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



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



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.



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.



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



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



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



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



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.



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.



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.



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>.

Bio-mimic Design of a Heart Valve

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