rewrite"It is well recognized that human bone tissue is a kind of hard connective tissue, which plays a crucial role in limb movement, metabolism, organ protection, hematopoiesis and calcium storage. Unfortunately, in recent years, there has been an increasing number of cases of bone tissue defects caused by factors such as viral invasion, tumor resection, traffic accidents and natural disasters. Accordingly, a series of normal and complex physiological activities involving bone tissue are bound to be severely hindered in patients. As a consequence, there is an urgent need to explore a fast and efficient treatment option for bone tissue regeneration and functional reconstruction.
Although the topic of bone defect repair has been favored as a research hotspot by scholars at home and abroad for decades, it has not yet been completely conquered and remains a major challenge to be addressed in the field of tissue engineering. Happily, thousands of studies have unanimously demonstrated that bone transplantation is the most promising strategy for dealing with this difficulty in clinical practice to date. Specifically, depending on the source of the material, bone transplantation is typically subdivided into autologous bone transplantation, allogeneic bone transplantation, xenogeneic bone transplantation, and synthetic bone transplantation. Among them, autologous bone transplantation is highly considered as an ideal therapeutic option due to excellent biocompatibility, violent osteogenic potential and low immune rejection. However, the extremely limited availability of autologous bone greatly hindered the implementation of this program. Both allogeneic and xenogeneic bone transplantation are essentially foreign bone implants for the patient, involving strong immune rejection and potential risk of disease spread. Given the above considerations, it is only natural for researchers to focus the task of repairing bone defects on synthetic bone transplantation, and they have been working on synthetic bone scaffolds for decades.
As raw materials for synthetic bone scaffolds, the so-called bone substitutes, perform a leading role in tissue engineering. In the late 1960s, Prof. Larry Hench defined the development of bone substitutes into three phases based on their ability to bind and react with human tissues in vivo, namely the bioinert material phase, the biodegradable material phase and the bioactive material phase. Table 1 summarizes typical representatives of each phase and their characteristics. It is worth noting that bioinert materials are not meant to be inert in the absolute sense, but are characterized only by insignificant reactions with body fluids.
Among numerous bioactive materials, in particular, bioactive glasses (BGs) are extensively employed for the repair of traumatic and pathological bone defects because of their excellent biocompatibility, fine bioactivity, outstanding angiogenic capacity, as well as strong angiogenic capacity. BGs are essentially a class of inorganic materials based on SiO2-Na2O-CaO-P2O5. Interestingly, they are gradually eroded in vivo or in simulated body fluids (SBF), releasing large amounts of active ions such as Na+, Ca2+, Si4+ and P5+. On the one hand, these ionic products induce the differentiation of progenitor cells to osteoblasts by stimulating the expression of osteogenic genes, and on the other hand, they are deposited on the glass surface to form bone-like hydroxyapatite (HA), which is able to form a strong bond with the host bone. In addition, BGs also possess a unique advantage that their physicochemical properties, degradation rates and biological functions can be modulated by adjusting or altering the glass components. Consequently, BGs have a wide range of research and clinical applications in the biomedical field.
In this work, we first retrospect the demand and success of BGs in bone tissue engineering, and then focus on their preparation methods, activity mechanisms, as well as reinforcement strategies. Subsequently, we collect several commonly used manufacturing techniques for BG-based scaffolds and insightfully summarize the great progress made in the field of bone defect repair by BG-based scaffolds in recent years. Furthermore, we also point out some complex challenges in their preparation and applications and provide some insights into these issues accordingly. In the end, the future directions of BGs are briefly envisioned. In summary, this paper provides a strong support for the dynamic development of BGs in bone tissue engineering." in an academic way