Identification of viscoelastic properties of Dacron aortic grafts subjected to physiological pulsatile flow.

In vascular surgery, most synthetic vascular grafts currently used for large vessels replacements are made of Dacron (polyethylene terephthalate; PET). In this study, the dynamic response of these synthetic arterial substitutes to physiological pulsatile conditions is investigated in depth. Experiments were performed on a mock circulatory loop developed to replicate physiological pulsatile pressure and flow. Two different models of Dacron grafts (branched and straight) were tested at various heart rate conditions. Results are presented in terms of cyclic axisymmetric diameter changes, hysteretic loops of the pressure-diameter change, and viscoelastic parameters, such as loss factor and storage modulus that are identified from the hysteresis loop. The amplitude of cyclic diameter change of the Dacron graft was found to be always below 0.2% for all the heart rates considered (from 57 to 187 bpm). The loss factor of the Dacron graft slightly increased with the heart rate; almost no effect of the pulse rate was observed on the storage modulus, which was identified to be around 100 MPa. Both glycerol-water mixture (i.e. the blood analogue fluid) and saline solution were used in the circulatory loop and results did not present significant differences between the two cases. This shows that the effect of the shear load on the dynamic response of Dacron grafts is negligible. A comparison between Dacron vascular implants and human thoracic aortas shows a large mismatch in their viscoelastic mechanical properties.

Biomechanical characterization of a chronic type a dissected human aorta.

Aortic dissection is one of the most lethal cardiovascular diseases. A chronic Type A (Stanford) dissected aorta was retrieved for research from a 73-year-old male donor without diagnosed genetic disease. The aorta presented a dissection over the full length, and it reached a diameter of 7.7 cm in its ascending portion. The descending thoracic aorta underwent layer-specific quasi-static and dynamic mechanical characterizations after layer separation. Mechanical tests showed a physiological (healthy) behavior of the intima and some mechanical anomalies of the media and the adventitia. In particular, the static stiffness of both these layers at smaller strains was three times smaller than any one measured for twelve healthy aortas. When the viscoelastic properties were tested, adventitia presented a larger relative increase of the dynamic stiffness at 3 Hz with respect to most of the healthy aortas. The loss factor of the adventitia, which is associated with dissipation, was at the lower limit of those measured for healthy aortas. It seems reasonable to attribute these anomalies of the mechanical properties exhibited by the media and the adventitia to the severe remodeling secondary to the chronic nature of the dissection. However, it cannot be excluded that some of the mechanical anomalies were present before remodeling.

Anisotropic fractional viscoelastic constitutive models for human descending thoracic aortas.

The generalized fractional Maxwell model, formulated for hyperelastic material within the framework of the nonlinear viscoelasticity with internal variables, is applied to identify viscoelastic constitutive equations from layer-specific experimental data obtained by uniaxial harmonic loading of ex-vivo human descending thoracic aortas. The constitutive parameters are identified by using a genetic algorithm for the optimal fitting of the experimental data. The accuracy of the fitted fractional model is compared to the fitted integer order model with the same number of Maxwell elements. The formulation of an original strain energy density function for anisotropic nonlinear viscoelasticity is introduced and constitutive parameters are obtained from the experiments.

Layer-specific hyperelastic and viscoelastic characterization of human descending thoracic aortas.

A layer-specific hyperelastic and viscoelastic characterization of human descending thoracic aortas was experimentally performed. Healthy aortas from twelve beating heart donors with an average age of 49.4 years, were received from Transplant Québec. Axial and circumferential strips were prepared from the specimens. They were dissected into intima, media and adventitia layers. Measurements of the opening angles were used to identify the circumferential residual stresses. Uniaxial tensile tests on axial and circumferential strips, together with the Gasser-Ogden-Holzapfel material model, were used to characterize the hyperelastic behaviour of the three aortic layers for each donor. Uniaxial harmonic excitations at different frequency, superimposed to initial stretch values, were used to characterize the viscoelastic behaviour. The storage modulus and the loss tangent were obtained for each layer in both directions; comparison to intact aortic wall was also performed. The generalized Maxwell model, within the framework of nonlinear viscoelasticity with internal variables, was used to obtain the constitutive material parameters. Results showed a positive correlation (p < 0.05 for circumferential media and adventitia) between stiffness and donor age for the three layers of the aorta in both axial and circumferential directions. A significant increase (around 50%) of the storage modulus (i.e. dynamic stiffness) was observed between the quasi-static value and loading at 1 Hz frequency, while further increase in frequency marginally affected its value. The loss tangent was only slightly influenced by the stretch value, which justified the use of the viscoelastic model adopted. Finally, similar loss tangent values were found for the three aortic layers.

Experiments on dynamic behaviour of a Dacron aortic graft in a mock circulatory loop.

Woven Dacron grafts are currently used for the surgical treatment of aortic aneurysm and acute dissection, two otherwise fatal pathologies when aortic wall rupture occurs. While Dacron is chosen for aortic grafts because of characteristics such as biocompatibility and durability, few data are available about the dynamic response of Dacron prosthetic devices and about their side effects on the cardiovascular system. In this study, a Dacron graft was subjected to physiological flow conditions in a specifically-developed mock circulatory loop. Experiments were conducted at different physiological pulsation-per-minute rates. Results show that, in comparison to an aortic segment of the same length, the prosthesis is extremely stiffer circumferentially, thus limiting the dynamical radial expansion responsible for the Windkessel effect in human arteries. The prosthesis is instead excessively compliant in the axial direction and develops preferentially bending oscillations. This very different dynamic behaviour with respect to the human aorta can alter cardiovascular pressure and flow dynamics resulting in long-term implant complications.

A Paper-Based Piezoelectric Accelerometer.

This paper presents the design and testing of a one-axis piezoelectric accelerometer made from cellulose paper and piezoelectric zinc oxide nanowires (ZnO NWs) hydrothermally grown on paper. The accelerometer adopts a cantilever-based configuration with two parallel cantilever beams attached with a paper proof mass. A piece of U-shaped, ZnO-NW-coated paper is attached on top of the parallel beams, serving as the strain sensing element for acceleration measurement. The electric charges produced from the ZnO-NW-coated paper are converted into a voltage output using a custom-made charge amplifier circuit. The device fabrication only involves cutting of paper and hydrothermal growth of ZnO NWs, and does not require the access to expensive and sophisticated equipment. The performance of the devices with different weight growth percentages of the ZnO NWs was characterized.

Viscoelastic characterization of woven Dacron for aortic grafts by using direction-dependent quasi-linear viscoelasticity.

In case of direction-dependent viscoelasticity, a simplified formulation of the three-dimensional quasi-linear viscoelasticity has been obtained manipulating the original Fung equation. The experimental characterization of the static hyperelastic behaviour, the relaxation, the dynamic modulus and the loss factor of woven Dacron from a commercial aortic prosthesis has been performed. An 11% difference of the reduced relaxation (after infinite time) between axial and circumferential directions has been observed for the woven Dacron. A very large increase in stiffness is obtained in case of harmonic loading with respect to the static loading. These findings are particularly relevant for dynamic modelling of currently used aortic grafts.