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Research Papers

Mechanical Properties of Arterial Elastin With Water Loss

[+] Author and Article Information
Yunjie Wang

Department of Mechanical Engineering,
Boston University,
110 Cummington Mall,
Boston, MA 02215

Jacob Hahn

Department of Mechanical Engineering,
Boston University,
110 Cummington Mall,
Boston, MA 02215

Yanhang Zhang

Department of Mechanical Engineering,
Boston University,
110 Cummington Mall,
Boston, MA 02215;
Department of Biomedical Engineering,
Boston University,
Boston, MA 02215
e-mail: yanhang@bu.edu

1Corresponding author.

Manuscript received July 13, 2017; final manuscript received December 12, 2017; published online February 12, 2018. Assoc. Editor: Jonathan Vande Geest.

J Biomech Eng 140(4), 041012 (Feb 12, 2018) (8 pages) Paper No: BIO-17-1307; doi: 10.1115/1.4038887 History: Received July 13, 2017; Revised December 12, 2017

Elastin is a peculiar elastomer in that it requires water to maintain resilience, and its mechanical properties are closely associated with the immediate aqueous environment. The bulk, extra- and intrafibrillar water plays important roles in both elastic and viscoelastic properties of elastin. In this study, a two-stage liquid–vapor method was developed to investigate the effects of water loss on the mechanical properties of porcine aortic elastin. The tissue samples started in a phosphate-buffered saline (PBS) solution at their fully hydrated condition, with a gravimetric water content of 370±36%. The hydration level was reduced by enclosing the tissue in dialysis tubing and submerging it in polyethylene glycol (PEG) solution at concentrations of 10%, 20%, 30%, and 45% w/v, which reduced the water content of the samples to 258±34%, 224±20%, 109±9%, and 58±3%, respectively. The samples were then transferred to a humidity chamber to maintain the hydration level while the samples underwent equi-biaxial tensile and stress relaxation tests. The concentration of 10% PEG treatment induced insignificant changes in tissue dimensions and stiffness, indicating that the removal of bulk water has less effect on elastin. Significant increases in tangent modulus were observed after 20% and 30% PEG treatment due to the decreased presence of extrafibrillar water. Elastin treated with 45% PEG shows a very rigid behavior as most of the extrafibrillar water is eliminated. These results suggest that extrafibrillar water is crucial for elastin to maintain its elastic behavior. It was also observed that the anisotropy of elastin tends to decrease with water loss. An increase in stress relaxation was observed for elastin treated with 30% PEG, indicating a more viscous behavior of elastin when the amount of extrafibrillar water is significantly reduced. Results from this study shed light on the close association between the bulk, extra- and intrafibrillar water pools and the mechanics of elastin.

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Figures

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Fig. 1

(a) Picture of an elastin sample enclosed in dialysis tubing and submerged in PEG solution. Sandpaper tabs were glued at the sides of sample with sutures looping around for mechanical testing. Four carbon dot markers were placed at the center of sample, and the position of the markers was traced by a camera during mechanical testing. (b) Schematic diagram of an elastin sample enclosed in dialysis tubing. An osmotic pressure between the inside and outside of the dialysis tubing causes water to leave the tissue.

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Fig. 2

A custom-built VPM hydration chamber integrated with a biaxial tensile testing device. The tissue remains in the humidity chamber during mechanical testing.

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Fig. 3

Average experimental results with fitting curves of (a) PEG concentration versus water content and (b) relative humidity versus PEG concentration. The R2 values represent correlation coefficients between the measurements and fitted results (n = 10 for control; n = 5 for 10%, 20%, and 30% PEG; and n = 4 for 45% PEG).

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Fig. 4

Water contents of samples at control and after LPM and VPM stages. The LPM samples were treated with (a) 10%, (b) 20%, (c) 30% (n = 5), and (d) 45% (n = 4) PEG solution (*p < 0.05).

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Fig. 5

Normalized dimension changes of PEG-treated elastin (n = 5 for 10%, 20%, and 30% PEG; n = 4 for 45% PEG). The dimensions of PEG-treated elastin were normalized to the values of the corresponding control group (*p < 0.05).

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Fig. 6

Swelling ratio versus water content of control (n = 10) and PEG-treated elastin (n = 5 for 10%, 20%, and 30% PEG; n = 4 for 45% PEG). Closed symbols represent average value (*p < 0.05).

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Fig. 10

Representative stress relaxation curves of elastin before and after PEG treatment. Water contents are 241%, 188%, 95% for control, and after 20% and 30% PEG treatment, respectively.

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Fig. 9

Ratio of the longitudinal to circumferential stretch versus water content of control (n = 10) and PEG-treated elastin (n = 5 for 10%, 20%, and 30% PEG; n = 4 for 45% PEG). All stretch values are paired with Cauchy stress of 79.95±0.25 kPa. Closed symbols represent the average data.

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Fig. 8

Normalized tangent modulus in the (a) longitudinal (long), and (b) circumferential (cir) directions of PEG-treated elastin (n = 5 for 10%, 20%, and 30% PEG; n = 4 for 45% PEG). The tangent modulus of dehydrated elastin was normalized to its corresponding control value when the sample is fully hydrated (*p < 0.05).

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Fig. 7

Average Cauchy stress versus stretch curves in the (a) longitudinal and (b) circumferential directions of control (n = 10) and PEG-treated elastin (n = 5 for 10%, 20%, and 30% PEG; n = 4 for 45% PEG). One-sided error bars of a standard error of the mean are shown in both directions (*p < 0.05).

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