Soft Tissue Fatigue Testing Fixture

Transcription

1 Project Proposal: Team #7 Soft Tissue Fatigue Testing Fixture Members: James M. Rollett Shashank A. Settipalli Austin S. McMann Client: Dr. Wei Sun University of Connecticut Biomedical Engineering and Mechanical Engineering Departments Arthur B. Bronwell Building Room #

2 Section Table of Contents Page Number Executive Summary Introduction Background Purpose of Project Previous Work Patent Search Project Description Objectives Methods Mechanical Designs Strain-Gauge Load Cells Data Acquisition and Software Budget 4.0 Conclusion 5.0 References.. 1

3 Executive Summary The main goal of this project is to construct a cyclical loading fixture for testing soft tissue that is compatible with an existing Bose Testbench system. This will be useful for testing tissues that may be components of a bioprosthetic heart valve derived from porcine or bovine or synthetic tissue. The client has requested that twelve samples can be tested at once while providing real time data acquisition and monitoring via NI Labview. This device will be capable of performing pure tensile or compressive testing as well as a combination of the two. This machine will run at 30 Hz for up to 200,000,000 cycles. The data will be sent wirelessly via a Bluetooth system. The primary method of measuring the loads on the tissue will be through strain gauges which will be incorporated into custom designed load cells. These load cells will also feature specifically designed filters and amplifiers to prepare the signal for NI Labview analysis. The tissue specimens and their fixtures will be submerged in a solution bath during testing. The fixture components will be constructed of commercially available polymers as well as some metal alloys. 1.0 Introduction 1.1 Background The client for this project is Dr. Wei Sun, a mechanical and biomedical engineering professor at the University of Connecticut. Much of his research has been focused on stress and strain analysis of soft tissue. This project is a continuation of his work on wear and durability testing for small soft tissue samples. Currently in his lab, there is a functioning biaxial testing device, as well as a uniaxial testing device capable of holding multiple specimens. A new design is needed for the uniaxial device that will offer easier tissue specimen setup, more accurate data acquisition, and the ability to monitor data as is it being gathered. The new device must also be able to perform cyclic tensile and flexural testing, as well as either pure tensile or flexural testing. These areas of improvement (along with other specifications) are the main focus of this new device. 1.2 Purpose of Project 2

4 The incidence of valvular heart diseases has been increasing quickly over the past few decades as a significant portion of the American population ages. These heart diseases include mitral/aortic valve stenosis and regurgitation. If left undiagnosed and untreated, these conditions may lead to myocardial infarctions and ultimately even death. The implantation of artificial heart valves has been considered as a remedy for many decades. Early mechanical heart valves of the 1950 s and the 1960 s have evolved into bioprosthetic heart valves (BHVs) of the present day. BHVs are an especially important development because they do not require anti-coagulant medication after implantation and generally exhibit better biocompatibility. Even though valves derived from porcine or bovine sources prove to be a better anatomical fit in the body, there is still an important need to test each and every device under the appropriate conditions. A typical functioning heart valve experiences a life-time of flexural and tensile stresses. An adequate replacement needs to thrive under these stresses. Therefore, an appropriate testing fixture needs to be designed in order to accommodate both flexural and tensile testing of a BHV tissue sample. In addition to durability, wear characteristics of the tissue sample must also be analyzed. An evaluation of the lifetime of a particular type of tissue is critical in discovering novel methods to improve the quality of life of millions of individuals with valvular diseases worldwide. It is for these reasons that an improved and effective fatigue testing fixture, in addition to the already existing Bose loading machine and software, would prove a valuable asset for analysis and evaluation in the department s tissue mechanics laboratory. 1.3 Previous Work Other devices capable of performing tensile or flexural testing have been created. At Clemson University in South Carolina a fatigue testing fixture was designed to perform cyclic tensile testing of up to twelve specimens at once. A schematic of one of the fatigue testing devices is shown in Figure 1.1. For each of the devices, one arm was fixed on the testing surface, while the other was free to move. The movable arms were connected to the testing device that applied the load. Figure 1.1 3

