In Part 1 of this series, we discussed the theory and science of wax infiltration in routine tissue processing. A key point stablished was that the basic paraffin molecule is a simple straight chain molecule composed of hydrogen and carbon atoms. However, for the highest efficiency in Histology application, certain ‘additives’, including polymers, are linked onto the paraffin molecule. This quite naturally increases the size of the molecule and thus, alters performance features such as speed of infiltration into tissues, microtomy enhancement, and uses at variable melting points. To visually illustrate the relationship of increasing the size of the paraffin molecule, with the rate of infiltration, consider the image below with the simple straight-chain molecule on the far left, and three chemically altered variations. All three variables stem from the original hydrocarbon molecule. It is plain to see how the three altered complexes could theoretically slow down wax penetration speed.

The technical definition of ‘polymer’ is a large molecule composed of repeating structural units generally connected by covalent chemical bonds.  The word is derived from the Greek words of poly (many), and meros (parts), further describing a long continuous molecule.  The word is often used in a general sense, but the term actually refers to a large class of natural and synthetic materials used for a variety of purposes.  In the commercial wax industry there is much discussion by suppliers in reference to various additives and how it optimizes their product for specific Histology performance.  In many cases certain terms such as polymer, resin, elastomers, and plastisizers have been used in a general sense without understanding that there is a great deal of overlap between the four definitions ‘and’ their use as an additive.  While these terms are frequently used by commercial vendors without specificity, here are the true definitions for the major additives in Histology waxes.

Plastisizer – An additive to the polymer that increases the fluidity of the material to which it is added.  It is often used as a softener for rubbers and resins to increase flexibility of the polymer.  This increases the capacity for a ribbon to be stretched out on the waterbath.

Resin – A hydrocarbon secretion of plants, particularly coniferous plants.  Thick and highly viscous, resins, particularly synthetic resins, are developed to give strength and a hardening quality to a polymer.  These tend to have a very sticky characteristic when embedding.  In microtomy it enhances ribboning strength.

Elastomer – A polymer consisting of elements such oxygen and silicon, in addition to carbon and hydrogen.  This is consistent with rubbers and have a characteristic of being able to resume its natural state after stretching and other forms of distortion from its natural state.

Polymer – A technical definition has already been presented.  The most visual enhancement of polymers in histological waxes is their ability to link section after section to form a continuous ribbon, and to achieve thinner sectioning as low as 2-3 microns.

Microcrystalline Wax – A higher molecular weight wax than most histology paraffin waxes. This prevents crystalline formations in the wax resulting in cracking.  A harder, more flexible wax, MCL waxes have a higher melting point making them more ideal for specific research applications requiring higher tissue processing temperatures.

As previously mentioned, there is considerable overlap in the definition and characteristics of the four additives presented.  Many of them on the surface seem to have the same qualities of flexibility, stretchability, ribboning strength, etc.  However, subtle differences in performance are what significantly distinguishes one wax blend of additives from another.  While our commercial vendors do a great service in providing general information on wax products, it behooves the Histology professional to know and understand the specific definitions used in polymers and additives; and through their own research and hands-on experience with each product, determine what wax blend works best for their application and personal technique.

Reference

• Brown, S., “The Art & Science of Histotechnology”, Lab Management Consultants, 2011.
• Brown, S., “The Basic Dynamics of Fixation & Processing”, Lab Management Consultants, 2008.
• Brown, S., “The Wax Museum”, Workshop Symposium-National Society for Histotechnology, Lab Management Consultants, 2008.
• www.poth-hille.co.uk, Microcrystalline Wax | Poth Hille (poth-hille.co.uk)
• Visser, G., “Waxes Used In Hot Melt Adhesives”, Sasol Wax Americas, Inc., 2007.
• Wright, J., “The Fundamentals of Mineral Base Oil Refining”, Noria Corporation, 2012.