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Tensile Testing: Effects of Melanin on Elastic Properties of Skin

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Tensile Testing: Effects of Melanin on Elastic Properties of Skin

Rahul Natesh

April 25, 2007

Melanin, the protein that colors the skin, is a polymer made of the monomer molecules indolequinone and dihydroxyindole carboxylic acid. Specialized cells called melanocytes located in the stratum basale of the epidermis secrete melanin. A recent study indicated that the carboxylic acid melanin precursors accumulate extracellularly (Chong et al.) Since growth factors and the extracellular matrix regulate the organization of the melanocyte cytoskeleton, it is suggested that they might also regulate precursor secretion and pigmentation through regulation of actin assembly that is involved in dendrite formation and melanosome movement (Abdel-Malek). Albinism is the genetic disorder in which an individual does not produce melanin. As such, an individual suffering from albinism may be susceptible to a weaker extracellular matrix since actin assembly and other critical functions may be affected. The goal of this experiment is to test if skin from an albino guinea pig would have differing tensile properties from skin from a pigmented guinea pig. When another experiment was conducted comparing the tensile properties of chicken skin, no significance in tensile properties was found due to large variances in the stiffness of the different skin samples (Appendix Table 1). Similar large variances could be present in the guinea pig skin that will be used in this experiment, and this could negatively affect the results.
Hypothesis and Objectives
As stated above, the primary objective of this experiment is to test for any differences in the tensile properties between albino and pigmented skin. Specifically, this experiment tests the forces required to rupture skin.
Hypothesis: The force required to rupture albino guinea pig skin will be significantly less than the force required to rupture pigmented skin.
1) Major Equipment

i. Instron Model 4444 benchtop materials testing machine.

2) Lab Equipment

i. Scalpel

ii. Cutting Board

3) Supplies

i. Scissors

ii. Calipers and Rulers

iii. Pieces of Foam (1/4 inch thick Confor Slow Recovery Polyurethane Foam)

4) New Purchased Materials

i. Albino Guinea Pig Skin (5 samples)

ii. Pigmented Guinea Pig Skin (5 samples)

The Instron machine will be used to stretch the skin samples till they rupture. Additionally, it will measure force applied and skin displacement throughout the trial. Initially, the pieces will be cut using the scalpel and cutting board. The scissors, calipers, and rulers are to trim and measure the final dimensions of the skin samples. The foam will be used as surrogate material, to become familiar with the Instron machine and the data acquisition process. The newly purchased items are required to conduct the experiment. Guinea pigs are regularly used as surrogate test subjects for humans in research. Moreover, the albino species is readily available for purchase at special research animal farms.
Proposed Methods & Analysis
The protocol is almost identical to the one conducted in Experiment 3 of the BE210 syllabus. Details about the set-up, calibrations, and modifications of the Instron machine can be found in the lab manual. There are two sample groups: albino skin and pigmented skin. There are 5 samples in each group. All samples will be subject to a crosshead speed of 100mm/min. This speed was chosen because it provided clear and concise data during the testing of the chicken skin. Also the size of the samples was set at 0.5in x 1.5in. This was chosen even though the clamp is 1in thick to allow for a sure grip across the skin. Trying to fit a hand-cut 1in skin sample in a 1in clamp can leave some edges hanging out. The groups will be compared statistically using a two-sample t-test. The type of two-sample t-test (Equal or Unequal Variance) will be determined using descriptive statistics on the sample group. A lower mean for albino skin rupture force coupled with a one-tail p-value of less than 0.05 would support the hypothesis that albino skin requires less force to rupture than pigmented skin.

  • Set up the Instron as indicated in the lab manual. (20 minutes)

  • Cut 5 pieces of 0.5 x 1.5 in. foam (to be used to become familiar with the protocol.)

