Transcript Hydrogen Storage Materials for Mobile Applications

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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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Background
Motivation
Intellectual merit & impact
Technical approach
Fabrication
› Testing
› Simulations
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Problems Encountered
Schedule
Future Work
Conclusions
References
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Stents reduce restenosis rates to 10-40% following angioplasty
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Desirable stent properties include:
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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.
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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
http://my.clevelandclinic.org/PublishingImages/
heart/stent_smart.jpg
Ratna et al. Recent advances in
shape memory polymers and
composites. J Mat Sci 43 (2008)
254.
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All stents currently approved for use by FDA are metallic
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Disadvantages:
Rapid expansion rates
› Compliance mismatching
› High manufacturing costs
› Limited areas available for drug loading
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Aim is to improve upon current stent technology through the
use of a reinforced shape memory polymer with unique
advantages
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We hoped to gain an increased understanding of
the strengthening mechanisms within a shape
memory polymer
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A reinforced shape memory polymer stent may be
a safer and more biomedically friendly device
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Stronger shape memory polymers will have
applications in many fields, not just stents
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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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Materials:
› Monomer: tert-butylacrylate (tBA)
› Crosslinker: Poly(ethylene glycol)n
dimethacrylate (PEGDMA)
› Photoinitiator: 2,2-Dimethoxy-2phenylacetophenone (DMPA)
› Reinforcement: Montmorillonite clay platelets
(Cloisite®)
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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
-0.2
––––––– 40-NR.001
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20-NR.001
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––––
––––– ·
––– – –
––– –––
0.3
0.1
Heat Flow (W/g)
Heat Flow (W/g)
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40P-3C
40P-2C
40P-1C
40P-0.5C
40P-NR
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35.17°C
-0.1
-0.3
37.25°C(H)
39.33°C
-0.5
23.89°C
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27.39°C(H)
30.84°C
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-0.6
-20
Exo Up
0
20
40
Temperature (°C)
60
80
Universal V4.1D TA Instruments
20% & 40% non-reinforced samples
-20
Exo Up
0
20
40
Temperature (°C)
60
80
Universal V4.1D TA Instruments
40% cross-linked samples
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Compressive modulus calculated from
tensile & flexural tests using method of
Mujika et al
Et
Ec 
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Et
2
1
Ef
Flexural modulus determined using TMA
Ef 
3
L F
4bh 3 s t
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Tensile modulus determined using tensile
tester
Tensile test specimen
Tensile test apparatus & furnace
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Tensile modulus at body temperature
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Compressive modulus calculation results
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Direct measurements
were also performed
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Two simulation categories:
› Reinforced SMP modulus determination
› Buckling analysis
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Stent designed as non-perforated cylinder
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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
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› 300mmHg (40KPa) differential pressure across stent
› Use 80KPa for a safety factor of 2
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Analyze for wall thickness when collapse
occurs
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› Buckling theory:
2E  t 
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
P* 
1   2  Do 
Timoshenko SA, Gere JM. Theory of elastic
stability. McGraw Hill, New York, New York 1961.
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Unfamiliarity with bioengineering issues
ANSYS
› Buckling
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Fabrication
› Bubbles, high wt% samples
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Tensile testing at body temperature
› Oven, heat gradients
Communication between committees
 Underestimated time for many processes
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Biocompatibility testing
› Cytotoxicity, thrombosis, platelet adhesion
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Further environmental tests
› Creep, erosion, wet strength
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More detailed simulations
› Uneven plaque distribution, non-cylindrical arteries
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Shape memory effect testing
› Strain recovery rates & recovery times
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Collapse press tests
Drug & gene loading investigations
Sterilization techniques
Clinical trials
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Testing revealed much lower than expected moduli for the
reinforced SMP material at body temperature due to its lower
than expected Tg
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The low modulus of the prototype material resulted in a
necessary stent wall thickness of 480µm, about twice as large
as is practical
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Must control Tg with two cross-linkers & maintain above body temp
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
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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;28:22552263.
 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(llactic acid) as a material for intravascular polymeric stents.
Biomaterials 1992;13:176-182.
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tBA
PEGDMA
Images from: www.sigmaaldrich.com
DMPA
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