Materials Fundamentals of Drug Controlled Release

Contents

2.1 Introduction of Materials Nanostructure

In Section 1.2 materials chemistry and processing technology were introduced and discussed in the context of the two main production routes to make glass materials for controlled release applications, that is, fusion and sol-gel. Due to its cultural and historical significance the word ‘glass’ is ubiquitous in many publications and is synonymous with a certain type of glassy material based around soda-lime glass—container-ware and window glass being the most common examples. However, glassy materials made by the two different processes, fusion and sol-gel, have different structural properties as a consequence of the molecular process involved during their formation. To fully understand the fundamentals of inorganic controlled release technology (inorganic CRT) materials science, it will benefit the reader of the book to introduce the theories and fundamentals of amorphous materials, in particular, on the physical aspects.

2.1.1 The Structure of Amorphous Materials

There is a general consensus that the word ‘glass’ signifies only that transparent material, which is mainly made from silica plus small amounts of other oxides, and is used for producing windows. We will transform this simple understanding on transparent materials into fundamentals of amorphous materials in view of their nano/microstructure.

Amorphous materials are generally regarded as non-crystalline materials. They are not new; the iron-rich siliceous amorphous materials recovered from the moon by the Apollo mission are some billions of years old. The history of man-made amorphous materials, the glass from silica, has been known for at least several thousand years. Although the science of the materials chemistry is well established, the material physics is not so well studied and understood to date. There is growing interest in the fundamental science of amorphous materials because most recently there has been great interest for their use for novel applications and technological areas. Healthcare is one of these areas including the materials science and technology for inorganic CRT, discussed here.

In general, non-crystalline amorphous materials possess randomness to some degree. The questions are what kind of randomness is present, and what is the best way to define it? There needs to be a standard based on which we can define amorphous materials and their structure. This standard is the perfect crystal, with the following definition:

In this book, we regard the repeating periodicity that is in all three dimensions. However, two-dimensional (2D)1,2 and one-dimensional crystals3 have been reported, in order to simplify the classical definition of a perfect crystal. The basic element of the perfect crystal is an atom or group of atoms, upon which the structure of the perfect crystal is built starting at the nanometre scale and growing periodically by an infinite extent and ending at surfaces (non-infinite extent), or at a defect, that is, foreign atoms or a dislocation of the crystal.

Considering the standard definition of a perfect crystal, we now have the following definition for an amorphous structure:

To understand the nature of long-range periodicity, Figure 2.1 shows schematically the periodicity of a perfect 2D crystal and the randomness of a 2D amorphous structure. It is seen that the non-crystalline structure consists of no translational periodicity (Figure 2.1b), which is in contrast to the crystalline structure where there is such translational periodicity (Figure 2.1a). Understanding the structural arrangement of atoms in a solid substance is essential for the understanding of its physical properties, and this is true for both crystalline and amorphous materials. Determination of a crystalline structure is made straightforward by measuring a ‘unit cell’, containing relatively few atoms in most cases. This unit cell is the fundamental building block upon which crystals are formed: by repeating in a periodical fashion the position of the unit cell in space. Such a procedure is impossible for a non-periodic amorphous solid, for which the unit cell is regarded as being infinitely large. Figure 2.2 shows a typical crystalline amorphous X-ray diffraction (XRD) profile, and Figure 2.3 shows an amorphous material XRD spectrum (detailed analysis of XRD will be introduced in Chapter 3).

The relative simplicity of the amorphous material XRD diffractogram belies the actual complexity of the system. The forms of randomness with which we are concerned create more uncertainty than a perfect crystal. Imaging the huge number of forms, scales and extents of amorphous randomness are all variables in view of the nanostructure. The structural variation of an amorphous material will at least affect the location, distribution, and interaction of any active ingredients present within the delivery matrices used in inorganic CRT applications.

In reality, the structure of amorphous materials is not complete randomness; they often have short-range order. Further still, some amorphous materials have considerable short-range...