5 Front and side view of clamps for securing and testing one tissue specimen, Sacks et al. Another device was created at the University of Pittsburgh. This device was able to perform both tensile and flexural testing on twelve tissue specimens at once. This is an important design feature in that it takes into consideration both the stretching and bending of the tissue. This allows the researchers to determine what type of impact these different types of forces each have on the tissue. The design was required to fit within the Bose ElectroForce 3200 test instrument, which is why the triangular setup was used. A CAD design of this device is shown in Figure 1.2. The arm of the Bose test instrument connected to the center of the fatigue testing devices as shown. As the arm moved up and down, the clamps attached to the arm moved as well. This applied equal force and displacement to each of the specimens throughout the experiment. Figure 1.2 4

6 CAD design of cyclic testing device, University of Pittsburgh s Tissue Mechanics Lab Patent Search Initial online patent queries resulted in numerous entries being found. The vast majority of these entries, however, dealt primarily with fatigue testing of entire bioprosthetic or mechanical heart valve replacements. There were a few patents that dealt specifically with tissue testing. Patent # : This patent applied by Baxter International Inc. dealt with an in vitro tissue testing device. The main purpose of the design here was to assess the hemo-compatibility of tissue samples when they are placed within a chamber that applies different pressures in a simulated fluid environment. 5

7 Patent # : The design put forth in this patent (issued in 2002) features a clamping mechanism to hold tissue samples and apply tensile and flexural forces. These operations are performed with the help of motors and small gears. Patent # : The main feature in this design is a testing chamber which houses a tissue sample and can mimic conditions within the body. Mechanical properties of the sample tissue are measured or calculated through optical and mechanical means. 2.0 Project Description 2.1 Objectives Design a soft tissue fatige testing fixture to meet specifications including: 12 samples Continuous data acquisition/monitoring Flexural and/or tensile testing Maximum 30 Hz, 200 million cycles Bluetooth communication Durable device Simple setup, easy to run 2.2 Methods A detailed approach to the final product can currently be broken down to three main domains: analyzing variations of different mechanical designs of the overall device and each functioning component, describing the function of strain-gauges and assessing their application to the current designs, and finally evaluating intermediate steps involved between acquiring data and displaying it, specifically in NI LabView Mechanical Designs The overall device will consist of the twelve fatigue testing fixtures, as well as the housing for the bath tissue and these testing fixtures. This setup will be connected to the arm of the Bose Testbench device, which will apply the load to the fatigue testing fixtures. 6

8 Each of the twelve fatigue testing fixtures will be identical in their design. A simple design of the device was created using SolidWorks as shown in Figure 2.2. Figure

9 SolidWorks design of proposed tissue testing device. Each fixture is essentially two clamps which will hold the tissue specimen in place, an arm to attach the custom made load cell, and attachments to the base of the device. The clamps will consist of two pieces of plexiglass. One piece will be the main body of the clamp. The second will be a smaller piece attached to the larger clamp section by a hinge. This clamp design is shown in Figure 2.2. Pin holes may be drilled into the edges of the plexiglass. This will allow pins to be inserted through the tissue to hold it in place during testing. Figure

10 SolidWorks design of a single clamp. The lower clamp of each of the twelve fatigue testing fixtures will be attached to the base of the device. This base will not be attached to the bottom of the bath tissue housing, allowing it to be moved by the arm of the Bose Testbench device. The upper clamp will be attached to a stationary arm. The stationary arm will have the strain gauge mounted on it to pick up the force applied to the tissue. It is important to secure the load cell on the arm that remains stationary. If the load cell was attached to the moveable arm, then the load cell will detect its own movement and distort the raw data. The upper clamps of each fatigue testing fixture will be secured to the bath tissue housing to keep them from moving during testing Strain-Gauge Load Cells The measurement of tissue deformation can be obtained by several methods. A load cell, a transducer that converts forces into electrical signals, can be easily implemented into a fixture. Vendors of custom-built strain-gauge load cells for tissue testing purposes have set the price range at $700-$900 for each load cell. Since the client requires the fixture device 9

