Designing i Reliability into Biomedical Devices: Finite Elelment Analyses for Biomedical Applications 2005 CAE Associates
Designing Reliability into Biomedical Devices: Finite Element Analyses for Bio-Medical Applications Peter R. Barrett Kenneth Brown Computer Aided Engineering Associates
There is Widespread Interest in Improving the Design Process Computer Aided Engineering Associates, Inc. 2
There is Widespread Interest in Designing Reliability! Design directly influences more than 70% of the product life cycle cost; companies with high product development effectiveness have earnings three times the average earnings; and companies with high product development effectiveness have revenue growth two times the average revenue growth."! "40% of product development costs are wasted!" source: Brenda Reichelderfer of ITT Industries http://www.isixsigma.com/library/content/c020311a.asp Computer Aided Engineering Associates, Inc. 3
Example Medical Device Development Cycle! From start until FDA approval, medical device evolution will typically include: Conceptual idea development (material selection, manufacturability) Check for patent infringements Develop initial drawings Preliminary analysis (FEA) Re-design / Optimization Prototype development and physical testing Pass final design analysis (FEA) and reliability evaluation Pass animal studies Include a delivery system design Meet packaging requirements Computer Aided Engineering Associates, Inc. 4
Sample Structural Design Issues (For Stents)! Maximize stiffness! Maximize surface area for drug coating! Maximize radiopacity! Minimize the delivery size so that it can fit on the smallest possible catheter! Maximize flexibility for ease in deployment! Meet strength and fatigue requirements Changing any one of these design variables may adversely affect others Example: increasing radial stiffness may decrease bending flexibility. Computer Aided Engineering Associates, Inc. 5
What Tools Can We Use to Design the Device?! Maximize stiffness to best support the vessel. Use Finite Element Analysis! Maximize surface area for drug coating. Use Finite Element Analysis! Maximize radiopacity for accurate stent deployment. Material selection! Minimize the delivery size so that it can fit on the smallest possible catheter. Design layout! Maximize flexibility for ease in deployment. Use Finite Element Analysis! Meet strength and fatigue requirements. Use Finite Element Analysis! Analysis procedures could potentially be automated via size / shape optimizations Computer Aided Engineering Associates, Inc. 6
Application to Medical Devices! Finite Element Analysis (FEA) is a requirement to achieve Food & Drug Administration s (FDA) approval for many devices that are implanted in the body.! FEA is used in the design, verification, and validation of medical devices through the use of thermal, fluid, electromagnetic and structural models.! Current analysis models are used to predict: Ultimate strength and fatigue failure loads. Flexibility Recoil or springback Vessel damage Computer Aided Engineering Associates, Inc. 7
Nonlinearities in Analysis! The structural nonlinearities inherent in medical device design are as follows: Material nonlinearities Plasticity Super-elasticity (Nitinol) Hyperelasticity (model of tissue, plastics, rubber materials etc.) Geometric nonlinearities Deployment of device Localized large strains Contact nonlinearities Part-to-part (self-contact) Stent Angioplasty balloon Part to body part Artificial Hip or Knee Needle Part to deployment device Needle Computer Aided Engineering Associates, Inc. 8
Finite Element Solution Procedure Stent Design! Solution steps: The polished laser cut tubing is starting point. Crimp onto balloon. Expansion to insertion diameter. Released to vessel diameter. Cyclic response for fatigue Radial compression simulation! Contact surfaces are used to simulate radial expansion and compression Computer Aided Engineering Associates, Inc. 9
Compression & Deployment Analysis Computer Aided Engineering Associates, Inc. 10
Example ANSYS Nitinol Material Input Computer Aided Engineering Associates, Inc. 11
Validation of Analysis with Testing 0.3 0.25 Base Model Finite Element Results Base Case Design Crush Data 0.2 Crush Froce (lb) 0.15 0.1 0.05 0 Computer Aided Engineering Associates, Inc. 12
How to Design a Better Device! Automated Finite Element Analysis Parametric input Design optimization Validated models! Rapid prototyping / testing Proof of concept Material property development Design confirmation Computer Aided Engineering Associates, Inc. 13
FEA Input Parameters! Geometry: 2-D drawing or 3D CAD model Geometry of test sample for validation Geometry of loading device! Material properties: Stress vs. strain Force vs. deflection for test data! Loading conditions: Pressure Temperature Acceleration Angular Velocity Cyclic loading for fatigue Computer Aided Engineering Associates, Inc. 14
Design Simulation - Structural! Parametric simulations allow variations of the design to be analyzed quickly and effectively in an automated fashion using variable input/output parameters.! A single base model is built and analyzed and perturbations about this design can be automatically performed. Computer Aided Engineering Associates, Inc. 15
Why Do Parametric Simulations?! Parametric simulations can be used to: Reduce design cycle Cost savings Man power Time to market Prototyping Design verification of existing or new designs Design optimization to design a better device. Sensitivity studies on product performance Quality assurance Set manufacturing tolerances Meet six sigma requirements Satisfy ISO 9001:2000 regulations Computer Aided Engineering Associates, Inc. 16
CAD Parametric Model Development Computer Aided Engineering Associates, Inc. 17
