Characterization and imaging of hydrated samples, and specifically biological samples, under electron microscopes is challenging as these operate under dry high vacuum conditions. Unless an environmental SEM is available, any moist or gas vapor found in the chamber causes severe electron scattering results in a significant increase of noise and loss of resolution. Drying of biological samples, on the other hand, can cause severe structural damage to samples due to the high surface tension force of water as it releases from the sample, causing the sample to crush. One very common method to overcome drying artifacts is performing a regulated drying procedure prior to imaging called: Critical Point Drying (CPD).

In this procedure, following chemical fixation, ethanol is gradually introduced into the sample replacing water, and then the ethanol is gradually replaced with liquid carbon dioxide (CO₂). Once completely infiltrated, the proper temperature and pressure are applied to reach a critical point where the liquid physical characteristics of CO₂ are indistinguishable from its gaseous state. At this critical point the CO₂ is removed as a gas without transitioning through a gas–liquid interface avoiding the surface tension that would damage the sample structure.

In the EM Unit we have Baltec CPD30 critical point dryer with specimen chamber (height ~ 35 mm, diameter ~ 22 mm). Metal specimen holders are available for drying larger samples and microporous specimen capsules are used as boat inserts for enclosing small samples.
This method is suitable for any material which can withstand 100% ethanol and Liquid CO₂.