11 to accommodate twelve tissue samples, the purchase of twelve custom-built load cells is too expensive for the current set budget. The picture below illustrates the size of a typical load cell. Fig Strain is defined as the fractional change a component experiences due to a force in a longitudinal direction. Strain, denoted by ɛ, is conceptualized in the figure below. Fig An average sized strain gauge (pictured left; Fig ) is actually a small component thinner than a sheet of paper. It consists of very fine wire in a coil which is subjected to the strain in the parallel direction. A strain gauge is usually placed on a carrier (a strip of thin metal) for support. The strain experienced by the test specimen is transferred indirectly to the gauge via the carrier. As the strain differences are felt, the resistance on the gauge changes slightly, resulting in a different output voltage if the gauge is connected to larger circuit. The most important parameter for any strain gauge is its gauge factor, which relates strain to changes in resistance. 10

12 ( ) A strain gauge is almost always incorporated into a Wheatstone bridge, which allows for differences in voltage to be readily observed. Figure on the left illustrates a basic schematic of a Wheatstone bridge. The voltages V EX and V o are related by the following equation. [ ] If one of the resistors is replaced by a strain-gauge (with a nominal resistor R G and straininduced change in resistance dr), the resulting schematic is named Quarter-bridge Circuit (illustrated in Figure below). Incorporating the definition of gauge factor and strain, the equation relating V EX and V o is slightly modified. ( ) A Half-Bridge circuit (Fig ) can also be implemented to account for compression as well tension. The additional gauge can be considered to have a strain induced resistance change of dr instead of dr. The relevant equation for this schematic is as follows: 11

13 The schematics and equations above do not account for the fact that wires and lead themselves have internal resistances and that these resistances are susceptible to temperature fluctuations. A three-wire connection (fig below) can be implemented to eliminate the effects of heat-dependent resistance fluctuations. Strain gauges typically require an excitation source (power supply; V EX ). Although V EX values vary from 3V to 10V, there is no industry set standard. The choice of reference resistors is vital in the ultimate sensitivity of the completed bridge. Remote sensing errors also need to be compensated especially if the bridges are far away from signal conditioners or power sources. Additional steps of filtering, amplification, and offset nulling can be carried out with the help of software Data Acquisition and Software In terms of data acquisition and software there are three main challenges to overcome. The first is controlling the Bose test bench system. The second is transmitting the data from the load cells to the computer. The third is manipulating and displaying the acquired data in NI Labview in real-time. The first problem is the most easily overcome. The Bose motor comes with software that allows it to be controlled from a personal computer. If this software is satisfactory to the client then the problem is solved, however if the client requires the motor to be controlled by Labview then it will be a much more formidable challenge. The second challenge is transmitting the data from our load cells to our computer. The client has requested that the data be sent via Bluetooth so a Bluetooth receiver and 12

14 transmitter will be necessary. Some additional pieces of equipment that are will be needed include a power supply, a microprocessor, and a NI Data Acquisition Board. The microprocessor will need to be programed to handle the custom demands of this project. Figure Data Acquisition Board Figure PIC Microprocessor The third problem is manipulating and displaying the data in real-time. This is important because the experiments can run for a period of several weeks. By displaying the data in real time a problem can be caught early, avoiding delays. To display the data a labview program will need to be written. It will most likely use the DAQ application of NI Labview to receive the data from a Data Acquisition board. The data will need be filtered, ideally before amplification. Some of the filtering may occur through the NI 13

