designing reliabilty into bio medical devices-medi2005

8/2/2019 Designing Reliabilty Into Bio Medical Devices-Medi2005

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Design in g Rel iab i l i t y i n t o Biom ed ica lDev ices: Fin i t e Elem en t An a ly ses f o r

Bio - Med ical App l i ca t ion s

Peter R. Barrett Kenneth Brown

Computer Aided Engineering Associates



Computer Aided Engineering Associates, Inc. 2

There i s W idesp read I n t e rest i n I m prov ing t he Design Process




There i s W idesp read I n t e rest i n Design ing Re l i ab i l i t y

! “Design directly influences more than 70% of the product life cycle cost;companies with high product development effectiveness have earningsthree times the average earnings; and companies with high productdevelopment effectiveness have revenue growth two times the averagerevenue growth."

! "40% of product development costs are wasted!"

source: Brenda Reichelderfer of ITT Industries http://www.isixsigma.com/library/content/c020311a.asp




Ex am ple Med ical Dev ice Deve lop m ent Cyc le

! From start until FDA approval, medical device evolution will typicallyinclude:

— 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




! 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.

Sam p le St ru ct u r a l Design I ssues ( Fo r St en t s)


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W hat Too ls Can W e Use t o Design t he Dev ice?

! Maximize stiffness to best support the vessel.

— Use Finite Element Analysis

! Maximize surface area for drug coating.


! Maximize radiopacity for accurate stent deployment. — Material selection

! Minimize the delivery size so that it can fit on the smallest possiblecatheter.

— Design layout

! Maximize flexibility for ease in deployment.


! Meet strength and fatigue requirements. — Use Finite Element Analysis

! Analysis procedures could potentially be automated via size / shapeoptimizations



App l i cat ion t o Med ical Dev ices

! Finite Element Analysis (FEA) is a requirement to achieve Food & DrugAdministration’s (FDA) approval for many devices that are implanted in thebody.

! FEA is used in the design, verification, and validation of medical devicesthrough 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



Non l in ear i t i es in Ana lys is

! The structural nonlinearities inherent in medical device design are asfollows:

— 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



Fin i t e Elem en t So lu t i on Procedu re – St en t Design

! Solution steps:

— The polished laser cuttubing is starting point.

— Crimp onto balloon.

— Expansion to insertiondiameter.

— Released to vesseldiameter.

• Cyclic response forfatigue

• Radial compressionsimulation

! Contact surfaces are usedto simulate radial expansionand compression



Com pr ession & Dep loym en t Ana lysi s



Ex am ple ANSYS N i t i no l Mate r ia l I np u t



Val i da t i on o f Ana lysi s w i t h Test i ng

0

0.05

0.1

0.15

0.2

0.25

0.3

C r u s h

F r o c e ( l b )

Base Model FiniteElement Results

Base Case Design CrushData



How t o Design a Bet t e r Dev i ce

! Automated Finite Element Analysis

— Parametric input

— Design optimization

— Validated models

! Rapid prototyping / testing — Proof of concept

— Material property development

— Design confirmation



FEA I npu t Pa r am e t e r s

! 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




Desi gn Si m u l at i o n - St r u ct u r a l

! Parametric simulationsallow variations of thedesign to be analyzedquickly and effectively in anautomated fashion usingvariable input/outputparameters.

! A single “base” model isbuilt and analyzed andperturbations about thisdesign can be

automatically performed.




! 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

W hy D o Pa r am e t r i c Sim u la t i ons?




CAD Param et r i c Mode l Deve lopm ent




Ex a m p l e I n p u t Par a m et e r s

! 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




Ex am p le Ou t pu t Pa r am e t e r s

! 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




Opt im izing t he Design Procedur e

! Once a parametric model exists, the design can be optimized usingautomated techniques.

AnalysisFile

Explore theDesign Domain

Optimize theDesign

Initial Design

Parametric Model & Loading

Solution

Parametric Results




Ex am ple Ana ly s is Goa l

! The goal of the analysis is to determine the configurations of the eyeglassframe that can tolerate being stepped on and still be capable of recoverymeeting stiffness design requirements.




Set t i ng u p t he Ana lys is Env i ron m en t

!

