Engineering small-caliber vascular grafts from collagen filaments and nanofibers with comparable mechanical properties to native vessels

Abstract

At the present time, there is no successful synthetic, off-the-shelf small-caliber vascular graft (<6 mm) for the repair or bypass of the coronary or carotid arteries. This stimulates on-going investigations to fabricate an artificial vascular graft that has both sufficient mechanical properties as well as superior biological performance. Collagen has long been considered as a viable material to encourage cell recruitment, tissue regeneration, and revascularization, but its use has been limited by its inferior mechanical properties. In this study, novel electrochemically aligned collagen filaments were used to engineer a bilayer small-caliber vascular graft, by circular knitting the collagen filaments and electrospinning collagen nanofibers. The collagen prototype grafts showed significantly greater bursting strength under dry and hydrated conditions to that of autografts such as the human internal mammary artery and the saphenous vein (SV). The suture retention strength was sufficient under dry condition, but that under hydrated condition needs to be further improved. The radial dynamic compliance of the collagen grafts was similar to that of the human SV. During in vitro cell culture assays with human umbilical vein endothelial cells, the prototype collagen grafts also encouraged cell adhesion and promoted cell proliferation compared to the synthetic poly(lactic acid) grafts. In conclusion, this study demonstrated the feasibility of the use of novel collagen filaments for fabricating small caliber tissue-engineered vascular grafts that provide both sufficient mechanical properties and superior biological performance.

Figure 1.

The collagen filaments were directly knitted into a tubular structure (B) on the lamb circular weft knitting machine (A).

Figure 2.

Diagram of the suture retention test set-up.

Figure 3.

Characterization of the 2-ply collagen filaments under dry (A) and hydrated (B) conditions. (C) Representative loadextension curves of dry and hydrated collagen filaments. (D) Swelling ratio of the uncrosslinked and crosslinked collagen filaments over 24 h.

Figure 4.

Dimensions of the collagen prototype grafts. (a) The single-layer knitted collagen graft (COL-K); (b) the bilayer collagen graft with the electrospun layer on the outer surface of the knitted layer; (c) the bilayer knitted and electrospun collagen graft (COL-KE). (A) Wall thickness of the prototype grafts; (B) Outer and inner diameter of the grafts; (C) pore size of the grafts.

Figure 5.

Microscopic images of COL-K (a) and (b), COL-KE (c) and (d), and PLA-K (e) and (f) (scale bar: a, c, e = 500 μm; b, f = 200 μm; d = 10 μm).

Figure 6.

(A) Bursting strength and (B) suture retention strength of the prototype grafts compared with the hydrated human internal mammary artery (IMA) and the human saphenous vein (SV) and the PLA-K reference graft. The data of the reference autologous grafts were obtained from [30]. * indicates statistical significant difference, p < 0.05, n = 3.

Figure 7.

(A) Radial dynamic compliance and (B) β stiffness index of the prototype grafts. * indicates statistical significant difference, p < 0.05, n = 3.

Figure 8.

Representative microscopic images of the collagen and PLA substrates and HUVECs attached at Day 3. (a) COL-K shares a similar structure with (c) PLA-K, but PLA-K has a smoother surface and larger surface area compared to COL-K due to the multifilament feature; (b) COL-KE, on the other hand, had a different morphology. A larger number of well-attached HUVECs grew on the collagen substrate (d), (e) compared to the PLA scaffold (f). Cells have been highlighted in red.

Figure 9.

Cell attachment and proliferation determined by alamarBlue^® assay over 9 days. *Indicates significant difference, p < 0.05, n = 3.