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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 higher ultimate 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 SteriStrips 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.
Chicken Skin - F vs. Displacement
Force (N)
40
trial 1
35
trial 2
30
trial 3
25
trial 4
trial 5
20
trial 6
15
trial 7
10
trial 8
5
trial 9
trial 10
0
0
10
20
30
40
Displacement (mm)
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-strips or sutures. To remedy this problem if it is to
occur, the laboratory group should use less steri-strip or 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-StripTM will 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
Chicken Legs
Specifications
thighs, with skin
Supplier
Fresh Grocer
Computational Pad
5 X 5 squares/inch,
8.5” X 11”
Black nylon, nonabsorbable, size 5-0
Office Depot
3mm X 75mm
3M United States
Total:
Sterile sutures
Steri-Strips
Safety Central
Cost
$71.20 for $0.89 per
lb
$17.97 for packs of
100 sheets
$1250 for single
needle & suture
packages
$200 for packs of 5
$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 nonabsorbable 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.