title: Fabrication of tubular constructs using hybrid extrusion printing and electrospinning for vascular tissue engineering applications
Creators: name: Fazal, Faraz
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: subject: synthetic grafts
subject: gelatin methacryloyl
subject: gelMA
subject: 3D printing
subject: electrospun nanofibers
subject: hydrogel printing
subject: vertical mandrel
subject: PLCL polymers
subject: fibre-reinforced grafts

description: Cardiovascular disease is the leading cause of mortality in the world, with the number of deaths rising every year. Coronary artery disease, peripheral artery disease, aneurysm, and strokes are the most prominent forms of vascular diseases. Bypass surgery is one of the techniques performed to treat these vascular diseases, which involves the re-routing of blood flow around the blocked arterial site using a vascular graft. Autologous grafts, such as saphenous vein and internal mammary artery, remain a gold standard for bypass grafting procedures. However, the removal of autologous grafts often results in the donor site morbidity. In some cases, the grafts are unavailable due to previous harvest or poor quality. Synthetic grafts represent an alternate option, however, they usually fail when used as small diameter bypass grafts. The mismatch between the mechanical properties of synthetic grafts and native arteries is the major cause of failure of these synthetic grafts made from PET and ePTFE. Therefore, there is an urgent and high demand to fabricate a vascular conduit having biomechanical behaviour similar to that of the native arteries. In this thesis, a new hybrid extrusion printing and electrospinning technique is presented to fabricate vascular grafts. A commercially available Creality3D Ender 3D printer was modified into a hybrid setup having a vertical mandrel, an extrusion printing head, and electrospinning heads. The setup is capable of fabricating layered structure using hydrogel and electrospun nanofibres. The gelatin methacryloyl (gelMA) hydrogel was selected for printing tubular constructs as it is widely used in the literature to encapsulate cells for bioprinting. Polycaprolactone (PCL) and Poly (L-lactide-co-ε- caprolactone) (PLCL) polymers were used to produce electrospun nanofibres for the reinforcement of hydrogel conduits. The setup was successfully tested to print grafts around the rotating mandrel. Bioprinting is an attractive technique to print tissues using hydrogels. However, printing of long tubular constructs from hydrogels remains a challenge. Additive-lathe printing method offers a solution to print grafts with high aspect ratios but printing hydrogels around the horizontal rods often leads to sagging, which results in the non-uniform wall thickness and subsequently variable mechanical properties. A new approach of printing around the vertical mandrel was used to fabricate gelMA grafts having uniform wall thickness. The printing parameters were selected to achieve the overlapping of two consecutive printed filaments, which resulted in the better bonding of filaments and good quality tubular constructs were fabricated. The tensile testing results revealed that the anisotropic properties of printed gelMA grafts were similar to those observed in the natural blood vessels. Moreover, no leakage was detected in the printed gelMA tubular constructs during the burst pressure measurement. Thus, a vertical additive-lathe printing method offers an attractive technique to print long vascular grafts using hydrogels. The burst pressure of the gelMA grafts printed on vertical additive-lathe printing setup was found to be lower than the normal haemodynamic blood pressures. Therefore, a reinforcement is required. A layer of electrospun nanofibres was collected over the printed hydrogel constructs to improve the mechanical performance. Various blends of Polycaprolactone (PCL) and poly(L-lactide-co-ε-caprolactone) (PLCL) polymer solutions were used to produce electrospun nanofibres. The printed bi-layered vascular grafts showed mechanical properties close to that of the native arteries. The tensile strength of the 12% gelMA constructs reinforced by 100/0 PCL/PLCL blend fibres was found to be greater than 3 MPa, which falls in the range of tensile strength value of native arteries (0.5 – 3 MPa for coronary arteries and 1.5 – 4 MPa for radial and mammary arteries). The fabricated vascular grafts showed a burst pressure of more than 2000 mmHg, which is within the range of burst pressure values for the human saphenous vein (1250 – 2476 mmHg) and radial artery (2001 – 2476 mmHg). Moreover, the compliance values of gelMA constructs reinforced by 100/0 PCL/PLCL (6.85 ± 1.01 %/100mmHg) and 75/25 PCL/PLCL (17.13 ± 7.35 %/100mmHg) were found to be similar to the muscular arteries (6.03 ± 3.39 %/100mmHg) and elastic arteries (16.21 ± 3.81 %/100mmHg), respectively. The cytocompatibility assessment showed that gelMA presented a bioactive surface for the endothelial cells to survive and grow. The newly developed hybrid setup has been utilized to fabricate fibre-reinforced vascular grafts. The mechanical performance of these constructs was found to be similar to that of native arteries. Also, PCL/PLCL fibres showed good cellular metabolic activity and gelMA surfaces were biocompatible for the HUVECs to grow. Thus, the presented fabrication method has a great potential to fabricate vascular grafts having biomechanical properties similar to that of natural blood vessels.