Metals
Metal is typically a hard and shiny material (an element, compound or alloy) with good electrical and thermal conductivity. They usually have the great plasticity to be industrially processed into ideal models for different applications. In recent decades, metals have shown promising applications in tissue engineering because of their outstanding mechanical properties, especially in bone regeneration. It has been shown that various metals are suitable for biomedical design from operating instruments to engineered scaffolds. Here we prefer to put more emphasis on porous metal materials, which meet the oxygen and nutrition demand in tissue engineering. In recent years, researchers have been looking into stainless steel, co-base alloy, memory alloy, electron etc., and the devolvement of porous metals has been setting the stage for the concept of osteointegration of metallic implants [11]. For example, Nover et al. [12] proved that porous titanium supported robust cartilage growth, making it a viable bone-like base material for the fabrication of engineered osteochondral tissue constructs, a clinically relevant alternative to allografts. Porous metal materials can be classified as independent-hole type materials and continuous- hole type materials. The independent-hole material has the features, such as lower density, better rigidity, higher intensity, excellent vibration absorption and sound absorption. Apart from these properties, the continuous-hole material owns the advantage of being permeable and ventilated as well.
However, the traditional metallic biomaterials have the tendency of corrosion and stress shielding in human body environment and are not the best candidate for bone engineered scaffolds. It is reported that fiberglass-reinforced fibers containing glass fibers and matrix metals have high tensile strength and elasticity while accelerating the rate of degradation [13]. Recently, magnesium alloys have been recognized as a new class of biodegradable biomaterials with interesting orthopedic engineering applications, as increased bone growth has been reported repeatedly [14-16]. The Fig. 2.2 shows the entangled 3D porous structures of magnesium scaffolds with different pores [17], and the magnesium scaffolds with different architectures also have been investigated. Witte et al. [18, 19] studied the possibility of an open-porous magnesium scaffolds used as temporary bone replacements. The enhanced formation of unmineralized ECM and the enhanced mineral apposition rate adjacent of the degraded magnesium scaffolds were accompanied by an increased osteoclastic bone surface, which resulted in higher bone mass and a tendency to a more mature trabecular bone structure around the magnesium scaffolds. It indicated that even fast degrading magnesium scaffolds showed a good biocompatibility and reacted in vivo with an appropriate inflammatory host response. That is to say, magnesium was a promising matrix selection. Zhang et al. [20] prepared the porous magnesium scaffolds using the fiber deposition hot pressing (FDHP) technology, magnesium as the matrix material. This kind of fiber-reinforced magnesium scaffolds, having the 3D interconnected microstructures with the porosity ranging from 33 to 54%, showed comparable elastic modulus and compressive strength to those of cancellous bone. Cheng et al. [21] successfully prepared two open porous magnesium scaffolds using the titanium nanotube as the space holder. The porosity and pore size could be easily, precisely and individually controlled, and the mechanical properties also could be regulated within the range of human cancellous bone by altering the orientation of pores without sacrificing the requisite porous structure. The animal tests demonstrated that the scaffold exhibited acceptable inflammatory responses and could be fully degraded in vivo. In addition, the scaffolds with larger pore size promoted early vascularization and up-regulate
Fig. 2.2 Tomography images of entangled architectured porous Mg (a and c) and the 3D spatial structures of the pores (entangled channels) in Mg (b and d). [17] (Adapted with permission from Ref. [17]. Copyright 2016 Elsevier Ltd)
collagen type 1 and OPN expression, leading to higher bone mass and more mature bone formation.
However, magnesium alloys have not get the FDA approval to date, which places a big obstacle in the path of magnesium scaffolds development for bone engineering applications. The researchers tend to use the materials that already have the FDA approval to guarantee the safety of scaffolds in the clinic. The researchers are unwilling to risk using new materials considering the time and financial cost. So the nanofibers or nanotubes are great choices to make up for this shortcoming.
