Nowadays, the necessity of the vascular grafts is gaining importance owing to the increasing number of patients in need of artificial vessel implantations due to cardiovascular diseases, hemodialysis treatments etc.
Tissue engineers use various laborious methods to produce synthetic blood vessels. Knitted or woven vascular grafts of polytetrafluorethylene (ePTFE), polyethylene terephthalate (PET) and polyurethane (PU) are successfully used in large diameters (> 6 mm). However, small diameter knitted or woven vascular grafts (< 6 mm) do not give convenient results due to early thrombosis formation, intimal hyperplasia, aneurysm etc. Hence there is still room at the small diameter level for research and further improvements.
An ideal vascular graft should perform all the functions of native vessels including morphological, structural, mechanical and biological features. Therefore, very similar to native vessels, nanofibrous vascular grafts are made up of layers with different functions, hence multilayered designs of the artificial grafts are superior to single layer designs. The design parameters are determined with respect to the structure and the mission of the building layers of the native vessels.
Among various applications for building artificial vessels to provide all these qualities by using multiple effortful techniques; electrospinning is a unique method that allows the design of all the layers at once.
Electrospun vascular grafts have a wide range of advantages over the other types of tissue engineered artificial vessels.
First of all, nanotopography of the electrospun mesh resembles ECM (extra cellular matrix) i.e. interconnected pores and high surface area providing endothelium formation and hence hinder arterial thrombosis.
Modifications like heparin incorporation, protein lining or polymer surface chemistry on the inner surface of the small diameter electrospun grafts enable spiral flow of blood as in the native vessels and minimizes related complications.
It is possible to maintain the design for SMC (smooth muscle cells) to grow, migrate, multiply and attach deep within the structure with controllable pore size and porosity of the assembly.
Electrospinning technology enables the construction of a vascular graft with required features by providing broad material options, flexibility in production parameters and adjustable composition and morphology.
The NS24 model electrospinning device was used to construct the artificial blood vessels shown in the images below in figure 1. The special designed mill collector with different diameter options has been assembled to the system and a continuous electrospun web has been coated on the rotating mill in a tubular form. A homogenous, uniform structure was obtained by the reciprocal movement of the collector mechanism. The rotating mills are made up of titanium to make it easy to remove the stand-alone tubular coating.
Figure 1: The electrospun TPU vascular graft with a diameter of 3 mm.
Figure 2: The set-up of rotating shaft collector assembled to the system.
Figure 3: The process of construction with a 3-nozzle feeding unit.
SEM images taken from the outer surface of an electrospun artificial vessel.
SEM images of the inner surface of an electrospun nanofibrous artificial graft :
SEM images of the cross-section indicating different densities mimicking the multilayer structure: