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1 The Unique Nature of Glass Alexis G. Clare Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, USA 1.1 WHAT IS GLASS? We tend to think of glass as a single material from which we manufacture many useful articles, such as windows, drinking vessels, and storage containers that can contain quite corrosive liquids, including aggressive laboratory chemicals. Therefore, the glass has to be quite corrosion- resistant and inert, including being able to maintain its optical properties while being in aggressive environments such as a dishwasher or extreme weather. For a material that we generally view as ‘‘delicate,’’ in terms of attack from chemicals, it can be quite resistant. Another extreme environment is the human body. An implanted medical device is subjected to a warm and wet environment with continual fluid flow and com- plex mechanicalCOPYRIGHTED loads, but perhaps more MATERIAL importantly there are many cells, some at work to reject foreign inert materials. They do this by encapsulating the materials with fibrous (scar) tissue. Hip replacements generally last 15 years or so, and in 2009 there was the story of a man who cut himself shaving and out of his chin fell a piece of glass. He had a lump under his chin, but he thought it was an abscess. In fact, 20 years Bio-Glasses: An Introduction, First Edition. Edited by Julian R. Jones and Alexis G. Clare. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd. 2 BIO-GLASSES: AN INTRODUCTION earlier he was involved in a car accident and a piece of the windscreen embedded in his chin, unbeknown to him. The inert glass would have been sealed off from the body by fibrous tissue and over the years it was pushed out through the soft tissue to the skin. This book will illustrate that, for biomedical applications, certain glasses can be active in the body and stimulate the natural healing of damaged tissues. The degradation of the glass is actually encouraged and plays an important role, allowing space for tissue regrowth and actively stimulating cells to produce tissue. Glass is actually far from being a single composition but rather is a state of matter, a subset of the solid state. A glass is a network of atoms (most commonly silicon) bonded to each other through covalent bonds with oxygen atoms. A silica-based glass is formed of silica tetrahedra (Figure 1.1) bonded together in a random arrangement. Window glass is usually based on the soda–lime–silica (Na2O–CaO–SiO2) system. Bioactive glasses also often contains these components, but in different proportions to inert glasses. Chapter 5 discusses the atomic structure in more detail. Si Figure 1.1 Schematic of a silica tetrahedron, the basic component of a silica glass. THE UNIQUE NATURE OF GLASS 3 Glass differs from what we think of as regular (crystalline) solids in a number of ways. A glass does not ‘‘melt’’ in the way a crystalline solid does. If we heat a pure single-phase crystalline solid, at some point the solid will melt with a well-defined melting temperature. Impurities will usually alter the melting point, and the presence of more than one crystal phase will lead to multiple melting points. Nevertheless, when melting occurs, there is an abrupt change from the solid to the liquid. If we attempt the same experiment with a piece of glass, we will not see a sudden change at a well-defined temperature, but we will see the solid ‘‘ease’’ into the liquid, probably a quite viscous liquid. The glass ‘‘transition’’ from the solid glass to the viscous liquid glass is an important property. Basically, the glass is an elastic solid below this transformation region and a viscous liquid above it. The structure of the solid has all the attributes of a liquid, except that the solid does not have the ability to flow on any meaningful time scale. The apocryphal story of cathedral windows in Europe being thicker at the bottom than they are at the top, having flowed due to gravity, is not true: the silicate glasses in windows are only going to flow on something approaching a geological time scale (unless things were really to heat up on Earth, in which case we would not be worrying about cathedral windows). What is even more curious about this glass transition (called Tg for short) is that, unlike a melting point, the range over which it happens and the temperature at which it starts very much depends on how the glass was made in the first place, the rate at which the glass was cooled, whether it had subsequent heat treatments, and so on. For most commercial glass used in medicine and biotechnology, if the glass is cooled from the melt faster, the overall glass structure will have a larger volume (lower density) than one that is cooled slowly. In terms of structure, solid glass and liquid glass look very similar. However, if you were able to take a photograph of the atoms showing their position, in a subsequent photograph of a liquid the atoms would have all moved, whereas in the glass they would be in much the same position as in the first photograph. Essentially, a glass is an elastic solid without the structural periodicity and long-range order of a crystalline material. It looks like a liquid but behaves like a solid. Why are not all solids like this? After all, it seems that there is a lot less rearranging involved in moving through the glass transition than there is in melting a crystalline material. Thermodynamics provides the clue: thermodynamically, systems are generally driven to the lowest energy (stable) state, so most solids would adopt the inherent order of the crystalline state, resulting in a lower potential energy for the solid. 4 BIO-GLASSES: AN INTRODUCTION However, kinetics occasionally gets the chance to overrule thermodynamics and will not allow the ordered structure to form if there is not enough time to arrange the atomic structure and establish the ordered state: hence the American Society for Testing and Materials (ASTM) definition of a glass as being a material that has ‘‘cooled from the melt without crystallizing.’’ The logical question would then be: ‘‘How fast would one have to cool for kinetics to overcome thermodynamics?’’ The speed of this would be dependent upon the composition. Silicate melts can cool relatively slowly without crystallizing (about 20 degrees per minute), whereas for a bulk metallic glass it is more like 2 degrees per second. So, if one has to thermodynamically trick a material into being a glass, what are the advantages? The word ‘‘glass’’ evolves from the Latin word glacies meaning ‘‘ice,’’ and by far the most utilized property of glass is its transparency, which comes as a result of its inherent isotropic nature. Although the atoms are not organized and are generally quite randomly arranged, the glass as a whole has a similar structure throughout. While glass can in principle be made from any mixture of atoms, the majority of commercial glasses are based upon silicates, and these have a transparency from just into the ultraviolet to somewhere in the infrared, with a transmission typically of up to 90%. The clarity of some types of glass used to make optical fibers is such that the fiber is transparent for miles and miles. A piece of window glass does not have 100% transparency in the visible mostly due to reflection loss. The reflection of light from glass depends upon the refractive index, which is the ratio of the speed the light moves through a vacuum compared to its speed in the material. Glasses with higher refractive index reflect more light, and this property is often used for aesthetic reasons. For example, the ‘‘lead crystal’’ that is often used in fine wine glasses is very sparkly due to the high refractive index of the lead-containing glass. The name ‘‘lead crystal’’ is a little deceiving, as glasses are certainly not crystalline – they are amorphous in structure. Reflection loss can be cut down by adding an anti-reflection coating, which is a coating that is based upon the destructive interference of light waves reflected from two interfaces. The limits of transparency in the ultraviolet and the infrared are governed by the electrically insulating natures of the glass and the type of elements and their bonds, respectively. Typically, the more electrically insulating a glass, the better ultraviolet transparency it exhibits. The heavier the elements and the lower the force constant of the bonds in the glass, the more infrared transparent the glass is. Another detriment to transparency is the existence of coloring ions. These are typically either transition metals or rare earths, the transition THE UNIQUE NATURE OF GLASS 5 metals being very strong coloring agents. Hence, for applications where thick optical paths are needed, then highly pure glass is required, because there are common contaminants such as iron in the silica sand used to make glass. The second most utilized property of glass is outstanding chemical durability in a number of different chemical environments. However, it should be noted that the chemical durability of glass is not always quite as outstanding as is sometimes believed. As will be discussed later on, glass does not always either require or desire high chemical durability. However, in comparison to many other materials, the chemical durability of glass is very good, and its isotropic nature lends itself to both the high durability and the control of the lower durability. One of the lesser extolled attributes of glass is its ability to be engineered to meet need.

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