hangingdrop

Hangingdrop (often written as hanging drop) is a term used in laboratory contexts to describe two related techniques that rely on a small liquid drop suspended from a coverslip for observation or growth.

In microscopy and cell biology, the hanging-drop technique is a method for preparing living samples for light microscopy. A tiny drop of the specimen is placed on the underside of a coverslip and then the coverslip is inverted onto a slide or chamber so that the drop hangs from the coverslip. This arrangement minimizes mechanical stress and evaporation, enabling observation of motility, morphology, and behavior of microorganisms or cells in a near-native environment. The technique typically uses volumes under a microliter and is compatible with phase-contrast, dark-field, or differential interference contrast illumination.

In structural biology and protein chemistry, the hanging-drop vapor diffusion method is used to grow crystals. A small drop containing protein solution mixed with precipitant is suspended from the underside of a coverslip over a reservoir of precipitant solution. Over time, water vapor diffuses from the drop to the reservoir, concentrating the solution in the drop until nucleation and crystal growth occur. Conditions such as protein concentration, pH, temperature, and precipitant type are adjusted to promote crystal formation. This method is widely employed to produce crystals suitable for X-ray crystallography and other structural analyses, typically taking days to weeks.

See also: wet mount, microscopy, protein crystallization, vapor diffusion.