  • Remove the skin from the guinea pig (Each group will have one albino and one pigmented guinea pig). Cut 5 pieces of skin of the same size as the foam surrogates from each guinea pig for a total of 10 skin samples per group. Store these samples in wet paper towels. (50 minutes)

  • Calculate the cross-sectional area of each sample using the calipers and ruler to measure its dimensions. Do this by placing all 5 skin samples together, measure it with the calipers, and then divide that value by 5 to find the average thickness. (15 minutes)

  • For each surrogate and skin sample, position the sample with the longer side vertical and the shorter side in the middle of the clamps so that an equal amount of the specimen is gripped on the top and the bottom. This is the no-load geometry. (5 minutes)

  • Apply all the samples (5 surrogate, 5 albino, 5 pigmented) using a 100mm/min crosshead speed. (90 minutes)

  • During loading and rupture, observe how the samples tear.

  • Using Matlab, plot a graph showing force vs. displacement and stress vs. strain for all skin and surrogate samples. The LabView software measured force and displacement during testing. Stress is calculated as force divided by cross-sectional area. Strain is calculated by dividing displacement values by gage length

  • Use a two-sample t-test assuming equal/unequal variance (determined by descriptive statistics) to compare the failure force between the two types of skin.

  • Calculate the stiffness and Young’s modulus using Matlab. Stiffness can be found using the best-fit line of the linear portion of the force-displacement curves. The linear portion of the graph is defined by the region in the first derivative of the best-fit line from the beginning till it starts decreasing.

  • Young’s Modulus was found in the same manner, only using stress-strain graphs.

  • Repeat the above two processes for all the skin samples.

Estimated time to run experiment (does not include running analysis): 3 hours.
Potential Pitfalls & Alternative Methods
A fallacy was discovered in the feasibility of the experiment as more research was conducted on the subject. The skin is made of several different layers as shown in Appendix Figure 1. The stratum basale is only a one-cell layer at the basement of the epidermis. Melanocytes make up about 10-25% of the total cells present in the stratum basale (SKIN). This means that melanocytes make up an even smaller fraction of epidermal cells overall. Hence, all the melanin produced may not be enough to affect the tensile properties of the skin.
An issue in the analysis portion of the experiment is that the group estimates the beginning and end of the linear potions of their force-displacement and stress-strain graphs. Without any scientific way of picking these points, this is a major source of inconsistency and error in the data. One way to modify this would be to take the derivative of the best-fit line, and stop including data when the derivative starts to decrease and eventually become negative. That eliminates the element of randomness that was involved before.
One more potential obstacle is possibility that there may not be enough skin on the guinea pig for the number of samples necessary. Due to financial constraints, smaller guinea pigs will be purchased (201-250 grams). This may or may not allow each group their allotted 5 pieces of skin. One solution to this would be to purchase retired-breeder guinea pigs. These are the guinea pigs that scientific farms used to breed the regular strains. They are cheaper in cost but their size varies. However the significant reduction in cost will allow each group to have two albino pigs and two pigmented pigs. Combined, there will definitely be enough skin for five pieces of each type.
The supplier is Charles River Laboratories, a leading provider of research animals for laboratories around the world.


Size (g)


Unit Cost

Total Cost

20 Hartley Guinea Pigs (Albino)





20 Hartley Guinea Pigs (Pigmented)







The back-up option would be to buy 40 of each type of pig, but they would be of the retired-breeder variety. The total cost for 80 of those pigs (40 albino and 40 pigmented) is $1636 @ a unit price of $20.45.


Trials (xhead speed in mm/min)











Foam 5






Foam 100






Skin 5











Skin 100






















Trials (xhead speed in mm/min)








Foam 5



Foam 100



Skin 5





Skin 100











Table 1. Stiffness values for specimen in each trial during Experiment 3 (Instron Tensile Testing: Structural and Material Properties of Chicken Skin).

Figure 1. Layers of the epidermis.

“Research Model Informational Resources: Hartley Guinea Pig References.” Charles River Laboratories. April 21, 2007. .
Chong Jin Loy, Siew Lan, Raman Govindarajan. “Probing Melanogenic Gene Expression in Keratinocytes” Asian Societies of Cosmetic Scientists.
Abdel-Malek, Zalfa. “Signaling Pathways in Pigment Regulation.” April 21, 2007. Newsletters/1997_3.htm
“SKIN.” April 21, 2007

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