International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
Study of Design Parameters of a Coronary Stent Based on
Poly-L-Lactide (PLLA)
Arun Harindran#1, Nidhi M. B *2
#
Student, Dept. Of Mechanical Engg, MBCET, University of Kerala
Pocket B2, House No. 32, Sector-16, Rohini, Delhi-110089, India
Abstract — Sedentary lifestyle and certain medical
ailments leads to deposition of plaque inside the
human coronary artery which is termed as
Atherosclerosis which if untreated, can prove fatal.
Critical patients are required to undergo surgery
wherein a Cobalt Chromium/ Nitinol (metal alloys)
scaffold is inserted into the narrowed artery to expand
it. This scaffold is known as stent. However these
stents may cause tissue damage and disrupt X ray and
MRIs due to their metallic nature. The mechanical
properties of PLLA differ from that of metals. Hence
a study on the design parameters of this PLLA stent
has been attempted. The advantage of PLLA stent
over metallic stents is its bio resorbable nature as it is
absorbable into human metabolism system within
months preventing restenosis and decreasing the
patients’ dependence on medications. This paper
proposes a Poly-L-Lactide (PLLA) based stent due its
bio compatibility. The effect of length of strut on the
dog boning of PLLA stents has been studied. For the
PLLA stent to be fit for medical use, dog boning
should be minimal.
Keywords — Expansion of stent, Simulation, Finiteelement method.
I. INTRODUCTION
Coronary diseases (CHD) is amongst the major
causes of deaths worldwide. An estimated 7 million
people die each year as a result of this disease [2]. It
is one of the most common and serious effects due to
unhealthy eating, sedentary lifestyle and ageing. The
accumulation of fatty deposits in blood vessel walls
narrows the passage for the movement of blood. The
resulting condition is called atherosclerosis. It causes
blockage of the coronary arteries and ultimately a
heart attack.
The Atherosclerosis (figure 1.1) consists in
the formation of plaque in the arterial walls.
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Fig 1.1 A) A normal artery with normal blood flow.
B) An artery with plaque buildup [4].
Conventionally, patients with severe
coronary heart problems are initially treated with drug
therapy and later, if necessary, with bypass surgery.
The introduction of stenting have opened new
possibilities for successful treatment of CHD as it is
far less invasive than conventional surgery. Rather
than creating a new passage for blood flow, like
bypass surgery, stenting opens or widen existing ones.
As depicted in Figure 1.3, the procedure is
simple:
•
A small incision is made in the femoral artery.
A Teflon coated catheter is inserted and threaded to
the plaque;
•
The catheter is moved over a guide wire so
that it can be accurately positioned in the blockage;
•
The balloon mounted on the distal tip of the
catheter is inflated.
•
The plaque is consequently compressed by
the balloon and the artery is restored;
•
The balloon is then deflated and removed
with the catheter, leaving the previously narrowed
vessel widened.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
made of steel, magnesium alloys, Cobalt Chromium
alloys etc.
These however are metallic in nature and pose
numerous side effects such as tissue scarring and
blood clots which pose a threat to patient’s life.
Consequently PLLA stents which are bio resorbable
are gaining momentum. These stents bio degrade
within a course of 6-24 months thereby eliminating
threats posed by metal stents.
Table I
Difference between the Stainless Steel and PLLA
stent
Steel
PLLA
Fig 1.2: Basic step representation of an angioplasty
procedure [4].
The stent acts as a support to the constricted
arterial wall. When the balloon is gradually inflated,
the stent expands accordingly. Once the stent is
deployed it is left there to prevent the constricted
artery from reclosing again. The stent’s wire mesh, in
its expanded state, allows to maintain a passage for
blood flowing there by reducing the contact between
the artery walls. Stents are generally crimped over a
folded balloon (three-or-six-folded balloon) so as to
reduce the crossing profile in order to reach each steno
tic region (as shown in figure 1.3).
It is metallic in
nature.
Is not absorbed
into human body.
It is a polyester and
is nonmetallic in
nature.
Is absorbed into the
human body in 6-24
months.
Remains in the
body for lifetime.
Remains in the body
for a relatively shorter
time.
May result in
blood clots or
scarring of tissue.
No adverse effects.
The study aims at studying the performance of a
conventional stent made of PLLA and thereafter
compare its performance with that of a Stainless Steel
stent. The conventional stents were modelled in a
CAD programs – Solidworks (SW) 2015 and analysis
performed in Comsol Multi-physics and SW 2015.
Different models of stents and their design parameters,
namely strut thickness and pressure deployment have
been compared in this study.
Fig 1.3: Stent insertion and deployment [4].
A: The catheter system is placed in steno tic region.
