Fabrication and Characterization of Electrospun Thermoplastic Polyurethane/Fibroin Small-Diameter Vascular Grafts for Vascular Tissue Engineering

Abstract

The demand for small-diameter blood vessel substitutes has been increasing due to a shortage of autograft vessels and problems with thrombosis and intimal hyperplasia with synthetic grafts. In this study, hybrid small-diameter vascular grafts made of thermoplastic polyurethane (TPU) and silk fibroin, which possessed a hybrid fibrous structure of an aligned inner layer and a random outer layer, were fabricated by the electrospinning technique using a customized striated collector that generated both aligned and random fibers simultaneously. A methanol post-treatment process induced the transition of fibroin protein conformation from the water-soluble, amorphous, and less ordered structures to the water-insoluble β-sheet structures that possessed robust mechanical properties and relatively slow proteolytic degradation. The methanol post-treatment also created crimped fibers that mimicked the wavy structure of collagen fibers in natural blood vessels. Ultrafine nanofibers and nanowebs were found on the electrospun TPU/fibroin samples, which effectively increased the surface area for cell adhesion and migration. Cyclic circumferential tensile test results showed compatible mechanical properties for grafts made of a soft TPU/fibroin blend compared to human coronary arteries. In addition, cell culture tests with endothelial cells after 6 and 60 days of culture exhibited high cell viability and good biocompatibility of TPU/fibroin grafts, suggesting the potential of applying electrospun TPU/fibroin grafts in vascular tissue engineering.

Fig. 1

Schematic of the customized electrospinning device (A). TPU/fibroin fibers were collected by a striated collector to form a fibrous sheet comprised of alternatively aligned and randomly oriented structures (B). Aligned fibers were collected in the grooves, while randomly oriented fibers fell on the ridges (B)

Fig. 2

FTIR test results for electrospun TPU-93A, fibroin, and TPU-93A/fibroin at 2 : 1 (F1T2) and 1 : 2 (F2T1) ratios. The peak shifts and the variation of intensities indicate a change in the material composition

Fig. 3

Small-diameter vascular grafts with 3.18 and 2.38 mm inner diameters made of electrospun TPU/fibroin sheets (A). The graft contains an aligned fibrous layer for the inner surface, upper-left, and a random-distributed layer at the outside, lower-right, (B)

Fig. 4

Electrospun samples of (A) pure TPU-93A, (B) TPU-93A/fibroin = 2 : 1, (C) TPU-93A/fibroin = 1 : 1, and (D) TPU-93A/ fibroin = 1 : 2

Fig. 5

Ultrafine nanofibers and nanowebs distributed on the regular electrospun TPU/ fibroin fibers (A). A nanoweb at a higher magnification (B). The crimped structure of the methanol-treated aligned- (C) and (D) randomly-oriented electrospun fibers

Fig. 6

The formation of 2D nanowebs was due to the fast phase separation of TPU/fibroin and HFIP. The formed bubbles were larger in (B) than (A) which led to a finer porous structure

Fig. 7

Mechanical properties of various TPU/fibroin blends including representative stress–strain curves, ultimate tensile strengths, and elongation-at-break values

Fig. 8

Cyclic tensile behaviors of vascular grafts made from TPU-93A/fibroin (A) and TPU-80A/fibroin (B) at a 1 : 1 ratio. Scatter data points represent the lower and upper bounds of human coronary arteries from experiments (van Andel et al., 2003)

Fig. 9

Endothelial cell culture results on electrospun grafts made of TPU-93A/fibroin blends at (A) day 6, (B) day 6 with Matrigel coating, and (C) day 60 (dots: nuclei)