Product design is an active area of research. In a top-down approach to product design, the required product properties dictate product development from the very basic choice of the functional material to use for a defined task up to the processing required to render the material in a suitable form. This article focuses on processing crystalline solids with a particular field of application in mind and shows how process conditions can be used both to generate a material with particular properties and to tailor these properties. The examples selected are biased towards novel pharmaceutical applications; the principles of the processes and the resulting product properties discussed can, however, be transferred to other areas where materials with similar functions are required. The case studies presented pertain to in situ coating of solid particles (cases 1 and 2) with a crystalline coating of a different chemical nature, to freeze casting for generating porous particles and, finally, to the potential for exploiting phase transformations to generate tubular crystalline solids that can function as micro-delivery devices for other materials. Potential advantages of all four processing techniques are the delivery of well-defined products with tailored properties that can be controlled by judicious choice of the process conditions.
Example 1: Forming and Coating the Solid Drug Simultaneously by Drop SolidificationAn in situ coating process [1][2][3] has the following prerequisites. The multi-component system from which the pastilles are formed must behave as a eutectic. For eutectic systems, one component can be isolated with a high degree of purity [4,5]. If a drop of this multi-component system is brought into contact with a colder environment, crystallization (nucleation and crystal growth) will commence on its surface [6]. If this drop is allowed to solidify in its entirety, the eutectic behavior ensures that the components are spatially separated. From this it follows that the higher-melting component, i.e., the component that crystallizes first, will preferentially accumulate on and near the surface of the pastille. As a consequence, the lower-melting components accumulate in the interior of the pastille. It is then conceivable that a eutectic mixture of an active pharmaceutical ingredient (drug substance) and a suitable coating material can be found that fulfils the above requirements. It then becomes possible to design and control the solidification in such a manner that a pastille is formed (see Fig. 1a) the exterior of which is predominantly composed of the coating material whereas the drug substance accumulates in the center of the pastille.In the following, the solidification of a drop of a binary melt will be described in detail. Matsuoka [7] summarized the binary solid-liquid equilibrium phase diagram types found in the International Critical Tables [8]. According to Matsuoka, it is
