Microtome

Important in science, microtomes are used in microscopy for the preparation of samples for observation under transmitted light or electron radiation.

Microtomes use steel, glass or diamond blades depending upon the specimen being sliced and the desired thickness of the sections being cut.

Microtomy is a method for the preparation of thin sections for materials such as bones, minerals and teeth, and an alternative to electropolishing and ion milling.

In the beginnings of light microscope development, sections from plants and animals were manually prepared using razor blades.

[5][6] In his Beschreibung eines Mikrotoms (German for Description of a Microtome), Wilhelm wrote: The apparatus has enabled a precision in work by which I can achieve sections that by hand I cannot possibly create.

Namely it has enabled the possibility of achieving unbroken sections of objects in the course of research.Other sources further attribute the development to a Czech physiologist Jan Evangelista Purkyně.

[10][11] Today, the majority of microtomes are a knife-block design with a changeable knife, a specimen holder and an advancement mechanism.

[17] A sledge microtome is a device where the sample is placed into a fixed holder (shuttle), which then moves backwards and forwards across a knife.

For hard materials, such as a sample embedded in a synthetic resin, this design of microtome can allow good "semi-thin" sections with a thickness of as low as 0.5 μm.

It allows the preparation of extremely thin sections, with the device functioning in the same manner as a rotational microtome, but with very tight tolerances on the mechanical construction.

As a result of the careful mechanical construction, the linear thermal expansion of the mounting is used to provide very fine control of the thickness.

To collect the sections, they are floated on top of a liquid as they are cut and are carefully picked up onto grids suitable for TEM specimen viewing.

Through the non-linear interaction of the optical penetration in the focal region a material separation in a process known as photo-disruption is introduced.

External to this zone the ultra-short beam application time introduces minimal to no thermal damage to the remainder of the sample.

In the laser microtome the laser-microdissection of internal areas in tissues, cellular structures, and other types of small features is also possible.

[19] The wedge profile knives are somewhat more stable and find use in moderately hard materials, such as in epoxy or cryogenic sample cutting.

Finally, the chisel profile with its blunt edge, raises the stability of the knife, whilst requiring significantly more force to achieve the cut.

Glass knives usually have small troughs, made with plastic tape, which are filled with water to allow the sample to float for later collection.

This is achieved by the inflow of a liquid substance around the sample, such as paraffin (wax) or epoxy, which is placed in a mold and later hardened to produce a "block" which is readily cut.

If the knife blade is at right angles (declination=90) the cut is made directly using a pressure based mode, and the forces are therefore proportionally larger.