The Properties Needed for an Ideal Scaffold
As mentioned above, satisfactory tissue engineering scaffolds should be able to replicate the biological functions and structural support of the native extracellular matrix. Firstly, the clinical success of the scaffolds is largely dependent on their material composition. One of the major concerns in scaffolds is the selection of appropriate raw materials. Various materials, such as organics, inorganics and metals, etc., have been tried to prepare scaffolds for many years. Actually, the selection of scaffold materials depends on the tissue type that they need to repair. For example, because calcium phosphate ceramics exhibit high degree of bioactivity, osteo- conductivity and wear resistance, they have been used to prepare hard tissue engineering scaffolds for decades.
However, although scientist have tried to evaluate the specific abilities of various single type of materials in order to search a perfect environment for the regeneration of tissues, most of the results are not satisfactory. In many cases, the scaffolds made of single type of material can hardly meet fully the requirements for the desired tissue repair. Therefore, biocomposite scaffolds have been more and more developed because they can combine all the advantages of every component whilst minimizing respective disadvantages. Hence, a composite usually has better properties than each component. On the other hand, based on the bionics principle, the tissue engineering scaffolds should be prepared as biocomposites because most of natural tissues are composed of more than one component, such as bone, cartilage, skin, nerve and blood vessel, etc. Especially, since many scaffolds made of single type of material have insufficient mechanical properties, other materials with high mechanical properties are normally needed to reinforce them.
Besides the composition, the second deciding factor for the performance of a scaffold is its structure. The architecture and microstructure of the scaffolds define the ultimate formation productiveness, shape, structure and properties of the regenerated tissue and organs. An ideal scaffold should possess porous structure, which had better contain both macropores and small micropores. The macropores facilitate the penetration and ingrowth of cells, blood vessels and tissues . The pore size may vary depending on the type of tissue engineering. The pore size should be normally at least 100 qm in diameter for cell survivability and successful desired tissue regeneration . In the case of bone engineering, the appropriate pore sizes are in the range of 200-350 qm . The porosity should be normally at least 80%. However, because the increase of the porosity unavoidably leads to the decrease of mechanical properties, the porosity should be decided according to the requirements of the regenerated tissue and the special implantation site. Furthermore, the pores should be interconnected to provide possibility for the necessary nutrient substances and oxygen to diffuse and arrive at the sites of the cells or tissues inside the scaffolds, and for the their metabolites and the scaffold biodegradation products to discharge out. Meanwhile, the micropores of smaller than 10 qm can concentrate more proteins that will improve further the bioactivity of the scaffolds and thereby promote the functions of the attached cells . Most importantly, the structure of the scaffolds should maintain its frame integrality during the whole period in vivo. to ensure the desired activities of the cells and satisfactory performances of the tissues. Finally, the external shape of the construct must be considered, particularly if the scaffold is customized individually .
As for its overall performance, an ideal scaffold for tissue engineering should possess all the qualities of a native extracellular matrix (ECM) and should function in the same way as that of ECM under physiological conditions. However, there is no clear and recognized guidance yet, according to which, the so-called ideal scaffold is defined. To date, numerous scaffolds produced from many types of biomaterials have been used to regenerate different tissues and organs, including bone, cartilage, blood vessels, nerves, skin, liver, etc. On this base, an ideal tissue scaffold should meet several performances. Since many scaffold-based tissue engineering approaches are still experimental, it is not yet clear and hard to say how to accurately design an so-called ideal scaffold. Additionally, each tissue type requires specific properties of the scaffolds. However, regardless of the tissue type, scaffold design should follow a set of minimum requirements [5, 32].