Critical point drying method

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

Tanaka K, lino A. Critical point drying method using dry ice. Stain Techno 1974 49 203-206. 

The main problems and difficulties of the microscopic observation by both TEM and SEM are how to prepare a membrane sample without any artefact. The first step of the preparation is a careful drying of the sample, and in order to avoid collapse of the original structure, the freeze-dry technique using liquid nitrogen or the critical-point drying method with carbon dioxide is usually employed. 

Ciyo-SEM methodology also facilitates the observation of highly hydrated systems. Harker and Sutherland [69] used the ability of cryo-SEM to preserve the structural integrity of the aqueous phase to characterize differences between mealy and non-mealy nectarines. The presence of juice on the surface of cells in non-mealy nectarines was observed after tensile tests produced a fractured surface. Such observations would not have been possible with conventional methods where dehydration and critical point drying are essential steps. A strong point to this study was the extensive use of other physical and chemical methodologies to help correlate textural difference based on storage parameters for nectarines. 

Since the 1980s a new microporous insulation material based on silicic aerogels is in a test phase. About 60 years ago Kistler from Stanford University developed a method to produce aerogels from silicic material without shrinking them He reacted waterglass with hydrochloric acid, washed out the sodium and chloride ions from the gel, substituted the water with alcohol and dried the gel in an autoclave above the critical point. Other methods begin with tetramethoxysilane for the production of monolithic aerogels. 

Critical point drying A method of drying delicate samples for electron microscopy by freeze-drying at the critical point of water. At the critical point of agiven substance, the densities and other physical properties of the liquid and gaseous states are identical. 

Following fixation, samples can either be viewed as whole mounts, using critical-point drying, or by other methods to dry the sample (Hayat, 1989), or... 

Where a cell surface antigen is of interest, and particularly where the cells are grown in vitro, a labelling method which avoids all the problems of fixation and embedding can be used. Antibody and conjugate incubations can all be completed prior to fixation and embedding for TEM or critical point drying for SEM. 

The drying preparation method in greatest use among biologists today is critical point drying (CPD). The method was first described by Anderson [420-422] and then by Hayat and Zirkin [423]. A clear explanation of the method and applications are given by Anderson [422, 424] and Cohen [425], while a somewhat more current review by Cohen [426] is recommended to the interested reader. 

Finally, for critical point drying of polymers a suitable series of fluids and conditions must be chosen that will not damage the specimen. Carbon dioxide dissolves in epoxies and in polystyrene. Apparently long term high pressure exposure and then a sudden release can turn these polymers into popcorn. Microscopic comparison of a material prepared by a variety of different preparation methods is the best way to uncover any possible artifacts caused by the specimen preparation method. 

Fig. 5.30 High resolution SEM images of a critical point dried and fractured FBI membrane reveal the fine structure quite clearly. The overview micrograph (A) shows the macrovoids (V) and the porous walls within the membrane. The robustness of the macrovoids suggests that the method is useful for observation of the in situ structure. The dense surface layer is composed of spherical particles (arrows) that are deformed into a dense monolayer while the support structure below is formed by a more open network of these spherical particles (B).

In the case of emulsions, latexes, some adhesives and wet membranes, the specimen of interest is wet, generally with water and must be dried prior to electron microscopy. The deleterious effects of air drying result from the stress of surface tension forces. The methods used by biologists [334] to avoid this problem are (1) the replacement of water with an organic solvent of lower surface tension, (2) freeze drying or (3) critical point drying. Some latexes can be fixed in their... 

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