Heart Valve
Introduction
The goal of this project was to design and prototype a model of an aortic heart valve, without actually using any human materials. This aortic valve was required to be made of thin materials to resemble an actual aortic valve, have at least two layers with one being more elastic than the other, and have the ability to recoil back to an “open position” after force is removed from it. Based on our previous research and experimentation on the human heart, its various parts, the best materials to use, and Young’s Modulus, we decided to attempt to build a model using plastic water bottles, straws, Play-Doh, and balloons. We had a general idea of what that model would look like and through a few redesigns and testing, we were able to develop our current, functioning prototype.
Background Information
Blood flow
Oxygen and carbon dioxide travels to and from tiny air sacs in the lungs, through the walls of the capillaries, into the blood. Blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs. Blood leaves the heart through the aortic valve, into the aorta and to the body. Blood flows from the right atrium into the right ventricle through the tricuspid valve. When the ventricle is full, the tricuspid valve shuts to prevent blood flowing backwards into the atrium. Blood leaves the heart through the pulmonic valve into the pulmonary artery and flows to the lungs.
Heart structure
The human heart is a four-chambered muscular organ, shaped and sized roughly like a man's closed fist with two-thirds of the mass to the left of the midline. The heart is enclosed in a pericardial sac that is lined with the parietal layers of a serous membrane. The heart is divided into four chambers consisting of two atria and two ventricles; the atria receive blood, while the ventricles pump blood. The right atrium receives blood from the superior and inferior vena cavas and the coronary sinus; blood then moves to the right ventricle where it is pumped to the lungs.
Valve Structure
The valves prevent the backward flow of blood. These valves are actual flaps that are located on each end of the two ventricles (lower chambers of the heart). They act as one-way inlets of blood on one side of a ventricle and one-way outlets of blood on the other side of a ventricle. When the left ventricle contracts, the mitral valve closes and the aortic valve opens. This is so blood flows into the aorta and out to the rest of the body. While the left ventricle is relaxing, the right ventricle also relaxes. This causes the pulmonary valve to close and the tricuspid valve to open.
Young’s Modulus and Valve Materials
Young’s Modulus Background
Young’s Modulus is the mathematical way to determine the elasticity of an object and its ability to return to its original shape, based on the stress and strain it can withstand. A higher Young’s Modulus indicates that the object has less ability to stretch and will mostly remain in its original shape, and a lower Young’s Modulus indicates that the object is more elastic but is less likely to return to its original shape. The general equation for Young’s Modulus is:
F = 𝒀 (∆𝑳/𝑳𝟎)𝑨
Young’s Modulus can also be found by dividing the stress by the strain, or setting up a ratio of stress to strain, using the equations:
σ = F/A and ɛ = ∆𝑳/𝑳𝟎
In these equations, F is defined as the Force in Newtons (N), A is defined as the cross-sectional area in meters squared (m^2), ∆𝑳 is defined as the change in length in meters (m), 𝑳𝟎 is defined as the initial length in meters (m), σ is defined as the stress in N/m^2, ɛ is defined as the strain which has no units, and Y is defined as Young’s Modulus in Pascals (Pa)
To learn more about the Young's Modulus of our selected materials, read our Materials Testing Report
Heart Valve Parts 1 and 2
Step 1, Choose materials:
Choose materials that you think you want to use and test them to determine the most suitable ones. When creating tissue that will act as an analog for real heart valve tissue, make sure to consider the physical properties of the material. You want the material to act similarly to real heart valve tissue.
Your model should mirror those properties as closely as possible. It may be helpful to compare the physical properties of your potential model to the physical properties of heart valves. You can find out more about those properties through online research.
Step 2, Test the materials:
Teams test their materials to gather information on their elasticity, which informs the material selection process.
Set up an elasticity-testing station(s) (see an example in Figure 1). IMPORTANT: Teams are responsible for setting up their own testing setups and for cleaning up after each test.
Testing can be accomplished by using an elevated structure (such as a ring or wire stand) to hang the material and then attaching mass to the end of the material.
Then compare the initial length of the hanging material to its length when different masses are added by calculating the change in length of the material for each mass added. The data collected is used to determine the Young's modulus of each candidate material.
Design a Heart Valve Parts 3, 4, and 5
Step 3, Design and Build Your Prototype
Once you have selected materials, begin the design and fabrication of your prototype replacement heart valves, which will also need to be tested.
Since your artificial valve material will be used in the place of real heart valve tissue, aim to make your model as similar to organic valve tissue as possible. Consider the design of the real valve when you are choosing and testing your artificial valve tissue.
You need to experiment with several different model ideas before designing one that meets the following specifications:
The model is thin like an aortic valve.
The model is made of at least two layers.
One of these layers is more elastic than the other.
The model recoils back to the "open position" after force is removed from it.
Step 4, Test Your Prototype
Test your models similarly to how individual materials were tested (as described in step 2). Additionally,design tests that mimic the action of a real heart valve functioning in a live body. For Example, hold the model stationary, pull it down some, and see if it recoils back to its "open" position. Remember: testing models and ideas is a vital aspect of the engineer design process. Here the intent is to test your models to see how they act under conditions similar to those in the body. For engineers, designing creative testing procedures is often part of the process.
Design and Construction
Initial and Final sketches
← First Prototype →
Prototyping Process
During this prototyping process we attempted several different methods to achieve our final product
Configured water bottles and straws in a manner that would allow for our “blood” (water) to flow through the model
These water bottles were acting as our means of pumping the blood through the valve, not our actual model
This did work, and will be incorporated into the final model
Used balloons and playdough to create our valve.
This did not work because we left the dough out to dry and it became hard and developed cracks, so the balloon would no longer pump with the water
Used a balloon wrapped in soft Play-Doh to create a cross section of the aortic valve, with the water bottles and straws set up in front, demonstrating how blood flows through the valve.
This was a good first attempt at our heart valve but ultimately lacked a clear distinction to a functioning heart valve
Our final prototype combined aspects from all of our initial tests: water bottle, straws and a balloon
We used a singular water bottle to catch the “blood” as it flowed through the heart valve
Our valve was a series of straws connected together, secured with the tip of a balloon to pinch the valve preventing the blood from flowing back into our balloon (back through the valve).
Final Prototype
To the right are videos of our working heart valve model
Below are pictures of the final model up close
IMG_5831.movIMG_5834.mov
