Digital volume correlation to understand the full-field strain distribution of hierarchical electrospun scaffolds for tendon/ligament regeneration

The regeneration of injured tendons/ligaments (T/L) requires scaffolds with a biomimetic structure and mechanics [1]. Electrospinning can mimic T/L from fibrils up to the whole tissue [1,2]. Recently, PLLA/Coll electrospun bundles and hierarchical scaffolds (EHS) demonstrated to drive fibroblasts morphology and alignment [3,4]. However, the full-field strain distribution, which drives the cellular growth/morphology on electrospun materials is totally unexplored so far. Digital volume correlation (DVC) has proven to be the best candidate [5]. This study aims at developing the first micro-CT in situ protocol, to investigate the full-field strain distribution of electrospun materials using DVC. Bundles and EHS of PLLA/Coll nanofibers were electrospun via a consolidated procedure [3,4]. The morphology of scaffolds was investigated by using SEM and micro-CT. The mechanical properties of scaffolds were defined with a tensile test. Then, an in situ micro-CT tensile tests was performed on them. After two reference scans, scaffolds were step-wise strained (bundles: 3%, 4%, 5%, 7%; EHS: 1.5%, 3%, 5%, 7%) and micro-CT scans acquired (bundles: voxel size =13 μm; EHS: voxel size = 9 μm) after 15 minutes of relaxation per strain step. DVC analysis (SPAM software) [6] was carried out. Sub-volumes of 36 (bundles) and 40 (EHS) voxels were used with a 50% overlap (spatial resolution 468 and 360 μm respectively). The uncertainty of the strain measurements was one order of magnitude lower, compared to the in situ test values, for all samples. Scaffolds resulted morphologically/mechanically comparable with the T/L hierarchical counterpart [1]. The DVC analysis revealed a progressive increment of the internal principal axial strains (ep1, ep3 and eD). Strains were caused by the rearrangement of nanofibers and bundles during the load. The strain fields were like those measured in natural tendon tissue [7]. These results will help to understand the full-field mechanics of electrospun scaffolds [8].