Results - Penn Engineering



Acid-Base Nylon Suture Deformation

Group 101 A-2

Kimmel, Lin, Rampuria, Wang

Final Project Report

Due: 28th April, 2006.

Background

The objective of this project is to test the feasibility of nylon as a suture material in different environments. While many surgeons today use metal or dissolvable staples, stitched sutures are still used as complements to the staples, and in many cases in place of the staples. However, since environmental conditions will vary in different tissues of the body, it is desirable to know how different aqueous environments will affect the mechanical properties of the suture material. For example, the stomach has a pH of 2, whereas the small intestine lumen pH is 9. This particular project focuses on the suture material nylon. Nylon is a polymer made up of a repeating dimer of adipic acid combined with Hexamethylene diamine, forming the poly-amide nylon. Because Nylon is a poly-amide, the polymer’s amide loci are vulnerable to acid hydrolysis, and can break up in strong acidic solution under the mechanism shown in Figure A1. Amide hydrolysis is also possible with an extremely strong basic environment, but such a potent basic environment is not found in the body.

In BE210 experiment 5, it was shown that the relative strengths of two sutures can be determined by comparing suture deformations in terms of displacement while in a loaded configuration. Expanding on the protocol of experiment 5 and incorporating acid base theory of experiment 2, the nylon suture mechanical properties can be tested in aqueous environments. The goal of experiment 5 was to test imaging as a means of obtaining mechanical measurements of sutures. This project applies the image analysis techniques of experiment 5 in the more dynamic setting of an aqueous environment.

Hypothesis, Goals and Aims

The aim of this project is to measure relative mechanical strengths of nylon in acidic, basic, and neutral environments. A sub-aim for the experiment is to test the suitability of the novel environmental testing chamber created for this experiment, as a manageable mechanical testing chamber. These aims lead to the over-arching goal of determining the usability of nylon as a suture in different parts of the body that may have caustic environments.

The hypothesis of the project is: given the chemical structure of the poly-amide polymer nylon, nylon sutures will be stronger in a basic or neutral environment, than it will be in an acidic environment (pH = 2). Since the basic environment simulated in the experiment is only pH = 9, there is no predicted relative difference in the deformation of the sutures in the basic environment with respect to the neutral environment. The relative strengths of the sutures will be measured as they were in experiment 5, by comparing suture deformations under strain.

Equipment

Major Equipment

• pH meter

• 10-gallon clear fish tank

• Underwater Webcam

• BE lab computer with Image analysis software

The pH meter is needed to make a solution with the desired pH, and also to continuously monitor the pH throughout the experiment, to make sure it is consistent. The computer software will be used to capture and analyze pictures for data.

Lab Equipment

• Metal hooks to hang clamps with wound sample on metal ruler.

• Clamps for wound surrogates used in Experiment 5

• 1kg and 2kg weights to be loaded on wound sample with clamps and metal hook.

• 12” aluminum rulers (5) to be used as support for hanging weights and sample.

• Wooden Block as camera stand.

• Permanent marker

• Thick rubber gloves

• Safety Goggles

The metal hooks will be used to hang the clamp on to the metal ruler bridge. These clamps will hold the nylon suture sample, and using another clamp, the 1kg and 2kg weights will be hung onto the nylon suture to cause deformation. The permanent marker will be used to make reference markings on the sample to analyze deformation. The wooden block will be used to place the camera inside the tank, and to bring it to the same height as that of the nylon sample, to help focus the images. Thick rubber gloves and safety goggles need to be worn at all times, since strong acid and alkali solutions will be handled.

Supplies

• Mesh material from Experiment 5, which will be cut into pieces measuring 6cm X 2.5cm to be tested as the surrogate wound.

• Nylon string from Experiment 5, which will be used to create the suture.

Newly purchased equipment

• CIC-220 640 X 480 Underwater Digital Webcam to take underwater pictures of sample deformation.

• 10-gallon clear Fish Tank (20“L x 10"W x 12"H)

• HCl concentrate, 10N, 1L (Ricca Chemical HCl)

• Sodium Hydroxide Concentrate, 0.1N, 1L

The underwater digital web camera is water-proof. It will be submerged inside the fish tank, to take pictures of the deformation of the suture sample. It is a webcam and will not need to be manually “clicked” to take pictures, but can be connected to a computer program to take pictures. This is also essential for this experiment, as it will increase precision, and reduce the need to keep hands inside the acidic/basic solutions.

Experimental Protocol

Preparing Suture samples

• Make 15 samples of wound surrogate with BE 210 lab mesh, each measuring 6cm by 2.5cm.

