bio-mimic design of a heart valve
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Improving Solutions to Valvular Heart Disease
Team Backdoor
Nash Anderson, Griffin Beemiller, Kyle Logan, Greg Olen, Blake Reller
Mate 310/350 Winter Quarter
3/17/2011
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Abstract
In order to test the theory of modifying the material of bio-prosthetic heart valves, an
experiment was conducted to determine whether elastomer films could be used as
replacements for the porcine tissue in bio-prosthetic heart valves. The elastic moduli and fatigue
resistance of the elastomers were tested to determine if they would be an acceptable
mechanical replacement. The results were found to be inconclusive due to improper testing
methods and small sample size.
Background/Project Overview
Valvular heart disease (VHD) is characterized by the presence of a defect or damage to
one of the four heart valves. The defect or damage may be congenital or acquired. The
damaged valve either becomes too narrow to open fully, preventing normal blood flow; or
unable to close completely, resulting in back flow. About 5 million Americans are diagnosed with
valvular heart disease each year [1]. In mild cases valvular heart disease can be treated with
medication, but in most cases the valve must be replaced or repaired.
Valvular repair is the best possible solution to VHD, but when a patient’s heart valve is
severely damaged, repair is not an option. In these cases the patient’s valve must be replaced
with a either a mechanical or a bio-prosthetic valve.
Mechanical heart valves are made out of pyrolitic carbon and can last an entire lifetime,
but the patient must take anticoagulant medication such as Warfarin on a daily basis. Without a
frequent regimen of anticoagulant medication the valve will clot at the mechanical hinges of the
valve resulting in failure. The valve hinges tend to clot due to their shape which causes turbidity
of flow.
Bio-prosthetic valves are the most common valvular replacement, being used in about
80% of patients today[2]
. These valves are made out of pericardium tissue (often from a pig), andclosely mimic the shape of actual heart valves. Because of this unique shape, these valves
have a minimal turbulence of flow. Therefore bio-prosthetic valves do not require anticoagulant
medication, which is why they are so popular amongst patients. The drawback to these valves is
that they need to be replaced every 10-15 years due to degradation and calcification of the
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pericardium tissue. The pericardium tissue degrades and calcifies because it is foreign tissue to
the body, even though it is treated with chemicals to decrease this effect.
Problem Statement
All replacement heart valves either have a limited lifespan or require the patient to take
anticoagulant medication for the remainder of their lives. The patient must choose which type of
valve is less inconvenient for them.
User Needs/ Current Solutions
The user needs a prosthetic heart valve that will not require long term medication
regimens and that will have a lifespan longer than 25 years. The valve must last long enough so
it will not have to be replaced in the majority of patients.
Design Requirements
● Bio Compatible - not incurring a toxic or detrimental immunological response.
● Resist blood coagulation without use of anticoagulant medication
● Undergo elastic shear deformation
● Maintain a seal that does not permit back flow
● Maintain elastic properties under cyclic shear load (>1.05 billion cycles)
Proposed Solution
Theory Behind Solution
The design of the bio-prosthetic valve has preferred flow characteristics in comparison to
all mechanical valves. The only problem with the bio-prosthetic valve is the tendency of the
pericardium tissue leaflets to degrade and calcify over time. A synthetic material would notdegrade, therefore if a synthetic material can be found which replicates the mechanical behavior
of the pericardium tissue; a valve could be designed with the flow characteristics of a bio-
prosthetic valve and the durability and lifetime of a mechanical valve.
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How Solution Meets Design Requirements and user needs
The proposed heart valve will not require the patient to be put on an anti-coagulant
regimen, and will outlast bio-prosthetic heart valves.
Explanation of design
The pericardium tissue used in bio-prosthetic heart valves will be replaced with an
artificial elastomer. The proposed heart valve will maintain the design and function of the bio-
prosthetic valve, but will last longer because the elastomer will not break down and calcify over
time.
Materials Science
Hypothesis
Elastomers closely resemble the mechanical properties of pericardial tissue. Therefore
they are a suitable material to mimic the function of a healthy heart valve.
Elastomers are chemical compounds whose molecules consist of several thousand
smaller molecules called monomers linked together by covalent bonds. These monomers repeat
and are linked together to form long chains. These chains have a backbone most often made up
of carbon bonds, either (C-C) or (C=C). These long carbon chains are highly flexible, disordered
and intertwined. The chains are flexible because rotation around (C-C) bonds allows the
molecules to take up many different configurations. [3]
In an elastomer’s normal state, it is highly disordered with a degree of high entropy. This
is the preferred state of the elastomer. When the elastomer is put under tensile stress, the
molecular chains are pulled into alignment and often take on aspects of a crystalline
arrangement. When the chains are lined up under a load, they are at a lower disorder resulting
in a lower entropy. Upon release they spontaneously return to their naturally disordered and
entangled state allowing the polymer to maintain its shape. [3] Both the deformation and the
subsequent recovery are time-dependent, suggesting that some part of their behavior is
viscous. Elastomers show a combination of elastic and viscous behavior known as
viscoelasticity. The degree of viscoelasticity is strongly dependent upon temperature and the
rate of deformation, as well as such structural variables as degree of crystallinity, crosslinking,
and molecular weight.
