Ultrahigh resolution scanning electron microscopy of biological materials.

In recent years, several ultrahigh resolution scanning electron microscopes (SEM) were successively developed. They were all equipped with a field emission electron gun and an objective lens with a short focal length, and showed a resolution better than 1 nm. With such instrument, not only intracellular structures but also virus, bacteriophages, and biological macromolecules were clearly observed. With improvement of the instrumental resolution, some unexpected problems came to the fore in the area of specimen preparation. The first is the metal coating of the specimen, because coated metal particles are plainly seen as rounded "pebbles" at very high magnifications. For observations at the high magnifications, therefore, uncoated and not conductively stained specimens were used. The second problem is contamination by the electron beam. This problem is complicated and remains unsolved. Although the ultra-high resolution scanning electron microscopy has just begun, it will surely open new research fields in biomedicine.