Project Proposal A System to Quantify 3D Spatial Deformation of Heart Valve Leaflets Team #8 Michael Hahn, Elizabeth Morgan, Oskar Perskaas Client Contact: Dr. Wei Sun, University of Connecticut, weisun@engr.uconn.edu
Executive Summary Heart valve disease is a significant cause of morbidity and mortality, and the primary treatment is valve replacement. Half of the aortic heart valves are replaced with bioprosthetic heart valves, but they continue to fail as a result of calcification and mechanical damage. Dr. Wei Sun, who has considerable research experience involving the biomechanics of heart valve biomaterials, has expressed interest in finding the source of the failure of bioprosthetic heart valves. Specifically, faulty design may be creating areas of increased strain causing unnecessary wear and early failure. To quantify the deformation of the leaflets, Dr. Sun proposes the construction of a device to apply various transvalvular pressures and analyze the motions of graphite markers on the leaflet surfaces. The device has a chamber to house the valve in, which would be attached to the mounting plate. A pump creates static and dynamic loading conditions, and a pressure transducer measures the transvalvular pressure. Multiple cameras are situated at various angles, positioned to observe a single leaflet at a time. Many graphite markers will be affixed to the leaflet surface, and the relative motions of the markers will be analyzed to produce local strain readings using a LabVIEW Virtual Instrument. Many components will be supplied by Dr. Sun, including cameras and pressure transducer. The remaining materials for the device will cost approximately THIS MUCH to acquire and construct. Once constructed and tested, it will be used to test many different kinds of bioprosthetic valves available. This information will reveal strain concentration patterns, if any, and hopefully provide valuable information that can be used to better the design of the valves. 1. Introduction Dr. Wei Sun is an assistant professor at the University of Connecticut in the Mechanical Engineering department. Dr. Sun received his Ph.D. in 2003 from the Bioengineering Department at the University of Pittsburgh. He worked as a Postdoctoral Fellow in the Mechanical Engineering Department at the Georgia Institute of Technology and then as a staff engineer at Edwards Lifesciences in the Heart Valve Therapy R&D Department. His research background includes designing new heart valve and annuloplasty prosthesis, modeling of biomimetic fiber scaffold materials as artery substitutes, and studying the biomechanics of heart valve biomaterials. Dr. Sun has done a lot of work with heart valves and is now interested in quantifying 3D bioprosthetic heart valve deformation during static and dynamic loading conditions. He would like to obtain data for several different kinds of heart valves that are available, including several that are currently on the market in the US, and the percutaneous valve (delivered via femoral artery) which is not currently approved by the FDA for use in the United States. Team 8, Page 2
This project will serve to design and construct a device that will be used to test several different heart valves and gather data related to their 3D mechanical deformation. Static and dynamic loading will be applied as multiple cameras observe and track graphite markers placed on the valve leaflets. LabVIEW will be used to create a Virtual Instrument that will acquire the images from the cameras and analyze the movement of the markers in 3D space. From the relative movements of the markers, strain will be calculated and areas of maximum strain will be revealed. This information will give insight into how the geometric design of the valves creates areas of concentrated strain (and the resulting stress), and will show which valve distributes the strain most effectively. The results might bring to light certain design characteristics that are more favorable than others, which may aid in designing a better valve that distributes the strains more evenly. Changing the design could reduce the concentrations of strain that the valve sustains with each loading cycle, reducing the overall wear. A valve with a better design could endure longer and in doing so improve quality of life for the patient and enable them to live longer with the implanted valve. The valve would have increased performance for longer, resulting in better quality of life for a greater time span. Since the bioprosthetic heart valves usually end up failing structurally, an improved design will reduce the failure rate of implanted valves, and function as a better treatment for aortic heart valve diseases. Although no device is currently being marketed for commercial use to our knowledge, Dr. Wei Sun has done work with a setup similar to the one proposed in this report. His work involved quasi-steady state loading of the heart valve at 40, 80, and 120 mmhg, using two cameras to track the movement of the markers. It is possible that three cameras will be used in the current device to ensure all markers are in the field of view, especially near the boundaries, and the third camera can act to increase accuracy when calculating marker positions. Dr. Sun only tested one particular bioprosthetic heart valve, while this project will involve the testing of several different heart valves, making it possible to directly compare the behaviors of various valves on the market. 2. Project Description 2.1. Objective The strain measurement system being designed is intended to house and test heart valves, both bio-prosthetic and natural tissue, of different sizes and geometries in order to analyze and evaluate form and function. The system will incorporate both physical testing as well as algorithmic analysis of the heart valve being tested. Both static and dynamic loading conditions will be implemented to test both the 3D motion of the valve leaflets as well as the transvalvular pressure. Team 8, Page 3
