Analysis of Material Choice and Geometry of Coronary StentsSara Beck, Josh Becker, Emily Burgess, Brian Della Mora, William Turri

MSE 365 Tuesday LabDepartment of Materials Science and Engineering, University of Michigan

Introduction

Metal Analysis

• Used Scanning Electron Microscopy (SEM) to visualize stents on amicroscopic scale

• Sample was mounted in Bakelite powder, ground, and polished to seemicrostructure

• Homogeneous microstructure observed• Etching needed for microstructure but not possible due to lack of supplies

• Composition of the BMS and DESanalyzed using EDAX and WD-XRF

• EDAX and WD-XRF were consistent• BMS and DES showed virtually the

same composition• WD-XRF data displayed was used to

identify the alloy choice as MP35N1

Component BMS (wt%) DES (wt%)Cr 19.9 19.6Co 31.0 30.8Ni 36.5 35.4

Mo 10.9 12.1

• Alloy has high strength, toughness, and corrosion resistance• Stents are wire drawn, plastically deformed into a sinusoidal shape, and

laser fused at certain junctions• Surface is then electropolished to ensure microscopic uniformity2

1 “Data Sheet: MP35N Alloy.” Specialty Steel Supply. 2008. Web.2 Loya, Mariana. Design and fabrication of advanced surface microstructures: surface modification of cardiovascular stent wires via RF plasma processing. Dissertation, University of California San Diego.Web.3 “Plastic Deformation During the Expansion of a Biomedical Stent.” Application Gallery. COMSOL Multiphysics. Web. 4 Ahmad, Hasan, et al. "Synthesis of Biocompatible Sterically-Stabilized Poly(2-(methacryloyloxy)ethyl Phosphorylcholine) Latexes via Dispersion Polymerization in Alcohol/Water Mixtures." Langmuir (2009): 11442-1449. Print.5 Udipi, K. “Development of a Novel Biocompatible Polymer system for Extended Drug Release in A Next-generation Drug-eluting Stent.” Journal of Biomedical Materials Research(2007): 1064-071. Web.6 “Indications, Safety, and Warnings.” Coronary Stent. Web.

Polymer AnalysisComputational Modeling

• Intended to run DSC and FT-IR to gather percent crystallinity, glasstransition, and functional groups

• Polymer coating was only 4.3 µm thick (not enough to run successfultests)

• A literature review was conducted to better understand properties of thepolymers and why they were chosen

• The two DES are the Endeavor and Resolute Integrity

• The coating needs to be both hydrophilic and hydrophobic to: Control drug release Maintain an interface with the blood vessel wall

• The release rate of Zotarolimus was optimized5

Cell Membrane

Hydrophilic Polymer

Hydrophobic Polymer & DrugStent

Summary

• The metal was identified as MP35N alloy processed by wire drawing• Diameter expansion comparison with theoretical values was consistent• Stent application increases circulation• BMS geometry can be simplified without losing functionality• COMSOL modeling shows importance of laser fusing in radial expansion• Gained an understanding of the polymer-drug system mechanism

Acknowledgements & References

Stent Polymer CoatingEndeavor Phosphorylcholine4

Resolute C19/C10/PVP5

• Polymers must be: Robust (rupture resistant) Biocompatible Hydrophilic

• Zotarolimus is a hydrophobic drugused to treat thrombosis andstenosis6

3

3.2

3.4

3.6

3.8

4

5 6 7 8 9 10 11 12 13 14 15 16

Inte

rnal

Dia

met

er (m

m)

Pressure (atm)

ExperimentalTheoretical

400

300

200

100

0

Von Mises Stress Distribution (MPa)

Future Recommendations• For testing of metal alloy and functional polymers, obtain bulk samples to

use for in situ material testing• Apply etchants to determine crystal structure and grain size• Model fluid flow more accurately using 3-D geometry• Determine radial expansion for BMS geometry

1.40

1.35

1.30

1.25

1.20

1.45

1.15

Flow Velocity (m/s)

• Heart stents are used to treat blood clots• Clots affect the flow of blood in vessels, leading to strokes or heart attacks• Blood flow modeled in COMSOL to compare clotted vessel to open vessel• When the blood clot is present, blood flow is significantly slower

Demonstrates decreased circulation• After stent implantation, circulation should return to normal

• Stents radially expand by applying an internalpressure to keep vessel open and alleviate clot

• The radial expansion of a stent was modeled3

Must plastically deform without failure Functionality of modeled geometry comparable

to BMS• Laser fusion:

Joins stent sections Reduces stress Decreases chance of stent failure

BMS Geometry

Special thanks to Michael Palazzolo at Medtronic for providing us with the stents and to Justin Scanlon for helping us set up experiments.

Images:"Coronary Stenting - Dr I.B.A. Menown MD FRCP Consultant Cardiologist." Coronary Stenting - Dr I.B.A. Menown MD FRCP Consultant Cardiologist. Web. Huang, Kenneth. "Mr. Heart." Future Science Leaders. 17 Jan. 2015. Web.BioLinx Polymer System. Biovisioning and Medtronic. Video.

Coronary stents are cylindrical instrumentsimplanted in blood vessels. These devices werefirst used in 1986 by Jacques Puel in France andhave since been optimized for the treatment ofthrombosis and stenosis. An in-depth study ofthe effects of material choice, geometry, andprocessing of bare metal stents (BMS) and twodifferent polymer coated drug-eluting stents (DES) made by Medtronic wascompleted.