1. Design and Validation of a Creep Testing System for Hydrogels Team: Christy Palmer, Megan Toth & Kelly Williams, Advisor: Glennys Mensing, Client: Weiyuan John Kao, University of Wisconsin-Madison, Dept. of Biomedical Engineering Abstract Interpenetrating networks (IPNs) have many biomedical applications such as wound healing. The efficacy of the IPN as a wound dressing is dependent on its mechanical properties. A creep testing system coupled with an environmental chamber was previously designed to evaluate the tensile creep properties of the IPNs. The goal of this project is to validate the pre-existing device following ASTM standard D2990 while modifying the device design and the test specimen fabrication protocol for improvements. A protocol was written to validate each component of the entire system. The protocol was followed to test three latex samples and three IPN samples. Background Interpenetrating Networks (IPNs) –Composed of cross-linked gelatin and poly(ethylene glycol) diacrylate –Used as a wound dressing Motivation Creep testing of IPNs: –Provides mechanical strength characteristics –Mimics tensile stress on IPNs due to wound contraction Problem Statement Complete design of tensile creep testing system coupled with an environmental chamber Establish protocol to make and test IPN samples Validate creep testing system following American Society of Testing Materials (ASTM) guidelines Design Specifications Temperature: +/- 2 o C pH: constant pH Force: +/- 1% of applied load Specimen Dimensions: – 11 mm gage length – 2 mm gage width – 1 mm thickness Grips Previously:Currently: Bottom and top grip clamp surfaces were sanded and lined with adhesive rubber and sand paper to minimize slippage of samples. Figure 5: The previous top grip has two components of force in the longitudinal direction. Figure 6: The current top grip has a single force in the longitudinal direction. LVDT Verification Measured (mm)LVDT (mm)% Error 5.35.3140.264 19.319.3140.070 33.733.5210.530 49.149.0610.079 72.472.370.041 Average % Error Sd of % Error 0.1970.210 To verify the extensometer, the LVDT core was marked and measured manually at 6 distances, while being held in that position. These measurements were compared with the output from the LVDT in Microsoft Excel from the WinDaq software. Weight AddedTime Lasted 14g5hr. 19min. 12g*4min. 13sec. 16g*16min. 12sec. 12g*11min. 23sec. Weight AddedTime Lasted 14g**5min. 10g2hrs. 7g1hr. 5g1hr. Tests in Air: Tests in PBS: *Weight added incrementally **Sample tested in DI water Weight Ranges Tested The IPNs were tested at different weight ranges. Initially tests were performed in air, and then tests were performed in a PBS solution. Future Work Develop a method to accurately measure the original length Mark the gage length on the samples to ensure consistency Vary stresses for each sample type Change sampling rate Fabricate and test more latex and IPN samples Develop a fabrication method for polyurethane samples Add method to change temperature of chamber Fabricate and test PEG 2K Epon Master and PDMS Stencil Teflon® Mold and IPN Samples Figure 7: The epon master is made on a silicone wafer and PDMS is poured over the mold. The stencil is then used to make the IPN samples. Figure 8: The Teflon® mold is sandwiched between glass slides to keep the thickness consistent. The IPN is fabricated between them. The Mold Previously: Epon Master and PDMS Stencil Did not meet ASTM dimensions Difficult to fabricate Expensive to make and maintain molds Currently: Teflon® Reusable Mold Meets ASTM dimensions Easy to fabricate Teflon® is inexpensive and molds are reusable The Samples IPN Samples: Formulation was modified to produce robust samples Used 600MW PEG and 15% gelatin Temperature was 60C for fabrication Samples were conditioned for 4.5hours Testing done at multiple weights Repeated testing done at 5grams for 3hours Latex Samples: Liquid latex was used to test chamber consistency Samples were fabricated in Teflon® molds Samples were assumed to be of constant age Repeated testing was done at 5grams for 3hours Latex Testing Results Figure 10: Latex samples were fabricated and aged for 24hours. Samples were then tested at 5grams for 3hours in PBS. Two out of three samples show highly consistent results Sample fabrication method is suspect for error of results Due to measurement error in the value of the original length, testing should be repeated Figure 11: The graphs shown represent the tensile creep testing results for three latex samples tested for three hours with 5 grams. Change in length in mm for three decades of time is shown. IPN Testing Results Future Work Develop a method to accurately measure the original length in order to calculate strain Vary stresses for each sample type Develop a fabrication method for polyurethane samples Add method to change temperature of chamber Fabricate and test PEG 2k Figure 9: The graphs shown represent the tensile creep testing results for three IPN samples tested for three hours with 5 grams. Change in length in mm for three decades of time is shown. Protocol A protocol was created for the validation of the creep testing system to provide a consistent method for the testing of materials, following ASTM standard D2990. The protocol is divided into manageable sections including Test Specimen Dimensions, Test Specimen Fabrication, Grip Maintenance, Force Calibration, Vibration Control, Data Collection, and Reporting of Data as seen in greater detail below. Figure 1: Image of the current design. The current design is composed of: an acrylic chamber, a pair of grips, counterbalance, granite block, a weight rack, an extensometer (LVDT), an analog to digital converter, and a computer for data acquisitioning. Testing All 3 samples show highly consistent results Due to measurement error in the value of the original length, testing should be repeated Counterweight Previously: Aluminum Cylinder Did not account for the buoyant force of the top grip Currently: Lead Slip Shots Accounts for the buoyant force of the top grip Mass is evenly distributed along wire mg T -kx Figure 3: Force body diagram of the creep testing system while undergoing force verification with the spring. Hooke’s law, F = -kx was used to determine the spring constant, k. Figure 2: Graph of Force (lb.) vs. Displacement of the spring from 6 grams to 24 grams with 2 gram increments. Spring constant of 0.0348 lb/in, 1.8% error. Force Verification Force was verified by comparing the k measured by the creep testing system to the actual k of the spring. Vibration Control Figure 4: Picture of the creep testing system with the granite block beneath it. A 12”x12”x4” granite block polished on both sides was purchased for absorbing bench-top vibrations.