The proposed strain measurement system will be comprised of a physical testing mechanisms which includes a fluid chamber which houses the heart valve being tested, a strain measurement system to capture the valve deformation, and a pressure transducer to measure the transvalvular pressure. Figure 1 shows an example of a strain measurement system design, similar to the one being proposed. A computer algorithm will numerically constructs all of the acquired data from the physical testing in order to calculate the deformation gradient, stress, and strain that the valve experiences under the different loading conditions. Figure 1. A dual camera system to capture leaflet deformation of a bioprosthetic heart valve. As previously described, several strain measurement systems have been designed with good success. However, the proposed system will take the previously developed systems to the next level, with the addition of a third CCD camera to track the valve deformations. In addition, cyclic and dynamic loading conditions will be tested. The proposed system will allow for stronger heart valve design by allowing the engineer to test the form and function of the developed heart valve before implantation. It is a readily available method for testing. Developed valves can be mounted to the system in manner similar to that of implantation, while both static and dynamic loading is applied. The numerical analysis then allows the engineer to quantitatively observe the function of the valve. When designing bioprosthetic heart valves, the goal is to reduce high stress concentrations. Since stress concentrations typically lead to failure, by reducing or eliminating them, the valve will show more success after implantation. Ultimately, patients with prosthetic heart valves should be able to live like those without the need for such mechanical aid. Therefore, the design of prosthetic valves should be able to function for years under different physical conditions and behavior. A strain measurement system would allow the testing of newly developed valves, aiding in the development of more efficient, longer-lasting, and all-around better mechanics. Team 8, Page 4
2.2. Methods There are several aspects to the proposed strain measurement system: a fluid chamber that houses a bioprosthetic heart valve, an optical strain measurement system to capture 3D leaflet cusp deformations under a variety of hydrostatic loading conditions, and a pressure transducer that simultaneously measures the transvalvular pressure. In addition, the system should be computer controlled with automatic data visualization and analysis. 3D motion of the leaflets should also be numerically constructed. The physical system of the system physically tests heart valves under both static and dynamic loading conditions. Each valve will be mounted to the device via a specific plate designed to house said valve. The goal is to mount the valves in a manner similar to when being implanted in the patient. Therefore, the tissue will be sutured onto the plate, which will then be mounted into the system. It is extremely important for each plate to be designed specifically for each type valve in order to ensure proper testing. During testing, a saline solution operating at 37 o C (body temperature) will be pumped through the valve via a closed-loop system to simulate the blood flow through the valve after implantation. Ideally, both steady flow and cyclic loading will be able to be implemented to acquire more data about the valve function. However, integrating cyclic loading conditions in the system would be a large task to accomplish due to the difference in physical set-up and data analysis; it would be a further step to take in the development process if the initial design works with little to no issues or flaws. During the flow, leakage will yield inaccurate results, and therefore, the mounting plate design is imperative. A tight seal must be made between the plate and valve tissue in order to eliminate any fluid leakage. Several plates will be machined for different types of valves (aortic, mitral, etc.) as well as different sizes. Figure 2 shows the four different valves in the heart, while Figure 3 shows several different heart valves. As can be observed, the geometry, design, and size of the valves are very different, and should be accounted for when testing; the need for specifically designed mounting plates is apparent. Note that since the goal of the measurement system is to simply test the form and function of the actual valve structure, there is no need to incorporate the entire environmental system of the heart (i.e., aortic arch). Team 8, Page 5
Figure 2. The four heart valves (1). The four heart valves are located at the entry and exit points of the heart chambers. They act as door which open and close in concert to keep the blood flowing in one direction. Valves affected by disease can compromise the flow of blood through the heart, and therefore, bioprosthetic valves are implanted. In a lifetime, heart valves will open and close more than two billion times, with the heart beating approximately 100,000 times a day (1). Therefore, the importance of a properly functioning bioprosthetic valve is of dire concern. (a) Edwards SAPIEN Transcatheter Heart Valve (26 mm and 23 mm sizes) (2). Some valves, such as the Edwards SAPIEN Transcatheter Heart Valve, are part of a stent structure, and therefore, require, not only a specialized mounting plate, but also a specialized mounting technique. Since a stented valve is not sutured in place when implanted, when testing, it should also not be sutured. Therefore, a tube-like device will be used as the mounting device when testing stented valves. Team 8, Page 6
