Design a Heart Valve, Part 3:

Introduction

Student teams use what they learned in the previous lessons and activities to research and choose materials for their model heart valves and test those materials to compare their properties to known properties of real heart valve tissues.

Once testing is complete, they choose final materials and design and construct prototype valve models, then test them and evaluate their data.

Based on their evaluations, students consider how they might redesign their models for improvement and then change some aspect of their models and retest—aiming to design optimal heart valve models as solutions to the overarching design challenge.

Learning Objectives

Students should be able to:

• Work through the steps of the engineering design process to examine a problem, research it and decide the best way to tackle it, design and create a solution, test the prototype solution and redesign as needed, and then report their findings.

• Research the materials that have been used in artificial heart valves in the past; identify possible materials for their prototype valve model designs.

• Test materials to determine their elasticity; compare those values to the elasticity of real heart valves.

• Construct prototype model heart valves, collect data from testing the model for its functionality compared to a real structure, and use the data to analyze the success of the model.

• Compile, summarize and present their research and designs to the class in verbal and written formats.

Remember the Challenge problem:

"You are a team of engineers for a bio-materials company that has a cardiovascular systems client who wants you to develop a model that can be used to test the properties of heart valves without using real specimens."

Let's Start!!!

Now that you have learned some background information about blood flow, heart structure and valve structure, it is time to start designing your replacement heart- valve models.

To accomplish this, you will be permitted to use any materials that you wish to create your models. Ideally, the materials should not cost much money, and could be collected from home. Types of material that you might consider using include latex or plastic from balloons and gloves, rubber pieces, tape, plastic, laminated cardboard, or any number of other household and found materials.

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 analogue 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.

As you do this part, remember the structure of aortic valves and what each layer of the valve does. Consider the trilaminar structure of aortic valves, and choose a number of materials that might be similar to the different layers of the valve. In particular, they should choose materials that possess similar properties to the ventricularis and the fibrosa layers.

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 any elevated structure (such as a ring or wire stand) to hang the material, and then attaching mass to the end of the material (see an example in Figure 1).

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.

Figure 1. An example setup to gather data to determine the elasticity of a material.

Following is what you need to determine, calculate and plot through your tests:

1. determine the Young's modulus of each material, applying what they learned from the associated lesson. Have students use software to create stress vs. strain graphs and determine lines of best fit; as an alternative, have students use graph paper and do this by hand. For each material, have teams:

2. create a stress vs. strain graph, plotting at least three data points determined by using at least three weights of different masses to measure and calculate stress and strain

3. determine the line of best fit based on the three points on the graph

4. determine the slope of the best fit line, which is the experimentally determined Young's modulus

5. Compare its calculated values to the known Young's modulus of heart valves (or even the different layers of heart valves) as part of choosing materials that would be best for the model. For reference, the Young's modulus value for a pig's heart valve tissue and some other materials are provided below:

6. conduct online research to find values for human heart valves or even the layers of the heart valves. (Tip: for some values check the suggested research links at the bottom of this page)

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.

IMPORTANT: Before building the model, you and the member of your group must sketch your design ideas and final design on paper.

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.

Step 5, Re-Design and Re-test as needed:

Based on the data that you obtained through testing, you may want to redesign your models and retest, as needed. Remember: Engineers usually revisit their proposed solutions many times to make sure they are the best solutions to the problems. For your re-designs, you might opt to choose different materials, look into ways to combine the materials differently, or alter the sizes of your model/materials—just as engineers would do. The cyclical improvement process that you experience is a good example of the iterative nature of the engineering design process.

Step 6, Digital Portfolio:

Once teams' models are built, tested and redesigned (as needed and as time permits), then they compile digital portfolios that summarizes and showcases the pertinent background information learned. This involves gathering together all the information they collected on the model, including materials chosen, research on tissue properties and behavior, design sketches and photographs, material testing results, and analysis of the model with recommendations for model use.

Assess the quality and completeness of your portfolios using the Digital Portfolio Rubric

(Refer to the Previous sections of this website for more background information on forces, elasticity, stress, strain, valve structure, valve tissue properties and Young's modulus.)

Materials List

Each group needs:

• Computer or tablet for Internet research and data graphing; Google sheets or Microsoft Excel® or similar software is recommended so students can graph data and determine lines of best fit.

You might also need (If not available in the maker-space bring from home):

• ring stand or equivalent

• hanging mass sets or known weights

• hooks for hanging materials from the ring stands or equivalent

• precise calipers and rulers, to measure very thin materials

• a variety of materials from which groups can make models, such as balloons, paper, plastic, rubber, latex gloves, hose/tubing, clear plastic sheets, laminated cardboard or other household, recycled or found materials etc...

• a variety of materials and tools that students can use to fabricate models, such as tape, glue, scissors, paper clips, hot glue guns etc...

Resources:

Following are some online data on the Young's modulus of human heart valve tissues:

• Development of a completely biological tissue engineered heart valve by Zeeshan Hayder Syedain, University of Minnesota, US. This PhD dissertation cites Young's modulus, also known as modulus of elasticity or elastic modulus, values for the fibrosa layer of the aortic valve as 23 MPa in the circumferential direction and 3.71 MPa in the radial direction and for the ventricularis layer as 9.55 MPa in the circumferential direction and 3.7 MPa in the radial direction: http://gradworks.umi.com/33/52/3352813.html

• The collagen organization in heart valves: a target for tissue engineered valves by A. Balguid et al., Eindhoven University of Technology, Netherlands. This poster summary of a paper provides an average Young's modulus value of approximately 20.9 MPa for human heart valves in the circumferential direction and approximately 2.1 MPa in the radial direction): http://repository.tue.nl/posters/738594.pdf

• A new approach to heart valve tissue engineering based on the modification of human pericardial tissue by Frantisek Straka et al., Institute of Physiology, Czech Republic. The abstract of this paper states that elastic modulus of approximately 13.1 MPa is comparable to native aortic heart valve: http://www.qscience.com/doi/abs/10.5339/qproc.2012.heartvalve.4.79

• St. Jude Epic heart valve bioprostheses versus native human and porcine aortic valves – comparison of mechanical properties by Martins Kalejs et al., Pauls Stradins Clinical University Hospital, Latvia. This abstract provides Young's modulus, also known as modulus of elasticity, for aortic valves from humans and pigs: http://icvts.oxfordjournals.org/content/8/5/553.full.pdf

• Mechanical properties of chordae tendineae of the mitral heart valve: young's modulus, structural stiffness, and effects of aging by Laura Millard et al., University of Birmingham, UK. The abstract of this paper provides Young's modulus values for the chordae tendineae of the mitral valve in pigs: http://www.worldscientific.com/doi/abs/10.1142/S0219519411003971?journalCode=jmmb