ABSTRACT
Bioactive glasses are the most promising bone replacement/regenerative materials because they bond to natural bone, are degradable, and stimulate new bone growth by the action of their dissolution products on cell. However, they lag behind other bioactive ceramics in terms of commercial success. In this chapter, we review the main developments in the field of bioactive glass and its variants, covering the importance of control of hierarchical structure, synthesis, processing, and cellular response in the quest for new regenerative synthetic bone grafts. Among the various strategies for the development of bioactive glasses, the sol-gel-derived mesoporous bioactive glasses (MBGs) can be foamed to produce interconnected macrospores suitable for the tissue ingrowth, specifically cell migration and vascularization and cell penetration. The combination of excellent textural parameters and
hence an enhanced in vitro bioactivity make the wormhole-like MBGs promising for applications in clinical orthopedics, controlled drug delivery, tissue engineering, etc. This chapter also focuses on the techniques that have been developed for characterizing the hierarchical structure of sol-gel glasses and hybrids, from atomic-scale disordered structures, through the covalent network between the components in hybrids and nanoporosity, to quantify the open macroporous networks of scaffolds. We also provide to the readers the recent advancement in this field from Hench’s 45S5 Bioglass®to new hybrid materials that have tunable properties and superior degradation rates. 3.1 Historical BackgroundTissue engineering has emerged as the most common approach for the regeneration and repair of tissues and organs lost or damaged as a result of trauma, injury, disease, and aging. It has the potential to overcome the shortage of living tissues and organs available for transplantation. In the past few decades, tissues such as skin, bone, and cartilage have been successfully regenerated. It is important to mention that more than 2.2 million bone graft operations are carried out worldwide every year to repair bone-related defects in orthopedics and dentistry [1, 2]. In this context, autografts are the most preferred choice for the treatment of bone defects, but their limited supply and donor-site morbidity lead to serious problems. Being a natural tissue, the successful remodeling of bone strongly depends on various factors such as blood vessel cells, progenitor cells, and growth factors. Alternatively, bone allografts have been developed, but they have many drawbacks such as they are expensive, have the risk of disease transmission, and possess poor mechanical properties [3-5]. Hence, synthetic biomaterials are ideal for bone substitutes. The commercial success of synthetic biomaterials is limited since they do not meet all the requirements of natural bone that for an autologous bone. More commonly, implants for bone replacement or fracture repair in load-bearing applications are made from metallic alloys for mechanical support; typically Ti6Al4V or Co-Cr alloys are used for joint/knee replacements [6, 7]. Although these metallic alloys are stronger and stiffer than human bone, they promote resorption by shielding the surrounding skeleton from its
normal levels and ultimately the implant becomes movable over a period of time. Subsequently, more products are designed to be bioactive such that they stimulate a specific biological response at the material surface, which leads to the formation of a bond between the tissue and the material. A typical example is synthetic hydroxyapatite (HAp), which has a chemical composition similar to that of the bone mineral. However, the slow biodegradability and low mechanical strength of HAp ceramics restrict their use in tissue engineering applications [8-10]. Bioactive glasses are one of the most attractive synthetic bone replacement materials available since they are potentially more bioactive than pure calcium-phosphate-derived materials. In 1969, Hench and his coworkers discovered that bone could chemically bond to glasses having certain compositions. This group of glasses is referred to as bioactive glasses or bioglasses [11, 12]. An important characteristic of these glasses is the formation of a hydroxycarbonate apatite (HCA) layer on their surface in aqueous solutions. Bioactive glasses can be produced in a wide range of forms, and they serve various functions in the body. The bioactive glass 45S5 Bioglass (46.1% SiO2, 24.4% Na2O, 26.9% CaO, and 6% P2O5, in mol%) was the first material produced by Hench in 1971 [11, 12]. After Hench’s discovery, various kinds of glasses have been synthesized by numerous research groups. However, no other bioactive glass composition has been found to have superior biological response over the original 45S5 glass composition. Due to the differences in composition, structure, and constituent phases, the bone-bonding properties of these materials differ from each composition. The relative bioactivity among different materials can be evaluated by measuring the rate of bone formation on the surface of the material. The 45S5 Bioglass, which was prepared by the conventional melt-derived method at the earlier stages, is most widely studied and used in clinical applications. The abbreviation indicates that it contains 45% SiO2 by weight and that the molar ratio between Ca and P is 5:1. Three key compositional features of the SiO2-Na2OCaO-P2O5 bioactive glasses distinguish them from traditional sodalime-silica glasses: (1) less than 60 mol% SiO2, (2) high Na2O and CaO content, and (3) high CaO/P2O5 ratio. These are the salient features that make the surface of bioactive glasses highly reactive when exposed to an aqueous medium.
