Electron microscopes require high vacuum for operation. If you put in a wet sample you might damage both the microscope and the sample. Drying such specimens in the air or under vacuum can drastically alter their structure or even destroy them completely; they should be dried using a gentler method.
The method used here is known as "Critical Point Drying" using the BAL-TEC 090 CPD, which prevents samples from collapsing during the drying process.
For water to evaporate, the liquid molecules have to cross the gas-liquid interface. As the liquid volume decreases, we find the interface pulling against the biological framework that the liquid is attached to. Delicate structures, like cell walls, tend to collapse by the surface tension as the interface moves by.
To bring the sample from the liquid to the gas phase without crossing a phase boundary, it is driven through the 'supercritical region'. The pressure and temperature are raised above the 'critical point' where the distinction between gas and liquid ceases to exist. Lowering the pressure now, (with the temperature still above the critical level) transforms liquid into gas without crossing an interface.
The critical point for water is 228.5 bar and 374oC. Such high pressure and temperature would destroy all biological samples. Liquid Carbon dioxide is a much more suitable liquid, with a critical point at 31.1oC and 73.8 bar, and it is used therefore as 'transitional fluid'. In some cases, acetone is first used to wash away all water, The acetone is then washed away with high pressure liquid carbon dioxide. The liquid carbon dioxide is then heated until its pressure goes beyond the critical point, at which time the pressure can be gradually released, allowing the gas to escape and leaving a dried product.