Unsimple matters of simple blades

Although the history of the microtome begins in the 1770s, it is believed that the first microtome was created by Mr. Wilhelm His Sr in 1865. Nevertheless, there will be a time for the full history of the most common device in pathology, but due to recent discussions with our partners, today I decided to introduce some knowledge about microtome knives. There are a huge number of them on the market, Factories compete in new technologies, well-known brands shift production to more economical facilities, and new producers come on their places. The flood of new, cheaper blades does not necessarily comes with the quality, and the price tempts the customer. But in such a case it is often an apparent saving.

On the left: A diagram of a microtome drawn by Cummings in 1770

On the right: Mr William His Sr.

How are they different? Why are they so expensive? After all, does it make sense to change well-tried knives?

I will try to answer that today.

The first criterion to be used is the hardness of the steel. According to my recent research, knives most often have a hardness in the region of 56-58 HRC. For those who don't know what the listed value is; it is a unit of measurement for Rockwell hardness scale, where a diamond cone with a tip angle of 120o presses with a load of 150 kg on the metal surface. There is a small indentation by which we can judge how hard the metal is. Probably this still doesn't mean much to you, so three examples:

- an axe 45-55 HRC,

- tool and high-speed steels 55-66 HRC.

- a really great kitchen knife - 63 HRC

Actually, what is affected by hardness and why not produce knives with 66 HRC?

It primarily affects the service life. The harder the steel, the more durable it is. However, this has its drawbacks. It is very difficult to process such a steel on such a small scale as the microtome blade (0.25 mm thick). Thus, at the same time, the price would be much higher than the currently available products. The values ​​of 56-58 are a big compromise, although on a scale where every single point is like a civilization leap, increasing the hardness to 60HRC could bring amazing benefits (in my humble opinion). Only that, unfortunately, no one would make money on it. How to compete when we can hear the price of 120 Euro instead of 50?

I know one thing for sure, in addition to durability, unmatched sharpness is also important. Here the differences are much bigger. At the first glance, I will present the types of sharpening:

- Strongly flat-concave / biconcave blades - flat-concave blades are those that are single-sided cutting knives. The cut side is heavily ground into a concave arc (unless there is a word for reverse arc) and the other side is flat. Double-concave blades have an arc-shaped cut on both sides. These knives are very sharp, suitable for soft tissue and routine work. However, they do not work very well for hard or even cartilage material. It would be ideal to work with these knives with material embedded in celoidine.

- Flat-concave blades - these are blades similar to the previous ones, but the cutting surface itself is thicker, which makes them more durable. Here, definitely the routine work with paraffin blocks comes into play. Nevertheless, these are still not knives that will perform well in continuous work with hard materials or paraffins with a high melting point (paraffin also affects the life of the blades).

- Wedge-shaped blade - these knives lose a bit of sharpness compared to the previous ones due to their simple shape. It certainly allows for high durability, also with harder materials, but these knives do not perform well in routine work. They will be perfect, among the others for frozen sections.

- Plane-shaped blades - the name may be a bit confusing. They are better referred to as tool-type blades, as the very tip of the cutting surface is ground at a different angle, making it look more like a trapezoid. What does this solution give you? Enormous durability and service life. These are the knives that should be used for bones.

Better, then, knives ground on both sides or ground on one side? The former are definitely more comfortable, but greater sharpness can be achieved with single-sided blades. It is enough to look at experienced carpenters who can cut several-meter-long sections with a thickness of 7-8 micrometers with a planer! Manually!

However, that is not all. Many manufacturers have their own patents for coating cutting surfaces. Of course, most of it is the company's secret, but there are also standard options, such as chrome or platinum coating, which allow, for example, to extend the service life. Some companies cover the cutting surface with teflon so that temperature changes do not increase friction. Of course, this is not the only way to prevent it.

One last thing about the sharpness of the knives. I realize that not many people know this, but the setting of our blades has a huge impact on how they cut. The worst part is that each microtome manufacturer has a different recommended table angle for the blade holder. Do you know what should be in your device? During the work, the angle can change a little bit day by day for a year, which we simply will not notice. A bit like a broken car mirror, when we want to look through it, and suddenly it turns out that it shows our wheels not the road. If you do not know the recommended angle, please ask your supplier. They will surely pass this knowledge on to you. Of course, you can also try it by yourself. You might really be surprised how many problems the correct angle can solve.

I didn't intencionally mention the blade cutting angle here. The vast majority of microtome knives have an angle of 34-35o. Does it matter? No. When the knife has an cutting angle above 40o or below 30o, then there is a good chance that we will not compensate it with the angle of the table for blade holder, making it impossible to work, and whether it is 34 or 35, it doesn't really matter. Ideally, every knife supplier should be able to position the microtome for his type of knife. Nevertheless, our habits, the specificity of work, often skills or familiarity with the technique have a huge impact on whether a given blade suits us. So is it worth checking out new knives? Always! Mainly because the technology of metalworking is also developing. Therefore, when preparing for a purchase, tests should always be performed, be it on empty paraffin blocks or during the routine work of all employees.

Hopefully today I shed some light on the intricate nature of microtme knives. A lot depends on those, and we often ignore important matters related to them, exposing ourselves to the loss of time during work.

Source:

1. Gelber D. Thin sectioning: details of technique. TheJ Biophys Biochem Cytol. 1957; 3(2): 311-316.

2. Diapath Microtome blades DiaCut Data Sheet

3. Personna microtome blades Data Sheet

4. Feather microtome blades Data Sheet

5. ThermoFisher Scientific microtome blades Data Sheet

6. Bancroft's Theory and Practice of Histological Techniques, 7th Edition (0702042269)

7. Chu JP, Diyatmika W, Tseng YJ, Liu YK, Liao WC, Chang SH, Chen MJ, Lee JW, Jang JSC. Coating Cutting Blades with Thin-Film Metallic Glass to Enhance Sharpness. Sci Rep. 2019 Oct 29;9(1):15558.