• Cut the surrogates laterally in half, creating two 3cm by 2.5cm pieces.

• Use Nylon string to make N-wave stitch sutures across the cut. The N-wave stitch, as the name suggests, drops down diagonally to the next-stitch hole, after the first vertical movement. Ensure that the stitch-holes are evenly spaced.

• Draw "+" markings 1cm above and below the stitches on the two halves of the sample. These are to be used as references for measurements of vertical deformation.

• Ensure consistency between samples in both geometry as well as markings, for statistically stronger results, when preparing the samples.

Preparing experimental solutions

• Wear gloves and goggles at all times when dealing with acid and base, and use different gloves for the acid and base environments.

• Prepare a pH = 2 solution using 10N HCl concentrate and tap water. Mix with a ratio of 35mL of 10N HCl to 35L of water.

• Prepare a pH = 9 solution using 10N NaOH concentrate and tap water. Mix with a ratio of 3.5mL of 0.1N NaOH to 35L of water.

• For the pH = 7 solution, use tap water.

Testing the pH effect

• For the 5 trials of each pH, fill the tank with solutions of pH = 2, 7, and 9.

• Standardize pH Meter with the pH = 4, 7, and 10 standard solutions before each trial. Ensure the standard solutions are from fresh stock.

• Position the pH meter in the tank, and record pH at 30 second intervals. Find average at end of trial.

• Position the wound surrogate and CIC-220 underwater camera in the 10 gallon tank, so the camera correctly captures an image of the suture hanging from the metal ruler support bridge. Begin timing once the sample is fully submerged.

• Set up the camera program, and take 3 sample shots of a calibration ruler, positioned next to the surrogate sample, for calibration of camera image dimensions. This is used to determine the correlation between image pixels and centimeters, vertically.

• Without disturbing the setup or the camera settings, take a picture of the sample and record vertical locations of the “+” markings.

• Calibrate the 1kg and 2kg weights with a spring scale inside the filled tank

• Load the surrogate sample at the 7 minute mark with the 1kg weight, and save picture when water settles. From pixel-centimeter scales established earlier, record vertical deformation measurements. Record measurements 3 separate times to determine precision.

• Load the sample with 2kg weight at the 15 minute mark, and save picture to make similar measurements.

• Ensure consistency between samples in terms of loading and measurements.

Comparison of results

• Perform two-sample T-tests assuming unequal variance comparing deformations for pH 2 to pH 7, pH 2 to 9, and pH 7 to 9. Make comparisons for vertical deformations at 1kg and 2kg.

• Determine precision of the measurements and report.

Results

The images taken of the loaded specimens are expected to look similar to the images analyzed in experiment 5 of BE210 as shown in Figures A2a and A2b. From these images the distances between the pen marked lines is expected to yield the deformation data for the suture.

It is expected that the deformation will increase when the load is increased from 1kg to 2kg, similar to what happened in experiment 5, when the load was increased from 0.5kg to 1kg. In addition, as presented in the hypothesis, the acidic environment trials will show greater deformation than the neutral or basic environment trials. It is expected that deformation measurements will show no significant difference between the basic environment and the neutral environment. Simulated results based on the results of experiment 5 are shown in Tables A3 and A4. Table A3 shows possible average and standard deviation data that the experiment can provide, and Table A4 shows the possible strain data for the sutures in the different environments. One such graph that would be constructed in analysis of data is Figure 1 shown below.

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Figure 1. Plot of average normal strains versus weight for the three environments with standard deviation as error bars.

From Figure 1, a number of suture properties can be deduced. Regression analysis of the plot will show whether or not there is a linear relationship between strain and increase in weight. In addition, the relative strengths of the sutures are depicted through the relative strains plotted in the graph. The greater the strain, the weaker the suture is. From above, the figure shows the acidic environment suture is expected to yield higher strains than the sutures in basic and neutral environments.

To compare the data sets, multiple two sample t-tests assuming unequal variances will be performed. First, a two-tailed t-test will be performed between the basic and neutral environment data sets, expecting a p-value > 0.4, to confirm that the alternate hypothesis that there is significant difference between the sutures in the two environments is not supported. Then a one-tailed t-test will be performed between the acidic and neutral data sets, expecting a p-value < 0.01, to support the hypothesis that the deformation in the acidic environment is statistically different from in the neutral environment. Then a similar one-tailed t-test will be performed between the deformation in the acidic and basic environments, expecting a p-value < 0.01, to support the hypothesis that deformation in the respective environments are statistically different.