(b) Biological valves (3). Biological valves come in many shapes, sizes, and are made of several natural and prosthetic materials. For this proposed system, porcine valves will be primarily used for testing. However, bovine and even human tissue is oftentimes used in biological valves. In addition, synthetic materials are also used to create such structures. Figure 3. Several different types of bioprosthetic heart valves. The purpose of the proposed measurement system is to test and analyze the physical deformation of the mounted heart valve under inspection. Therefore, several methods must be implemented in order to capture such deformations. To capture the 3D motion of the leaflets, an inflation method corresponding to thin-walled tissues will be utilized. Such a method requires small markers and several high-speed digital cameras to trace the marker motion to infer the associated inflation and membrane strain. Previous measurement systems have used two borescopes fitted with CCDs cameras to track the marker movements on the tissue. The borescopes are able to be placed close to the tissue without being blocked by the external machinery. A third camera will ideally be incorporated and tested in the final design. However, with an additional camera, several aspects of the system such as the algorithm used to analyze the gathered data will need to be altered in order to incorporate the additional information. Therefore, the benefits will be weighed against the cost of incorporating the third camera in order to determine whether its addition is worth the additional work it would require. A pressure transducer will also be utilized to measure the transvalvular pressure. When heart valves are closed, they experience the highest pressure. The bearing load at that point is the highest, and acts on the valve at a pressure of about 120 mmhg (for the average, healthy adult). In the closed state, the chamber is static and backflow and backpressure will be mimicked. The left ventricle has about 7 mmhg of pressure in the closed state. Therefore, the difference between the bearing side of the valve and the left ventricle side of the valve will be used to calculate the transvalvular pressure. In addition to the closed loading mode (when the valve is shut), a second loading mode, when the valve is open, ideally will also be analyzed. When the valve is open, there is little load acting on the leaflet structures due to the ability for the solution to flow freely through the chamber. Team 8, Page 7
Therefore, during the open loading mode, the general shape of the valve and valve leaflets will be analyzed. All of the data gathered from the physical testing of the mounted heart valves will be reconstructed and analyzed through a computer algorithm. A DLT (direct linear transformation) standard algorithm will be utilized to reconstruct the 3D motion of the leaflets as well as calculate and analyze the stress and strain of the valve under both static and dynamic loading conditions. Since such strain measurement systems have been previously develolped, standard algorithms already exist and will be utilized and manipulated to fit the proposed design. LabView is the software of choice for this measurement system. In addition, Abaqus will be utilized to analyze any stress concentrations that the tested valves may experience. The Fung Model will be used to calculate the deformation gradient of the heart valve being tested. The deformation gradient is defined as: [ ] [ ] (1) Where [ ( ) ( ) ] (2) And [ ( ) ( ) ] (3) The remaining unknowns in equations (1) can be defined as the following: (4) (5) For n=1, 2, 3, and 4. While X 1 and X 2 can be defined as: Team 8, Page 8
( ) ( ) (6) The Green strain (E) and second Piola-Kirchhoff stress (S) can then be calculated, respectively, for further analysis of the heart valve behavior: (7) ( ) (8) Where F is the deformation gradient, I is the identity matrix, and N is the nominal stress. A previously written MatLab code by one of the group members will aid in the coding for the strain measurement system. However, the code as of now is for biaxial testing, and will need to be slightly manipulated to fit the inflation testing being performed by the proposed system. 3. Budget: The strain measurement system project is still in the very early stages of design, making it difficult to gauge the ultimate cost of the final product. However, since Dr. Sun has graciously given us access to his lab equipment, large components needed for the final design (i.e., the pressure transducer and CCD cameras), will be of no cost (and instead, will be on a loan basis). Additionally, Dr. Sun has contacts with other research professors both at the University of Connecticut as well as other colleges and universities, allowing for additional equipment to be acquired on a needed basis. Therefore, the overall cost of the measurement system will be minimal in comparison to what it would be if the large components were not supplied. A table outlining the components, materials, and costs can be found below in Table 1. Table 1. Components, Materials, and Costs of the Strain Measurement System Component Material Price Rate Estimated Price Fluid chamber.220 inch clear acrylic Plexiglas $5.84 Sq. Ft $24 Fluid Saline $4.99/500ml $20 Rotating base $30 Mounting post $20 Interchangeable valve mounts $25 7 Rigid borescope Supplied Fluid pumping system Supplied Pressure transducer Supplied Cameras Supplied Team 8, Page 9
Heart valves Computer hardware/software Supplied Supplied Total: $119 The goal of the design is to keep the total cost to an absolute minimum. Therefore, by taking advantage of the equipment supplied to the project (by both Dr. Sun and his colleagues), the total cost is estimated at around $120.00, which can be concluded as low enough to not need a design change. However, with each design addition/change, Dr. Sun will be contacted to ensure that the cost is within the expected budget (which remains an unfixed number). 4. Conclusions: The strain measurement system we design will be able to capture 3D leaflet cusp deformations using both static and dynamic loading conditions. Using DLT and Marker techniques, the motion of the leaflets can be numerically reconstructed. This data can lead to the generation of 3D Stress concentration images using Abaqus. The transvalvular pressure will also be determined using a pressure transducer. The system will be computer controlled using LabVIEW, and contain automatic data visualization and analysis. There is currently no market for systems that quantify 3D spatial deformation of heart valve leaflets. Only a handful of researchers have attempted to build their own systems, and all of them have had significant limitations. We will be drawing heavily on Dr. Sun s previous research and experiments to create a superior system. With our rigorous experimental validation of numerical results and with Dr. Sun s accurate material model in describing the leaflets material properties, we plan on being able to measure and predict heart valve leaflet deformation more accurately than any other system known to date. Team 8, Page 10