NX CAE Symposium 2013 Andrew Jabola, Application Engineer, Saratech Inc. Shannon Gott (Ph. D. Candidate) & Masaru Rao (Assistant Professor), University of California - Riverside Planar Titanium Stent Design A Modern CAE Environment: Enabling Smarter Decisions
Agenda Objective Background: Motivation for Stenting Planar Stent Challenges FEA: The Key to Redesign Post-processing Comparison Against Physical Test Data Lessons Learned 2
Objective Develop and refine titanium micromachining techniques to create nanopatterned titanium stents Solve current stent limitations with physical means Hypothesis: rationally-designed surface nanopatterning will enhance desired vascular cell responses relative to uncontrolled surfaces 3
Background: Heart Disease Heart disease is the leading cause of death in the U.S. Most common form of heart disease is cardiovascular disease (CVD) CVD is caused by atherosclerosis, characterized by plaque build up Image from Texas Heart Institute: http://www.texasheart.org/hic/topics/cond/carotidarterydisease.cfm 4
Background: CVD Treatment Three treatment options Drugs Catheter assisted procedures: Stenting Coronary artery bypass surgery Image from National Heart Lung and Blood Institute: http://www.nhlbi.nih.gov/health/dci/diseases/stents/stents_all.html 5
Background: Stents Today s stents have a variety of problems Stents damage artery wall lining Potential complications Bare-metal stents (BMS) : Restenosis smooth muscle build up Drug-eluting stents (DES) : Thrombosis blood clotting Images from Curfman GD et al., NEJM. 2007;256:1059-1060 6
Planar Stent Fabrication Nanopatterning fabrication techniques are inherently planar Using an approach demonstrated by Takahata & Gianchandani, we have circumvented this limitation Pattern Fabrication 7
Planar Stent Expansion High uniformity of stent struts! Planar to 3D stent transformation a) 80 µm thick, deep-etched planar Ti stent b) Tapered needle weaved through to create compact cylindrical geometry c) Balloon-mounted stent d) Stent after deployment in 3 mm I.D. mock artery 8
Planar Stent Radial Stiffness Testing Radial stiffness is important for patency Displacement control with load response recorded Current Ti stents possess about half the radial stiffness of commercial stainless steel stents Possible reasons for lower radial stiffness: Material Properties Less material Stent Force-Displacement Data Design 9
FEA Objective Analyze and Correlate current planar stent design by Takahata against physical test data Optimize future designs using FEA and verify using physical test Takahata Design Future Design 10
Analysis Challenges Highly Nonlinear Analysis Large Displacement/Large Strain Difficult Contact Analysis both in Expansion and Crushing Nonlinear Material Properties 11
Analysis Objective Expand Stent to match deployed configuration Stent is expanded to 3 mm in diameter Crush Stent to correlate against physical test data Stent is crushed back to 1.5 mm for correlation 12
Analysis Setup Pre/Post NX 8.5 Advanced Simulation Solver NX NASTRAN 8.5 Advanced Nonlinear (ADINA) 601/129 NL Transient Solution Solution Timesteps (time/time steps) Expansion (100 s/410 time steps) Expander Relax (25 s/50 time steps) Crush (75 s/200 time steps) 13
Analysis Setup Material Grade 1 Commercial Pure Titanium Isotropic, Plastic, Setup with a NL Stress-Strain Curve Stress-Strain Curve determined through tensile testing by UC Riverside Isotropic Hardening 14
Stress (MPa) Analysis Setup Stress-Strain Curve Data points were reduced from actual test data 300 Titanium Stress-Strain Curve 250 200 150 100 50 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Strain (mm/mm) 15
Analysis Setup Stent Modeled using solid elements (~20K) Reduced to a single link for reduction in computation time 16
Analysis Setup Needle/Balloon Modeled using Plate Elements Enforced Displacements used to expand 17
Analysis Setup Crushing Mechanism Modeled using plates and RBE2s 18
Analysis Setup Elements Overall Element size is 0.015 mm Linear Solid (HEX/WEDGE) and Plate Elements used Plate Region is also the same resolution for contact considerations Element Count: 216732 elements 19
Analysis Setup Boundary Conditions Stent Stent was constrained only in Y at ends Y+ 20
Analysis Setup Boundary Conditions Balloons Enforced Displacements used on balloons 1) Balloons Initial Translate in Z directions to initially expand stent 2) Center Plate Radial expands to fully deploy stent 21
Analysis Setup Boundary Conditions Initial Expansion 22
Analysis Setup Boundary Conditions Full Deploy (3 mm) 23
Analysis Setup Boundary Conditions Plates Enforced Displacements at RBEs used for crushing 1.5 mm 24
Analysis Setup Boundary Conditions Animation 25
Analysis Setup Contact Conditions Frictionless Contact Contact Birth/Death Used for expansion and crushing 26
Analysis NX NASTRAN Adv. NL (ADINA) 601/129 Transient Analysis Solution Memory: 5.4 GB 27
Post-processing Test Correlation Indicators Visual Deformation Crushing Reaction Force 28
Deformation Analysis Results
Results Deformation - Comparison 30
Results Deformation - Comparison 31
Results Deformation - Comparison 32
Results Reaction Force Crushing Force Processed as N/mm of link length (link length 0.8 mm) Reaction Force taken at end of RBE 33
Results Reaction Force FEA vs. Physical Test 34
Lessons Learned Analysis was product of many runs Following is list of important parameters and lessons learned while running SOL 601/129 These can be applied to many other NL analyses 35
Lessons Learned General Tips Step-by-Step Analyze in pieces, don t try and setup entire analysis in one shot Time Step Optimization Place Time Steps only where you need them Automatic Time Stepping is a must (should be a default) 36
Lessons Learned Contact Parameters and Mesh Resolution Line Search Turn this on to help with contact problems Mesh Resolution Meshes must match between contact to get most accurate force results Contact Damping is required, but needs to be tuned, otherwise results can be odd 37
Future Work Future Work is being carried on by Shannon Gott of UC Riverside Optimization of design to increase stiffness Fatigue Life Analysis 38
Conclusion Correlation between physical test data and FEA was achieved using NX NASTRAN and NX CAE Many lessons learned that are applicable to many nonlinear situations Provides Basis for future work 39
Q/A 40
References 1. J. Lu, M. P. Rao, N. C. MacDonald, D. Khang and T. J. Webster, Acta Biomaterialia, 2008, 4, 192-201. 2. P. Vandrangi, S. C. Gott, V. G. J. Rodgers and M. P. Rao, presented in part at the 7th International Conference on Microtechnologies in Medicine and Biology, Marina Del Rey, CA, April 10 12, 2013, 2013. 3. A. W. Martinez and E. L. Chaikof, Wiley Interdiscip. Rev.-Nanomed. Nanobiotechnol., 2011, 3, 256-268. 4. M. F. Aimi, M. P. Rao, N. C. Macdonald, A. S. Zuruzi and D. P. Bothman, Nat. Mater., 2004, 3, 103-105. 5. E. R. Parker, B. J. Thibeault, M. F. Aimi, M. P. Rao and N. C. MacDonald, J. Electrochem. Soc., 2005, 152, C675-C683. 6. K. Takahata and Y. B. Gianchandani, J. Microelectromech. Syst., 2004, 13, 933-939. 41