTRODUCTION

Atherosclerosis is a progressive disease of the large arteries [1]. It is the primary cause of heart disease and stroke, and is the underlying cause of about fifty percent of all deaths in westernized societies. Atherosclerosis is characterized by the accumulation of lipids and fibrous elements in the large arteries. It is also distinguished by abnormal thickening and hardening of arterial wall resulting in loss of elasticity [2]. Studies have shown that endothelial dysfunction precedes and thought to play a major role in the development of atherosclerosis. Endothelial dysfunction is a condition where endothelial cells (ECs), cells that form a thin membrane that lines the inside of blood vessels, do not function properly [3]. The endothelium functions as a selectively permeable barrier between blood and tissue [1]. Damage to the endothelial cells leads to an increase in permeability across the membrane to macromolecules such as low-density lipoprotein (LDL) and are ideal sites for lesion formation. This can lead to atherosclerosis because a primary initiating event in atherosclerosis is the accumulation of LDL.

Endothelial cells are mainly affected by three mechanical factors; pressure from pulse, shear stress from blood flow, and strain due to the elasticity of the blood vessel [4]. Endothelial cells require these mechanical factors to function at certain levels. Deviation from these levels can result in damage to the cells and lead to disease. The arterial wall is constantly subjected to both the flow-induced shear stress and wall strain. The vascular wall stiffens as it is stretched. This nonlinear elastic response may be a factor for plaque formation.

The goal of this project is to characterize local strain stiffness responsible for changes in endothelial cell function. In order to do this, synthetic models that mimic arterial geometry and mechanical properties will bedeveloped. Developing a non-involve method to characterize strain would be beneficial in examining how strain effects endothelial cell function.

METHODS

Model Development. Sylgard Elastomer was used to create the models that simulate human tissue [5]. The mechanical properties of Sylgard 184 were tested for various mixing ratios of Sylgard 184. Mixing ratio of 1:10, 1:20, and 1:30 created for this project. The ratios correspond to the curing agent and the base by mass. For the 1:10 ratio, there was one gram of curing agent to every nine grams of base. The Sylgard 184 base was poured into a container and weighed on a mass balance. While the container remained on the mass balance, the curing agent was slowly added until the desired mass ratio was achieved. The mixture was stirred and placed under a vacuum to remove any air bubbles.

Once the air bubbles were removed, the mixture was used to make Models A and B (Figure 1). For Model A, the sylgard mixture was poured into a petri dish, approximately 2mm in thickness. Grounded coffee was distributed over the sylgard. The DIC needed particles of random geometry to track in order to calculate strain. Grounded coffee was chosen since it could be made into the model (would not come off like ink), easily accessible, and could reasonably control the size. The model was placed in an incubator at 37[degrees]C to cure overnight. After curing, the model was removed and cut into a rectangle approximately three inches by a half an inch. This model was used for a control test. For Model B, the mixture was poured over a slow turning rod, one-fourth inch in diameter, over heat. Grounded coffee was distributed over the turning rod. After curing, more of the Sylgard 184 mixture was poured over the rod, creating another layer covering the coffee. After curing, the model was carefully removed from the rod. Model B simulates the correct geometry of the artery and were used to test strain due to fluid.

VIC-3D Measurement System. Type B models were used to test the strain due to hydrostatic pressure variation. This was done through the use of the VIC-3D Measurement System. This system consists of a pair of digital cameras that utilizes Digital Image Correlation, DIC, and a computer with VIC-3D software [7]. DIC is a method of measuring deformation on an object surface. This method involves tracking random patterns created by small areas of particles of random geometry (grounded coffee) on an object surface during deformation.

Development of a control test apparatus. A control test was needed to compare the strain calculated by the DIC and the theoretical strain. A linear actuator with a clamping system and a flat sample, shown in Figure 2, was used to perform this test. The linear actuator was programed to move back half an inch in a second and forward half an inch in a second.

RESULTS

The theoretical strain was calculated by using the first image taken, before the linear actuator started moving, and the image where the linear actuator was fully compressed, sample fully extended.

The length from image 1 and the length for image 2 were measured. The strain was then calculated by subtracting the two lengths, dividing it by the original length, and multiplying by a hundred for a percentage.

Length 1 (L) was 1.12 inches and length 2 (L') was a 1.302 inches. So the change in length ([DELTA]L) was 0.182 inches, making the strain 16.25%. Figure 4 was last image before the linear actuator was fully compressed. The region between yellow and orange was 16.25% strain, calculated by DIC.