The expected p-values from the one-tailed t-tests above support the hypothesis that the deformation in the acidic environment will be greater than in the other two; and in turn support the final result that the nylon suture is stronger in basic and neutral environments than it is in an acidic environment.

Potential Pitfalls & Alternative Methods/Analysis

The principal risk in this experiment is that of the nylon sutures breaking under the weight or possibly dissolving, which would yield no data. In the unlikely event of this happening, the weights should be decreased to prevent mechanical suture failure. There is also a drawback that there is a risk of distortion of the image in the solution because the medium is a dense fluid. However, because the solution should be of uniform density, a webcam submerged in the solution should correct for most of the distortion of the image in the water. In addition, it may be assumed that the water, being uniform, will distort all samples equally. The primary concern, however, is that since the camera is submerged underwater, the field of view of the camera is decreased, so particular care must be taken to ensure the suture remains in the analyzable view of the camera.

Another problem may be the resolution of the image; in the previous labs, there were some issues with the precision of the image calibration, leading to uncertainty over whether especially small deformations are actual deformations or just variations in the image analysis. All deformations must be checked against the precision of the image to ensure the deformation is valid.

Another drawback to the experimental setup is that there is numerous interaction required with each of the samples. Each sample must be loaded manually with both the lighter and heavier weights underwater. In doing so, there is a risk of moving the camera position as well as the suture position. As stated above, since the camera has a reduced field of view, keeping the camera in a stationary position throughout the experiment is extremely important. In addition, the interaction required will probably take a great deal of time; however, the large size of the fish tank will allow the testing of multiple samples in parallel. This should allow the experiment to be performed in the allotted six hour time frame.

Because this lab primarily depends on the quality of image yielded from the webcam, it is crucial that these images be sharp and clear to accurately determine deformation. In the event that the image data is blurry or out of focus due to the water, an image enhancement program may have to be applied to sharpen the image to yield the deformation. In the event that this is not possible, and the images taken are still blurry, the experimental protocol may have to be modified. Rather than taking the images underwater, the sutures may just be left to soak and deform for a set time, as stated above, in the solution before being pulled out and photographed in air (with a similar apparatus to that of Experiment 5) to accurately determine deformations. Another possibility would be to photograph the suture deformation through the glass of the tank. This would increase the refraction and distortion in the images, especially while photographing from the outside, through the glass and into the solution. If properly recalibrated, the image analysis virtual instrument could compensate for this. By putting a ruler at the same point as the sutures, the distortion of the glass and water can be quantified. While these methods are not ideal, they ensure that in the event of lack of high quality data, there will still be some data that will be available for analysis.

Budget and Justification

The major purchases necessary for this experiment are:

• Underwater Digital Web Camera

o Model CIC-220 (640 X 480 pixels)

o Price: $80 from

The underwater digital webcam has the same image resolution as the camera present in the lab, but this one is uniquely required in this experiment since it is water-proof. The camera will be immersed in solution, and since this is a webcam, it can take pictures while connected to the computer, without manually ‘clicking’ to take the picture.

• 10-gallon clear Fish Tank

o Measurements 20“L x 10"W x 12"H

o Price: $13 from

The large size of this clear tank assists setting up of camera and nylon sample inside, while leaving enough distance between camera and sample to allow for focusing of image.

• Staedtler Fairgate 12" Combination Aluminum Ruler

o Price: $15.75 – 5 rulers @ $3.15 each

o Available at

A stack of 5 of these rulers can be taped together, and placed across the width of the top of the tank, which is 10” wide. Using this support to hang the sample makes the need of expensive metal bars redundant.

• Hydrochloric Acid Concentrate

o 1L bottle, 10N strength

o Price: $30 from Ricca Chemical HCl

o Available through Fisher Scientific

• Sodium Hydroxide Concentrate

o 1L bottle, 0.1N strength

o Price: $18 from Fisher Scientific

Strong acid and base concentrates are required to dilute to 35 liters of acidic and basic solutions that will be needed to fill up the fish tank. 1L each of 10N HCl and 0.1N NaOH is enough for this experiment to be repeated up to 20 times.

The total cost for the experiment will only be $156.75 of the $2000 allotment.

Appendix

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Figure A1. Mechanism for the acid hydrolysis of an amide.[1]

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Figure A2a. Before loading weight Figure A2b. After loading 1 kg weight

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Table A3. Estimated experimental data for vertical suture deformation.

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Table A4. Strain calculations from estimated deformation data.

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