PCI & AimRadial 2018 | Biomechanics of biodegradable stents: a primer for cardiologists - Rosaire Mongrain

1. Biomechanics of biodegradable stents:
a primer for cardiologists
McGill
PCI – AIM RADIAL 2018
Rosaire Mongrain
Professor
Department of Mechanical Engineering
McGill University

2. 1. Stent scaffolding function:
Mechanical Properties (Elasticity & Stiffness)
Sy: Yield stress
Su: Ultimate
tensile stress
E : elastic modulus
(Young) in Pa
Metals
E
Cold drawing
neckingElasticity
and
Stiffness
(radial
strength)
Plastic
deformation
 E
strain strain
Radial
Strength:
global
structural
property
Creep:

3. • Stent Fracture classification 2 Severe fractures
• SF correlated with:
• Anatomical location (tortuosity)
RCA (57%) > LAD (34%)
• Stent design and lesion types
SF in middle of single stent
SF in extremities of long stents
Type I, II DT≈ 172d, Type V  1800d
1. Stent scaffolding function:
Mechanical Properties (Fatigue)

4. Peuster et al, 2006 Biomaterials 27
(2006) 4955–4962
Google images
Illustration of a grain
structure of a 130 micron
316L strut
Grain size
For a given material, the smaller
the grain the higher the strength
Usually, small grains is
associated with low ductility
Rule of thumb: should ensure 8-
10 grains across a strut
Usually 30 microns
1. Stent scaffolding function:
Mechanical Properties (Grain size and strength)

5. IDEAL MECHANICAL PROPERTIES:
Elastic modulus E ↑ (to limit stent recoil)
Yield strength Sy ↓ (<200 Mpa) (to lower balloon pressure
(small grain size < 30 mm) for stent expansion)
Ultimate strength SU↑ (> 300 Mpa) (for stent longevity)
Ductility p ↑ (> 15%) (for stent longevity under
under heart pulsation)
EXt3 ↑ (radial strength) (for buckling resistance)
For BMS, the stent should last 400 x 106 cycles (fatigue) (FDA -
ISO 10993). For resorbable stent, 10-20 X 106 cycles.
Poncin P, Proft J, Materials & Processes for Medical Devices Conference 8-10 September 2003
1. Stent scaffolding function:
Mechanical Properties (Ideal properties)

6. Bioresorbable
poymeric stent
Bioresorbable polymeric stents
Igaki-Tamai stent, poly-L-lactide polymer, 2003
“ Thick-strut, Ormiston & Serruys
“..bulky implants, resistance to flow
-Modulus of elasticity
2 orders of magnitude < metals
Young : 3-9 Gpa vs 193 GPa for Steel
Ormiston, JA, Serruys PWS, Circ
cardiovasc inter, 2009, 2, 255-260
170 mm
Absorb BVS PLLA 150 mm (acute
recoil 6.7 % ± 6.4 %)
1. Stent scaffolding function:
Mechanical Properties (Strut thickness & radial force)

7. 1. Stent scaffolding function:
Mechanical Properties (Stent Materials)
Zinc stent (Michigan TU)
Mg stent (Magmaris Biotronik)
Waskman et al, 2009 JACC 2(4), doi 10.1016/j.jcin.
2008.09.01 5
Early recoil

8. Material Sy
MPa
Su
MPa
ε
%
CR
mmyr-1
Grain
size μm
316L 190 490-580 40 - 12-30
L605 310 680-1140 30 - 30
Fe-35Mn 230 550 30 0.44 <100
WE43 195 280 2 1.35 10
PLLA 3-3.7 50-80 5-10 - -
PGA 7 55-110 15-20 - -
2. Bioresorbable Metals
(Material properties comparison)
3 CLASSES: 1) Mg (Mg, Mg-Al, Mg-Ca)
2) Fe (Fe, Fe-Mn, electroformed-Fe, …)
3) Zn (wrought, cast, Zn-Mg, Zn-Ca, Zn-Sr)
4) Amalgamation of galvanic couples)

9. Mg Sy
MPa
Su
MPa
ε
%
CR
mm/yr-1
Mg 20 86 13 4.07
AM608 - 220 3 2.78
WE43 19.5 280 2 1.35
AE21 125-
130
235 7 1.17
Mg-Ca 136 240 11 1.71
Mg-Zn 247 280 22 0.92
Zn
(2013)
Su
MPa
ε
%
CR
mmyr-1
Zn 37-
120
65 0.02
Zn
wrought
440 10 -
Zn cast 175-
425
13 -
Zn-Mg -
Zn-Ca 20-
265 20
-
Zn-Sr -
2. Bioresorbable Metals
(Material properties comparison)
Bowen et al., Adv Healthcare Mate, 2016, 5, 1121-1140
Hyuk In S et al, Acta Biomater, 60, 2017, 3-22
Hermawan H, Mantovani D, Acta Biomater, 2013, 85-85-92
Moravej et al, Acta Biomater, 201 1726-1732
Mariot et al, Powder Technology, 294, 2016, 226-235
www.minitues.com, www.azon.com

10. AMS-1 Mg alloy stent 93%wt Mg + 7%wtRE 165 mm, DREAMS-1,
120 mm, DREAMS-2, 125 mm, Magmaris 150 mm
2. Bioresorbable Metals
(Material properties comparison)
Asbhy Charts
Magnesium
Steel

11. •Three-dimensional
•Fully developed inlet
•Time dependent
•Rigid walls
•Non-Newtonian fluid
(Carreau-Yasuda)
•Time-averaged Re =
190
•Wormersley = 2.8
Benard, N., et al., 2006.Computational Approach to Estimating the Effects of Blood
Properties on Changes in Intra-stent Flow. Annals Biomed. Eng. 34(8): 1259- 1271
3. Stent design and hemodynamic disturbances
(Struts effects)
Stent
geometry

12. Qmax=300 ml/min
Secondary flow before the strutVelocity along green line
3D mesh with hexahedral elementsFlow conditions
Faik I. et al Biomedi
Mat 2: S28-S37, 2007
3. Stent design and hemodynamic disturbances
(Struts effects)
Hemodynamics

13. Wall shear rate
distribution at
peak forward flow
Shear rate
distribution in an
artery cross
section at peak
forward flow
Shear Rate (1/s)
Shear Rate (1/s)
3. Stent design and hemodynamic disturbances
(Strut effects)
WSS

14. ShearRate(s-1)
Non-embedded (apposed) struts
Square Circular Elliptical Tear Drop
Mejia J., Mongrain R., Bilal R., Leask R., Bertrand O.F., Biomedical Engineering Online, May 2009
Thin struts: less injury
(Rogers C, Edelman ER.
Circulation. 1995 15;91(12):2995-3001)
3. Stent design and hemodynamic disturbances
(Strut effects)

15. 3. Stent design (Summary)
Polymers << Steels
Light Metals < Steels
Small grain needed for strength
Thin struts needed to minimize wall injury and
hemodynamic disturbances
Streamlined struts better than blunt struts

16. http://www.rustbullet.com.au/technical/how-it-
works/introduction-to-corrosion-and-process/
Macro-galvanic corrosion
http://www.nauticexpo.com/prod/mgduff/ships-sacrificial-
anodes-for-ballast-tank-25367-239716.html
The m-galvanic corrosion amalgamate concept
4. New Concept for a Metallic Biodegradable
Stent (Micro Galvanic Corrosion)
Anodes
Cathode
Any binary
selection with
a galvanic
potential gap

17. 3. Cold Spray for stent manufacturing
(Stent Fabrication Process)