Due to the high water content, fixed biological samples must undergo a gentle drying process. In this way, e.g. shrinkage or rupture of membranes, triggered by the evaporation of water in the vacuum, is prevented. In the first step, water is substituted by a series of increasing alcohol dilutions (30%, 50%, 70%, 90% and 100%), e.g. with ethanol. Afterwards, the alcohol, which has a much lower surface tension than water, is replaced by hexamethyldisilazanes (silane) and the sample is air-dried (fume cupboard). A more complex method than air-drying with silane is "critical point drying". Here, liquid carbon dioxide (CO2) is introduced into a pressure chamber. When the pressure and temperature exceed a "critical point", the pressure is reduced again and the dried sample is removed at normal pressure. Other drying methods are lyophilisation and vacuum drying for samples with low water content.