TENSILE TESTING: Elastic Properties of Chicken and Other Skin Types

Rachel Howitt

BE210 Final Project

Submitted April 25, 2007

BACKGROUND

Mechanical properties of skin differ based upon the particular location and organism in which the skin is found. It is important to knowhave accurate values of elastic (Young’s) modulus forforpurposes of closing skin wounds by suturing, skin grafts, skin flaps, and other methods in orderfor recovery to be most successful.[1]skin for purposes of choosing suture stitch, among other reasons. If sutures and skin are characterized by drastically different elastic moduli, further injury may be causedthe modulus of the suture is much higher than the modulus of skin, tension may cause formation of a hypertrophic (abnormally raised) scar.[2]

Skin from different body parts differs in anatomical structure. The dermis is the layer which contributes most of the skin’s mechanical properties, and is specialized for various tasks: Ffor example, the highest concentration of nerve fibers is found at the fingertips, and the dermis layer is thickest at the palms and soles of feet.[3] As such, skin should possess different values for Young’s modulus at these different locations.

Previously, the difference in Young’s moduli along different directions (long axis vs. along circumference) of chicken legskin was investigated. Young’s modulus was found to range from 1.67 to 7.88 MPa, depending on the specific sample and direction tested.[4] In a study performed in the BE210 laboratory, tensile axis orientation was not found to contribute significantly to differences in modulus, although other studies indicate such significant differences.[5] Since difference according to location was not previously investigated in the undergraduate laboratory previously, it is fitting to analyze the issue now.

HYPOTHESIS / OBJECTIVE & AIMS

The purpose of this experiment is to determine and compare elastic properties of varying skin types from a single organism, namely, chicken. Young’s (elastic) modulus of skin from legs, wings,and breast will be determined using an Instron 4444 testing machine. The proposed hypothesis is that Young’s modulus of chicken skin at each of the three locations will be significantly different (p < 0.05) from one another. If significant differences are found, it will also be possible to order the skin based from least to greatest modulus.

This experiment will impart knowledge of Instron usage, and determination of stress and strain according to given equations from force and displacement. Educational goals include ability to apply statistical operations such as ANOVA and t-tests. The experiment will be performed using chicken skin since it is easily obtained and approved for use by undergraduates. However, students should also gain awareness of how (and if) skin’s mechanical properties differ between organisms, in addition to differing by location.

EQUIPMENT

major equipment

Instron 4444 benchtop materials testing machine

This instrument will provide measurement of force and displacement, from which Young’s modulus can be calculated.

lab equipment

caliper

ruler

These measurement tools will be used to determine dimensions of the skin samples being tested.

supplies

scalpel

scissors

cutting board

latex gloves[RH1]

paper towels

Supplies are necessary to remove chicken skin from meat, then to cut to skin to appropriate sizes for testing in Instron. Paper towels are used to line Instron clamps to prevent slipping. Gloves are convenient for students who are too squeamish to touch chicken barehanded.

newly purchased equipment

5 chicken legs

5 chicken wings

5 chicken breasts

The necessary meat samples are to be purchased at a nearby grocery store. Skins will be removed, and their elastic properties will be determined. Prices provided in Budget section.

PROPOSED METHODS & ANALYSIS

Refer to BE210 Laboratory Handout, Spring 2007, Tensile Testing: Elastic Properties, p. 5-7 for general procedure. Modifications are provided below.

A. Specimen Harvest and Preparation (15 min.)

  • Samples should be cut into 1.5 × 6 cm pieces
  • Two samplesshould be obtained from each piece of chicken. Three pieces may be obtained from the breasts if pieces are large enough.
  • Ensure that all samples are cut in the same orientation from any one type of chicken skin. Record the orientation (e.g. chicken legs’ 6 cm side along the vertical direction, had the chicken been standing).

B. Instron Set-Up and Surrogate Tensile Testing

  • There is no necessity to use a surrogate in this experiment, since previous research has allowed establishment of protocol.
  • Instron will be run at 25 mm/min with a sampling rate of 20 points per second
  • In unloaded conditions, top and bottom clamps should be positioned approximately 3 cm apart.

C. Instron Specimen Tensile Testing (4 min. per sample, or about 2 hours)

  • Load specimens in the Instron by first closing the top clamp, allowing skin to hang freely in place, then closing the bottom clamp once skin has been uncurled (carefully utilizing the hands of a second group member, since it is unadvisable for a clamp to be closed on one’s fingers).
  • Once sample has been loaded, but before running the Instron, obtain the following measurements:
  • Use a caliper to measure depth.
  • Use a ruler to measure exact width (approximately 1.5 cm) across the middle of the sample.
  • Use a ruler to measure exact length from bottom of the top clamp to top of the bottom clamp.

Determination of Young’s Modulus

  • For each sample, obtain a value for Young’s Modulus using Matlab as follows:
  • Convert displacement data to strain by dividing by the original (pre-stretched) length
  • Convert force data to stress by dividing by a product of the original depth and width
  • Graph stress vs. strain
  • Curve-fit the linear portion of the graph using a first degree equation, and designate slope as Young’s modulus. Linear portion begins after the toe region and endsat the elastic limit on the stress-strain graph. Ensure linear portion was obtainedby finding R2 value > 0.98.