Example Input Parameters! Automated Finite Element simulation scripts Parametric modeling Strut diameters, radii, widths Number of repeating segments Tubing thickness Geometry generation Scripted based on parameters 2D base revolved to 3D Material property input Elastic moduli Non-linear material modeling Material testing and validation Loading and boundary conditions Displacements Couples Pressures Inertial loads Computer Aided Engineering Associates, Inc. 18
Example Output Parameters! Parametric results data can also be extracted Displacements Reaction forces Radial stiffness Bending flexibility Stresses Single max. value Average values Strains Extent of permanent deformation Elastic springback Computer Aided Engineering Associates, Inc. 19
Optimizing the Design Procedure! Once a parametric model exists, the design can be optimized using automated techniques. Initial Design Parametric Model & Loading Solution Parametric Results Analysis File Explore the Design Domain Optimize the Design Computer Aided Engineering Associates, Inc. 20
Example Analysis Goal! The goal of the analysis is to determine the configurations of the eyeglass frame that can tolerate being stepped on and still be capable of recovery meeting stiffness design requirements. Computer Aided Engineering Associates, Inc. 21
Setting up the Analysis Environment! Meshing, material properties, boundary conditions, and solution controls are defined in a solution tool that is linked directly to the CAD system! Toggling between the design and analysis tools is possible at any time Computer Aided Engineering Associates, Inc. 22
Initial Solution to Debug Model! Prior to the sensitivity study, an initial solution is generated by solving in the simulation environment.! This is not a required step, but it is always recommended, particularly for debugging the nonlinear solution. Computer Aided Engineering Associates, Inc. 23
Postprocessing of Hystersis Nitinol Super Elastic Material Computer Aided Engineering Associates, Inc. 24
Setting up the Sensitivity Study Environment! Once the initial solution is resolved, input and output variables for the sensitivity study are defined in the simulation environment.! Input and output variables are defined by checking the box to the left of the parameter.! In this case we specify the defining geometric parameters as the input variables Computer Aided Engineering Associates, Inc. 25
Results from the Sensitivity Study Environment! The output variables for this study are the Von Mises stress and the deformation in the load direction.! The stress can be used to simulate the elastic response, while the displacement is used to measure stiffness Computer Aided Engineering Associates, Inc. 26
DOE Method Analyses Computer Aided Engineering Associates, Inc. 27
Optimization; Design for Six Sigma Output; Histograms; Probability Tables Input Distributions Computer Aided Engineering Associates, Inc. 28
DOE Solution! Sensitivity of Max. Von Mises stress vs. frame thickness and frame bottom radius are illustrated in the figure! As expected the larger the bottom radius & thinner frame produces the lowest stress values! Specific data is extracted from the Response Tabs Computer Aided Engineering Associates, Inc. 29
Conclusions! Sensitivity results (example on right) provide valuable data for the design engineer to quickly determine the most relevant design parameters! Optimization routines can be used to develop more robust designs! Probabilistic evaluations can be used to quantify tolerances required to meet quality control Computer Aided Engineering Associates, Inc. 30
Inferior Vena Cava Filter Example Fluid Structure Interaction! Inferior Vena Cava (IVC) Filters prevents the passage of large lifethreatening emboli to the lungs.! Treatment is an alternative to anticoagulant therapy! Analyses provide: Understanding of the filter-vesselflow interaction Opportunity for design iterations to modify flow field, limit filter deformations reduce filter stresses, etc. Computer Aided Engineering Associates, Inc. 31
Structure Geometry and Mesh! Structural Geometry from CAD Model (Pro/E, Unigraphics, Design Modeler, etc.)! Computational mesh generated in Ansys! Mapped mesh using extrusion and sweep meshing techniques! Boundary Conditions created in ANSYS GUI. Computer Aided Engineering Associates, Inc. 32
Structural Boundary Conditions! Similar to structural analysis modeling! ¼ Symmetric model with symmetry BC s! Vessel supported on Ends to prevent rigid body motion! Fluid structure interaction defined with surface loading! Total time and time step setup matches CFX input! Forces passed from CFX to ANSYS! Displacements passed from ANSYS to CFX Computer Aided Engineering Associates, Inc. 33
Fluid Geometry and Mesh! Flow domain generated by ANSYS Design Modeler! Computational mesh generated by ANSYS CFXMesh! 4 filter legs attached to vessel walls! 12 legs unattached! 150 K elements ¼ symmetry of model Computer Aided Engineering Associates, Inc. 34
Filter CFD analysis Solid surfaces act as interface between fluid and solid domains CFD solution provides unsteady pressure loads on solid surfaces Solid deformations constitute new boundary for CFD Two-way interactions, fully coupled iterative solutions at each time level Computer Aided Engineering Associates, Inc. 35
Inlet pressure pulse Pressure = 100mmHG Duration = 1.0 s Analysis ends at 1.5 s Computer Aided Engineering Associates, Inc. 36
CFD Analysis Results! Blood flow model Density as a function of unsteady pressure pulse, weakly compressible Non-Newtonian fluid! Moving mesh capability to accommodate deformed solid boundaries Filter face and legs Vessel wall Computer Aided Engineering Associates, Inc. 37
Flow animations Computer Aided Engineering Associates, Inc. 38
Structural Analysis Results - Deformations Computer Aided Engineering Associates, Inc. 39