2012 Proceedings - International Tissue Elasticity Conference

065 NON–INVASIVE MECHANICAL STRENGTH MONITORING OF VASCULAR GRAFT DURING
BABOON CELL CO–CULTURE USING ULTRASOUND ELASTICITY IMAGING.
D Dutta 1 , KW Lee 2 , RA Allen 2 , Y Wang 2,4 , J Brigham 3 , K Kim 1,2,4 – Presented by JM Rubin .
1 Heart and Vascular Institute, University of Pittsburgh Medical Center, Pittsburgh, PA, USA;
2 Bioengineering Department, 3 Civil and Environmental Engineering Department, 4 McGowan
Institute for Regenerative Medicine, University of Pittsburgh, Pittsburgh, PA, USA.
Background: Mechanical strength is a key design factor in engineered vascular grafts for maintaining high
burst pressure while preserving compliance of the native vessels. To date, most assessments are made
invasively at the end of cell culture; hence, such measurements cannot be correlated with cell growth.
Aims: The aim of this study is to monitor the compliance of a polymer–based vascular graft as fibroblast
and smooth muscle cells (SMC) grow on it using non–invasive ultrasound (US) elasticity imaging (UEI).
Methods: Fibroblasts and SMCs isolated from baboon artery were co–cultured on a porous tubular graft
inside a bioreactor. The graft was fabricated from a biodegradable elastomer [1]. Flow of cells was
maintained using a pulsatile pump, and the pressure was recorded. At the end of days 0, 6 and 13, US
radiofrequency frames were captured over one complete pump cycle, and 2D phase–sensitive speckle
tracking was applied to estimate the displacement field, from which the average compliance of the graft
was determined. Using this average compliance, and the geometry obtained via B–scan, a finite element
(FE) model of the scaffold was created and solved for the experimentally observed pressure conditions.
Direct mechanical pressure–diameter test using laser micrometer was also performed, and the results
were compared to US speckle tracking results.
Results: Axial displacement maps indicating overall distensions of the graft wall are shown superimposed
on B–scan images in Figure (a).
Average compliance of the graft at
the end of days 0, 6 and 13 are
plotted in Figure (b). The FE
displacement field in Figure (c)
qualitatively matches with the
displacement field obtained via US
speckle tracking thereby confirming
that the average compliance value is
representative of the overall
mechanical strength of the graft. The
average compliance of the graft (in
%/100mmHg) after days 6 and 13
was found to be 52 and 22.5,
respectively, through direct
mechanical pressure–diameter tests,
which matched closely with the
speckle tracking results (50 and
20.2, respectively, as presented in
Figure (b)).
Conclusions: The feasibility of non–invasively monitoring the compliance of a vascular graft with cell
growth via UEI is demonstrated. The stiffness increase with multiplication of flibroblasts and SMC agrees
well with the mechanical measurements and previous scientific observations. An inverse problem
formulation to obtain the compliance distribution in the graft is underway that will help correlate any
non–uniformity in graft compliance with non–uniform cell growth.
Acknowledgements: This study was supported in part by NIH 1R21EB013353–01 (PI: Kim). Small animal imaging
system (Vevo2100) was supported through NIH 1S10RR027383–01 (PI: Kim).
References:
[1] Lee KW, Stolz DB, Wang Y: Substantial Expression of Mature Elastin in Arterial Constructs. Proc Natl Acad Sci
USA, 108, pp. 2705–10, 2011.
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