Meshing, material properties,boundary conditions, andsolution controls are definedin a solution tool that is linked

directly to the CAD system

! Toggling between the designand analysis tools is possible

at any time




I n i t i a l So l u t i on t o D ebug M odel

! Prior to the sensitivitystudy, an initial solutionis generated by solvingin the simulationenvironment.

! This is not a required

step, but it is alwaysrecommended,particularly for debugging

the nonlinear solution.

P i f H i N i i l S El i M i l




Pos tp rocess ing o f Hys te rs i s N i t i n o l Sup er Elast i c Mate r ia l




Set t i ng up t he Sensi t i v i t y St udy Env i r onm en t

! Once the initial solution isresolved, input and outputvariables for the sensitivitystudy are defined in thesimulation environment.

! Input and output variables are

defined by checking the box tothe left of the parameter.

! In this case we specify thedefining geometric parameters

as the input variables




Resu l t s f r om t he Sensi t i v i t y St udy Env i r onm en t

! The output variables for thisstudy are the Von Mises stressand the deformation in the load

direction.! The stress can be used to

simulate the elastic response,while the displacement is used

to measure stiffness




DOE Met ho d Ana ly ses




Opt im iza t i on ; Design f o r Si x Sigm a

Input Distributions

Output; Histograms;Probability Tables




DOE – So lu t i o n

! Sensitivity of Max. Von Misesstress vs. frame thicknessand frame bottom radius are

illustrated in the figure

! As expected the larger thebottom radius & thinner frameproduces the lowest stress

values

! Specific data is extractedfrom the Response Tabs




Conc lus ions

! Sensitivity results (exampleon right) provide valuabledata for the design engineerto quickly determine the mostrelevant design parameters

! Optimization routines can beused to develop more robustdesigns

! Probabilistic evaluations canbe used to quantify tolerancesrequired to meet qualitycontrol




I n f e r i o r Vena Cava Fi l t e r – Exam p le Flu i d St r uc t u r e I n t e ract i on

! Inferior Vena Cava (IVC) Filters

prevents the passage of large life-threatening emboli to the lungs.

!

Treatment is an alternative toanticoagulant therapy

! Analyses provide:

— Understanding of the filter-vessel-flow interaction

— Opportunity for design iterations tomodify flow field, limit filter

deformations reduce filterstresses, etc.




St ru ct u r e Geom et r y and Mesh

! Structural Geometry fromCAD Model (Pro/E,Unigraphics, DesignModeler, etc.)

! Computational meshgenerated in Ansys

! Mapped mesh usingextrusion and sweepmeshing techniques

! Boundary Conditionscreated in ANSYS GUI.




St ru ct u r a l Boun dary Cond i t i on s

! Similar to structural

analysis modeling! ¼ Symmetric model with

symmetry BC’s

! Vessel supported onEnds to prevent rigidbody motion

! Fluid structure interactiondefined with surfaceloading

! Total time and time stepsetup matches CFX input

! Forces passed from CFXto ANSYS

! Displacements passedfrom ANSYS to CFX

Fl id G d M h




Flu id Geom et r y an d Mesh

! Flow domain generated

by ANSYS DesignModeler

! Computational meshgenerated by ANSYSCFXMesh

! 4 filter legs attached tovessel walls

!

12 legs unattached

! 150 K elements ¼symmetry of model

Fi l CFD l i




Fi l t e r – CFD ana lys is

• Solid surfaces act asinterface between fluidand solid domains

• CFD solution providesunsteady pressureloads on solid surfaces

• Solid deformations

constitute newboundary for CFD

• Two-way interactions,

fully coupled iterativesolutions at each timelevel

I l t l




I n l e t p r essu r e pu l se

Pressure = 100mmHG

Duration = 1.0 s

Analysis ends at 1.5 s

CFD A l i R l t




CFD An aly s is Resul t s

!

Blood flow model — Density as a function of

unsteady pressurepulse, weakly

compressible — Non-Newtonian fluid

! Moving meshcapability toaccommodatedeformed solidboundaries

— Filter face and legs

— Vessel wall

Fl i t i




Fl o w a n im at i on s

St t l A l i R l t D f m t i



St r u ct u r a l Ana ly s i s Resu l t s - D ef o r m a t i ons

designing reliabilty into bio medical devices-medi2005

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