B: As the balloon is gradually inflated, the stent
expands and is placed into the arterial wall. C: After
deflation, the stent is left in situ, preventing the plaque
from narrowing.
II. RESEARCH PROBLEM
Coronary heart disease is one of the leading cause
of mortality today.
The treatment varies from
administering drugs, bypass to angioplasty. Metals
and alloys have been the primary choice of doctors to
treat CHD. Conventionally used stents include stents
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III. PLLA MATERIAL
PLLA or the Poly L Lactic Acid is a melt-processable
semi-crystalline thermoplastic.
It is formed
biologically by fermenting renewable carbohydrate
feed stocks. PLLA is usually stable under normal
conditions but degrades slowly under humid
environments (temperatures above its glass transition
temperature 55°C). Its chemical resistance is limited.
However it has good resistance to solvents. Its
mechanical properties are similar to polystyrene. It is
gaining momentum in the medical field such as
bioresorbable stents, stitches and anchors. The
material properties are as mentioned in Table II and III:
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Table II
Properties of Steel 304 [7]
Young’s modulus
193 GPa
Shear modulus
73 X 106 MPa
Yield strength
207 MPa
Density
7.86X10-6kg/mm3
Poisson’s ratio
0.27
Table III
Fig 4.1 Palmaz Schatz Stent (Cordis, USA)
Properties of PLLA [14]
Young’s Modulus
3600 N/mm2
Density
1250 kg/m3
Ultimate Tensile Strength
189 N/mm2
Poisson’s ratio
.36
Yield strength
60 N/mm2
The Palmaz Schatz (PS) is one of the earliest stents
design and also one of the most common ones. The
figure 4.1 represents just one cell of the entire
geometry.
.
IV. STENT MODEL
A Stent is a meshed tubular structure usually having
repetitive patterns over the entire coil and extends
longitudinally. The stent model is created is
Solidworks 2015.
The modelling procedure is as follows1. Initially half the strut is modelled in 2D.
2. The half strut is then extruded.
3. Using the Mirror command the entire single
strut is formed.
4. Using the Mirror command again a pair of
struts is created.
5. The bridges are then created.
6. The structure is then copied axially.
7. The cell geometry is then wrapped over a
cylinder.
8. The circular pattern feature then creates the
entire loop of the stent.
9. Final geometry is then combined to form
closed coil.
Fig 4.2: S 670 (Medtronic, USA)
Fig 4.3 ION stent (Boston Scientific)
The stent dimension of the Palmaz Schatz are used to
model stent geometry cell. The dimensions have been
listed by manufacturer Cordis, USA.
The dimensions are described in the table IV
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
The loads were applied directly to the inner surface of
the stent to allow expansion.
Table IV
Dimensions of the stent [2]
Category
Number of cells
66 (33X2)
Cell size
1.1128 mm2
Cell areas
73.448mm2
Outer diameter of the stent
3 mm
Inner diameter of the stent
2.9 mm
Length of stent
20 mm
A. Boundary Condition
The boundary condition is the application of a force
and/or constraint. In Solidworks boundary conditions
are stored within what are called Fixtures. They are
created using the Advanced Fixtures option in
Solidworks.
The ends of the stent are free from
constraints. The stent is supposed to expand radially.
Consequently a radial expansion of the stent is
prescribed.
The Advanced Fixture options is selected with the
cylindrical faces selected to expand radially to 2mm.
For the stent cell geometries the end faces are fixed
and the expansion pressure is applied to the inner wall
of the stent as shown in figure 4.4. The top face of the
stent is subjected to pressure exerted by the arterial
plaque.
Fig 4.5 Stent expansion pressure acting on inside
wall.
Pressure is exerted by the arterial plaque inside the
narrowed artery. This pressure is exerted over the
outer face of the stent as shown in figure 4.6 and is
taken as 780 KPa.
Fig 4.6 Atheroscopic plaque pressure on top of
stent.
Fig 4.4 Boundary condition imposed on the stent
cell.
B. External Loads
A stent expands radially when subjected to pressure
from the inside due to balloon expansion. The radial
expansion is due to the balloon expansion with
prescribed diameter. The arthroscopic pressure acting
on the stent is also considered.
C. Meshing
Mesh generation is the process of generating a
polygonal mesh that approximates a geometric domain
under study. In order to obtain the most accurate
results the mesh has to be fine. The optimum size can
be selected using mesh convergence study.
•
The geometries are meshed with Tetrahedron
Mesh. (Figure 4.7).
•
Fine Mesh has been used after convergence
studies.
Pressure acts
•
On the inner wall as shown in figure 4.5;
•
On the outer wall of the stent as shown in
figure 4.6. The clinically used stent deployment
pressure of 14 atm, 15 atm, and 16 atm were used.