Statistical Analysis

  • Perform an ANOVA test (single-factor, alpha = 0.05) between Young’s moduli of each type of chicken skin.
  • If p < 0.05 for ANOVA test, perform t-tests (two-sample assuming equal variances) between data for each pair of chicken skin types (three pairs overall)
  • If p < 0.017 for any pair, conclude significant difference in elastic modulus.[RH2]
  • In pairs where significant differences exist, order the skin from least to greatest modulus.

POTENTIAL PITFALLS & ALTERNATIVE METHODS

As previous researchershave observed, chicken skin samples have a propensity to slip from the Instron clamps in which they are held. This problem was alleviated in previous experiments by holding a portion of the skin sample, which extended beyond the clamp, using a pair of tweezers. Such an approach is not ideal, however, because the sample may actually be pulled by tweezers to a degree that compromises accuracy of its measured properties.[6] In this experiment,this anticipated pitfall will be avoided by lining the clamp with pieces of paper towel, which impart greater friction than the existing rubber clamp linings, thus alleviating slippage.

It is also possible that ANOVA results will indicate no significant difference among any of the sample groups, in which case there is no necessity to perform further t-test analysis. This, however, is not enough to disprove the idea of differing modulus based upon location – many chicken skin locations were not tested here, and other organisms may exhibit greater differentiation of mechanical properties between skin locations.One should, however, obtain additional chicken skin samples from other locations (e.g. Additionally, ANOVA results may signify a difference among samples which is not evident upon comparing any two groups To obtain p < 0.05 one may further pursue measurements of chicken skin from other locations (e.g. thigh, neck, sole of feet), or one can obtain skin from a wholly different organism.

There may also be a great amount of variation in modulus within one type of skin sample. Such variation is minimized by ensuring all samples are cut in the same direction, and further controlled by obtaining a minimum of two samples from any one piece of chicken. However, since skin comes from a living organism and its manufacturing is not a “quality-controlled” process, great opportunity for variation exists. The variation between different chickens may cause lack of statistical significant difference, even though samples from multiple locations of one unique organism could have been significantly different. To avoid this pitfall, five pieces of skin could be harvested from one whole chicken. Sizes of measured skin pieces may have to be adjusted in order to ensure adequate sample size (minimum N=5).

Finally, it is possible for chicken skin to exhibit a fracture curve which makes it impossible to identify Young’s modulus at all. This scenario was not observed in the related BE210 experiment[7], where all fracture curves had obvious linear portions (see Appendix Figure A1), but has been observed in three-point testing of chicken bone[8] (see Appendix Figure A2) where R2 < 0.98 over any attempted linear region. Such curves may need to be disqualified for lack of identifiable modulus value.

BUDGET.

purchases / amount* / unit costs / total cost / supplier
chicken leg / 50 pieces / $2.98 / package of 4 / 13 packages × $2.98 = $38.74 / The Fresh Grocer
4001 Walnut St.
Philadelphia, PA
chicken wing / 50 pieces / $1.79 / package of 6 / 9 packages × 1.79 = $16.11
chicken breast / 50 pieces / $4.99 / package of 8 / 7 packages × 4.99 = 34.93
total / $89.78

* Each group needs five pieces, and there are ten groups in the class.

REFERENCES

[1] H. R. Chaudhry, B. Bukiet, M. Siegel, T. Findley, A. B. Ritter and N. Guzelsu. “Optimal patterns for suturing wounds.”Journal of Biomechanics, Volume 31, Issue 7, July 1998. pages 653-662.

[2]ibid.

[3] Scott L.Hansen,Stephen J.Mathes,David M.Young. “Skin and Subcutaneous Tissue.” Schwartz’ Principles of Surgery. Mc-Graw-Hill: 2005.

[4] G. Gorospe, R. Howitt, Z. Lin, A. Stein. “Tensile Testing: Elastic Properties.” Bioengineering Laboratory II, 2007.

[5] Niitsuma K, Miyagawa S, Osaki S. “Mechanical anisotropy in cobra skin is related to body movement”. European Journal of Morphology, 2005 Oct-Dec; 42(4-5):193-200.

[6] G. Gorospe, R. Howitt, Z. Lin, A. Stein. “Tensile Testing: Elastic Properties.” Bioengineering Laboratory II, 2007.

[7] ibid.

[8]G. Gorospe, R. Howitt, Z. Lin, A. Stein. “Bending: Bone Fracture.” Bioengineering Laboratory II, 2007.

APPENDIX


Figure A1. Stress-strain responses for each of five longitudinal strips of chicken skin in uniaxial tension.Young’s modulus was obtained from curve-fitting with R2 > 0.98 for each trial. /
Figure A2: Force-displacement graph for four samples of chicken bone during three-point testing. For trial 3 it is impossible to obtain a curve-fit equation where R2 > 0.98, unless only a very small portion of the graph is used.

[RH1]is that what they're called?? maybe check online if a more official name

[RH2]check to see what we did in exp #4 (the last week) when it didn't look like there was any sig diff...