MECHANICAL OPTIMIZATION OF VASCULAR BYPASS GRAFTS A Thesis Presented to The Academic Faculty By Luc Felden In Partial Fulfillment Of the Requirements for the Degree Master of Science in Mechanical Engineering Georgia Institute of Technology May 2005 MECHANICAL OPTIMIZATION OF VASCULAR BYPASS GRAFTS Approved by: Dr. David N. Ku, Advisor The George W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Dr. Alexander Rachev, Co-Advisor The George W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Dr. Elliot L. Chaikof Department of Surgery Emory University School of Medicine Date Approved: March 28, 2005 ACKNOWLEDGEMENTS I wish to express my genuine gratitude to my advisor Dr. Ku, for offering me an opportunity to work in his lab and for his guidance in my research. I particularly wish to thank Dr. Rachev for his daily assistance on this project. I sincerely thank Dr. Chaikof for his review and comments. I thank you all for your help and friendship along this year. My heartfelt appreciation goes to all my lab mates from whom I learned more about the American way of life. I am deeply grateful to my family and friends for the support and love they have provided me while I was abroad. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS...............................................................................................iii LIST OF TABLES............................................................................................................vii LIST OF FIGURES...........................................................................................................ix LIST OF SYMBOLS or ABBREVIATIONS...................................................................xii SUMMARY ....................................................................................................................xiii CHAPTER 1 INTRODUCTION........................................................................................1 1.1 Atherosclerosis..............................................................................................2 1.2 Treatments.....................................................................................................3 1.3 Problems with current grafts.........................................................................4 1.4 Theories on Intimal Hyperplasia...................................................................5 1.5 Hypothesis...................................................................................................10 1.6 Objectives...................................................................................................10 CHAPTER 2 EXPERIMENTAL DESIGN AND METHODS........................................11 2.1 Design concept............................................................................................11 2.2 Specification of mechanics-matching grafts...............................................11 2.3 Mathematical model....................................................................................12 2.4 Design.........................................................................................................19 2.4.1 Design optimization.........................................................................................19 2.4.2 Material requirements......................................................................................21 2.4.3 Identification of the SEF of a particular form W=W(I1)..................................23 2.4.4 Example of first invariant, biocompatible, hydrophilic material.....................26 2.4.5 Example of host artery parameters..................................................................27 2.5 Building elastic tubes..................................................................................30 iv 2.6 Identification and verification tests.............................................................30 2.6.1 Sample placement............................................................................................31 2.6.2 Initial inner diameter measurement.................................................................31 2.6.3 Preconditioning................................................................................................32 2.6.4 Pressure-outer diameter measurements............................................................33 2.6.5 SEF parameters................................................................................................38 2.6.6 Model validation..............................................................................................39 2.7 Sensitivity analysis......................................................................................40 2.8 Burst pressure..............................................................................................41 2.9 Selection algorithm.....................................................................................41 CHAPTER 3 RESULTS...................................................................................................46 3.1 Choice of material.......................................................................................46 3.2 Pressure-diameter curves of the tube for identification..............................47 3.2.1 Initial geometry................................................................................................47 3.2.2 Cross-section....................................................................................................49 3.2.3 Volume.............................................................................................................50 3.2.4 Comparison......................................................................................................51 3.2.5 Differences.......................................................................................................51 3.3 SEF parameters...........................................................................................54 3.4 Model validation.........................................................................................56 3.5 Design of mechanics-matching grafts.........................................................58 3.6 Sensitivity analysis......................................................................................61 3.7 Possible range of fabrication.......................................................................65 3.8 Selection of one prototype..........................................................................67 3.9 Picture of the prototype graft #5.................................................................69 3.10 Deviation from prediction...........................................................................69 3.11 Burst pressure..............................................................................................73 CHAPTER 4 DISCUSSION.............................................................................................74 v 4.1 Summary of accomplishments....................................................................74 4.2 Comparison with ePTFE, Dacron, SVG, arteries.......................................74 4.3 Limitation of approach................................................................................78 4.4 Need for in vivo test of IH..........................................................................82 4.5 Clinical advantage.......................................................................................83 4.6 Conclusions and recommendations.............................................................84 APPENDIX A ”Predicted Mechanical Responses of PVA hydrogel Grafts” Program..86 APPENDIX B ”Design outcomes” Program...................................................................89 APPENDIX C ”Selection of the best Manufactured Graft for the patient” Program......92 REFERENCES.................................................................................................................96 vi LIST OF TABLES Table 2.1: Mechanical properties of a canine coronary artery [16] 22 Table 2.2: Mechanical properties of current synthetic grafts (Goodfellow Corporation, Gore & Associates Inc) 22 Table 2.3: Vessel dimensions and circumferential stretch ratio at physiological condition (100mmHg,lz =±1.610.17 ) [54] 27 Table 2.4: Material constants of the canine carotid artery assuming zero stress under no load condition [54] 28 Table 2.5: Mechanical responses of the CCHA calculated from the membrane model 29 Table 2.6: Human LCCA mechanical response [16] 29 Table 3.1: Thickness of current graft to be compliant 46 Table 3.2: Range of thickness of a possible compliant tube made of PVA hydrogel and corresponding stiffness. 47 Table 3.3: Pressure-diameter relation (“cross-section” measurement) 49 Table 3.4: Pressure-diameter relation (“volume” measurement) 50 Table 3.5: Differences in the diameter measurement between the two experiment setups 52 Table 3.6: Mechanical responses of the tube for SEF identification (lz=1.0) 54 Table 3.7: Experimental and theoretical mechanical responses of the tube for SEF identification with an axial stretch ratio of 1.038 56 Table 3.8: Mechanical responses of the IOG for a CCHA calculated from the membrane model 59 Table 3.9: Design outcomes for a human coronary host artery 61 Table 3.10: Mechanical responses of the reference and representative sample canine host artery 66 Table 3.11: Manufactured Graft initial dimensions and predicted compliance 67 vii Table 3.12: Theoretical and experimental mechanical responses of the prototype 70 Table 3.13: Other