Heart Valve Model
As a group, we were tasked with creating a replica of the aortic heart valve, and producing a model of our design. To do this, the project was broken into steps. We first conducted research into the functions and composition of the heart. After learning more about its properties and function we then began our search for materials that we could use that could best replicate the valve. We selected materials, tested them, and finalized our choices. We then began the design phase. Using sketches and paper models we brainstormed different ideas that we could use to create a working valve. We tested these sketches for accuracy by using paper models. We selected a design, and commenced our building phase with the materials that we had already chosen. After building, we ran tests to see the functionality. We ran into problems and errors, so we made adjustments and ran more tests. Finally, we concluded with a functioning prototype that accurately represents the aortic valve.
Analysis of Steps Completed
The overall purpose of this project is to design and construct an accurate representation/model of the aortic valve inside the heart. This task has been broken into 6 Parts. Step one focuses on the process and decision of choosing which materials to use. To ensure that the materials that we chose would be most accurate in representing the functionality of the valve, we had to select materials that we thought would have parallel properties to those of the aortic valve. The second step focuses on the testing of these materials, after selecting which ones had the similar properties. The testing phase consisted of gathering data on the chosen materials to figure out which ones would be the most effective for our model and writing down the data. To do this, we found the lengths (initial, final, and change), strain/stress (under force/weight), thickness, and areas. Using this data, we were able to select the best materials to use in the later parts of this project. To test these materials, we used a spring scale with different weights, and recorded the change in the length of each material tested. This information was used to assess the elasticity of the object. After we were able to determine the elasticity, we were able to judge whether or not the material would be able to withstand the necessary stress and strain that it would be subjected to while acting as the aortic valve. Step three focuses on the design process and construction of our prototype. To ensure that the materials that we chose would be most accurate in representing the functionality of the valve, we worked with the materials we tested in steps one and two, where we selected materials that we found to have parallel properties to those of the aortic valve. The testing phase consisted of gathering data on the chosen materials to figure out which ones would be the most effective for our model and writing down the data. We started our design process by creating sketches of our model that we discussed. As we drew, we used the techniques we had learned from the orthographic drawing tutorials. From these sketches, we began construction. We built our first prototype using paper so that we could get a general idea for the real model. Then, we used the materials that we had selected to create our first model. After completing step three, we began step four, which was testing our prototype. We chose to test our model with water, as it best represents blood. After our first test, we noticed that there was a tear in the balloon, which is one of the main parts of our model. This is where we began step five. We made the necessary adjustments and continued to test our prototype again, and completed our first model. Step five consisted of fixing and implementing the necessary adjustments.
Below are images of our prototype.
Research on the Heart
When cardiac tissues contract, it pushes blood throughout the chambers of the heart. The valves inside the heart allow the blood to flow in one direction and does not allow it to return (cardiac regulation). Blood flow through the heart and body follows this path: Deoxygenated blood enters the heart from the superior and inferior vena cava via the right atrium, then passes through the tricuspid valve into the right ventricle. Next, blood is pumped through the pulmonary valve into the pulmonary arteries, which lead to the lungs, blood picks up oxygen. Oxygenated blood leaves the lungs and returns to the heart via the pulmonary veins. This blood enters into the left atrium, and passes through the bicuspid (mitral) valve into the left ventricle. The pumping action of the heart pushes the blood out of the left ventricle through the aortic valve into the aorta, which is ultimately responsible for leading oxygen rich blood into the body. The two valves tricuspid and mitral valve prevent blood backflow from the ventricles into the atria. The leaflets of each of these valves are anchored into place by strands of mostly collagen and elastin called chordae tendineae, which prevent the valves from opening the wrong way into the atria.The two valves that are directly outside of the heart (pulmonary and aortic valve) are both tricuspid valves (possessing three leaflets each) and are not anchored in place by tissue. Each of the valve leaflets relies on its tissue structure to withstand the pressures exerted by blood flow. Aortic valves between the left ventricle of the heart and the aorta. They are semilunar valves composed of three leaflets. During diastole (when the ventricles relax), the valve closes to prevent regurgitation of the blood back into the heart. During systole (when the heart contracts, moving blood into the blood vessels), the aortic valve opens, systole permitting blood to move into the aorta. This sequence of events repeats with each cardiac cycle, (average 60 times per minute). Human aortic valve leaflets are composed of three distinct tissue layers (trilaminat). The ventricularis layer faces the left ventricle. The spongiosa layer is the middle valve layer. The aortic side of the leaflet is called the fibrosa layer. Endothelial cells cover these layers, forming a cell monolayer that protects the valve. The fibrosa layer is composed mostly of collagen. It aligns in a certain way during the backflow of the diastole, allowing the valve to elongate as it closes, forming a complete seal between the left ventricle and the aorta. The alignment of collagen fibers in the fibrosa layer also gives the now closed valves strength to withstand the backward flow of the blood from the aorta which prevents regurgitation. The ventricularis layer of the aortic leaflet is composed mostly of elastin, a protein with the ability to stretch out with stress, but return to its original shape once the stress is removed. When the heart relaxes, blood pressure in the valve forces the leaflets to close. As the collagen in the fibrosa layer aligns to let the valve leaflet stretch and completely seal the blood vessel, the elastin in the ventricularis layer stretches. When the backflow ceases (because the ventricle contracts during systole), the pressure from the backflow eases, permitting the elastin to recoil and thus causing the leaflets to fold up and open the seal so that blood may flow into the aorta. The spongiosa layer, located between the ventricularis and the fibrosa, is composed of GAGs (glycosaminoglycans), which help to align the collagen and lubricate the movements of the ventricularis and the fibrose during valve leaflet movement.
