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TECHNICAL PAPERS: Soft Tissue

Viscoelastic Testing Methodologies for Tissue Engineered Blood Vessels

[+] Author and Article Information
Joseph D. Berglund1

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332; Georgia Tech-Emory Center for the Engineering of Living Tissues, Atlanta, GA 30332

Robert M. Nerem

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332; School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332; Georgia Tech-Emory Center for the Engineering of Living Tissues, Atlanta, GA 30332

Athanassios Sambanis2

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332; Georgia Tech-Emory Center for the Engineering of Living Tissues, Atlanta, GA 30332athanassios.sambanis@chbe.gatech.edu

1

Currently at: Medtronic Vascular, 3540 Unocal Place, Santa Rosa, CA 95403.

2

Corresponding author.

J Biomech Eng 127(7), 1176-1184 (Jun 06, 2005) (9 pages) doi:10.1115/1.2073487 History: Received July 13, 2004; Revised June 06, 2005

In order to function in vivo, tissue engineered blood vessels (TEBVs) must encumber pulsatile blood flow and withstand hemodynamic pressures for long periods of time. To date TEBV mechanical assessment has typically relied on single time point burst and/or uniaxial tensile testing to gauge the strengths of the constructs. This study extends this analysis to include creep and stepwise stress relaxation viscoelastic testing methodologies. TEBV models exhibiting diverse mechanical behaviors as a result of different architectures ranging from reconstituted collagen gels to hybrid constructs reinforced with either untreated or glutaraldhyde-crosslinked collagen supports were evaluated after 8 and 23 days of in vitro culturing. Data were modeled using three and four-parameter linear viscoelastic mathematical representations and compared to porcine carotid arteries. While glutaraldhyde-treated hybrid TEBVs exhibited the largest overall strengths and toughness, uncrosslinked hybrid samples exhibited time-dependent behaviors most similar to native arteries. These findings emphasize the importance of viscoelastic characterization when evaluating the mechanical performance of TEBVs. Limits of testing methods and modeling systems are presented and discussed.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

Mechanical analog of a three-parameter constitutive mathematical model. Also known as the standard viscoelastic solid (SVES), this model consists of two elastic components (R1,3p and R2,3p) and one viscous component (η3p). Diagrams illustrate the effects of each parameter on the resulting stress relaxation (SR) and creep profiles.

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Figure 2

Mechanical analog of a four-parameter constitutive mathematical model consisting of a Maxwell unit (R1,4p and η1,4p) connected in series with a Voigt unit (R2,4p and η2,4p). Diagrams illustrate the effects of each parameter on the resulting stress relaxation (SR) and creep profiles.

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Figure 3

Yield and failure energies of TEBV models after 8 and 23days of in vitro culturing. Yield energies (a) were defined by the work required to stretch constructs to the point of yielding during uniaxial tensile testing. Failure energies (b) were defined by the work required to stretch constructs to the point where stress levels had dropped to 50% of their maximum values. Ratios of failure to yield energies (c) signify the extent of plastic deformation that took place prior to failure. ∗p<0.05 vs corresponding time point of traditional collagen construct.

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Figure 4

Representative stress relaxation and creep profiles illustrating the diverse viscoelastic behaviors of traditional (×), UnXL (◻), and Glut (+) TEBV models and of native arteries (엯). Insert depicts the relaxation profiles of traditional and Glut TEBVs for the entire step stress relaxation time course.

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Figure 5

Best fit mathematical representations of TEBV models and native arteries using three and four-parameter constitutive models. While the three-parameter model characterized Glut TEBV relaxation profiles relatively accurately, it failed to represent the viscoelastic behaviors of the other TEBVs and native arteries. The four-parameter model accurately represented the viscoelastic profiles of all samples.

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Figure 6

Best fit four-parameter constitutive model elastic moduli and coefficients of viscosity of native arteries and TEBV models. Mechanical parameters were determined from stress relaxation and creep analysis data after 8 and 23days of in vitro culturing. ∗p<0.01 vs corresponding test and time point of traditional collagen construct, ‡p<0.01 vs corresponding test of native artery, †p<0.01 vs creep.

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