Electrospun hemocompatible PU/gelatin-heparin nanofibrous bilayer scaffolds as potential artificial blood vessels

Abstract

Cardiovascular diseases are one of the most frequent causes of death all over the world. Particularly in recent years, coronary heart diseases showed a significantly increasing trend. Nowadays artificial blood vessels have become an important implant for the treatment of a serious stenosis or occlusion of blood vessels in clinic. Synthetic polymers, such as polytetrafluoroethylene and polyurethanes, have been successfully used as large diameter artificial blood vessels, but long term patency of small artificial diameter vessels (<6 mm) is not satisfactory due to thrombosis and intimal thickening. With the development of regenerative medicine, tissue engineering strategies were explored to bioengineer blood vessels. An artificial scaffold shall be applied to generate for the basis to create a small diameter blood vessels. The inner layer of such vascular scaffolds is designed to form a continuous layer of endothelium to prevent thrombosis and consecutive clogging. Furthermore, it is essential that the bioengineered scaffolds should have adequate mechanical properties similar to those of native vessels. Electrospinning technique is known as an efficient processing method to manufacture nanofibrous structures. This processing method is being explored as well for drug release systems, wound dressings, and scaffolds for bone regeneration. A non-woven nanofibrous structure shall mimick the nano structure of fibrous extracellular matrix components. Moreover a tubular geometry of desired diameter could be created by electrospinning for seeding smooth muscle cells and endothelial cells as basis to bioengineering a vessel. Polyurethanes (PUs) have been widely applied in many fields, especially as biomedical materials owing to their excellent elasticity, mechanical properties and biocompatibility. PUs often posses a micro-phase separated structure, which is thought to contribute to hemocompatibility of these polymers. Recently, we reported about nanoor microfibrous PU-membranes and tubes with a uniform structure and controllable fiber diameters by electrospinning. But the low hydrophilicity and cell affinity of PUs became a bottleneck when they were used as scaffolds for artificial blood vessels. Gelatin is a natural degradable polymer derived from collagen. Gelatin has many merits, such as biodegradability, biocompatibility, and commercial availability at relatively low cost. Gelatin has been found to improve spreading and proliferation of endothelial cells. Therefore, it has been widely explored for medical applications such as scaffolds and microspheres for tissue-engineering and drug delivery systems. Heparin is a highly-sulfated linear glycosamine, which plays a critical role in the regulation of the blood clotting cascade and therefore has been widely used as an anticoagulant. The application of antiproliferative agents to the localized adventitial surface of injured blood vessels has been previously found to be effective in reducing stenosis. The ideal artificial blood vessel should have appropriate mechanical properties, compliance matching of nature vessels, non-thrombogenicity, high tissue compatibility. Constituents based on biopolymers and PUs could contribute to the mimicry of the characteristics of native blood vessels meanwhile the PUs nanofibrous scaffolds could improve the mechanical properties of the scaffolds. In this paper, we aimed to develop a bilayer nanofibrous scaffold, which mimicks the morphological and mechanical properties of a native blood vessel. A bilayered construct was prepared by sequential deposition of gelatin layer and PU layer by electrospinning on a rotating mandrel-type collector. Bilayered tubular scaffolds composed of elastic PU fibers as the outside-layer and hemocompatible gelatin-heparin fibers as the inner-layer. Heparin retained its biological activity after the electrospinning process. The release of heparin over an appropriate period of time in vitro was achieved, which makes the tubular scaffold a potential candidate as scaffold for artificial blood vessels.