Faculty advisor: Bob TranquilloThis research was supported by the Undergraduate Research Opportunities Program (UROP)

Patel, Puja

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Fatigue Testing of Transcatheter Heart Valve Commissures
Puja Patel
Tranquillo Laboratory, Department of Biomedical Engineering
University of Minnesota - Twin Cities
Background
Fatigue testing of new heart valve designs is performed to verify 
the valve’s capability of withstanding stresses that it will endure 
during millions of heart cycles. Professor Tranquillo’s lab in the 
Biomedical Engineering Department is developing transcatheter heart 
valves that use a nitinol wire frame, nitinol stent, and engineered 
tissue. In order to optimize anchoring sutures between engineered 
tissue and wire frames, fatigue testing is conducted on entire valves 
in accelerated wear testers, which can be time consuming and costly. 
Because a patient’s heart goes through millions of cyclic stresses 
and changes throughout life, it is important to create engineered 
valves that can undergo the varying levels of repeated stress and 
force. One current testing method is conducted post-implantation with 
the use of exercise and heart monitoring (Piérard & Lancellotti, 2007). 
However, sometimes the stress testing post-implantation can be too 
late, which may be dangerous for the patient. 
Therefore, there are also up to 50 different inspections and tests 
conducted on manufactured heart valves, that include microscopic 
analysis, size analysis, and fatigue testing (M. Sheppard, 2016). The 
main method used for fatigue testing currently involves a 
pressurization fixture that tests the entire heart valve in order to 
determine if the heart valve fails at certain stress levels. The valves 
are then monitored for cracks, tears, and other issues (M. Sheppard, 
2016). This test can be inefficient because an entire valve is needed 
in order to conduct testing, and each valve can be costly to create. 
Materials/Methods
Fatigue Testing Chamber Mechanism
The fatigue testing chamber works by using the linear actuator to 
convert energy into a linear motion. The linear motion of the actuator 
was used to pull on the tissue. One end of fishing wire was attached to 
screws that connected to the moving base of the linear actuator, and 
the other end was attached to a plastic clip. The red arrow shown on 
Figure 3 displays the direction of the movement of the base of the 
linear actuator. The plastic clip was then used to clip onto the leaflets 
of the tissue sample. The base of the tissue sample and the wireframe 
were also attached to another plastic clip that was fixed, in order to 
hold the tissue in place. The linear actuator was then able to pull the 
clip back and forth, which in turn pulled the tissue back and forth. 
Fatigue Testing Chamber: Design 1
The first design that was created utilized plastic clips glued to the 
base. A fishing wire was attached to the second plastic clip, which 
attached to the tissue as shown in Figures 5 and 6 (below). However, 
when using the linear actuator, the tissue was pulled by an upward 
force. This is the opposite of what occurs at a leaflet in the body, so 
the design had to be changed.  
Fatigue Testing Chamber: Design 2
The second design created was a modification of the first design. 
The plastic clips were attached to a bar so that the clips could be 
hung upside down. This way, the tissues could be held upside down 
as well, and therefore, when the linear actuator pulled the on the 
second clip in an upward motion, it was actually pulling the tissue in 
a downward motion. This motion was more similar to the actual 
motion of leaflets in heart valves. Figures 7 and 8 display the new 
design. 
Dynamic Strain Measurements
Three main measurements were collected from the videos of the 
different samples placed in the testing chamber, total tissue length 
when flexed, total tissue length when not flexed, and deflection. The 
total tissue length measurements were then used to calculate tissue 
strain. The tissue strain values, which were found to be in the range 
of 2-6% were then converted into maximum stress values, which 
were found to be in the range 16-77 kPa.
Further Testing
Further testing can be done using the chamber in order to confirm 
the benefits of using a mesh skirt. Further testing can also be done on 
suture patterns that are used to attach the mesh skirt or the wire 
frame. The mesh skirt was able to protect the tissue, but there could 
still be tears in the tissue later on if the suture pattern put too much 
stress in one location of the tissue. This could be tested with different 
suture patterns, and running the fatigue testing for longer periods. In 
order to improve the fatigue testing chamber, the tissue could be fixed 
more securely by fixing the back of the tissue to the clip or the base of 
the testing chamber. This would allow for more strain on the tissue 
because the tissue would not be able to bend and flex as much as in 
the previous two designs.   
Post-Test Tissue Analysis
Conclusions
References
Post-test tissue analysis was conducted in order to determine if 
there was any tissue damage. The following images display tissue 
