A Hybrid Electrospun-Extruded Polydioxanone Suture for Tendon Tissue Regeneration

Abstract

Many surgical tendon repairs fail despite advances in surgical materials and techniques. Tendon repair failure can be partially attributed to the tendon's poor intrinsic healing capacity and the repurposing of sutures from other clinical applications. Electrospun materials show promise as a biological scaffold to support endogenous tendon repair, but their relatively low tensile strength has limited their clinical translation. It is hypothesized that combining electrospun fibers with a material with increased tensile strength may improve the suture's mechanical properties while retaining biophysical cues necessary to encourage cell-mediated repair. This article describes the production of a hybrid electrospun-extruded suture with a sheath of submicron electrospun fibers and a core of melt-extruded fibers. The porosity and tensile strength of this hybrid suture is compared with an electrospun-only braided suture and clinically used sutures Vicryl and polydioxanone (PDS). Bioactivity is assessed by measuring the adsorbed serum proteins on electrospun and melt-extruded filaments using mass spectrometry. Human hamstring tendon fibroblast attachment and proliferation were quantified and compared between the hybrid and control sutures. Combining an electrospun sheath with melt-extruded cores created a hybrid braid with increased tensile strength (70.1 ± 0.3N) compared with an electrospun only suture (12.9 ± 1 N, p < 0.0001). The hybrid suture had a similar force at break to clinical sutures, but lower stiffness and stress. The Young's modulus was 772.6 ± 32 MPa for the hybrid suture, 1693.0 ± 69 MPa for PDS, and 3838.0 ± 132 MPa for Vicryl, p < 0.0001. Hybrid sutures had lower overall porosity than electrospun-only sutures (40 ± 4% and 60 ± 7%, respectively, p = 0.0018) but had a significantly larger overall porosity and average pore diameter compared with surgical sutures. There were similar clusters of adsorbed proteins on electrospun and melt-extruded filaments, which were distinct from PDS. Tendon fibroblast attachment and cell proliferation on hybrid and electrospun sutures were significantly higher than on clinical sutures. This study demonstrated that a bioactive suture with increased tensile strength and lower stiffness could be produced by adding a core of 10 μm melt-extruded fibers to a sheath of electrospun fibers. In contrast to currently used sutures, the hybrid sutures promoted a bioactive response: serum proteins adsorbed, and fibroblasts attached, survived, grew along the sutures, and adopted appropriate morphologies.

Keywords: biomaterials; polymeric scaffolds; tissue engineering.

FIG. 1.

Material characterization of the hybrid electrospun-extruded suture using SEM and Hg porosimetry. (a) Cross-sectional μCT scan showing the 12 filament electrospun sheath with 4 melt-extruded cores. Scale bar 200 μm. (b) SEM image showing suture morphology with both electrospun and melt-extruded fibers visible on the surface. Scale bar 100 μm. (c) Representative pore size distribution of sutures measured by Hg porosimetry. The two most frequent pore diameters of the electrospun and hybrid suture were between 0.5 and 5 μm and between 50 and 100 μm. The Vicryl curve showed minimal intrusion, with a peak between 20 and 50 μm, and the PDS curve displayed no intrusion. (d) Porosity and average pore diameter of sutures using Hg porosimetry. Overall porosity (%) was significantly higher for the electrospun suture compared with the hybrid suture (p = 0.0018) and compared with Vicryl (p < 0.0001). (e) Average pore diameter (nm) was significantly higher for electrospun suture compared with the hybrid suture (p = 0.0079) and compared with Vicryl (p = 0.0015). Statistical significance was determined at *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001. Error bars represent standard deviations; n = 3 for all conditions. μCT, microcomputed tomography; NI, no intrusion; ns, not significant; PDS, polydioxanone; SEM, scanning electron microscopy.

FIG. 2.