15 Labview software while some may need to occur in the circuit before transmission. It will take trial and error to determine the best configuration. Because the data is likely to be very small voltages it may be necessary to amplify the data before sending it into NI Labview. This can be accomplished by using an operational amplifier to increase the voltage. Some experimentation will be necessary to determine the optimal gain. 3.0 Budget The initial budget proposed in this section is a rough estimate. It can be divided into three sections: budget allocated for the materials required for building the fixture itself, budget set aside for strain gauges and associated wiring/resistors, and finally budget specified for data acquisition components such as the Bluetooth wireless module and a PIC microprocessor. Fixture: Equipment Possible Vendors Estimated Cost Plexiglass or Acrylic Home Depot, Lowe s, Sears, $ Dimensions available: 3ft x 5ft 4ftx3ftx0.22in, etc Mansfield Supply Silicone Sealant Petco, WalMart, PetSmart etc. $10.00 Ti alloy hinges, screws, Home improvement stores, $30.00 washers, nuts, pins online Load Cells: Equipment Possible Vendors Estimated Cost Strain Gauges (15-20) Various online vendors $40 Amplifiers/Filters Various online vendors $20 Strain gauges, by themselves, are relatively cheap. Additional components include filtering and amplification devices. Data Acquisition, Wireless Components 14

16 Equipment Possible vendors Estimated Cost Bluetooth Module Various $60 PIC Microprocessor Various $10 Insulated Wires, Resistors various $10 Currently, there are no specifications listed for a temperature controlled bath system or a filtering system. These inclusions can be implemented if later budget assessments allow. The following devices, components or software are already available in the laboratory or can be borrowed from other labs or departments. National Instruments LabVIEW software Bose TestBench system NI Data Acquisition devices Computer consoles for monitoring data and controlling Bose TestBench Additional Wiring and Resistors Simulated Body Fluid can be synthesized with relative convenience Tissue samples can be provided for testing and troubleshooting 12V or 24V Power Supply for all the strain gauges Power supply for Bluetooth Module 4.0 Conclusion The main impetus for designing and building this device is to allow more accurate and useful tests to be performed simultaneously on multiple soft tissue samples. This will be achieved through specific design choices, such as the positioning of the load cells and the allowed range of motion of the movable base. Placing the load cells above the tissue attached to the stationary top section prevents the load cells from moving during testing. Keeping the load cells stationary will prevent any unwanted movement during testing. The load cells won t pick 15

17 up any unwanted forces that would be caused by this movement. This will keep the data as uncontaminated as possible. Allowing the base to be moveable within a large range of motion allows for the option of tensile, flexural, or both types of testing. This will increase the amount of data that can be gathered about both of these types of forces and how they each affect the tissue. The knowledge gained from this type of testing could eventually lead to improved designs of bioprosthetic heart valves and other tissue engineering endeavors. A better understanding of how tissue is affected by fatigue testing will help tissue engineers design valves that can withstand the forces applied while in use for much longer than the current bioprosthetic valve lifetime of ten to fifteen years. This device will eventually help improve the quality of life of patients receiving tissue derived organ replacements. 5.0 References [1] A.Mirnajafi et al., Effects of cyclic flexural fatigue on porcine bioprosthetic heart valve heterograft biomaterials, Dept. Bioengr., Univ. Pittsburg, Pittsburg, PA, Tech. Rep. Aug [2] T.L. Sellaro et al., Effects of collagen fiber orientation on the response of biologically derived soft tissue biomaterials to cyclic loading, Dept. Bioengr., Univ. Pittsburgh, Pittsburg, PA, Dept. Bioengr., Clemson Univ., Clemson, SC, Edwards Lifesciences, Irvine, CA, Tech. Rep. Apr [3] Amrani et al., In vitro tissue testing system, U.S. Patent 5,902,937, May 11, 1999 [4] Vesely et al., Tissue fatigue apparatus and system, U.S. Patent 2002/ A1, July 25, [5] Love et al., Device and method for testing tissue, U.S. Patent 2002/ A1, Jan. 31, [6] NI Developer Zone, (May, 2010). Measuring Strain with Strain Gages. [Online]. Available: [7] Bose Corporation-ElectroForce Systems Group, (2006). New Soft Tissue Fatigue Testing Technique Allows Researchers to Build Better Heart Valves. [Online]. Available: 16

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