Design and Validation of Creep Testing System for Hydrogels. Team: Megan Toth& Kelly Williams, Advisor: Glennys Mensing, Client: Weiyuan John Kao, University of Wisconsin-Madison, Dept. of Biomedical Engineering. mg T -kx Force Verification Figure: Graph of Force (lb.) vs. Displacement of the spring from 6 grams to 24 grams with 2 gram increments. Spring constant of lb/in, 1.2% error. Figure: 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. Vibration Control Figure: 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. 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 standards. 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. 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 as standards 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 Modify existing protocol and creep testing system to provide optimal experimental conditions. Test different formulations of IPNs in an 11 week study. 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 Figure: 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. 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 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 : The previous top grip has two components of force in the longitudinal direction. Figure : The current top grip has a single force in the longitudinal direction. LVDT Verification Measured (mm)LVDT (mm)% Error Average % ErrorSd of % Error 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. LVDT Light weight Durable Fits onto existing weight rack Counter weight compensation LVDT Must maintain constant weight Can apply weight slowly and evenly Must not enter LVDT core The Mold: Teflon® Reusable Mold Teflon® is inexpensive and molds are reusable Meets ASTM dimensions Easy to fabricate Figure 7: The Teflon® mold is sandwiched between glass slides to keep the thickness consistent. The IPN is fabricated between them. Marking Gauge Length At this step, food coloring is used to mark a set gauge length on the IPN The Samples IPN Samples:Legend: 4 Formulations Room Temperature PBS Solution pH 7.4 +/ G/60PEGdA G/60PEGdA 2k 50G/50PEGdA G/50PEGdA 2k G = Gelatin PEGdA = polyethylene glycol diacrylate 600 = 600 MW PEG 2k = 2k MW PEG All solution ratios are given in weight % All formulations are 10% Gelatin Creep Testing Figure 8: IPN Testing Results. The average test results are shown for each formulation tested with an n = 3. Tests were conducted for three hours in PBS at room temperature and pH of 7.4. Table 1: IPN Raw Testing Data. Data for all three trials of each of four formulations are given. This data corresponds to the graph in Figure. Add method to change temperature of chamber Vary stresses for each sample type Test formulations containing a modified gelatin backbone Test formulations containing modified PEGdA IPNs containing 600MW PEGdA under went more creep than 2k samples. Formulations containing 600MW underwent more creep at lower gelatin/PEGdA ratios The creep chamber is capable of testing various formulations of IPNs consistently Christy Palmer Kao Lab Glennys Mensing PhD WJ Kao PhD