30 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



Part 2 



Embedment of Biological Specimens in 

 Plastic Blocks 



Specimens of biological material that are capable of being dehydrated 

 without undesirable alteration of color or shape, and are not detri- 

 ment ally affected by the medium in which they are mounted, can be 

 preserved so that their natural appearance is retained indefinitely by 

 embedment in transparent plastic blocks. Plate 2 illustrates the diver- 

 sity of specimens that can be preserved in this way. In the preserva- 

 tion of specimens by this method, the most satisfactory results have 

 been obtained by employing a plastic-forming embedding medium that 

 can be applied in a fluid state and then be converted through polymeri- 

 zation into a hard, durable solid without the use of pressure or very high 

 temperature . 



The procedure for embedding biological specimens in blocks of 

 methacrylate plastic comprises the following principal steps: (1) Re- 

 moval of inhibitor from monomer; (2) addition of catalyst to monomer; 

 (3) preparation of partially polymerized casting sirup; (4) casting plas- 

 tic base in mold for specimen; (5) dehydration and preparation of 

 specimen for embedment; (6) embedding specimen and polymerizing 

 plastic around it; (7) heat-treating polymer to prevent subsequent 

 surface crazing; (8) removing cast block containing specimen from mold; 

 (9) machining and polishing finished block. 



The essential steps in the embedment of objects in polymerizable 

 resin-forming substances involve the use of certain patented processes 

 (27 , 28, 29, 30, 32) z for which licenses may be required when the work 

 is to be done on a commercial basis. It is therefore recommended that 

 the holders of the patents be consulted in all cases where licensing ar- 

 rangements appear to be in order. 



Various polymerizable substances, including methacrylic esters, 

 phenol-aldehyde and urea-aldehyde condensates, allyl compositions, and 

 alkyd-styrene mixtures, have been reported to be more or less satis- 

 factory for use in this connection. The embedment of organic material 

 in methyl methacrylate, polymerized while surrounding the objects 

 suspended in it, was carried out by Randolph in 1934, and described in 

 his patent (31) . The applicability of this procedure to the preservation 

 of biological specimens was pointed out by Hibben (8) and preliminary 

 results obtained in the embedment of agricultural specimens were 

 reported by Knight (10). Preservation of biological material by em- 

 bedment in phenol-aldehyde and urea-aldehyde resins was described by 

 Brunner and Scheele (24) . The employment of allyl ester compositions 

 was reported by Brous in 1945 (o) . Maleic anhydride and styrene mix- 

 tures for the embedment of biological specimens were described by 

 Gerhart (26). Acrylic vinyl compositions reported to be especially 



3 These patents are all held by E. I. du Pont de Nemours & Co. (Inc.) and limited 

 licenses for their use in connection with specimen embedding may be obtained under 

 suitable circumstances on a royalty-free basis. 



