April 28, 2005
Group T2B
Section 102
Brandon Chen
Stephen Cifelli
Robert Metter
Ek Kia Tan
Uniaxial Suture Strength Testing
A. BACKGROUND
In Experiment 5 (Imaging Techniques for Displacement Measurements), it was determined that sutures using the running locked stitch experienced less displacement per unit of applied force than those with the interrupted stitch (0.0015 mm/g vs. 0.0032 mm/g – see Appendix). However, the fabric being sutured was consistently observed to rupture before the stitches themselves failed. Therefore conclusions derived from the data were relevant to the suture system as a whole (i.e. fabric and stitchings), but were not representative of the thread material by itself.
The proposed experiment will alleviate this problem by eliminating the possibility of structural failure in the material being sutured. To this end, we will utilize rubber that possesses a much higher tensile strength (209 psi) than that of the suture thread (48 lbs.), which will cause the thread to fail before the rubber. Thus, we will obtain data that is independent of the material being sutured; failure force (the maximum force applied to the sample) will be solely dependent on the material properties of the string and the mechanics of the suture technique. Experiments 3 (Tensile Testing: Elastic Properties) and 5 will be effectively combined, as we propose to obtain tensile data with a uniaxial tensile test using the Instron machine for each suture sample. This will generate a nearly continuous set of force vs. displacement data rather than the isolated data points of Experiment 5. In that experiment, the force was increased by predetermined increments, which made it impossible to pinpoint the exact failure force. By using the Instron to make closely-spaced measurements while approaching the failure force, we will be able to measure failure force with greater accuracy through the force vs. displacement graph.
B. HYPOTHESIS AND SPECIFIC AIMS
Hypothesis
The running locked stitch will be able to withstand a significantly greater force than the interrupted stitch before rupturing.
Aim 1: Construction of suture samples
· Suture using (1) running locked and (2) interrupted stitch techniques
Aim 2: Characterization of suture samples
· Uniaxial tensile test using Instron
· Construction of Force vs. Displacement graph
· Determination of Failure Force (Maximum Applied Force) from Force vs. Displacement graph
C. EQUIPMENT
Major Equipment
· Instron Model 4444 – This machine is used to perform uniaxial tensile test on the suture samples because it can apply force at a uniform rate on the samples.
· LabView software loaded on a Pentium Personal Computer – Data from the Instron will be collected for the construction of a force vs. displacement plot. This would enable us to specify the maximum force on the plot for each sample.
Lab Equipment
· Weight Set (0.5 kg, 1.0 kg, and 2.0 kg) – The weights are used to verify the load cell transducer settings in the Instron.
Supplies
· Rubber (Ultra-Soft Polyurethane) – Two pieces of rubber (0.75 in. x 1.25 in.) will be sutured together to create a sample to be tested in the Instron. The tensile strength of the rubber (209 psi) is much larger than the tensile strength of the thread material (48 lb) used to suture the two pieces together. This would allow us to properly gauge the strength of the running locked stitch and the interrupted stitch as the thread material would rupture before the rubber. The rubber samples are sized to fit the dimensions of the jaw faces (mentioned below).
· Thread (Polished Cotton Fiber Rope, 0.058” in diameter) – Thread material used to suture the two rubber samples together. As mentioned above the tensile strength of the thread material is 48 lb.
· Scissors – Device used to cut rubber to its proper dimensions.
· Needles – Device used to thread stitches to combine the two pieces of rubber.
· Ruler – Device used to measure the dimensions of the rubber.
· Pencil – Device used to outline the dimensions of the rubber.
Newly Purchased Equipment
· Screw Side Action Grips – This new grip would be used to replace the one provided on the Instron machine in lab to ensure that that there is no sample slippage while performing the experiment. The Screw Side Action Grips have a rated capacity of 5 kN for samples with thicknesses of up to 0.6 inches. This capacity is more than sufficient to rupture both the rubber and thread before slippage as their tensile strengths are only 209 psi and 48 lb, respectively.
· Serrated Jaw Faces for Flats – This device is used in conjunction with the Screw Side Action Grips to provide a proper fit with the Instron and a tighter grip on the samples.
D. PROPOSED PROTOCOL AND METHODS
· Cut rubber into 20 pieces measuring 0.75 in. x 1.25 in. Suture 5 pairs on the short sides using the running locked stitch and the other 5 pairs using the interrupted stitch.
· Use 5 stitches for each sample with equal separation ~ 0.15 in.
· Calibrate the Instron machine by comparing the measured load value to the actual weight of the provided masses (0.5 kg, 1.0 kg, and 2.0 kg).
· Load the LabView Virtual Instrument “Instron.VI” software and set the crosshead speed to 60 mm/min and sampling rate to 5 samples/second.
· Place one of the running locked stitch samples in between the clamp and “Jog” the sample until it is taut and note the height between the clamps. This is defined as the no-load position.
· Rename the file to “Running 1” and using the Instron 4444, run a uniaxial tensile test on the sample to obtain force vs. displacement data.
· Upon rupture, make notes about the specimen, rupture position (whether the stitches break simultaneously or separately), or shape that is pertinent.
· Repeat for the other 4 running locked samples and also the 5 interrupted stitch samples. Rename the file after each trial to prevent losing the previous saved file.
· Analyze the graphs obtained to determine the maximum force that each sample could withstand. Load the data into Excel and use the maximum function (on the force data) to determine each suture’s maximum force.
· Perform a paired one-tailed t-test between the running locked stitch group and the interrupted stitch group to determine if there is a significant difference in the maximum suture strength and to determine which stitch is stronger.
