Rewrite “In 1969, Prof. Larry Hench and his colleagues first developed 45S5 Bioglass® (46.1SiO2-24.4Na2O-26.9CaO-2.6P2O5, in mol%). Later, they discovered that 45S5 showed excellent bioactivity and degradation properties in solution environments, and when implanted in bone defects in vivo, it could also bind to the surrounding host bone activity and assist in bone tissue repair, which created a huge sensation in the medical field. From then on, a variety of BGs were designed and prepared one after another. In 1997, Maria Brink adjusted the original proportions of the 45S5 components and newly introduced small amounts of alkali metal oxides K2O and MgO to obtain 13-93 BG. Compared with 45S5, 13-93 BG had a slightly slower degradation rate, but higher mechanical strength and better processing properties, while no significant distinctions were observed in terms of bioactivity. Subsequently, Huang et al. substituted partially or entirely the SiO2 content of 13-93 BG with B2O3 and maintained the same amount of modifiers to obtain 13-93 B1, 13-93 B2, and 13-93 B3 in sequence. They noted that the ion release rate, degradation rate, and conversion rate of HA in phosphate aqueous solution were improved to varying degrees for the B2O3-substituted BGs compared to 13-93 BG. Nowadays, related scholars are still committed to the research and development of BGs, and they synthesize new BGs by adjusting the original composition of existing BGs or introducing other oxides into the glass system. Table 2 enumerates the composition of some frequently-used BGs in bone tissue engineering.
2.2 Preparation methods of BGs
Generally, the synthetic channels for BGs mainly cover melting-quench (MQ) method and sol-gel (SG) method. The former is a conventional preparation method with the advantages of high efficiency, simple operation, controllable process, short preparation cycle and large-scale production, which was followed in the synthesis of early BGs such as 45S5 and 13-93. The latter is an emerging method for preparing BGs since the 1990s, which is characterized by low energy consumption and equipment requirements, typically represented by products such as 58S and 80S. A large amount of literature indicates that BGs prepared by different methods vary in structure, physical properties, degradation characteristics, and bioactivity.
2.2.1 MQ method
The schematic diagram of preparing BGs by MQ is shown in Figure 1. In general, firstly, the corresponding oxide or carbonate powders are calculated and weighed according to the designed composition ratio of the glass. Next, after thorough stirring, the mixture is transferred to a platinum crucible. Then, the crucible is heated in a muffle furnace, and the muffle furnace stops warming up and maintains a constant temperature for 2-4 hours until the temperature rises to the point where the mixture is completely melted (~1200-1600°C). Finally, the molten glass liquid is rapidly quenched using the steel plate cold pressing method to obtain bioactive glass blocks. Whereafter, the glass blocks can be crushed, ball-milled and sieved to hold a specific particle size according to the actual application requirements.
BGs prepared by MQ method feature excellent mechanical properties, good processability and adjustable degradation. However, the components of such glasses are difficult to control precisely. There are two main reasons for this phenomenon. On the one hand, small amounts of impurities adhering to the crucible surface can readily enter the system during the melting of powder materials, which undoubtedly alters the original composition of the glass, and on the other hand, the serious loss of volatile components during the melting reaction tends to result in the component segregation to a large extent in the multivariate system. In addition, when the SiO2 content in the glass system is excessive (over 60%), the degradation characteristics and bioactivity of BGs will be drastically decreased or even lost. Hence, the MQ method has certain limitations in the preparation of BGs.
2.2.2 SG method
The schematic diagram of preparing BGs by SG is shown in Figure 2. Firstly, tetraethyl orthosilicate (TEOS), distilled water and dilute nitric acid solution are added into a clean beaker in a certain ratio and next stirred for 1 h with the help of a magnetic stirrer. Then, triethyl phosphate (TEP) is added to the mixed solution and stirring is continued for 1 h. After that, an appropriate amount of nitrate is added to the solution and the stirrer is removed after another 2-3 h of stirring. Finally, after aging (~ 2-3 d at room temperature), drying (~ 2 d at 70°C) and calcination (~ 2-3 h at 600-800°C), SG derived BGs are obtained.
Such BGs are produced in the liquid phase by hydrolytic condensation reactions, ending up with a highly homogeneous glass composition. More attractively, they tend to be characterized by large specific surface area and high porosity. When applied in bone tissue engineering, the large contact area with physiological body fluids helps to facilitate the solubilization of active ions and the formation of HA. Therefore, BGs prepared by this method are usually famous for their high bioactivity. However, SG derived BGs also have certain inherent drawbacks, such as internal susceptibility to phase separation, low mechanical strength, and easy agglomeration of powder particles.
Figure 3 summarizes the physicochemical properties and bioactivities of the BGs synthesized by these two methods. It is well known that the bioactivity of BGs depends on their specific surface area. However, it is worth noting that a large specific surface area is not absolutely favorable for bone tissue regeneration and repair. One powerful explanation for this viewpoint is that it also significantly increases the possibility of uncontrollable degradation of BGs simultaneously.” in an academic way