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Keywords:

soft tissue biomechanics

Abstract:

The quality of tissue engineered vascular grafts should rival native vasculature. This study evaluates the effect on the mechanical properties of stretching developing graft material inside the peritoneal cavity. At the University of Queensland, a device has been developed that, when inserted into the peritoneal cavity, attracts cells around a tubular scaffold to generate autologous arterial grafts. The device is capable of cyclically stretching (by means of a pulsatile pump) developing tissue to increase the mechanical strength of the graft. Pulsed (n=8) and unpulsed (n=8) devices were implanted for 10 days in lovenaar sheep (n=8). Pulsation occurred for a period of 5-8 days before harvest. Thick unadhered autologous tissue with cells residing in a collagen extracellular matrix was produced in all devices. From each implant, as well as from normal carotid arteries, an axially and a circumferentially oriented strip of approximately 10 by 5 mm was cut. These samples were then mounted on a uniaxial tensile test bench (INSTRON 5567) and submitted to cyclic loading at gradually increasing levels of elongation (crosshead speed of 1 mm/s), followed by elongation until rupture. The acquired data was processed to obtain the Cauchy stress and the logarithmic strain. All samples were tested within 48 hours after explantation. Different variables were extracted from each experimental data set: the stress level at 20, 30, 40 and 50% strain, the tangent stiffness at these strain levels and the stress and strain at which failure occurred. These variables showed a significant (p=0.049) increase of failure strength, as well as a significant (p=0.028) increase of failure strain for pulsed samples compared to controls. No significant differences were found for stress or tangent stiffness at the different strain levels. Testing direction (axial versus circumferential) had no significant effect on the mechanical properties, neither for the pulsed nor the unpulsed samples. Summarized data of the failure stress and failure strain are shown in Table 1 for pulsed and unpulsed samples, and for carotid artery samples. This study has shown that unadhered tissue tubes with increased mechanical strength in response to pulsation can be produced with every implant after a period of 10 days. However, these tissue tubes require a more fine-tuned exposure to pulsation to be suitable for use as vascular grafts. Axial Circumferential Non-pulsated Pulsated Carotis Non-pulsated Pulsated Carotis Failure strength (MPa) 0.09 ± 0.05 0.22 ± 0.15 5.42 ± 1.40 0.078 ± 0.033 0.17 ± 0.25 4.29 ± 0.92 Failure strain (-) 0.52 ± 0.13 0.71 ± 0.17 0.87 ± 0.22 0.604 ± 0.105 0.70 ± 0.21 0.79 ± 0.19 Table 1: Failure properties of unpulsed and pulsed vascular grafts and of normal sheep carotid arteries.