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In clinical practice, synthetic vascular prostheses, such as dacron fabric grafts and expanded polytetrafluoroethylene[1-2], polyethylene terephthalate[3] and polyurethane[4] have been developed to supplement the limited supply of native graft materials. They perform reasonably well in high-flow and low-resistance conditions, for instance in large peripheral arteries, but are not suitable for small caliber arterial reconstructions. They may also act as a foreign body in the patients[5]. The possibilities of thrombus induction, embolism and occlusion require extensive anticoagulant/ antithrombotic control, with resulting complications. It is therefore desirable to develop an artificial vessel with lower cost and good biocompatibility in order to overcome these problems.
The application of tissue engineering technologies to the construction of artificial blood vascular grafts has been reported recently[6-12]. Research has demonstrated that tissue engineered vessel construction provided good anti-thrombolitic ability and could accelerate self-repair and tissue growth within a natural matrix scaffold seeded with vascular endothelial cells and with smooth muscle cells[13-14]. Three-dimensional sheets of these cells, with associated extracellular matrix, were rolled over a mandrel to form tubes. Then, the inner tube was seeded with endothelial cells[15]. However, the manufacture of these vessels takes at least 3 months to achieve and longer if the patient’s own cells are used. None of these approaches have resulted in an appropriate tunica media structure, incorporating circumferentially aligned smooth muscle cells between elastic lamellae with functional contractile abilities. In addition, a major difficulty with any organ developed in an ex vivo culture situation is that with time the cells alter their phenotypes as well as their immunogenic properties due to great changes in their external environment. Researchers pointed out that the extracellular matrix (ECM) of porcine origin might be of use as a substitute to artificial scaffolds because of its very low immunogenicity. Many papers have been published about methods to construct vascular scaffolds, such as glutaraldehyde cross-linking, freezing under lower temperature, lyophilization, radiation and decellularization. Wilson et al[16] first applied surfactant and trypsin as decellularization reagents for canine common carotid artery, successfully preparing an acellular vascular scaffold. The ideal acellular method should provide specific properties, for example, complete cell extraction, non-immunogenicity, intact collagen and elastic fibrous structure of ECM, and sufficient mechanical strength.
The aim of this paper was to prepare a novel tissue engineered blood vascular scaffold using acellular porcine aorta with a novel modification, so as to improve the biocompatibility and mechanical properties.

Triton X-100 is a mild decellular reagent, which can remove all cells and sustain the intact ECM structure. In the study, group Ⅲ (with 1% Triton X-100 treatment for 84 hours) had complete cellular extraction efficiency, whilst retaining the integrity of collagen and elastic fibers, and adequately maintaining the structure of the vascular wall. The treatment methods for the other 3 groups all resulted in complete decellularization, but inevitably disturbed ECM structures and also directly induced the degradation of protein fibers. Tension test data demonstrated that the breaking strength of group Ⅲ was superior to the other groups.
In addition, sufficient mechanical strength is also an important feature along with low immunogenicity. The mechanical properties of the ECM are largely a consequence of its collagen fiber architecture and kinematics. Some published papers reported using glutaraldehyde, formaldehyde, EDC and epoxy chloropropane as a chemical cross linker to increase the matrix strength. However, chemical reactions can change the ECM molecular structures and properties and some reagents exhibit cellular toxicity effects, which reduce the viability of the cells being seeded. The special fiber arrangement of ECM leads to orthotropic mechanical behavior of the scaffold, with the preferred fiber direction showing greater stiffness and strength than the cross-preferred fiber direction[17]. The collagen fiber arrangement of the ECM should therefore not be destroyed during modification. The physical approach based on lyophilization and thermal cross-linking can change the mechanical characterization of the acellular vessel matrix. SEM results illustrated that the matrix fibers changed into a bulky rope-like fibrous complex from the initial thin and small fibers present before the modification. The matrix morphology was a result of fibers becoming closer to each other under the higher temperatures, not a molecular chemical reaction. Mechanical test data showed the tensile breaking strength value increased after successive lyophilization and thermal cross-linking. This effect was most pronounced in group Ⅲ, where the tensile strength reached 1.70 MPa after 12 hours of thermal cross-linking.
Vascular endothelial cells can provide not only a physical barrier on the vessel surface, but are also very important factors in regulating vessel flexibility and maintaining the balance of the blood coagulation-fibrinolysis system. The formation of endothelial cell layers with normal morphology and function is the mark of successful tissue engineered blood vessels. Human umbilical cord vascular endothelial cells are accessible and good for a candidate as a type of seed cell in tissue engineered vascular construction. 24 hours later, seeded endothelial cells were shown to have adhered to and spread on the scaffold surface by hematoxylin-eosin staining and SEM images analysis. Most of the endothelial cells displayed a flat shape, showed high cellular viability and distinct differentiation characteristics. After 7 days of in vitro culture, the seeded endothelial cells proliferated in large amounts, fully covered the scaffold surface and formed a thin layer structure similar to natural blood vessel intima tissue. This proves that the endothelial cells derived from human umbilical cord vein have good biocompatibility with the acellular matrix from porcine aorta.
For tissue engineered vascular constructs, especially for arteries and large caliber vessels, the formation of a smooth muscle layer in vitro is also a very important research area. Under physiological conditions, vascular smooth muscle can keep the flexibility and contraction function of vessels.
At the same time, the smooth muscle cells are interacting with endothelial cells and the synergistic action between the two cellular types plays a key role in maintaining the structural stability of vascular wall and its physiological functions. In addition, the morphology and function of vascular endothelial cells cultivated in vitro under static conditions are very different from those of cells cultured under dynamical blood flow in   vivo [18]. Therefore, further research in co-culture of vascular smooth muscle cells and endothelial cells in vitro and effects of dynamical surroundings on tissue engineering blood vessels constructs are pressing needs. We intend to investigate this in subsequent research in vascular tissue engineering.

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