Open tissue processors:
An early automated version of manual processing, the “dip and dunk” tissue processor automatically moved tissue cassettes to various reagent containers. Most models were unsafe without the use of a fume hood since the open reagent containers allow evaporation of chemicals, such as xylene. Alcohol also absorbed water from the air, making it difficult to keep alcohol concentrations close to 100% as needed for complete dehydration. They also lacked pressure, vacuum, and temperature control and therefore processing took much longer than today’s "closed" models.
Closed tissue processors:
Tissue cassettes remain in a closed chamber and reagents are drawn in from external containers and pumped back out. Since reagent containers are closed, including the main processing chamber, exposure to reagents is more limited and chemical evaporation is small, making it safer than "open" processors. Closed processors, like the one shown in the lower image, have the ability to maintain vacuum and pressure on the reagents being pumped in and out of the closed chamber. Temperature is also well controlled for faster and more consistent processing.
Microwave processors:
These special laboratory grade processors make use of microwaves to either directly affect tissues/reagents or indirectly warm processing reagents for faster processing. Some models require manual reagent changes between processing steps and careful temperature monitoring. Some models of microwave processors do not use conventional reagents for processing, but use proprietary reagents to achieve fixation, dehydration and clearing. Some more recent models are very similar to the standard closed-chamber processors with the addition of gentle tissue/reagent heating for rapid turnaround time. Tissue size must be carefully controlled for optimal microwave processing. Routine tissue sections must be less than 3 mm in thickness.