Our research notes on the important properties to consider when testing heart valves:
Heart valve leaflets are thin and flexible tissue. Leaflets go through billions of open/close cycles in their life. They are pushed open during systole by the force of flowing blood
They are flexible but stiff which is important for supporting the “weight” of the blood flow
Important property: Mechanical Anisotropy (the directionality of the mechanical properties. Collagen fibers are aligned circumferentially in the valve leaflets which make valves in this direction mechanically stronger. Stretching a leaf valve in the circumferential direction makes it then the same leaflets stretched radially. The valve has a trilaminar structure which is an important aspect of valve biomechanics. Function of the valve influences structure and structure determines function. Fibrosa (collagen-rich layer):responsible for mechanical strength of the valve. Spongiosa, (glycosaminoglycan-rich layer): acts as lubricant and shock absorber. Elastin-rich ventricles: responsible for the stretching that occurs during each cycle. The elastin is relaxed when the valve is open (systole) but stretches so that a leaflet can coapt during diastole.Interview question: What methods do you use to test these properties in the lab?
When testing macro-mechanical properties, the lab uses a biaxial strain device(It takes sample valve leaflets and applies known loads(stresses) to the sample in the radial and circumferential directions at the same time). Strain(stretch) of the valve is measured in response. Data comes from how compliant or stiff the valve in each direction is.
A special microscope called an atomic furse microscope (AFM) is especially designed for controlling a nanoscale tip and sample interaction. Stiffness is collected based off how the tip moves over the sample, how much force is applied, and the size/shape of the tip. This is a good technique for correlating mechanical properties with protein content
Movat's pentachrome stain: shows clearly the 3 layers in aortic valve to compare stiffness between layers.
Research on Materials Tested/Data Collection
Elasticity: describes the property of a material in which a material return to its original shape after stress has been applied to the material
Regarding elasticity, if a material does not recoil it is a product of deformation
Stress can be described as the force on an object and (causes temporary deformation until the stress is released from an elastic material) is typically correlated with tension
Young’s Modulus is a constant that varies depending on material→ describes the relative response of a material due to stress (in other words it is the ratio of stress to strain)
F = 𝒀( ∆𝑳/ 𝑳𝟎 ) 𝑨
Where
Force = applied force to a structure
Y = the constant of Young’s Modulus determined by material
∆𝑳 = the change in length of the material
𝑳𝟎 = the initial length of the material
A = the cross sectional area of the material
Stress: σ= F/A
Strain: ɛ=∆𝑳/𝑳0
Both stress and strain are directly manipulated to achieve our Y constant and can be rewritten as Y= ( ∆𝑳/ 𝑳𝟎 ) / (F/A) which clearly shows the ratio of strain and stress
Y= ( ∆𝑳/ 𝑳𝟎 ) / (F/A)
Strain/ Stress
Data Collection:
We collected data in a spreadsheet, and organized our data by each material we tested. We tested a total of 3 different materials and took measurements at 3 different masses.
Procedure:
We collected materials:
Weights, masses
3 different rubber bands
String
Spring scale
Ring stand
Ruler (measurement device)
Measured the initial length of the rubber band
Attach the spring scale to the end of the ring stand
Attach each of the 3 weights separately and record the Force in Newtons. Record in spreadsheet.
Hang the rubber band from the spring scale and measure the final length of each rubber band with all three different masses. There should be 3 total final lengths.
Calculate the differences between each of the final lengths and the initial length. This is your change in length also known as ∆𝑳
Repeat steps 5 and 6 for each rubber band
Use data collected to calculate the Young’s Modulus
How to calculate Young’s Modulus: We collected this data in order to calculate the Young’s Modulus to describe the elasticity of a material.
We can use the derived equation: Y= ( ∆𝑳/ 𝑳𝟎 ) / (F/A) with Y being the Young’s Modulus.
Method of Construction
Our design process consisted of sketching and paper models. Our building method operated on a “trial and error” mindset. We began construction by assembling our materials. (see below for list).