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The three geometries that have been considered are
commercially available stents the different materials
that are commercially used for manufacture of stents
were chosen for the study. The conventional materials
were Stainless Steel (SS 316L), Cobalt Chromium
Alloy (Co-Cr), Platinum Chromium Alloy (Pt- Cr),
Nitinol (Ni- Ti) and Tantalum (pure metal).
The PS stent has a rectangular geometry with 2
bridges arranged circumferentially along the sides.
When the stent expands the loop expands and due to
the bridges all the connecting loops expand
circumferentially.
Fig 4.7 Meshed PS Stent
After the meshing was performed the total nodes of
the in the PS stent model (figure 4.7) was 15342. The
number of elements in the stent model is 7896.
The S670 stent also has geometry similar to the PS
stent however the number of bridges in the stent is
four
.
The ION stent has the geometry in the shape of ―X‖
without any bridges. The ―X‖ shape consists of
repetitive units which extends circumferentially,
V. RESULTS AND DISCUSSION
Fig 4.8 Meshed S670 stent
The degrees of freedom for the S 670 stent model
(figure 4.8) is 50,829. Number of Nodes in the model
is 17,129. The total number of elements comes out to
be 9,199.
The preliminary stress analysis was conducted in
Comsol Multiphysics.
The maximum stress
concentration was in the corners of stent where it is
connected to the bridge. The PLLA stent has lesser
stresses than its steel counterpart. Both the stents have
Von mises stress lesser than their respective ultimate
tensile strengths.
Consequently they both are
deformed plastically but don’t undergo failure. Both
the stents have a linear dependence on pressure over
time resulting in increasing dog boning.
Fig 5.1 Stress in Steel Stent
The maximum stress concentration can be seen in
the helix adjoining the bridge. The highest Von Mises
stress obtained is 450 Mpa which is below the UTS of
steel as shown in figure 5.1. The bridges of the stent
have the lowest stress concentration which is around
50 Mpa. The arms of the stent have mid-range stress
ranging from 150-250 MPa.
Fig 4.9 Meshed ION stent.
The total number of nodes for ION stent (figure 4.9)
is 13583. The number of elements for the ION model
is 7830.
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Table VI
Comparison Of the results
Steel
PLLA
High stress
concentration at apex
Presence of dog boning
is less.
Fig 5.2 Maximum stress concentration at the corner
of helix
Dog boning is more
prominent.
The same analysis was conducted with varied stent
strut thickness and the results (Table 5.3) are as
follows:
The above figure 5.2 is a zoomed version of a
single strut.
Stent
Thickness
Fig 5.3 Stress in PLLA stent.
The PLLA stent also shows higher stress
concentration at the helix similar to steel. The highest
stress encountered in the stent is 148 MPa as shown in
figure 5.3. The arms of the strut experience lower
stress. However the ends of the PLLA stent expand
more than its central region denoting the presence of
dog boning.
During the expansion process, the radial
displacement and dog boning increased gradually as
the pressure increased.
The Table V and VI shows comparison between the
stresses.
Lower stress
concentration at apex
Table VII
Thickness v/s Dog boning
Stress
DOGBONING
0.1mm
1.48 X 108
N/m2
24.23
0.09mm
1.45 X 108
N/m2
22.89
0.08mm
1.42 X 108
N/m2
20.31
The stent thickness vs. the stress is shown in figure
5.4. There is a linear relation between the stent
thickness and the stress developed.
Table V
Stresses in Stent (in N/mm2)
Steel
PLLA
4.5 X 108
1.48 X108
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Fig 5.4 Stent thickness v/s Stress
After performing 5 iterations with decreasing stent
strut thickness, 0.05mm exhibited lowest dog boning
for the PLLA. Consequently the PS stent was
remodeled with strut thickness as 0.05 mm and the
stress analysis performed again to give results shown
in figure 5.4
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The Von Mises stress developed in PS stent as a
function of time is shown in figure 5.8
Fig 5.5 Stent Thickness vs. Dog boning
The figure 5.5 shows the graph representing the
effect on the dog boning of the stent. Almost linear
dependence of the dog boning on the stent thickness
can be established from the graph.
Fig 5.8 Von Mises Stress vs. Time
Fig 5.6 The PS stent with reduced dog boning.
The maximum stress that the stent
encounters while expanding is 173 MPa.
The
displacement prescribed is 2mm (figure 5.6). The
stent cell geometries of different commercially
available metallic stents was performed using the
PLLA material to find the stress concentration at
different deployment pressure 10 atm , 12 atm, 14 atm.
The Von Mises stress in PS stent at 10 atm pressure is
170 Mpa as shown in figure 5.6.
Fig 5.9 Von Mises vs. Displacement
The displacement goes up till to 0.51 mm.
