Instron Uniaxial Tensile Testing

Instron Uniaxial Tensile Testing:

Mechanical Failure Properties of Suturing and Steri-StripTM

BE 210

Final Project Proposal

Date Due: 4/28/06

Group number: 101B5

Ricky Chung, Aron Gera, Seungyon Kim, Raina Wallace

I. Background

In experiment 3, Instron Uniaxial Tensile Testing, an Instron 4444 was used to test the tensile properties of either chicken skin or a blue foam material. Under the application of uniaxial load, the structural body shows deformation until it ruptures. In experiment 3, it was shown that blue foam has significantly stronger properties than chicken skin, based upon ultimate strength, failure force, failure displacement, and stiffness. This builds a base for this experiment, which involves using the Instron 4444 for testing and distinguishing the adhesive properties of suturing and steri-strip application. Because this experiment is not testing for material properties, only failure displacement and ultimate strength are adequate measures. Failure displacement measures the elasticity of the individual adhesion methods, and ultimate strength will be used to distinguish their ability to withstand tensile forces. Both of these properties are important when treating a wounded patient and thus this lab has very real-world applications that will give the lab participants an understanding of the basic mechanics behind wound treatment.

Performance of surgical suturing techniques and application of Steri-StripTM are both formulated to optimize a number of mechanical outcomes, including gap-width of a wound and strength resisting either a load or wound separation under load. Thus it is necessary to distinguish between these adhesive methods for effective medical decision making. For a general background on the materials, the Steri-StripTM is an adhesive, sterile, and flexible wound closure strip produced by 3M Health Care Professionals, and is appropriate for low tension wounds. From experiment 5, Imaging Techniques for Displacement Measurements, it was concluded that double-sided suturing is statistically stronger than single sided stitching. Therefore, the double-sided suturing technique is used to be compared with the Steri-StripTM,a professional medical care product. Also, the InstronTM Model 4444 table-top mechanical testing machine is used to apply uniaxial force to the chicken skin samples.

II. Objectives & Hypothesis

1. Objectives

•To determine the two sample’s unique ability to withstand tensile forces by observing ultimate strength data.

•To determine the relative elasticity of both adhesion methods through observing average failure displacement.

•Ultimately, the tensile and elastic properties of the two adhesive methods will depict the unique adhesion applications of the two methods. For example, sometimes elasticity is more important that ability to withstand forces (perhaps if wound is located at site of little load application).

2. Hypothesis

•Under the load application, the skin samples with Steri-StripTM will have a significantly higherultimate strength than the samples with a suture technique.

•The skin samples with double sided stitching will have a significantly higher average failure displacement.

III. Equipment

Major equipment:

• Instron model 4444

The Instron Model 4444 uses force versus displacement to accurately measure the ultimate strength and failure displacement on the chicken skin that is repaired with sutures and Steri-strips.

Lab Equipment:

• Scalpel, Scissors, cutting board
• Calipers & rulers
• Weight set (500g, 1kg, 2kg)

The scalpel, scissors, & cutting board are used to remove the chicken skin from the bones. The caliper & rulers ensure that the suturing method and placement of the Steri-Strips is uniform. The weight set is used to calibrate the Intsron 4444.

Supplies:

• Paper towels
• Trash
• Soap

The supplies will be used for clean-up and to avoid messes.

Newly Purchased Equipment:

• Chicken legs
• Staedtler Engineering Computation Pad, 5 X 5 squares per inch
• Sterile sutures
• Steri-Strips

The sutures & Steri-Strips are of medical grade and will be used on chicken skin to provide a realistic model of the strength of wound healing products. The computational pad is used to assist in measuring and accurately sewing the sutures. The chicken skin will remain on the bones to prevent drying and brittleness.

III. Proposed Protocol and Methods

1. Sample Harvesting & Preparation

•Remove the skin from the chicken thighs and cut the chicken skin pieces into 10 pairs (20 total pieces) of 1in. by 1in. rectangle samples. These pieces are specifically sized to ensure that the mechanical properties of the suturing/taping are being tested instead of the properties of the chicken skin.

•Place the cut skin (in pairs) on wet paper towels to prevent them from drying out and becoming brittle.

•Take a sheet of the grid paper and cut them into 2in. by 2in. squares. This will be used to ensure uniform suturing of chicken skin samples.

•Place a pair of chicken skins so that the touching edges meet along the middle line. Suture the skins together with the double sided method by using the first line away from the middle. The needle should go through the intersecting grid lines, for a total of 4 holes on each piece of skin. Repeat for 4 other samples.

•Cut the Steri-strips so they are half the original length (~37.5mm). The rest of the chicken skins (10) are “taped” together in pairs using 4 evenly spaced Steri-strips for each sample. Once again, the edges of the chicken skin should be as close as possible to reduce the variance in displacement measurements.

2. Instron Set-up and Specimen Tensile Testing

•Follow part B of the protocol in Exp #3: Tensile Testing of Chicken Skin to calibrate the Instron 4444 machine.

