PROJECT STATEMENT AND SPECIFICATIONS THREE POINT BENDING DEVICE FOR FLEXURE TESTING OF SOFT TISSUES TEAM 4 Michael Harman Minh Xuan Nguyen Eric Sirois CLIENT CONTACT Wei Sun, Ph.D. Assistant Professor UCONN BME and ME Department Arthur B. Bronwell Building Rm. 203 Phone: (860) 486 0369 Fax: (860) 486 5088 E mail: weisun@engr.uconn.edu
STATEMENT OF NEED In the development of implantable devices that interact mechanically with the body s native tissue, simulations between implant and host are often developed. The accuracy of these simulations depends heavily on the level of detail used in the characterization of any tissue involved (both the host tissue and any tissue present in the new device, as in bioprosthetic heart valves). For this reason, the Biomechanics Lab at the University of Connecticut has a need for a three point bending device that is capable of testing the flexure properties of native and engineered soft tissue. The mechanical properties of soft tissues can usually be quantified and characterized by tensile mechanical testing, such as uniaxial or biaxial testing. However, tensile testing cannot provide accurate quantification of the mechanical behavior of soft tissues in the low strain region and with different layers of fibers. Flexure testing, on the contrary, supplements tensile testing by focusing on the mechanics of the tissues exhibiting very low strain. Therefore, a request is made for the construction of the three point bending device capable of flexure testing and calculating the flexure rigidity, bending stiffness, transmural strains, and transverse shear stiffness of soft tissues. INTRODUCTION & OVERVIEW The client and his research team in the Biomechanics Lab are currently conducting studies on the mechanical properties of various soft tissues, primarily heart valves. Their lab contains a biaxial testing machine, which is frequently used to determine the stress and strain response of tissues. Data collected via biaxial testing are fundamental in quantifying and validating the nonlinear elastic, anisotropic nature of the tissues. Biaxial testing, however, is limited because it treats the test specimen as a homogeneous material. Soft tissues, such as blood vessels and heart valves, are heterogeneous and consist of multiple layers of fibers arranged in different networks. For example, heart valve leaflets are tri layered with the ventricularis layer composed of a network of collagen and elastin fibers, the spongiosa layer composed of proteoglycans, and the fibrosa layer composed of a dense network of collagen fibers. When biaxial testing is performed on the leaflet, the collected data is unable to indicate how the different layers of the leaflet response to the applied load because, as previously mentioned, the leaflet is treated as homogeneous. Flexure testing, however, is an effective method of evaluating the force deformation relationship of different layers of the soft tissue. It is capable of rendering the different layers deformed by different amounts in different regions. This is essential and critical in analyzing the effect of applied loads on the different layers of the tissue. Moreover, since soft tissues have very low bending stiffness, flexural deformation provides a sensitive approach to evaluating the mechanical properties of the tissue, especially in the low strain range, which is very difficult if using tensile mechanical testing. 1
Thus, the client has requested for the construction of a three point bending device. The primary purposes of this device are to perform flexure testing and compute the flexure rigidity, bending stiffness, transmural strain, and transverse shear stiffness of a soft tissue specimen. Specifically, the device will be run and controlled by a computer program that will be written specifically for the device. The program will allow the user to apply a load to a tissue specimen submerged in saline solution at body temperature. The deformation of the tissue will be tracked by a high resolution camera and computed, along with the amount of load applied, by the program. The data collected will be used by the program to calculate the flexure rigidity, bending stiffness, transmural strain, and transverse shear stiffness. REALISTIC CONSTRAINTS Environmental: For proper tissue mechanical response, the tissue must be maintained in conditions that simulate the in vitro environment. As such, a phosphate buffered saline solution, maintained at 37 ± 1 C is required. Phosphates will be added to a normal saline solution to reach a ph of 7.4. The environment will be mildly corrosive. Manufacturability: Many of the parts required for system operation will be extremely small, and thus expensive to manufacture precisely. In some cases trade offs will have to be made to lower manufacture costs at the expense of device accuracy. Whenever possible, parts will be manufactured in house (at the University of Connecticut machine shop). Sustainability: Whenever possible, non corrosive parts will be chosen. A review of the literature has indicated though, that some corrosive parts will be required. All potentially corrosive parts should be periodically replaced. The method chosen for measuring tissue displacement should be calibrated prior to each use. Any components subject to friction should be lubricated periodically to prevent wear and