· If there is significant difference between the suture strength of the running locked stitch and the interrupted stitch, then the p value of the one-tailed t-test would be less than 0.05. If the p value is larger than 0.05, no conclusion can be drawn.
E. ANTICIPATED RESULTS
The Instron machine will measure the load on each sutured rubber sample as it is displaced from its original position. From this data, a force-displacement curve can be constructed for each sample. There will be two groups of five samples each: one group sutured with an interrupted stitch and the other sutured with a running locked stitch. The data from each of the samples will be plotted as force vs. displacement. Figure 1 shows a sample graph for the interrupted stitch and Figure 2 shows a sample graph for the running locked stitch.
Figure 1: Force-displacement plot for rubber Figure 2: Force-displacement plot for rubber
samples sutured with an interrupted stitch. samples sutured with a running locked stitch.
From each graph, the maximum force applied to each sample can be determined, it is simply the peak force of the force-displacement plot. The maximum force will be determined for each sample using the “Max” function in Excel. Possible sample data for this is included in Table 1. A paired one-tailed t-test will be used to compare the maximum force between the interrupted stitch and the running locked stitch. A potential p value is in Table 1. It is expected that the running locked stitch will be able to withstand a significantly greater force than the interrupted stitch before rupturing, so a value of p<0.05 is anticipated.
Table 1: Maximum force values (with average and standard deviation) for rubber samples sutured with the interrupted stitch and the running locked stitch, and the p-value for their comparison.
F. POTENTIAL PITFALLS AND ALTERNATIVE METHODS
1. Suture Variability
Suture variability, as seen in Experiment 5, will be difficult to eliminate completely. There will be five stitches in every sample, as dictated by the protocol, but inconsistencies in distances between each suture, and the lengths and tightness of each suture is expected to have an impact on the resulting rupture forces. Specifically, variability in the running locked stitch could result in an uneven distribution of force on a certain part of the thread. For the interrupted stitch, these factors could cause one stitch to break before the others.
Ideally, all stitches should be evenly spaced, at an equal distance from the center, and have uniform tightness. In Experiment 5, the relative standard deviation for running locked stitch data was 10.02% and 18.40% for the interrupted stitch (see Appendix). With meticulous preparation and practice in stitching, the precision in the data can be improved upon greatly.
2. Testing materials before experiment
The materials used in this experiment were chosen for their specific properties, but some parts of the experimental procedure may still need adjustment. For example, an effective distance for the stitches from the end of the rubber piece must be determined so that the stitches do not tear the rubber before they break themselves. If the rubber tears, the stitches must be moved farther from the end of the rubber piece.
3. Analysis of rupture force
Each trial should be observed carefully at the sutures for any irregularities or patterns in relation to the force vs. displacement graph. These observations could help explain why a certain stitch technique is stronger than the other. The stitches could break all at once or one at a time. If a type of suture deviates from its normal rupture pattern, the sample could have been poorly stitched and may need to be discarded. The sample could also have been loaded unevenly, causing more force to be placed on one side of the stitch. Conducting one to two trial runs may help in determining the fashion in which each type of stitch is expected to break. Ideally, only samples for which the stitches break uniformly should be considered when comparing the strength of the stitch types.
G. BUDGET
Purchase: Ultra-Soft Polyurethane Rubber
Cost: $10.93 per sheet (2 sheets needed per group), total for 20 groups: $437.20
Supplier: McMaster-Carr (#8824T125)
Specifications: size – 6 inches x 4 inches x 0.06 inches (enough for twenty 1.25” x 0.75” pieces), tensile strength – 209 psi
Justification: The rubber samples must be strong enough to withstand more force than the thread used for the sutures. This rubber can withstand forces up to 209 psi, compared to the 48 pounds for the thread. The difference in strength is adequate for proceeding with tensile testing.
Purchase: Polished Cotton Fiber Rope
Cost: $5.15 per spool (1 spool needed per group), total for 20 groups: $103.00
Supplier: McMaster-Carr (#1931T51)
Specifications: length – 500 feet, diameter – 0.058 inches, tensile strength – 48 pounds
Justification: The thread must be strong enough to withstand some force, while weak enough to rupture before the rubber. This thread can withstand a force of up to 48 pounds, less than the rubber (209 psi), but high enough to perform an effective experiment.
Purchase: Screw Side Action Grips
Cost: $820.00 per pair
Supplier: Instron (#2710-205)
Specifications: rated capacity – 5 kN, specimen thickness – up to 0.6 inches
Justification: In Experiment Three, the rubber samples slipped out of the Instron grips before rupturing. These grips should be strong enough to hold the rubber, since the force applied by the grips (5 kN) is much higher than the force that will be needed for the stitching to rupture.
Purchase: Serrated Jaw Faces for Flats
Cost: $290.00 per pair
Supplier: Instron (#2702-141)
Specifications: size – 2 inches x 1.2 inches, specimen thickness – up to 0.6 inches
Justification: Jaw faces are required to use with the Screw Side Action Grips. These jaw faces are an appropriate size for the samples, and the serrated shape should be effective in holding the rubber tightly.
Total Cost: $1650.20
H. APPENDIX
Table 2: Slope of data points for each sample in displacement vs. load plots for fabric sutured with running locked stitch and interrupted stitch from Experiment 5, along with the average and standard deviation for each suture type. A paired t-test was used to compare the slope of the running locked stitch samples to that of the interrupted stitch samples. The p value indicates that the running locked stitch slopes (displacement/force) are significantly less than the interrupted stitch slopes.