Materials List:
Stucco tape
Balloons
Water bottle
Scissors
Paper
Water
X-Acto Knife
Cardboard circle
Simplified Construction Process
Step One- sketches of our model. See pictures below.
Step Two- We created paper models to help us visually represent the different angles that our materials would have to bend at. This helped us to see our design in a 3D sense, and to make sure that it would actually be doable. We tested our sketches in this method until we found a design that would be effective and constructable.
Step Three- We chose a water bottle as the body of our model. Because of the ability to compress the plastic casing with our hands, we found that we were able to simulate the pressure that the aortic valve would undergo when compressing. By using this bottle, our model would be more accurate in its ability to represent this force,
Step Four- We cut the water bottle to size, and made careful adjustments to smooth the edges of the bottle so not to tear the thinner materials such as the balloons and tape.
Step Five- We proceeded to take the cardboard circle and size it to the circumference of the opening of the water bottle. We cut the circle in half, so that it lined up with the edges of the opening and left half of the circular opening uncovered.
Step Six- We covered the half-circle piece of cardboard with stucco tape so that it would be sturdier and water-resistant.
Step Seven- We attached the piece of cardboard covered with the tape to the opening of the water bottle, so that half of the circle was uncovered, and the other half was covered.
Step Eight- Taking the balloons, we cut them open and stretched one over the other half of the bottle, which was not covered by the cardboard circle.
Step Nine- We secured all of the coverings with stucco tape.
Step Ten- We sized and cut a second balloon in quarters to create leafs as the second layer.
Step Eleven- We attached these quarters of the balloon over the opening, as well as the cardboard and the stretched balloon. We layered the leaves so that ⅓ of the quarter falls on top of the other. In this way they will open when water is poured through one end, but will prevent water from permeating the model when attempting to enter from the other end. It is in this way that our model accurately represents the function of the valve.
Summary of Function:
When squeezed, our valve will open to allow water to pass through. This accurately represents the pressure that would be exerted on the valve when it contracts to allow blood to flow through. To prevent water from entering the opposite side, we created flaps that layer on top of each other and guard the opening. It is in this way that our model is only permeable from one side, and stimulates the functionality of the aortic valve.
Trial Data
Numerical Data:
We ran into 2 problems when creating our model and along with our explanations we have numerical data that helps to explain the progression. The percent for the contracted heart represents the percent effectiveness and the percent for the closed heart valve represents the percent error in the returning blood. This data proves that our model was relatively effective in transporting blood to and stopping blood from returning.
First Tests:
Our first model we noticed after testing was not effective and realized this was due to a whole in the balloon as stated in our results above. The water was not flowing from the tube that was supposed to contract and as a result the contracted percent effectiveness was insufficient. The hole had a smaller effect on the water that got back through the heart valve when it was closed.
Second Tests:
Our second data table shows the progress from the first revision. The data shows a clear improvement from the contracted percent effectiveness with the fixing of the hole in the balloon. The closed data remained constant because no changes were made to this piece of the model. In revision 2 and table 3, you can see the final progression and change in the closed percent error data. →
For our second revision the pieces of balloon blocking the water from returning through the heart valve were not secure and this is why we suspected a 5-10 % error in the returning “blood” and fixed this by stabilizing the pieces with extra stucco tape and this made the whole valve more sturdy as represented in our data.
Third, and Final Tests:
To test our model we poured 235 mL of water through both ends of our model for a total of 4 trials. We measured the amount of liquid that made it through the ‘valve’ to the other side and related this to the original amount to calculate a percentage.
Images of Sketches
Conclusion
We have been able to definitely conclude that our design and prototype functions as desired, and accurately represents the aortic valve and its functions. Using materials that best replicate the properties of those that make up the valve, we constructed the four main components of our model. We first used the plastic casing of a water bottle for the body of the model. This allowed us to exert compression on the prototype, which replicates the contraction of the aortic valve. This also allowed us to ensure that our model would withstand the pressure and forces that a valve would be subjected to while in use. Next, we used cardboard and stucco tape to create a cover for the opening of the bottle so that liquid would not be allowed to pass through without compression. We use a balloon stretch across the remaining open portion of the bottle, to allow a space for water to pass when compressed. Finally, we created four flaps that cover the opening so that water cannot pass through in the incorrect direction, much like the aortic valve. In the aortic valve, there is a closing so that the blood cannot pass through in the wrong, or “opposite” direction. Our flaps, which were layered on top of each other, functioned in the same way. After solving an error involving our materials, we were able to replace the damaged section and create a functioning prototype for our aortic heart valve.
Overall, our group completed the project and produced a successful prototype collaboratively. In reflecting on our methods as a group, it should be noted that during our next project, we will focus on better time management and the adherence to a time line to complete everything by the due date. Other than that, our group communicated effectively and worked well together.
link to padlet- https://padlet.com/mifong1/6ginmd168z9zxpv9