TOOL - Microtome Blade Guide

When aiming for excellent sections and great blade durability, it is important to match blades to the job.

At high performance levels, blades are good for fairly specific tasks:

• 'Select' for 2µm sections of soft tissue, but with little durability
• Feather R35 is similar to our 'Select' or 'Silver' blades
• 'Silver' for 2‐4 µm sections and improved durability
• 'Extremus' for 4µm and thicker sections, excellent durability
• 'Diamond' is for hard materials, even small bones
• 'Gold' is particularly useful for cryo sections

Disposable Blades

For technicians who have never used disposable microtome knives, or for those who occasionally experience problems, the following tips may be helpful. 

Holder

The disposable microtome knife holder is the single largest factor in optimising the results of a disposable knife. If the disposable knife is not adequately secured, vibrations will occur and poor section quality will result. Secondly, a holder that becomes damaged (the slightest nick at the leading edge of the holder, front or back) or has a build up of residue paraffin inside the holder, will limit the holding capability and affect cutting performance.

Clearance Angle

A change in holder or disposables may require an adjustment to the microtome clearance angle setting. This setting may range between 5 and 12 degrees. Trial and error is the best method for determining your optimum angle.

Embedding

When embedding, use slightly smaller pieces of tissue. If a larger piece is used, embed the tissue at an angle. Mount hard tissue at the upper portion of the specimen block.

Tissue Preparation

Pre-chill your block in a mixture of ice and water (slush) before cutting. The combination effects of cooling and moisturising minimises or eliminates compression. Especially before cutting hard tissue, moisturise the face of the tissue for approximately five seconds with a compress of warm water, then cool the block with a tissue-freezing aerosol. These tissue preparation procedures will definitely improve section quality.

Technique

A slightly slower and more consistent microtome speed is required when using disposables. Poor section quality and irregular tissue dimension are evident if speed is too rapid.

Compression Myth

It is a myth that a compressed ribbon is evidence of a distorted tissue section. Although severely compressed ribbons may yield unsatisfactory results, it is also true that the finest tissue sections result from ribbons that are deemed overly compressed. The miracle wonders of the warm water bath process soften and flatten compressed ribbons into perfect tissue sections, without detectable distortion in tissue size or cell disruption.

Water Bath Temperature

A temperature setting of 40 to 42ºC is recommended, especially if the tissue is ultra-thin. Higher bath temperatures may dissolve the paraffin, making it difficult to mount the tissue on a slide.

Edge Quality

Variations in edge quality are not uncommon for disposables manufactured today. Variables such as microscopic defects in steel, the high speed production processes itself and the frailty of human judgement is sometimes difficult, if not impossible, to control. Accordingly, some edges may require a short break-in period to achieve the perfectly honed edge, while others may simple require a small shift in cutting position of the knife.

Profile Descriptions

Low Profile Disposable: Used as a general purpose knife: for cutting paraffin sections. It is dimensionally smaller than a high profile knife and is generally sold with a coated edge. It only fits in a low profile holder. Usual dimensions are 76mm x 8mm x 0.25mm.

High Profile Disposable: Used as a general purpose knife: for cutting paraffin sections. It is slightly heavier and higher than a low profile knife and is sold either as a coated or uncoated edge. It only fits in a high profile holder. Usual dimensions are 76mm x 14mm x 0.3mm.

Edge Types

Uncoated: The edge facets have not been coated. As a result, cutting performance is somewhat limited and cutting resistance and tissue compression may be experienced. However, for may applications uncoated edges perform very well and at an economic price. Uncoated edges perform better if a heavier duty disposable knife is used.

Coated: The edge facets have been coated with an edged enhancing substance such as teflon®, polymer, ceramic or titanium nitride (TiN). The edge enhancement reduces the coefficient of friction, making the edge more lubricious. Tissue slides off the cutting edge easily, TiN specifically increased the longevity of the edge.