Dynamic mechanical analysis in small diameter electrospun vascular grafts

Abstract

Small diameter vascular grafts out of electrospun material offer a close match to the biomechanical and structural properties of native blood vessels. For an optimal adaptation of electrospun prostheses to native vessels, the mechanical behaviour as it would occur in vivo has to be predicted. Dynamic measurements are therefore favourable. Aim of this work was to establish a protocol for performing force controlled dynamic mechanical analysis (DMA) measurements for dynamic characterisation, at physiological pressure range, by the use of DMA setup and common dynamic loading. Furthermore, common dynamic tests were performed in order to compare the information gained between these tests and DMA measurements. For the experiments, five electrospun grafts of three different wall-thicknesses (w.t) (150, 200, 300m) with inner diameter of 3 mm were prepared. A BOSE® Electroforce system ( Instruments ElectroForce Systems Group, New Castle, DE, USA) was used for tensile tests on ring-shaped specimens. The rings were loaded in circumferential direction by two steel pins. First, the specimens were loaded dynamically at force controlled mode by triangle and sinus waves. The dynamic loading consisted of 10 working cycles at frequency of 1Hz from 0.05N to 0.18N, corresponding to a physiological pressure of 50 to 150mmHg. Then, the samples were loaded in DMA mode with a logarithmic frequency sweep with 10 points from 1 to 100 Hz, with mean level of 0.18N, dynamic amplitude of 0.15N and a mean level rate of 0.1N/sec. The dynamic tests for both triangle and sinus waves showed differences in the resulting force-displacement graphs. From the force-displacement graphs the stress-strain curves were produced by implementing two different approaches for strain calculation of circular grafts. From the stress-strain curves the elastic modulus was calculated. Grafts of 150m had an E-modulus of 0.20MPa, whereas for 200m and for 300m the stiffness was 0.18MPa and 0.13MPa respectively. The results from the DMA show the behavior of the samples over the varying frequency. DMA measurements provide a variety of metrics as results for structural characterization. The mean dynamic stiffness for the 150m samples was 0.62N/mm, for 200m 0.94N/mm and for 300m 1.13N/mm. The area of the hysteresis loop decreased at higher wall thickness. Dissipated energy was 0.16N*mm for samples of 150m w.t, 0.01N*mm for 200m, and 0.1N*mm for 300m. The phase angle was 8.5degrees for 150m, 7degrees for 200m, and 6.5degrees for 300m. The DMA measurement also showed an increase in storage and loss stiffness at higher loading frequency. The structural behavior of small diameter ring-shaped specimens was measured by DMA and common dynamic loading experiments. The measurements showed that DMA is applicable for small ring-shaped electrospun samples. The viscoelasticity at varying frequencies at physiological loading was obtained, which gives more detailed information about the frequency dependent dynamic behavior. This allows a better characterization of the basic structural behavior in comparison to common dynamic tests.