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In order to be useful for various applications, elastomers
must be strengthened by cross-linking the polymer chains. With a
low frequency of the branching cross links, a soft rubbery material
is produced. Silicones and polyurethanes can be cast this way,
using low-viscosity liquid precursors with reactive end groups.
If an elastomer is stretched, as shown in Figure 1, energy
is stored in it. Just as in the application of a slingshot, the
elastomer used in the propulsion mechanism will snap back into
place after being stretched.
Materials
To pick the materials for testing a CES plot from the biomaterials database was created
looking for low Young’s Modulus (.8-12MPa) [2] and high Fatigue Strength. Fatigue Strength will
be one of the most important factors in the decision because the material will have to withstand
over 1 billion cycles. Young’s Modulus was chosen because it is closely related to Shear
Modulus through the equation:
Shear Modulus: G = E / [2(1+v)]
v = poisson’s ratio = - εt / ε = (lateral or transverse strain) / (axial strain)
For elastomers v = ~ ½ ( G = ~ .333333E)
After research on availability of materials and consideration of the CES plot, shown in
Figure 2, Thermo Polyurethane and PDMS were chosen for testing.
Figure 1: The figure (A) portrays a molecular view of an
elastomer in its preferred highly disordered state with high
entropy. Picture (B) shows the elastomer when stretched tobecome partially crystalline with aligned chains. The dots in
the picture represent crosslinking between chains. 1
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Thermo Polyurethane:
Thermo Polyurethane, shown in Figure 3, is a polymer formed through step-growth
polymerization.
Step-growth polymerization refers to a type of polymerization mechanism in which bi-
functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers
and eventually long chain polymers. Many of these are naturally occurring polymers but some
synthetic polymers exist such as polyesters, polyamides, polyurethanes, and many more. Due
Figure 2: Biomaterials database CES plot comparing Fatigue Strength v. Young’s Modulus. A limit
was set to exclude non biocompatible materials.
Figure 3: Schematic of a monomer of a polyurethane molecule. 5
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to the nature of the polymerization mechanism, a high extent of reaction is required to achieve
high molecular weight.
The polyurethane chain is a complex structure. Due to the presence of benzene rings,
the structure has hard and soft areas within the chains. This results in a structure that will
organize into stronger, less flexible areas and other areas that are weak and elastic. The stiffer
areas are the result of the benzene rings from multiple chains lining up and stacking on top of
each other. The weaker areas in the material are the areas where the benzene rings have not
lined up, and they form a regular disordered elastomer structure. These softer areas will stretch
and result in the elastic properties of polyurethane, whereas the benzene alignment results in
the material’s strength. When stretched, the soft area’s double bonded oxygen molecule forms a
hydrogen bond with a methyl from another chain within the structure, resulting in higher strength
between chains.
Polydimethylsiloxane (PDMS):
Silicones are inert synthetic compounds, formed through chain growth polymerization.
Chain growth polymerization is when unsaturated monomer molecules add onto a growing
polymer chain one at a time. The structure consists of an -O-Si-O-Si- “backbone” replacing the
common -C-C-C-C- in carbon-based elastomers. This results in a linear polymer with lower
bond angles than the common carbon backbone, making the polymer viscous. This requires the
polymer chains to be crosslink in order to form silicone rubber and useful in engineering
applications. In crosslinking, methyl groups are substituted by vinyl groups to form crosslinking
sites between entangled chains. Crosslinked polymers have good stability of rubber properties
over large temperature range of -50C to 200C. Silicones are chemically resistant and have good
sealing capability. They are commonly used in biomedical applications for seals and o-rings.
Figure 4: Schematic of a monomer of a PDMS molecule. 6
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Silicone chains have a simple structure with low bond angularity and evenly bonded
methyl-groups that surround the chains. These molecular properties result linear molecules.
Silicone molecules will thus slide past each other easily when, for example, a tensile load is
applied. This is why the material results in a lower elastic modulus than that of polyurethane.
Testing
Objective
The goal of our testing procedures was to obtain values of elastic modulus for
elastomers PDMS and TPU and compare these values to those recommended for elastomers
being used in this application. For elastomers, the shear modulus can be approximated to be
one third of the elastic modulus. It was also an objective to observe if these values for elastic
modulus would be changed after putting the samples through multiple cycles of fatigue.
