Design A Heart Valve

Abstract

For this lab our goal was to design a model of the aortic heart valve. Our model needed to mimic a real valve, so it was essential that the model was thin, had two elastic layers, one layer being more elastic than the other, and would naturally recoil back to open position after force was removed from it. Throughout the lab we ended up brainstorming three ideas and pursuing the last one that came to mind. Our heart valve model was made out of half a plastic water bottle, one heavy resistance band, one balloon, rubber bands, and scissors. The resistance band was the inside layer, while the balloon was wrapped around it to make sure no “blood” was able to escape while traveling through the valve. The two layers created an open cylinder, so blood entered through the cup and left at the end opening of the two layers. When no force was being exerted on our model blood flowed consistently through the valve. However, when we exerted force downwards on the two elastic layers towards a central point, this caused the bands to stretch and meet together at the point, closing the once open hole. This immediately stopped the flow of blood in our model. When we released the downward force the two layers naturally went back upwards, reopening the hole. This allowed the trapped blood to escape, and continue throughout the valve. Our model ended up being very effective, as it met all the criteria and had a similar functionality to the aortic heart valve.

Background

The heart is a muscular organ located just slightly behind and to the left of the breastbone. The heart works everyday pumping blood throughout the cardiovascular system, a complex network of arteries and veins. The aortic valve is located right above the left ventricle. The aortic valve is the main outflow valve of the left heart. In simple words it moves blood throughout the heart, which allows us to function as living every day.​​

When blood pumps during systole (the phase of the heartbeat when the heart muscle contracts and pumps blood from the chambers into the arteries) blood is then forced to move around the heart and throughout the various vessels that are associated with blood flow. Blood exits the left ventricle and then must pass through the aortic valve. Mechanical heart structure of the heart valve and blood flow causes the valve to open. The blood flow and the force that comes with it causes the elastin in the ventricular layer to “relax” and recoil back to an open position. The valve then experiences laminar flow (parallel flow of a liquid) from the ventricle side through the aortic valve to the aortic side.

During diastole (phase of heartbeat when the heart muscles relax and allow the chambers to fill with blood), the ventricles relax which enables the blood flow to change. This change of blood flow causes a backflow of blood into the heart. This applies a force on the aortic valve which causes it to close. This force exerted on the aortic side of the heart valve (fibrosa layer) to slightly move and reinforce the valve. Collagen, now rearranged, causes the elastin in the ventricularis to stretch out. This enables the three leaflets to reach out in the middle which completely seals the valve. This prevents further backflow of blood. The blood now on the aortic side begins to move in oscillatory flow, which is in more circular, scattered flow.

Step 1:

To understand how we chose materials, you need to look at the first steps of the project.

Our goal: Determine the most suitable materials to use in our heart model by determining the young modulus of different materials and comparing it to Young Modulus of the tri-layer structure in the aortic valve leaflets.

• Note: Young Modulus measures how easily a material stretches or deforms.

We began to determine materials that as a team we thought would be the most suitable for our heart valve. These materials will act as an analogue for real heart valve tissue, so it is essential that we consider physical properties of the material compared to the three layers of the aortic leaflets.

We also took into consideration the trilaminar structure of the aortic heart valve, which can be seen in the image above. Taking all of this into consideration, we used all of this to gather elastic materials that we thought would closely mimic the young modulus of the tri-layers in the aortic valve.

We ended up choosing four materials to test: Light Resistance Band, Long Resistance Band, Heavy Resistance Band, and a white balloon. We picked out these materials for a few reasons. First they gave us a wide variety of elasticity. This enabled us to have a good chance of being able to match a material’s young modulus to one of the tri-layers' young modulus. This would give us a super accurate material to mimic the movement that happens in the leaflets of the aortic heart valve. They also had a lot of surface area which we could use in different ways to mimic an aortic heart valve through our model. We began to test our four materials to calculate their approximate young modulus by following the procedure below!

Step 2:

To test our four materials we set up an elasticity-testing station. An example of this type of model can be seen below.