•Set the speed of the crosshead and the direction of movement of the Instron machine to 60mm/min and upward, respectively.

•Run the Instron machine with the 5 sutured skin samples and the 5 Steri-strip samples, following part C of the protocol in Exp. #3. Before running each trial, tabulate the initial geometric properties (width and depth to find cross-sectional area) of the sample. Record the displacement of an edge of the sample according to the force loaded for each trial.

•Plot the force-displacement curves and tabulate the mechanical failure properties (ultimate strength and failure displacement) for both sutured sample tests and Steri-strip sample tests. The ultimate strength is the maximum load placed on the sample while the failure displacement is where the sample ruptured.

•To determine if the failure properties for suturing is statistically different from the failure properties for Steri-strips, perform a t-test for both ultimate strength and failure displacement.

V. Anticipated Results

This lab uses the Instron 4444 to gather data in the form of Force vs. Displacement plots. From the plots of the individual trials, it is possible to determine the ultimate strength and failure displacement of the material in each trial. Because ultimate strength represents the object’s ability to resist tensile forces, it can be used as the performance criterion to compare the resilience to stretch of the two adhering methods. The failure displacement will be used to calculate the elasticity of the adhesion method. The results of the data from the ten trials of each set of samples should resemble the curves in Figure 1.

Figure 1: Force vs. Displacement Curve for Chicken Skin from Experiment 3

The ultimate strength can be obtained by determining the highest force value for each plot and the failure displacement can be obtained by determining the distance at which the specimen ruptured. Due to the non-uniform nature of chicken skin, its failure properties will already have an innate variance. Depending on how well each group is able to stitch the chicken skins together for each sample, it is expected that the variance will decrease.

After the distinct adhering method’s average ultimate strength (and standard deviation) is determined, a t-test must be performed to establish whether the data for the two samples is significantly different. It is postulated that the Steri-strips will have a higher ultimate strength, representing their better ability to withstand tensile stresses.

The measurements of failure displacement and ultimate strength will be an indication of the unique adhesive properties of the specimen. It is expected that the suturing will have a higher failure displacement than the Steri-strips. Therefore, since the suturing is expected to have a higher failure displacement but lower ultimate strength, the average slope of the linear part of the curve is expected to be smaller than for Steri-strips, which is expected to have a higher ultimate strength but smaller failure displacement.

VI. Potential Pitfalls

There are uncertainties that could cause this laboratory to go awry. The biggest uncertainty is whether the chicken samples, when places in the Instron 4444 machine, will actually rip at the site of their adhesion. It is a possibility that the chicken skin samples would rip at other sites, and thus any measurements yielded from this data would not represent the properties of the steri-stripsor sutures. To remedy this problem if it is to occur, the laboratory group should use less steri-stripor suture intervals for the interface between the two adjoining halves. This will make the adhesion site weaker and thus more likely to tear at this point. Also, the group might try lowering the crosshead speed to ensure more consistent results.

A similar problem is that due to the gelatinous nature of the chicken skin, there is a possibility that the Steri-StripTMwill not optimally adjoin the two chicken skin samples. To cope with this problem, it will be necessary for the group to dry the chicken skin samples with paper towel. For consistency, the group should also attempt to make the chicken skin of the same relative dampness for the suturing samples. Because the lab is measuring the adhesive properties and not the material properties of the chicken skin, drying the skin should not misrepresent the results in any way (assuming the load application is fine).

Another problem may be that the suturing of the skin samples is too time-consuming given the six hour window. The chicken skin samples may be very hard to control for such fine work, and thus might require ample time. This again can be dealt with the application of less suture intervals for the interface between the two chicken skin halves. This can greatly cut down on the time required for the chicken skin suturing.

VII. Budget

The following table lists the various supplies with their specifications, supplier, and cost for all of the necessary purchases to conduct this laboratory.

Supplies / Specifications / Supplier / Cost
Chicken Legs / thighs, with skin / Fresh Grocer / $71.20 for $0.89 per lb
Computational Pad / 5 X 5 squares/inch, 8.5” X 11” / Office Depot / $17.97 for packs of 100 sheets
Sterile sutures / Black nylon, non-absorbable, size 5-0 / Safety Central / $1250 for single needle & suture packages
Steri-Strips / 3mm X 75mm / 3M United States / $200 for packs of 5
Total: / $1,539.17

Justification:

The newly purchased items are significant to the nature of the experiment. The chicken legs provide the skin needed to test the hypothesis. The chicken thighs are cheaper than drumsticks and the skin remains flat instead of curved. The computational pad provides a measuring tool to assist with suturing. The chicken can be sutured with the paper to provide a precise grid, then it can be removed once the suturing is done. The sterile sutures and Steri-Strips are both actual products used in wound healing. The non-absorbable and 5-0 size for sutures are characteristic of sutures used on delicate skin, such as the face. The Steri-Strips are regularly used in the place of sutures and staples. These products of medical grade will provide a realistic comparison of the effectiveness of wound treatment products.