*New Trends in Bioactive Glasses for Bone Tissue: A Review DOI: http://dx.doi.org/10.5772/intechopen.100567*

**Table 2.**

*Composition of bioactive glass and glass ceramics (% weight) [18].*


The biomaterial versus tissue interface, which is established by implantation, is almost inevitably a blood ÷ material interface and the initial events are dominated by the absorption of blood proteins on the implant surface. At this contact it was established that a series of biological processes: Adsorption and desorption of biological growth factors, in the HCA layer, which determines the activation of stem cell differentiation; The action of macrophages, which phagocytose local residues, allowing cells to grow; Attachment to the bioactive surface of stem cells; Differentiation of stem cells with the formation of bone growth cells, called osteoblasts; Osteoblasts generates extracellular matrix with bone formation; Crystallization of the phosphate inorganic matrix by embedding bone cells in a living composite structure (**Figure 4**) [19].

The chemical and topological properties of the implant surface strongly influence the properties of the biolayer and this influence must be understood and controlled in order to optimize the biocompatibility of the material used. Relevant in the study of biocompatibility is the fact that proteins and cells have nano- and micrometer sizes, which requires extremely delicate approaches. Of equal importance are the properties of cells, for example, their ability to communicate via the extracellular matrix with signal molecules (molecules used in the process of living cell synthesis). During tissue healing, numerous bioactive signaling molecules control tissue formation, and some proteins have demonstrated the ability to stimulate healing near the implant. All these mechanisms contribute to the response of the tissues to the implant and can determine whether the body accepts the implant or not, whether it is biocompatible.

Japanese researchers have tested the effect of surface area on bone proliferation. Three types of biomaterials were compared: bioactive glass, dense hydroxyapatite and glass ceramics. Each material was implanted in a 6 mm diameter hole, which was drilled into the bone of an adult rabbit's leg. Bioactive glass has been found to produce

**Figure 4.** *Bioactive glass surface reaction [19, 20].*

bone tissue and is subsequently resorbed much faster than the other two materials, both of which have a lower surface reactivity than glass.

The rate of bone growth around an implanted material depends in part on the rate of dissolution of the silica network and therefore it is very good to determine as accurately as possible the system in which the oxide composition of the bioglass.

Alkaline content plays an important role in the stability of bioglass. From this point of view, two categories are distinguished: bioglass with rich alkaline content and bioglass with poor alkaline content. The latter are characterized by a high degree of decomposition over time, during bone reconstruction. This type of bioglass has been used in maxillofacial applications and in the chaining of the inner ear bones.

Most determinations were made with glasses based on 6 oxides: SiO2-Na2O-K2OCaO-MgO-P2O5, as it was found that the bone binds to materials with a wide range of compositions in this system. Soft tissue binding occurs for a much smaller range of compositions.

There are three basic compositional requirements for silico-chalco-sodium glasses to bind to hard tissue. These are: less than 60% SiO2 (mol), high content of Na2O and CaO, high CaO / P2O5 ratio. The level of bioactivity is strongly dependent on the relative concentrations of ions.

The most successful bioactive glass is the one that contains P2O5 between 6 and 15%.

In the diagram of the SiO2-CaO-Na2O ternary system (6% P2O5), some materials form a bond with the bone in 30 days. Other glasses bind to the soft tissue, some of the glasses are almost chemically inert and others are resorbable and dissolve in 10 to 30 days.

Bioglasses from another part of diagrams, from a technological point of view, are not forming glass and have not been tested as implant materials. Until now, it has been considered that in order to be bioactive, glasses and glass-ceramics must contain both CaO and P2O5, which are the component oxides of hydroxyapatite.

Ohura and collaborators have shown that glasses in the CaO-SiO2 system without P2O5, as well as those containing very small amounts of P2O5, form a layer of hydroxyapatite on their surface when immersed in simulated body fluid (SBF). In contrast, under the same conditions, the glasses in the SiO2-free CaO-P2O5 system do not form the hydroxyapatite layer. It follows that bioactive compositions can be obtained in the CaO-SiO2 system rather than in the CaO-P2O5 system.

Bioactive glasses usually have weak strength and resilience properties, which is why they are reinforced with metal fibers made of stainless steel, titanium and Co-Cr alloys. As a result of the reinforcement with metal fibers, the volume of defects and the residual tensions decrease, and the microcracks produced are below the critical length and have rounded extremities.
