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
Background Motivation Intellectual merit & impact Technical approach
› Fabrication› Testing› Simulations
Problems Encountered Schedule Future Work Conclusions References
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 2002;11:137-147.
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/heart/stent_smart.jpg
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
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
Reinforced SMP stent designed through:
› Fabrication of prototype reinforced SMP material
› Mechanical testing of prototype material
› Computer simulations of reinforced SMP stent response
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
tBA & PEGDMA distilled with hydroquinone/methyl-ester 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
UV broad-spectrum light used in photopolymerization
Post-bake performed for 3hrs at 70°C
Cost:› Group spent $910.63 in researching › Materials $0.23/stent, total cost $3.05/stent
Tg determined using DSC› Only non-reinforced samples had obvious
transition› 20% - 38°C, 40% - 27°C
20% & 40% non-reinforced samples 40% cross-linked samples
27.39°C(H)
23.89°C
30.84°C
37.25°C(H)
35.17°C
39.33°C
-0.6
-0.5
-0.4
-0.3
-0.2
Hea
t Flo
w
(W/g
)
-20 0 20 40 60 80
Temperature (°C)
40-NR.001––––––– 20-NR.001– – – –
Exo Up Universal V4.1D TA Instruments
-0.7
-0.5
-0.3
-0.1
0.1
0.3
Hea
t Flo
w
(W/g
)
-20 0 20 40 60 80
Temperature (°C)
40P-3C––––––– 40P-2C– – – – 40P-1C––––– · 40P-0.5C––– – – 40P-NR––– –––
Exo Up Universal V4.1D TA Instruments
Compressive modulus calculated from tensile & flexural tests using method of Mujika et al
Flexural modulus determined using TMA
12
f
t
tc
E
E
EE
t3
3
fsbh4
FLE
Tensile modulus determined using tensile tester
Tensile test specimen Tensile test apparatus & furnace
Tensile modulus at body temperature
Compressive modulus calculation results
Direct measurements were also performed
Two simulation categories:› Reinforced SMP modulus determination› Buckling analysis
Stent designed as non-perforated cylinder
Modulus determination simulated using small block of SMP material
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:
3
o2 D
t
1
E2*P
Timoshenko SA, Gere JM. Theory of elastic stability. McGraw Hill, New York, New York 1961.
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
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
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
Yakacki CM, Shandas R, Lanning C, Rech B, Eckstein A, Gall K. Unconstrained recovery characterization of shape-memory polymer networks for cardiovascular applications. Biomaterials 2007;28:2255-2263.
Stoeckel D, Bonsignore C, Duda S. A survey of stent designs. Min Invas Ther & Allied Technol 2002;11:137-147.
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 1992;13:176-182.
PEGDMA DMPAtBA
Images from: www.sigmaaldrich.com
Tensile test at body temperature
Tensile test at room temperature
Displacement in x-direction
Displacement in y-direction
Displacement in z-direction
Typical failed buckling analysis resulting displacement