High-resolution x-ray tomographic morphological characterisation of electrospun nanofibrous bundles for tendon and ligament regeneration and replacement

Repair of ligaments and tendons requires scaffolds mimicking the spatial organization of collagen in the natural tissue. Electrospinning is a promising technique to produce nanofibers of both resorbable and biostable polymers with desired structural and morphological features. The aim of this study was to perform high-resolution x-ray tomography (XCT) scans of bundles of Nylon6.6, pure PLLA and PLLA-Collagen blends, where the nanofibers were meant to have a predominant direction. Characterisation was carried out via a dedicated methodology to firmly hold the specimen during the scan and a workflow to quantify the directionality of the nanofibers in the bundle. XCT scans with 0.4 and 1.0 micrometer voxel size were successfully collected for all bundle compositions. Better image quality was achieved for those bundles formed by thicker nanofibers (i.e. 0.59 micrometers for pure PLLA), whereas partial volume effect was more pronounced for thinner nanofibers (i.e. 0.26 micrometers for Nylon6.6). As expected, the nanofibers had a predominant orientation along the axis of the bundles (more than 20% of the nanofibers within 3° and more than 60% within 18° from the bundle axis), with a Gaussian-like dispersion in the other directions. The directionality assessment was validated by comparison against a similar analysis performed on SEM images: the XCT analysis overestimated the amount of nanofibers very close to the bundle axis, especially for the materials with thinnest nanofibers, but adequately identified the amount of nanofibers within 12°.