Design and analysis of a burst strength device for testing vascular grafts.

The design and analysis of a device to measure the burst strength (strength under a state of pure radial internal pressure) and compliance of vascular grafts and flexible pressurized tubes is presented. The device comprises three main sections, viz., a clean air-dry pressure controller, a test specimen holder, and automated software for control and data collection. Air pressure is controlled by means of a valve and a dedicated mechanism allowing reaching up to 120 psi in increments of 1 psi, and recording pressure changes with 0.04 psi resolution. The circumferential strain is determined by measuring the radial displacement of the vascular graft using an optical arrangement capable of determining a maximum radial displacement of 10 mm with 0.02 mm resolution. The instrument provides a low uncertainty in compliance (±0.32%/100 mm Hg-1) and burst strength measurements. Due to its simplicity, the device can easily be reproduced in other laboratories contributing to a dedicated instrument with high resolution at low cost. The reliability of the apparatus is further confirmed by conducting finite element analysis, elasticity solutions for pressurized cylinders, and testing of small diameter vascular grafts made of a commercial aliphatic polyurethane tested under radial internal pressure.

Prediction of circumferential compliance and burst strength of polymeric vascular grafts.

The circumferential compliance and burst strength of vascular grafts are predicted through the conically modified von Mises and elasticity theories, providing an analytical closed form solution for both parameters. Besides the graft's radii, the model for circumferential compliance depends solely on the elastic modulus and Poisson's ratio of the polymer material, and its accuracy was verified by finite element analysis and measurements. The analytical expression of the burst strength requires accurate determination of the material's tensile and compressive yield stress, which were carefully obtained by using digital image correlation measurements in uniaxial tensile and compressive tests of the constitutive material. The average measured circumferential compliance and burst strength of an 8mm graft made of a commonly used biomaterial, Tecoflex® SG-80A, are 1.05%/100mmHg-1 and 34.1psi (1763mmHg) and the proposed analytical predictions fall within the experimental scattering. Thus, it is shown that the circumferential compliance and burst strength of vascular grafts can be analytically predicted by knowing the elastic and yield material properties accurately, without needing to actually test the graft under radial pressure. This is a major advantage which can aid in the design and tailoring of vascular grafts.