Development, characterization and feasibility of industrialization of a novel hybrid semi-degradable vascular access for haemodialysis

Early-stage renal disease patients desperately need haemodialysis to compensate the loss of natural blood filtration and maintain system homeostasis. However, for decades vascular accesses, required for haemodialysis, have been the bottleneck of this procedure, limiting the kidney transplant wait time (3.7% chance a year) to an average of 1000 days. When possible, arteriovenous fistula is the choice to create a vascular access for long-term haemodialysis. Although, functional fistulae have the highest success rates in the long-term due to its purely native tissue composition, two thirds never reach the maturation point to allow haemodialysis – failing in the short-term. At this point synthetic grafts fill the gap, providing immediate access (24-72h after minor surgery) – early cannulation. However, being fully composed of synthetic bioinert material they are not able to biointegrate or match compliance of adjacent anastomosed vessels, eventually failing in the long-term. Following on what fails with fistulae and synthetic grafts, it is clear how both have complementary advantages (and disadvantages). Therefore, in this work an ideal vascular access approach is envisioned as one that behaves as a graft in the short-term and as a fistula in the long-run. This way providing the benefits of a readily available and predictable graft for early cannulation, which slowly converts into native vascular tissue; adapting and evolving according to local haemodynamics in a similar fashion as the artery and vein connected by the vascular access. In this work, principles of in situ tissue engineered are followed, using electrospinning, to achieve a vascular graft that performs as a bioactive scaffold, hosting the colonisation of vascular cells that trigger tissue development. Enzymes released during this process degrade most of the scaffold, allowing progressive and controlled remodelling from scaffold to vascular tissue. However, an elastic artificial bioinert mesh remains embedded in the tissue, ensuring optimal immediate recovery from haemodialysis punctures and continuously intermediating haemodynamic divergences between artery and vein. Silk fibroin and polyurethane were the chosen materials to develop this hybrid semi-degradable vascular graft due to their respective bioactivity and elasticity. In this sense, the first milestone was to achieve an electrospun blend of these two materials – Silkothane® material – to serve as the core structure of the graft. Physicochemical analyses demonstrated the maintenance of the characteristic features of fibroin and polyurethane upon solubilisation, blending, electrospinning and post-processing with ethanol or methanol. Envisioning their possible application as semi-degradable substrates for haemodialysis, tubular meshes were further characterized, showing sub-micrometric fibrous morphologies, tuneable mechanical properties, permeability before and after puncture in the same order of magnitude as commercial grafts currently used in the clinics. The development of the Silkothane® graft followed as a subsequent milestone to reach. Considering the demonstrated haemocompatible and cell adhesive properties of silk fibroin, the graft design concerned three electrospun concentric layers. Thus, composed of a thick core layer of Silkothane® material in between two thin layers of pure electrospun fibroin, providing a continuous presence of this cell adherent material across the Silkothane® graft’s entire structure. The full three-layered graft, influenced by the polyurethane presence, ensured mechanical properties that are a determinant factor for the success of a vascular access (e.g., vein-graft compliance matching). The Silkothane® graft demonstrated early cannulation potential in line with self-sealing commercial synthetic grafts, and a degradability driven by enzymatic activity. Moreover, the fibroin-only layers and extracellular matrix-like morphology, presented by the graft, revealed to be crucial in providing a non-haemolytic character, long clotting time, and favourable adhesion of human umbilical vein endothelial cells with increasing viability after 3 and 7 days. Accordingly, the proposed approach represents a promising step forward towards an in situ tissue engineering vascular access and its potential for vein-graft anastomosis stability, early cannulation, and biointegration. Finally, to ensure industrial feasibility of the Silkothane® graft, a pilot plant electrospinning system able to scale up manufacturing, while increasing quality, was developed for clinical validation and initial commercial application. Results from directly comparing the current and the upgraded electrospinning systems revealed a 5-fold drop in cost per Silkothane® graft, placing it well below average production cost per commercial synthetic grafts, at a rate of six 30 cm-long grafts fabricated per day. In summary, the promising findings achieved in this thesis provide the means to fabricate a pioneering in situ tissue engineering vascular access, addressing all needs from the commercial, industrial and – potentially – clinical perspectives, while aiming to finally eliminate the half-century old haemodialysis bottleneck, which is failing vascular accesses.