Design of Experiment
Values for elastic modulus would be obtained using an Instron Tester. An instron tester
that was more sensitive to strain would have been ideal but was unavailable. The variables in
the experiment were chosen to be materials, amount of cyclic fatigue, and thickness of
elastomer film. Controls included shape of the sample, temperature, and rate of deformation.
These values can be seen in:
Input Variables
Factors: Levels
Materials: PDMS and TPU
Thickness: .01 in and .02 in
Fatigue: 0 cycles and 8,000 cycles
Fatigued Samples
Samples subjected to shear bending for 8,000 cycles, a cycle being one shear
bend to 90 degrees and back to 0 degrees.
ControlsRate of deformation: 500mm/min
Geometry of sample: Gage length: 1 inch Width: 1 inch
Temperature: 25°C
Response Variable
Elastic modulus
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Expected Results
For a material to be considered for the heart valve application, it must have no difference
in elastic modulus between fatigued and non fatigued samples. It is expected that the PDMS
elastomer will have no significant change in elastic modulus due to its ease of chains sliding
past one another at low stresses, and that it will out-perform the TPU samples.
Results of Test
A statistical analysis of our results in terms of main effects and interactions of variables
can be seen below (Figure 5).
Both of these plots show that the only factor that had a significant effect on our data was
the material of the samples. When the values we obtained were compared to the
recommended values for elastomers in this application, both of our materials fell short.
Obtained E Values:
PDMS = 0.003 MPa
TPU = 0.15 MPa
Recommended elastic modulus values [2] for heart valve leaflet:
> .8 MPa< 12 MPa
Figure 5. Main Effects and Interaction Plots of our data generated by Minitab software.
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Figure 6: The figure above depicts one of the trials of tensile testing TPU. Line A portrays the slope that was used to
obtain our value of the elastic modulus. Line B is the line that was likely used to obtain the recommended values for
elastic modulus, as well as the elastic modulus values found in CES.
Discussion
Our results did show, as expected, that PDMS has a lower modulus than TPU, but that
is as far as we are able to conclude due to lack of power in our experiment. The obtained values
for the elastic modulus of the polymers did not compare closely to the recommended values for
elastomeric elastic modulus2, nor the pericardial tissue values. This result is likely due to the
location of the obtained slope on the stress strain diagram. Figure 6 depicts one of our
experimentally obtained stress strain diagrams for TPU. As shown in the figure, the difference
the location of the line results in a very different slope. This difference results in our obtained
values of elastic modulus being much lower than the recommended. There are few experiments
on elastomers that depict an elastic modulus from the initial slope (approx. <25% strain) which
is the area in question for our application.
Due to the uncertainty of the elastic modulus values and our low sample size in testing,
there is considerable room for improvement in our methods and results. A promising place to
begin improving would be to establish a better foundational knowledge of he high-cycle fatigue
behavior of elastomers. There is scarce information in this area since the vast majority of
elastomers are not used in applications where cyclic fatigue is a factor (particularly above 1
billion cycles).
It is also possible that we could have obtained more valid results by changing our test
method. This would include testing more samples of each material and testing them differently.
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As opposed to simple tensile tests, generation of hysteresis curves for the elastomers would
yield useful information concerning cyclic elastic loading and unloading. These are obtained by
measuring the force of the elastomer as it returns to its original arrangement. The hysteresis
curves would yield useful information pertaining to the elastomer’s ability to conform back to its
original formation after an applied load.
Conclusions
Neither PDMS nor TPU can be accepted for use as heart valve leaflets based on our
findings. However, these materials should not necessarily be ruled out either. With more
accurate testing tools and methods, valid elastic modulus values could have been attained,
which could yield a definitive answer as to whether our samples of PDMS and TPU would
exhibit acceptable mechanical properties for use in heart valves.
We recommend further mechanical testing of the elastomers. Once a material is found
with sufficient mechanical properties we suggest testing surface coagulation properties as well
as the fluid dynamics of a prototype elastomer heart valve.
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Sources
1Chemical Composition and Structure of Elastomers." Elastomer Chemistry. 19 Feb. 2011.
<http://www.standard-gasket.com/tech_specs/elastomer_chemistry.htm>.
2“Elastomeric Sheet Materials for Heart Valve and Other Prosthetic Implants.” US Patent, July
20, 1982.
3"Heart Disease: Heart Valve Disease." MedicineNet. 22 Feb. 2011.
<http://www.medicinenet.com/heart_valve_disease/article.htm>.
4 Interview with Dr. Luke Faber on February 3, 2011
5"Polyurethane." Wikipedia The Free Encyclopedia. 3 Mar. 2011. Wikipedia.
<http://en.wikipedia.org/wiki/Polyurethane>.
6"Silicone Rubber." Caojunbang. 11 Feb. 2011. <http://caojunbang.centerblog.net/5-107-rtv-
silicone-rubber>.