Strain Measurements in Model B. For this test, hydrostatic pressure variation was used to induce strain. Milk was used as the fluid in this test to create the necessary contrast needed by the DIC. A two meters tall tube was connected to the sample, 1:10 ratio, and filled with milk. When a valve was opened it released the fluid. The change in the height of the fluid created pressure. From this, the theoretical strain could be calculated.

For this test, the valve was opened for two seconds and the height of the fluid changed by 1.5m. Modulus of elasticity for Sylgard 184, 1:10 ratio, was 1.45MPa [5]. The density for milk was 1.032kg/L. The radius of the sample was 4mm and the thickness was 1mm. So the theoretical hoop strain was 4.19% and the theoretical longitudinal strain was 2.09%. The highest strain the DIC calculated was about 2.1% strain in the yy-direction. More pressure or a lower modulus of elasticity was needed to see a higher percentage of strain.

DISCUSSION

The goal of this project was to develop a non-involve method to characterize strain that could be used to characterize local strain stiffness responsible for change in endothelial cell function. Throughout this project, synthetic models that mimic arterial geometry and mechanical properties that could be tested with DIC were successfully made. Also, a control test apparatus was successfully built. The biggest challenge faced was getting the models to work with DIC due to size of particles and contrast. The challenge of the size of the particles was overcome by grinding coffee and using mesh to apply it. The contrast was solved by using milk and whiteout applied so that it would not crack when the sample was stretched. These type of experiments are needed in the development of a noninvasive technique to characterize the strain on EC cells and to confirm computational data. Noninvasive techniques are important because these are safer and cheaper than other techniques. Since computational data is theoretical, experimental data is used to confirm for it is more real world. Digital Mechanical Analysis tests will be done on Sylgard 184 samples in order to determine more of its mechanical properties. During this project, a successful control test to compare the strain calculated by the DIC and the theoretical stain was developed. Also, was able to come up with a method to test and tested strain due to fluid on synthetic models that mimic arterial geometry and mechanical properties.

CONCLUSIONS

During the duration of this project, synthetic models that mimic arterial geometry and mechanical properties were made and tested with DIC. A successful control test was developed in order to compare the strain calculated by DIC and the theoretical strain. Also, was able to test strain due to fluid on synthetic models that simulated arterial geometry and mechanical properties. Overall, these results demonstrate the potential to measure strain on endothelia cells by use of DIC.

REFERENCES

[1] A. J. Lusis, "Atherosclerosis," Nature, vol. 407, no. 6801, pp. 233-241, 2000.

[2] "Endothelial Function Testing," Cedars-Sinai, [online] 2013, http://www.cedars-sinai.edu/Patients/Programsand- Services/Womens-Heart-Center/Services/Endothelial-Function-Testing.aspx

[3] R. N. Fogoros M.D., "Endothelial Dysfunction," ( About.com), [online] 2011, http://heartdisease.about.com/od/coronaryarterydisease/g/Endothelial-Dysfunction.htm

[4] J. Ohayon, A. M. Gharib, A. Garcia, J. Heroux, S. K. Yazdani, M. Malve, P. Tracqui, M.-A. Martinez, M. Doblare, G. Finet and R. I. Pettirgrew, "Is arterial wall-strain stiffening an additional process responsible for atherosclerosis in coronary bifurcations?: an in vivo study based on dynamic CT and MRI," American Journal of Physiology, vol. 301, no. 3, 2011.

[5] Martin, Bryn; Kotsakos, Tom; Stevens, Justin; Nicolaon, Sebastien; Cespedes, Steven;, "Quantificaion of the Modulus of Elasticity and Dynamic Properties of Sylgard for Various Mixing Ratios, 2003-2006", (neurohydrodynamics), [online] 2011, http://www.neurohydrodynamics.com/wiki/index.php?title=Quantification of the Modulus of Elasticity and Dynamic Properties of Sylgard for Various Mixing Ratios, 2003-2006

[6]"Introduction to Dynamic Mechanical Analysis (DMA): A Beginner's Guide", (PerkinElmer), [online] 2013, http://www.perkinelmer.com/CMSResources/Images/44-74546GDE IntroductionToDMA.pdf

[7] "The VIC-3D Measurement System", (Correlated Solutions), [online] 2013, http://www.correlatedsolutions.com/marketing/Brochures/VIC-3D%20System%20Specs.pdf

Emily Gould, Nicholas Carroll, Dr. Gail D. Jefferson, and Dr. Saami K. Yazdani

Department of Mechanical Engineering, 150 Jaguar Drive, Shelby Hall, Mobile, AL 36688
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Author:Gould, Emily; Carroll, Nicholas; Jefferson, Gail D.; Yazdani, Saami K.
Publication:Journal of the Mississippi Academy of Sciences
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2014
Words:1637
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