Hollow and solid whiskers are formed during the solvent exchange from the dimethyl sulfoxide or dimethylformamide solvates of dexamethasone acetate to the sesquihydrate by immersion of the initial phase in water. Detailed investigations using mainly optical and scanning electron microscopies lead us to propose a growth mechanism of the hollow whiskers. Six criteria appear to be necessary for the formation of thin tubular crystals during the solvent exchange. To reproduce the crystallization occurring at the surface of the initial crystal, an experimental assembly was made with a syringe and a glass filter. When the six conditions are fulfilled, this technique permits the production of 100% of hollow whiskers.

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