INTRODUCTION

A curable polymer replaces tissue fluid in silicone plastination . Silicone polymers and chemicals, other than the Biodur™ products , have been developed to carry out the silicone plastination process (Henry et al., 2001; Smodlaka et al., 2005a). The basic components are common products of the silicone industry and may be used to some degree interchangeably with caution (Henry et al., 2002). Principally, methods of placing the silicone into the biological specimen are the same: an intermediary solvent (acetone/methylene chloride) is removed by a decrease in pressure to extract the solvent (von Hagens, 1979a; 1979b; 1986; von Hagens et al., 1987; Henry and Nel, 1993). Hence a tissue void is produced and the polymer-mix is drawn into the cells. Each variant produces high quality durable plastinates.

The basic difference in methodology, excluding the polymers , is the sequence in which polymer, catalyst chain extender and cross-linker are combined. The basic cold S10 process combines the silicone polymer with the catalyst and chain extender to serve as the impregnation-mixture (von Hagens, 1986). This yields an unstable impregnation-mixture which commences to thicken upon mixing. Therefore, it is prudent to keep the mix in the cold to prevent premature thickening (increased viscosity) . As noted in the earlier manuscript on S10, impregnation with this mix may be done also at ambient-temperature . However, the pot-life of the mix is greatly reduced. Uniquely , the Dow™/Corcoran/ Room-temperature method combines the silicone polymer with the cross-linker (Glover et al., 1998). This mix yields a stable impregnation-mix even when kept at room temperature. Therefore, there is no need to keep the impregnation-mix in the freezer for storage or for impregnation . Recent experience has demonstrated that both the Biodur™ and the North Carolina products may also be used with this methodology.

Chemicals used in  these  "generic"  silicone­ plastination techniques are the  same or at least  similar to those used in the classic Biodur™ S 10 Cold­ temperature  plastination  process:

• Acetone,
• Silicone polymer ,
• Cross-linker, to form a 3-D meshwork of the elongated silicone molecules by side to side
• Chain extender, to promote the silicone molecules to form longer-chain silicone molecules,
• Catalyst, to prepare the silicone molecules for elongation and cross-linkage,

The sequence of combination of these products is the difference in these two methodologies. The general steps of silicone plastination are described earlier in this volume. This manuscript will highlight the differences with respect to this variant process which occur during impregnation and curing of the plastinated specimens.

MATERIALS AND METHODS

The basic steps of plastination are  used for each plastination technique:

Steps I and 2 (Specimen preparation and Dehydration) are the same for all plastination methods, even with the Dow methodology. Please refer to: "The Sl0/ 15 Plastination Cold-temperature Technique" for this information . Adequate specimen preparation and thorough cold dehydration are essentials for producing good plastinates.

The more common Dow™ products for silicone plastination are:

PR10 and PR14 (lower viscosity): Silicone polymer Ct30 and Ct32 : Catalyst and chain extender, premix Cr20 and Cr22: Cross-linker

Step 3. Forced Impregnation:

Replacing the volatile solvent (acetone) in a biological specimen with a curable polymer. For this to occur, the same criteria as for the classic Biodur™ S10 plastination process must be met, including thorough cold dehydration.

Impregnation equipment: Similar to the Biodur™ S10 requirements - Vacuum chamber and pump (oil or dry); Specimen basket; Vacuum gauge; Bennert manometer; Needle valves. One exception:  no  deep  freezer  is needed for impregnation or storage of impregnation­ mixture. However for dehydration , freezers are highly recommended.

Preparing reaction-mixture: Dow™ PR 10 or PR14 polymer is mixed with Cr20 or Cr22 (Cross-linker and Chain  extender)  at  100:10  to  prepare  the  reaction- mixture. Thorough stirring is recommended. One of the two main differences is how the reaction-mixture is to be handled. The reaction-mixture is stored and used at room temperature (Glover et al., 1998; Glover, 2004; Raoof, 2004). It is stable (does not become viscous) for at least ten years at room temperature .

Adjusting vacuum: The speed for lowering of pressure in the vacuum chamber is very fast when compared to the Biodur™ S10 Cold-temperature Technique. The viscosity of the PR10 polymer is 20% less than the viscosity of S10 polymer. The impregnation-mixture is not reactive, as long as it is  not exposed to catalyst. Therefore, the polymer-mix remains fluid. As well, it is used at room temperature so there is no increase of viscosity from placing the mixture in a  cold environment. When monitoring  bubble  formation,  a rapid boil is the standard. If in doubt, go slower, to prevent incomplete impregnation with  resulting shrinkage. Pressure is decreased by  closing  the  air intake valves.

