Investigations on the formation of hollow acicular crystals and their usage as container systems

The coating of sensitive compounds is of high interest for food and pharmaceutical industry. While most of the coating materials form amorphous layers around the material, studies on crystalline coatings are rare. In the presented work, the progress in the research on hollow needle crystals and, moreover, innovative results concerning their usage as coating material are introduced. Hollow crystalline needles have mesmerized scientists from different disciplines for decades and have initiated research on the development of new production methods and the clarification of their formation mechanisms. This work focuses on acicular crystals, which are produced by solvent-mediated phase transitions and develop a cavity along their longest axis during their growth. Since the first reports on tubular crystals formed via a solvent-mediated phase transition from solvates to hydrates, the field can be widened to hydrate-to-anhydrate and anhydrate-to-hydrate transformations. Novel crystallization methods are presented and successfully introduced as fast and reproducible production processes for hollow crystalline needles. By means of these methods new needle-forming compounds are described as potential container systems. Moreover, the insights into the crystallization process provide first evidence on the cavity formation mechanism. It can be shown that by the utilization of these crystallization methods, a direct proof for a successful filling of the hollow needle crystals is feasible. The combination of the simple crystallization methods and the, moreover, introduced filling procedure represent the first steps in the development of hollow crystalline needles as protective container systems in pharmaceutical and food industry