*Review* **Recent Advances in the Application of Characterization Techniques for Studying Physical Stability of Amorphous Pharmaceutical Solids**

**Yanan Wang 1,†, Yong Wang 1,†, Jin Cheng 1, Haibiao Chen 2, Jia Xu 1, Ziying Liu 1, Qin Shi 1,\* and Chen Zhang 2,\***


**Abstract:** The amorphous form of a drug usually exhibits higher solubility, faster dissolution rate, and improved oral bioavailability in comparison to its crystalline forms. However, the amorphous forms are thermodynamically unstable and tend to transform into a more stable crystalline form, thus losing their advantages. In order to investigate and suppress the crystallization, it is vital to closely monitor the drug solids during the preparation, storage, and application processes. A list of advanced techniques—including optical microscopy, surface grating decay, solid-state nuclear magnetic resonance, broadband dielectric spectroscopy—have been applied to characterize the physicochemical properties of amorphous pharmaceutical solids, to provide in-depth understanding on the crystallization mechanism. This review briefly summarizes these characterization techniques and highlights their recent advances, so as to provide an up-to-date reference to the available tools in the development of amorphous drugs.

**Keywords:** amorphous drug; characterization methods; crystallization; physical stability; molecular mobility
