Characterization of Complex Drug Formulations Using Cryogenic Scanning Electron Microscopy (Cryo‐SEM)

Abstract The physicochemical properties of complex drug formulations, including liposomes, suspensions, and emulsions, are important for understanding drug release mechanisms, quality control, and regulatory assessment. It is ideal to characterize these complex drug formulations in their native hydrated state. This article describes the characterization of complex drug formulations in a frozen‐hydrated state using cryogenic scanning electron microscopy (cryo‐SEM). In comparison to other techniques, such as optical microscopy or room‐temperature scanning electron microscopy, cryo‐SEM combines the advantage of studying hydrated samples with high‐resolution imaging capability. Detailed information regarding cryo‐fixation, cryo‐fracture, freeze‐etching, sputter‐coating, and cryo‐SEM imaging is included in this article. A multivesicular liposomal complex drug formulation is used to illustrate the impact of different cryogenic sample preparation conditions. In addition to drug formulations, this approach can also be applied to biological samples (e.g., cells, bacteria) and soft‐matter samples (e.g., hydrogels). © Published 2022. This article is a U.S. Government work and is in the public domain in the USA. Basic Protocol 1 : Cryo‐fixation to preserve the native structure of samples using planchettes Alternate Protocol : Cryo‐fixation to preserve the native structure of biological samples on sapphire disks Basic Protocol 2 : Sample preparation for cross‐sectional cryo‐SEM imaging Basic Protocol 3 : Cryo‐SEM imaging and microanalysis