samples that were put through testing with and without mesh, in order 
to compare damage done. The important features are shown.    
Using the post-test analysis it was determined that the mesh skirt 
provided sufficient protection to the tissue (Figures 11-14). Both tissue 
samples that used the mesh skirt showed no damage (Figures 11 and 
13), and both tissue samples that did not use a mesh skirt showed 
damage, including holes and tears, where the wire frame was 
attached or touched the tissue (Figures 12 and 14). The range of 
tissue strain values was found to be 2-6%, as shown in Table 1. 
These values for strain that was applied to the tissue corresponded to 
16-77 kPa of stress applied to the tissue. These values can be used 
to determine if the fatigue testing chamber accurately applied the 
correct amount of strain on the tissues, and thus determine if the 
fatigue testing chamber is an effective way to test tissue samples and 
commissures of heart valves.  
Hasan, A., Ragaert, K., Swieszkowski, W., Selimović, Š., Paul, A., Camci-
Unal, G., … Khademhosseini, A. (2014). Biomechanical properties of native and 
tissue engineered heart valve constructs. Journal of Biomechanics, 47(9), 1949–
1963. doi:10.1016/j.jbiomech.2013.09.023
Huber, J. E., Fleck, N. A., & Ashby, M. F. The selection of mechanical
actuators based on performance indices. Retrieved from http://www-
mech.eng.cam.ac.uk/profiles/fleck/papers/85.pdf
Linear Actuator Guide. (2016). Retrieved October 7, 2016, from
Anaheim Automation,
http://www.anaheimautomation.com/manuals/forms/linear-actuator-guide.php
M. Sheppard, L. (2016). How artificial heart valve is made - material,
making, history, used, processing, parts, components, steps, history. Retrieved 
October 8, 2016, from How Products are Made, 
http://www.madehow.com/Volume-6/Artificial-Heart-Valve.html
Piérard, L. A., & Lancellotti, P. (2007). Stress testing in valve disease.
, 93(6), . Retrieved from
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1955215/
In order to create an efficient and simple system for conducting 
fatigue testing on the commissures of the heart valve, a linear actuator 
was used to simulate the pulsing movement of the valve cycle. The 
linear actuator is a device that converts energy into motion in a 
straight line. By utilizing the linear actuator to pull the tissue back and 
forth, the leaflet motion of the heart valve was simulated. 
In order to simplify the fatigue testing chamber, instead of testing 
the entire valve, only a small section of the engineered valve tissue 
was tested. This involved sectioning the tissue and cutting the 
wireframe into segments so one commissure post could be tested at a 
time, as shown in Figures 1 and 2. Sutures were used to attach the 
wireframes to the tissue, along with mesh fabric for some samples. 
The tissue was held using plastic grips. The black dashed line in 
Figure 2 displays where the wire frame and sutures would be located. 
Testing was conducted with three different tissue samples, during 
three different weeks. Each test ran for 7 to 8 days, and the linear 
actuator moved 5 mm in each direction. The linear actuator completed 
90 cycles per minute. Therefore the tissue samples and wireframes 
were tested for about 907,200 cycles to 1,036,800 cycles. Videos 
were recorded for each of the tissue samples and measurements 
were taken. Post-test analysis was also conducted in order to examine 
any tissue damage that occurred.  
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Desig
n/We
ek
Tissue Total Tissue 
Length (not 
flexed) 
(mm)
Total Tissue 
Length 
(flexed) 
(mm)
Tissue
Strain 
Deflection (mm)
1-1 1, mesh 
skirt
9.184 9.692 0.054 0.758
2-2 1, mesh 
skirt
11.408 10.876 0.048 0.899
2-2 2, mesh 
skirt
10.759 10.954 0.018 0.795
2-3 3, mesh 
skirt
8.818 8.946 0.014 1.804
2-3 3, no 
mesh 
skirt
10.772 11.025 0.023 0.525 
2-3 2, no
mesh 
skirt
8.897 9.445 0.060 1.819 
Average 9.973 10.156 0.063 1.10
Standard 
Dev.
1.035 0.826 0.055 0.515
Figure 11: Tissue 2, Design 2, 
Week 2, with mesh
Figure 12: Tissue 2, Design 
2, Week 3, without mesh
Figure 9 and 10: The images above show the tissues analyzed 
post-test from the third week of testing. From left to right: Tissue 3 
with mesh, Tissue 2 without mesh, and Tissue 2 without mesh. 
Figure 3: Design 1, side view
Figure 6: Tissue attachment 
Figure 7: Tissue attachment 
Figure 8: Design 2, back-view  
Figure 5: Design 1, side view
Figure 4: Tissue attachment, Design 2
Table 1: Strain and deflection values for each tissue sample  
Leaflet 1 Leaflet 2
Figure 14: Tissue 3, Design 
2, Week 3- without mesh
Figure 13: Tissue 3, Design 
2, Week 3- with mesh
Without MeshWith Mesh
Leaflet 1
Leaflet 2
Commissure
Figure 1: Full valve in a stent Figure 2: Sectioned tissue
Acknowledgements
Thank you to Zeeshan Syedain, UROP for funding this work, and the 
Minnesota Partnership for Biotechnology and Medical Genomics Grant 
(to RTT) for the main funding of this project. 


Fatigue Testing of Transcatheter Heart Valve Commissures

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Fatigue Testing of Transcatheter Heart Valve Commissures

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