Procedure:

1. Gather four materials that you want to test. Gather three different weights with all different masses which will be used to collect data.

2. Hang your first testing material from the ring stand and measure/record its initial length. Then attach the lightest weight to the bottom of the material and measure/record its final length. Repeat this process for all three weights.

3. Then repeat this whole process for all materials you want to test.

4. Now that you have this data use the formula sheet (shown below) to determine Force, change in length, cross section area of material, stress, and strain.

5. Using all this data make a graph comparing the stress vs. strain. Find the best fit line and its equation. The slope of the LBF is the Young Modulus. Record each young modulus for each material tested.

L0 = Initial Length

ΔL = Change in Length

F = Force = Y(change in length/initial length)A

Y= Young Modulus = stress/strain

A = cross sectional area

Stress = F/A

Strain = ΔL/L0

Stress is proportional to strain

Brainstorming

1. Our first idea was to create a tube with three elastic bands crossing over each other in the center. When force was applied to each of the bands pulling outwards, they would tighten and close the hole in the center. When force was released the bands would recoil and return back to “open position”.

2. Our second idea was to have two bands running through the center of a water bottle. Each band would have a small hole cut in the center of the band. When force was exerted, pulling the bands outwards from either direction, that would cause the band to tighten, closing the hole in the center. When force was released, the bands would recoil and return back to “open position”.

Testing

We decided to test Option 2 as our first option. The materials we would use included: one plastic water bottle, one super exercise -heavy resistance band, one balloon, scissors, and a screwdriver. We created our first model and began to test its efficiency! (Photos at bottom of the page!)

Our first option was definitely a bust. The hole was able to open and close but water seeped through the sides of the resistance band and the design looked sloppy overall. Plus it would continue to somewhat fall apart when we tried using it. We then decided to try a different technique. This idea came to our team after our first trial.

Brainstorming (Take 2)

3. Our Design Brainstorm! This design would have two elastic bands making a cone shape. When no force was being exerted onto the layers, a hole remained at the bottom, where the “blood” could easily flow through. When force was exerted on the bottom of the bands, they were pulled downward and caused the hole to shrink and close. When force was released, the bands would recoil and return back to “open position”.

Results

Our Heart Valve Model! (Photos at bottom of the page!)

Conclusion

Concluding this lab, we were able to create a functioning aortic heart valve! Through multiple brainstorms and tests we were able to determine the best shape for our model and best elastic materials being the heavy resistance band and white balloon. Our heart valve model was made out of two elastic layers that created an open cylinder, so blood entered through the cup and left at the end opening of the two layers. When no force was being exerted on our model blood flowed consistently through the valve. However, when we exerted force downwards on the two elastic layers towards a central point, this caused the bands to stretch and meet together at the point, closing the once open hole. This immediately stopped the flow of blood in our model. When we released the downward force the two layers naturally went back upwards, reopening the hole. This allowed the trapped blood to escape, and continue throughout the valve. We concluded this was the most effective model design because it accurately mimicked the functionality of the aortic heart valve. The two layers we chose both had the closest young modulus to the three layers found in the aortic leaflets which created an accurate model when it came to elasticity. Our model was also extremely effective. Blood in our valve was able to easily flow through the valve and stop immediately when force was applied. It met all the criteria and had a similar functionality to the aortic heart valve, which can be seen through the models exceptional performance.

Reflection:

While reflecting on our work for this lab, there are a few things that we might consider changing if we were to do this experiment again. First, I would say testing more materials. While we used a few different types of therabands and balloons, which are very elastic and worked well for our design, testing things with less elasticity might have been a unique way of designing a heart valve, given that we know a heart is not as elastic as a balloon. Trying something like stretchy clothes from a shirt or leggings could have been other materials we could have tried. Another thing we could have done is sketching more design ideas, and more in detail. Although we did do designs and we attempted using a few, having everyone in the group draw more sketches and then testing most of those ideas might have improved our final design. Overall, the group collaborated well with each other, and we took a creative approach on creating a heart valve.