unnecessary error. These measures will increase the repeatability of data obtained using the system. Ethics: The budget for this project is limited. As such, all purchased parts will be thoroughly researched so that the most reliable, and yet most cost effective, component is chosen in each case. All funds spent will be used solely for the purchase of parts and services. Whenever possible, parts will be manufactured in house to conserve funds. Finally, the client will be kept informed of the initially planned budget along with periodic updates and consulted prior to any necessary deviations from the budget. Health and Safety: The device will be routinely exposed to excised animal tissue. Surface materials should be chosen such that they are smooth and capable of being sterilized after each use. Materials chosen should also be non toxic and non destructive to the tested tissue. 2
time. Political and Social: There are no political or social constraints on the project at this OTHER DATA The client, Dr. Wei Sun, is currently an Associate Professor in the BME and ME department at the University of Connecticut. His research area focuses on understanding, through experimental and computational approaches, the biomechanical interactions between percutaneous devices and surrounding body tissues for the treatment of heart valve diseases, such as mitral regurgitation and aortic stenosis. He is currently working on constructing finite element models of body tissues and percutaneous devices to simulate tissue device interactions when the device is implanted. Critical to this is a comprehensive and accurate description of material behavior. Experimental testing is, thus, necessary to provide data for the comprehension of the dependence of tissue stress on strain and the effect of small deformation on the different layers of tissue. The construction of the three point bending device will contribute tremendously to this understanding of tissue behavior and help enhance Dr. Sun s research. Aside from Dr. Sun, the following three published journal articles, and possibly more in the future, will be frequently consulted for information on previously constructed three point bending device. These articles will be used to understand how a three point bending device was built, what materials were needed, and how the mechanical properties of the tissues (i.e. flexure rigidity, bending stiffness, transmural strain, and transverse shear stiffness) were calculated. QUESTIONS 1. Mirnajafi, Ali, et. al. The Effects of Collagen Fiber Orientation on the Flexural Properties of Pericardial Heterograft Biomaterials. Biomaterials 26 (2005) 795 804. 2. Yu, Qilian, et. al. Neutral Axis Location in Bending and Young s Modulus of Different Layers of Arterial Wall. The American Physiological Society (1993) H52 H60. 3. Gloeckner, Claire, et. al. Effects of Mechanical Fatigue on the Bending Properties of the Porcine Bioprosthetic Heart Valve. ASAIO Journal (1998). Which programming language should be used? Does one specific language (LabView or C++) provide an advantage in either capability or ease of interface with the user? Do other such 3 point bending devices exist? How do they accomplish the goal of flexural strain measurement? On the tissue being measured, how far from any outside contact should measurement of strain be taken? 3
For image acquisition, will motion tracking of the tissue be required to move the imaging apparatus with the tissue, or can the specimen be imaged over the entire testing range without it? What is the margin of error for the reproducibility of data? Should there be one program for the controlling of external devices and another program for the post processing of data, or should both be included in one main program? The data should be saved in what type of format (ex. Excel or Text)? Should the device be calibrated before every test or just once? TECHNICAL SPECIFICATIONS Physical: Tissue Mounting Size Thickness 100 400 µm Width 3 5 mm Length (maximum) 30 mm Minimum image points on tissue specimen 30 Tissue Displacement Distance In direction of tissue curvature 0.5 mm Against direction of tissue curvature 2 mm Time 5 seconds Mounting Bath Length 1 foot Width 6 inches Height 2 inches Electrical: Input voltage Input current 1φ, 115V AC 5 A Mechanical: Applied Load (maximum) Force Measurement Accuracy Motor Torque Resolution 10 grams 0.001 grams 0.42 lb ft 1.8 degree 4
Software: User Interfaces Keyboard, Mouse Hardware Interfaces Monitor, Data Acquisition Equipment, Camera Resolution Capability (minimum) 640 x 480 pixels Spatial Resolution 30 µm / pixel Communication Protocols USB, PCI, PXI Computer Requirements Operating System XP or higher Processor 2 GHz Intel Core 2 Memory 1 GHz Environmental Optimum Operating Temperature 37 ± 1 C Optimum Operating ph 7.4 Operating Environment Indoor Safety The system contains fluid at elevated temperature with a slightly basic ph. The system should never be left in operation unattended. While working with the system, operators should wear gloves, closed shoes, and long pants. Maintenance All potentially corrosive parts should be periodically replaced. The method chosen for measuring tissue displacement should be calibrated prior to each use. Any components subject to friction should be lubricated periodically to prevent wear and unnecessary error. 5