Design, Fabrication and Characterization of Piezoelectric and Dielectric Materials for Tubular Electrospun Grafts

The development of tubular grafts is a hot topic in research, especially in the biomedical field due to the high request of tissue replacements for recovering from specific diseases. The ideal graft, in addition to providing a mechanical stability and stiffness, should be able to be hosted by native tissues, and to promote cell growth. This latter property has been found particularly enhanced by piezoelectric materials.<br/>In this study, we developed millimetric tubular grafts made of dielectric or piezoelectric materials, namely Polycaprolactone (PCL) and Polyacrylonitrile (PAN) .<br/>We fabricated millimetric tubular grafts using an electrospinning setup after preparing the solutions as follows. The PCL solution was prepared with a concentration of 15% w/v in a Tetrahydrofuran/ Dimethylformammide mixture (1:1 w/w), while the PAN solution was prepared with a concentration of 15% w/v in Dimethylformammide, by magnetic stirring at 300 rpm overnight at room temperature. Solutions were kept under gentle stirring at room temperature prior to usage.<br/>The morphological characterization was performed optically using a stereomicroscope for measuring the macro-features, and a scanning electronic microscope to assess the micrometric features of the grafts. Wettability of the surfaces was assessed through the measurement of the water contact angle (WCA) after depositing a drop of distilled water on planar samples. The mechanical properties of the tubular grafts under radial expansion were obtained through a dedicated system that was designed to measure the deformation of the fibrous mesh structure as a function of the internal pressure. In contrast, the measurements of the d₃₁ piezoelectric coefficient of the produced meshes were conducted using a setup specifically developed in-house. This setup involves clamping a stripe of the nanofiber mesh between a rigid support and a flexible steel cantilever, which serves as a sensitive force gauge. An electric field is thus applied orthogonally to the mesh thickness by electrically biasing two parallel metal plates. Pooled poly(vinylidene fluoride) (PVDF) film was used as a standard control, whereas electrospun fibers of PVDF/BaTiO₃ and poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT-PBT) as a positive and negative control, respectively.<br/>PCL and PAN tubular grafts were produced with an average diameter of 8 mm. Fiber size was different depending on the constitutive material: image analysis revealed that PAN electrospun fibers had a mean diameter of 0.30 ± 0.04 µm; instead, PCL showed a broader diameter distribution with a mean value of 0.67 ± 0.39 µm. Pore equivalent diameters of PAN and PCL meshes were 0.32 ± 0.18 µm and 0.51 ± 0.37 µm and their average porosities 50% and 42%, respectively. Both PAN and PCL meshes displayed an appreciable porosity, i.e., 50% and 42%, and pore size of 0.8 and 2.0 µm², respectively. From the mechanical standpoint, the PAN scaffold resulted less deformable than the PCL one, and therefore more resistant, for pressures above 20 kPa. Upon an inner expansion leading to a 18% circumferential deformation, the PAN tubular scaffolds were able to hold 42% higher pressure (i.e., 39 kPa) than PCL ones (i.e., 27 kPa). The two polymers gave rise to fibrous meshes with different wettability, measured WCA, being PCL highly hydrophobic (WCA = 129° ± 21°) and PAN hydrophilic (WCA = 20° ± 6°). Measurements of the piezoelectric coefficients of the produced fiber meshes showed that PAN scaffolds showed a higher piezoelectric coefficient d₃₁ = 20.0 ± 16.0 pm/V than PCL scaffolds (d₃₁ = 1.12 ± 0.25 pm/V).<br/>This work shows a new approach to develop electrospun tubular grafts made of either dielectric or piezoelectric materials, with interesting morphological and mechanical properties that may be exploited in several biomedical applications, including the replacement of vascular and intestine tracts.