Impact of the testing protocol on the mechanical characterization of small diameter electrospun vascular grafts
Graphical abstract
Introduction
The development of successful small diameter vascular grafts is still a challenging task (Huang and Niklason, 2014; Song et al., 2018). Ideal properties of a vascular substitute include suitable surgical and mechanical properties, biocompatibility and off-the-shelf availability (Woods and Flanagan, 2014; Pashneh-Tala et al., 2015; Dimitrievska and Niklason, 2017). A key challenge for producing functional vascular grafts with long term patency rates is to replicate the complex mechanical behavior based on the viscoelastic properties of native vascular tissue (Greenwald and Berry, 2000; Macrae et al., 2016). Electrospinning is an established method to produce tailored vascular grafts (Ercolani et al., 2015; Sarker et al., 2018). This production process allows the adjustment of mechanical properties of the final construct (Bergmeister et al., 2013). Different materials, micro- and macrostructural modifications, additives and modifications of the electrospinning process are used to adjust and optimize mechanical properties (Nezarati et al., 2014; Awad, Niu, Ali, Morsi, Lin). Mechanical tests are important in evaluating and optimizing the designed constructs and thereby improving in vivo behavior of the graft. Especially the compliance of the artificial conduit has to be optimized, as a compliance mismatch to native tissue is involved in the mechanisms that induce intimal hyperplasia (Mugnai et al., 2013). Numerous mechanical tests and measurement protocols have been published in the past, from measuring basic material properties up to complex, multiaxial tests for establishing constitutive models (Macrae et al., 2016; McClure et al., 2012; Arroyave et al., 2015; Holzapfel and Ogden, 2010). The knowledge of significance and limitations of mechanical tests is important to help in the choice of proper tests and testing conditions. This is also important in the interpretation and comparison of previously published data, which can be challenging because of the different testing protocols that have been used. Several groups systematically described the influence of boundary conditions on the measured results, like the influence of sample geometry (de Gelidi et al., 2017; Waldman and Lee, 2005; Legerlotz et al., 2010), sample gripping (Sun et al., 2005), sample preservation (O'Leary et al., 2014; Caro-Bretelle et al., 2015; Hemmasizadeh et al., 2013), or preconditioning effects (Hosseini et al., 2014; Tonge et al., 2013). Tensile strength and vessel's compliance are the most common measured values, but also measurements on viscoelastic behavior and long term performance like creeping are increasingly performed.
Aim of this study was to systematically investigate the influence of the measurement protocol on the measured mechanical properties. Due to viscoelastic behavior, differences in sample's compliance were expected at dynamic sample loading compared to a common quasistatic tensile test. A measurement protocol was established which allows to investigate these differences. The samples were loaded at two different temperatures under dynamic and quasistatic conditions. The influence of preconditioning cycles and the influence of a high number of loading cycles was further investigated. Additionally effects of wall thickness variations of the fabricated specimens on the mechanical behavior were evaluated by the use of two different commonly used synthetic polymer materials (Pashneh-Tala et al., 2015; Grasl et al., 2010; Thottappillil and Nair, 2015) with three different wall thicknesses. Compliance and tensile strength were evaluated and compared between the different samples and the different load cases.
Section snippets
Electrospinning of vascular grafts and sample preparation
Vascular grafts were produced out of two different synthetic polymer materials using a previously described electrospinning setup (Grasl et al., 2010, 2013) without additional electrodes to create randomly oriented fibers. One material was polyurethane (PUR), (Type Pellethane 2363-80A, Lubrizol Advanced Materials, Inc., Ohio, USA), the other was Poly(L-lactide) (PLLA), (Type 81273, Sigma-Aldrich, Vienna, Austria). The steel mandrels were precoated with a thin layer of polethylene oxide (PEO),
Vascular graft characterization
Fig. 3 shows SEM images of the inner and outer surface for the three different graft types and the two materials.
In Table 1 the mean wall thicknesses, void fractions and fiber diameters of the inner and outer surface in the different test groups are shown. The wall thickness measurements showed that the samples were produced with a consistent, uniform wall thickness in all graft types. Highest variations were seen in the 400 μm types.T he calculated void fraction increased at higher wall
Discussion
The applied measurement protocol allowed the comparison of the compliance measured in a quasistatic tensile test and under cyclic conditions. A significant difference between the compliances was seen, when the measurement was performed on a previously unloaded sample or when the sample was already in a preloaded state. The influence of nonlinear material behavior on the measured elasticity of arteries was also described by others (Chirinos, 2012; Fung, 2018), as the so called incremental
Conclusion
The measurements showed that with standard quasistatic tensile tests alone, no prediction of the in vivo compliance is possible in viscoelastic electrospun vascular samples. It is important to start the measurement from a preloaded condition to avoid overestimation of the vessel's compliance. Measurements at 37 °C are necessary, as significant differences in tensile strength and compliance were measured compared to room temperature. By electrospinning a complex fiber network is generated and
CRediT authorship contribution statement
Martin Stoiber: Conceptualization, Investigation, Writing - original draft, Visualization, Software. Christian Grasl: Conceptualization, Investigation, Writing - review & editing, Visualization. Katharina Frieberger: Formal analysis, Data curation, Visualization, Software. Francesco Moscato: Methodology, Writing - review & editing, Supervision. Helga Bergmeister: Methodology, Writing - review & editing. Heinrich Schima: Methodology, Writing - review & editing, Supervision.
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