Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 90

The term glass commonly refers to inorganic materials that have an amorphous structure. Although the structure of a glass has short‐range order arising from bonding of an atom with its immediate neighbors, the structure has no long‐range order. Silica can be produced in both crystalline and amorphous states and a comparison of their structures provides a useful illustration of the difference between a crystalline material and a glass (Figure 3.9). Although silica glass is composed of SiO₄ tetrahedra with corner‐sharing oxygen atoms as in crystalline silica, its three‐dimensional structure is composed of a network with no long‐range order.

A problem with silica glass is that it starts to soften only at high temperature (above ~1200 °C), making it difficult to form the glass into objects with desirable shapes. Consequently, metal oxides are commonly added to silica during the production process to obtain a glass that can be formed and shaped more easily. These metal oxides, referred to as network modifiers, add cations to the glass structure and break up a fraction of the Si–O–Si bonds. In quantities of a few percent to a few tens of percent by weight, the network modifiers often include sodium oxide (Na₂O) and calcium oxide (CaO), but a variety of network modifiers are used in many glasses to achieve the desired properties. Figure 3.11 illustrates the structure of a glass with one of the simplest compositions, formed by adding Na₂O to silica and referred to as a sodium–silicate glass. Overall, the positive charge of the sodium cations balance the negative charge of the oxygen atoms that are no longer bonded to two neighboring silicon atoms, called non‐bridging oxygen atoms.

Figure 3.11 Schematic representation of the structure of a sodium silicate glass.

Silicate glasses, composed of the SiO₂ glass‐forming network, are the most commonly used glasses, accounting for over ~95% of the tonnage in industrial and commercial applications. A common silicate glass composition is one that is used for windows of buildings, composed of ~70% SiO₂, ~15% Na₂O, and ~10% CaO by weight, plus minor amounts of other oxides. Borate glasses, composed of the B₂O₃ glass‐forming network, and borosilicate glasses, composed of interspersed SiO₂ and B₂O₃ networks, also see considerable use commercially. Phosphate glasses, composed of the P₂O₅ glass‐forming network, are used to a more limited extent.

Glasses, nondegradable or bioactive, find limited use as biomaterials, mainly to treat diseased or damaged tissues and organs where their compositional flexibility and ease of forming provide attractive benefits (Chapter 7). Radioactive glass microspheres, for example, are used to treat liver cancer. Bioactive glasses, such as the silicate glass designated 45S5, are used to a limited extent to heal bone defects while bioactive borate glass in a microfibrous form is used to heal skin wounds.