Shape Memory Polymer for Stent Applications

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Shape Memory Polymer for Stent Applications

• Jeremy Cheng
• Sara Clementson
• Grant Hatcher
• Hilary Lane
• Ryan Mulholland
• Swezin Than Tun

• ENMA490 Capstone Final Report
• May 13th, 2009

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Acknowledgments

• Dr. Al-Sheikhly
• Dr. Bigio
• Dr. Briber
• Dr. Bonenberger
• Dr. Ethridge
• Dr. Fourney
• Dr. Kofinas
• Dr. Phaneuf
• Dr. Seog
• Mr. John Abrahams
• Mr. Michael Kasser
• Mr. Tom Loughran
• Mr. Xin Zhang

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Outline

• Background
• Motivation
• Intellectual merit impact
• Technical approach
• Fabrication
• Testing
• Simulations
• Problems Encountered
• Schedule
• Future Work
• Conclusions
• References

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Background

• Stents reduce restenosis rates to 10-40
  following angioplasty
• Desirable stent properties include
• High radial strength
• Good compliance matching with arterial walls
• Biocompatible
• Radio-opacity for visualization during X-ray,
  MRI, etc.
• Contain drugs and/or genes for additional therapy

Stoeckel D, Bonsignore C, Duda S. A survey of
stent designs. Min Invas Ther Allied Technol
200211137-147.
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Background (contd)

• Shape memory polymers are based upon different
  conformations of polymer chains at different
  temperatures
• Because shape memory effect is not due to a phase
  change, strains up to 400 are recoverable

Ratna et al. Recent advances in shape memory
polymers and composites. J Mat Sci 43 (2008) 254.
http//my.clevelandclinic.org/PublishingImages/hea
rt/stent_smart.jpg
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Motivation

• All stents currently approved for use by FDA are
  metallic
• Disadvantages
• Rapid expansion rates
• Compliance mismatching
• High manufacturing costs
• Limited areas available for drug loading
• Aim is to improve upon current stent technology
  through the use of a reinforced shape memory
  polymer with unique advantages

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Intellectual merit impact

• We hoped to gain an increased understanding of
  the strengthening mechanisms within a shape
  memory polymer
• A reinforced shape memory polymer stent may be a
  safer and more biomedically friendly device
• Stronger shape memory polymers will have
  applications in many fields, not just stents

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Technical Approach

• Reinforced SMP stent designed through
• Fabrication of prototype reinforced SMP material
• Mechanical testing of prototype material
• Computer simulations of reinforced SMP stent
  response

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Fabrication

• Materials
• Monomer tert-butylacrylate (tBA)
• Crosslinker Poly(ethylene glycol)n
  dimethacrylate (PEGDMA)
• Photoinitiator 2,2-Dimethoxy-2-phenylacetophenone
  (DMPA)
• Reinforcement Montmorillonite clay platelets
  (Cloisite)
• Samples made with 0, 0.5, 1, 2, 3
  reinforcement (by weight) at both 20 40
  crosslinking

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Fabrication (contd)

• tBA PEGDMA distilled with hydroquinone/methyl-es
  ter remover
• tBA, PEGDMA, DMPA (0.1 wt), Cloisite mixed
  and injected into mold made of 1/16 viton gasket
  between two glass slides coated with Rain-X as a
  releasing agent

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Fabrication (contd)

• UV broad-spectrum light used in
  photopolymerization
• Post-bake performed for 3hrs at 70C
• Cost
• Group spent 910.63 in researching
• Materials 0.23/stent, total cost 3.05/stent

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Testing

• Tg determined using DSC
• Only non-reinforced samples had obvious
  transition
• 20 - 38C, 40 - 27C

20 40 non-reinforced samples
40 cross-linked samples
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Testing (contd)

• Compressive modulus calculated from tensile
  flexural tests using method of Mujika et al
• Flexural modulus determined using TMA

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Testing (contd)
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Testing (contd)

• Tensile modulus determined using tensile tester

Tensile test specimen
Tensile test apparatus furnace
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Testing (contd)

• Tensile modulus at body temperature

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Testing (contd)

• Compressive modulus calculation results
• Direct measurements
  were also performed

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Simulations

• Two simulation categories
• Reinforced SMP modulus determination
• Buckling analysis
• Stent designed as non-perforated cylinder
• Modulus determination simulated using small block
  of SMP material

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Simulations (contd)

• Buckling analysis based on constant, uniform
  pressure on exterior of stent wall
• Max pressure from Agrawal et al
• 300mmHg (40KPa) differential pressure across
  stent
• Use 80KPa for a safety factor of 2
• Analyze for wall thickness when collapse occurs
• Buckling theory

Timoshenko SA, Gere JM. Theory of elastic
stability. McGraw Hill, New York, New York 1961.
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Problems Encountered

• Unfamiliarity with bioengineering issues
• ANSYS
• Buckling
• Fabrication
• Bubbles, high wt samples
• Tensile testing at body temperature
• Oven, heat gradients
• Communication between committees
• Underestimated time for many processes

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Initial Schedule
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Final Schedule
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Future Work

• Biocompatibility testing
• Cytotoxicity, thrombosis, platelet adhesion
• Further environmental tests
• Creep, erosion, wet strength
• More detailed simulations
• Uneven plaque distribution, non-cylindrical
  arteries
• Shape memory effect testing
• Strain recovery rates recovery times
• Collapse press tests
• Drug gene loading investigations
• Sterilization techniques
• Clinical trials

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Conclusions

• Testing revealed much lower than expected moduli
  for the reinforced SMP material at body
  temperature due to its lower than expected Tg
• Must control Tg with two cross-linkers maintain
  above body temp
• The low modulus of the prototype material
  resulted in a necessary stent wall thickness of
  480µm, about twice as large as is practical
• 480µm wall thickness reduces flow rate to 58
  original flow rate
• Simulations as performed were sufficient to show
  general trends in the behavior of the material
  but accuracy could be improved with more advanced
  version of software
• Difficulties due to unfamiliarity with software

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References

• Yakacki CM, Shandas R, Lanning C, Rech B,
  Eckstein A, Gall K. Unconstrained recovery
  characterization of shape-memory polymer networks
  for cardiovascular applications. Biomaterials
  2007282255-2263.
• Stoeckel D, Bonsignore C, Duda S. A survey of
  stent designs. Min Invas Ther Allied Technol
  200211137-147.
• Timoshenko SA, Gere JM. Theory of elastic
  stability. McGraw Hill, New York, New York 1961.
• Agrawal CM, Haas KF, Leopold DA, Clark HG.
  Evaluation of poly(l-lactic acid) as a material
  for intravascular polymeric stents. Biomaterials
  199213176-182.

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Questions?
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Supplement
PEGDMA
DMPA
tBA
Images from www.sigmaaldrich.com
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Supplement
Tensile test at body temperature
Tensile test at room temperature
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Supplement
Displacement in x-direction
Displacement in y-direction
Displacement in z-direction
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Supplement
Typical failed buckling analysis resulting
displacement