Fig 5.10 Von Mises stress in S670 stent
The maximum Von Mises Stresses developed in the
S 670 stent is 440 MPa as figure 5.10.
Fig 5.7 Von mises stress in PS stent.
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Fig 5.11 Von Mises Stress vs. Time
The S670 stent’s Von Mises stress development as
a function of time is shown in figure 5.11.
Fig 5.12 Von Mises vs. Displacement
The Von Mises Stress in ION stent at 10 atm is 150
MPa as shown in Figure 5.7.
Fig 5.14 Von Mises Stress vs. Time
Figure 5.14 shows the Von Mises Stress developed
as a function of time. Stress is developed in the stent
due to the deployment pressure for expanding it
radially and the pressure exerted on the outer walls of
stent by the atheroscopic artery. Von Mises stress is
developed in different parts of the each stent geometry
which are shown in respective figures 5.14. The
maximum stress concentration is along the arms of the
PS and the S 670 stent. However this is not the case
in ION stent. The maximum stress region along the
ends of the stent. Also the stresses developed in the
stent varies linearly with the deployment pressure
acting on the inside face of the stent.
Table VIII
Deployment Pressure v/s Failure
At 10 atm Pressure
If failure occurs
PS Stent
NO
S 670 Stent
YES
ION stent
NO
At 12 atm Pressure
PS
NO
S 670
YES
ION
ALMOST FAILS
At 14 atm Pressure
PS
YES
S 670
YES
ION
YES
It is evident from the analysis (table VIII) that PS
stent is the most suitable stent cell geometry for PLLA
based stent because under deployment pressures of 10
atm and 12 atm the stent doesn’t fail as Von Mises
stress induced is below the Ultimate Tensile Strength
of the PLLA material (189 MPa).
Fig 5.13: Von Mises Stress in ION stent.
The maximum Von Mises stress developed in ION
stent is 131 MPa as shown in figure in 5.13.
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VI. CONCLUSIONS
Coronary stents have different cell geometries.
These cell geometries determine the stent performance
and are responsible for the optimum stress distribution.
In this work three commercially available stent cell
designs namely the Palmaz Schatz, ION and S670
stents were analyzed to find the optimal cell design
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
which would be the most suitable for PLLA material.
[12]
In this work, the effect of 2 design variables- stent
strut thickness and the stent deployment pressure has
been studied. The Von Mises stress concentration in
the Steel and the PLLA were plotted. It is found that
the stress concentration at the helix of PLLA stent was
lower in comparison to that of Steel. However the
result indicates that PLLA stent has the presence of
dog boning which arises from geometrical variables.
For the stent to be fit for medical usage the dog boning
in PLLA stent has to be absent or minimal. After
establishing the linear relationship between stent strut
thickness and the dog boning, the thickness of PS stent
was reduced to 0.05 mm from 0.1mm and reduction of
the unwanted phenomenon was achieved. Further
analysis was conducted to find the best suitable design
for a cardiac stent based on PLLA material. The aim
was to select the best stent cell geometry based on the
different clinically used stent deployment pressure.
From the analysis conducted using the deployment
pressures of 10 atm, 12 atm, 14 atm, it was found that
for a bio degradable PLLA stent, the Palmaz Schatz
geometry is the most suitable amongst the three
geometries as it doesn’t fail for deployment pressures
of 10 atm and 12 atm respectively.
[13]
[14]
[15]
Results of Biodegradable Poly-l-Lactic Acid Coronary Stents
in Humans‖ (Circulation. 2000; 102:399-404.).
Kristi Basu, Pranab Ghosh, Abhijit Chanda, ―Study of Stent
Deformation and Stress Developed at Different Stent
Deployment Pressures‖.2012 COMSOL Conference.
Frank Harewood, Ronan Thornton, ―Step Change in Design:
Exploring Sixty Stent Design Variations Overnight‖. Altair
Engineering Limited, 2005.
Thamarasee M. Jeewandara , Steven G. Wise Martin
K. C. Ng, ―Biocompatibility of Coronary Stents‖ Materials
2014, 7(2), 769-786.
Niels Grabow, Carstenm Bunger,
Christine Schultze,
Kathleen schmohl, David P. Martin, Simon f. Williams,
Katrin Sternberg
and Klaus-Peter Schmitz
, ―A
biodegradable slotted tube stent based on poly(L-lactide) and
poly(4-hydroxybutyrate) for rapid balloon-expansion, Annals
of biomedical engineering.‖ January 2008.
Therefore, the objective of the work which was to
find the optimal geometry for the bio resorbable
PLLA stent which could be used for the most
clinically used stent deployment pressure was
established as the Palmaz Schatz stent.
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