Mechanical testing of hybrid and control sutures. (a–d) Tensile properties of braided electrospun, hybrid, and control surgical sutures. Force at break (N) was lowest for the electrospun suture and highest for Vicryl, but all significantly different from each other (p < 0.0001); breaking strain (%) was lowest for Vicryl compared with all sutures (p < 0.0001); ultimate stress (MPa) of the electrospun suture was lower than the hybrid (p = 0.008), PDS (p < 0.0001), and Vicryl (p < 0.0001) sutures; Young's modulus (MPa) was lower for electrospun and hybrid compared with both surgical sutures (p < 0.0001). Statistical significance was determined at *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001. Error bars represent standard deviations; n = 5 for each condition.

FIG. 3.

Protein corona composition of hybrid and control sutures. (a) Principal component analysis of protein coronas formed on electrospun filaments (Es) and melt-extruded yarns (Ext), as well as on surgical sutures PDS (PDS) and Vicryl (V). There is less separation between electrospun filaments and melt-extruded yarns than compared with, and between, PDS and Vicryl. (b) Heat map of differentially expressed proteins in the coronas of PDS sutures, electrospun filaments (Es), and melt-extruded yarns (Ext). These three materials were incubated in four serum samples (M1, M2, F1, and F2) for 1 h before mass spectrometry analysis of bound proteins. There were five main clusters of bound proteins detected, totaling 338 differentially expressed proteins. Cluster 1 increased in intensity with electrospun filaments and melt-extruded yarns; cluster 2 increased in intensity with extruded filaments only; cluster 3 increased with electrospun filaments only; cluster 4 increased with electrospun and PDS sutures; and finally, cluster 5 increased with PDS sutures only. There was little variation between the four serums tested. Log10 scale; red represents increasing intensity and green represents decreasing intensity.

FIG. 4.

Tendon fibroblast attachment and metabolic activity on hybrid and control sutures. (a) Initial attachment was higher on hybrid (28 ± 7%) and electrospun sutures (24 ± 7%), compared with on Vicryl (10 ± 5%) and PDS (2 ± 0.1%), p < 0.0001. Vicryl also had a significantly higher initial attachment compared with PDS (p = 0.002). (b) Cell number on hybrid and control sutures was compared on days 1, 3, and 7. On days 3 and 7, there were more cells attached to the hybrid and electrospun suture than to PDS and Vicryl (p < 0.0001). There was no significant difference between electrospun and hybrid sutures or between Vicryl and PDS sutures on days 3 or 7. (c) For each material, the change in cell number was measured relative to day 1. On day 3, there was an increase in cell number on hybrid sutures (p = 0.0247) and electrospun sutures (p = 0.0003) and a decrease on PDS sutures (p = 0.0082). On day 7, there was an increase in cell number on hybrid and electrospun sutures (p < 0.0001 for both), PDS sutures (p = 0.0364), and Vicryl sutures (p = 0.0007). Statistical significance was determined at *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001; Error bars represent standard deviations; n = 3 for each condition.

FIG. 5.

SEM images of healthy tendon fibroblasts seeded on hybrid and control sutures. The left column shows the unseeded sutures; the middle column shows the sutures cultured for 4 days; the right column shows the cells cultured for 7 days. (a–c) Hybrid sutures. (a) Hybrid sutures without cells. (b) Cells show preferential attachment to electrospun over melt-extruded fibers at 4 days. (c) Cells are growing and migrating to attach to the melt-extruded fibers at 7 days; however, their morphology is not as clear as on electrospun fibers. (d–f) Electrospun control suture. (d) Electrospun suture without cells. (e) Cell attachment to electrospun fibers with individual cells visible. (f) Significant cell attachment and elongation (shown with yellow arrows) along fibers at 7 days. (g–i) PDS control suture. (g) PDS sutures without cells. (h) No visible cells at 4 days. (i) Some marking on the suture at 7 days, but unclear if these are cells. (j–l) Vicryl control suture. (j) Vicryl sutures without cells. (k) Some cells potentially attached at 4 days, but difficult to distinguish from the suture coating. (l) Some potential cell coverage at 7 days, with cells exhibiting a rounded morphology. Scale bars are 50 μm for (a–e and g–k and 20 μm for f and i).