Impregnation  regimen:

Day 1: The dehydrated and degreased specimens are removed from the solvent (acetone), excess solvent is drained and the dehydrated,  solvent-filled  specimens are placed m the room-temperature polymer impregnation-mixture. Submerge the specimens immediately to prevent solvent evaporation from their surface and hence, surface drying. Close  the port  of the vacuum chamber with the glass and let the specimens accommodate/equilibrate  over night.

Day 2: Warm the vacuum pump a few minutes. Seal the vacuum chamber and lower pressure (around Yi atmosphere) until bubble formation becomes rapid. Stabilize pressure at the level which maintains a rapid boil. Monitor pressure and decrease the pressure as needed to maintain a rapid boil.

Day 3: Continue to monitor bubble formation (solvent extraction) maintaining a rapid boil by decreasing pressure (incrementally  close  needle  valves)  as needed.

Day 4a: Small specimens will likely finish impreg­ nation as noted by cessation of bubble formation. Tum off pump and bring specimens to atmospheric pressure. Proceed to Step 4 - "Curing/hardening".

Day 4b: Large specimens or a large quantity of specimens. Continue to monitor solvent extraction (watch bubble formation) and maintain a rapid boil by incrementally closing the needle valves.

Day 5 or plus X: Continue to monitor solvent extraction (watch bubble formation) and maintain a rapid boil. When impregnation is completed,  as  noted  by cessation of bubble formation, tum off pump  and bring specimens to atmospheric pressure.

Rule: If bubbles are actively rising to the top of the polymer and bursting, impregnation is not finished! Impregnation will be complete when needle valves are closed and no more rapid bubbling is noted and when pressure reaches <5mmHg. Nearly complete evacuation of acetone/solvent is necessary to avoid incomplete impregnation of the specimen with the polymer-mix which may lead to shrinkage.

Specimen removal and drainage of polymer-mix : Follow  SIO protocol.  Since  keeping  the  reaction-mix cold  is  not  necessary ,  specimens  are  drained  for  an extended   period   directly   into   the   room-temperature plastination  chamber.

Step  4.  Curing  (Hardening  or  Cross-linking) of  the impregnation-mix within the specimen to make it dry: Equipment for curing:

• Absorbent paper: To wipe excess  polymer-mix from the
• Paint brush or Mist bottle: To apply the Ct30 or Ct32 to the
• Foil (plastic wrap): To seal the specimen in an air­ tight environment and keep the  Catalyst  next  to the impregnated  specimen.

Curing/Cross-linking is a two-step process:

a. Drainage of the excess silicone-mix. Drain excess polymer-mix from the specimens into the plastination chamber.

b. Catalyzing, cross-linking of the silicone polymer molecules . This reaction occurs because as the catalyst (Ct30 or Ct32) is applied to the impregnated specimen, it prepares the PR10 or PR14 silicone molecules in the impregnation-mix to react with the cross-linker (Cr20 or Cr22) in the impregnated specimen. Catalyst is applied to the specimen surface via a mist bottle or brush. Then the catalyst covered specimen is wrapped in The next day foil is removed and the specimen checked to see if cured (dry). If not completely cured, the specimen surface is wiped and catalyst is applied to the specimen . The specimen is rewrapped with foil. The following day the specimen is checked for complete curing. If curing is complete, the specimen is ready for use. If curing is not complete, more catalyst is applied and the specimen is wrapped again in foil, etc.

RESULTS

The PR10/PR14 specimens are durable and some flexibility is exhibited by thinner specimens (Glover et al., 1998; Glover, 2004). The specimens are dry and odorless. They are not models but the actual specimens. Numerous specimens and whole cadavers have been successfully plastinated using the room-temperature technique

DISCUSSION

Plastinated specimens are the real specimen and not a model (Figs. 1-6). All specimens are free of offensive odor and dry (Latorre et al., 2001). One feature of the finished product is a less than transparent surface which conceals intricate surface cellular detail (Henry et al., 2004; Smodlaka et al., 2005a; 2005b). Room temperature specimens may be used as teaching  aids both in the class room and the clinical setting (Latorre et al., 2001; Raoof et al., 2006). They may be compiled and used as a library of specimens for normal , exotic and pathological anatomy (Henry, 2005) and in research (Raoof, 2001).

Figure 1. Lateral view of left canine coxal region - Dow Room-temperature technique .	Figure 2. Median sagittal view of human kidney - Dow Room-temperature technique .	Figure 3. Auricular surface of canine heart - Dow Room­ temperature technique.
Figure 4. Bovine kidney - Dow Room-temperature technique	Figure 5. Lateral view of left bovine uterine horn - Dow Room-temperature technique	Figure 6. Atrial surface of canine heart - Dow Room­ temperature technique