American Laboratory News Edition
     Volume 30, Number 7, Page 10, March 1998


Microplate Stirring Technology
    By Patrick Cleveland


The MICROPLATE approach to analytical measurement and solution manipulation has become the technique of choice for many life science research and clinical testing laboratories. In the modern laboratory, the demand for increased productivity is a major concern for laboratory managers, and the use of microplates has made a significant contribution to increasing sample throughput. Depending on the size, the microplate permits 96, 384, or more samples to be handled in parallel, providing substantial enhancements in the output of data. Moreover, the standardization of design has permitted automation of operations. Apart from the improvement in reproducibility that always accompanies automation, the microplate approach enormously reduces the labor cost per analysis. Nowhere have the benefits of microplate technology been more evident than in the drug discovery field. Automated high-throughput screening systems utilizing microplates are present in all major facilities. To increase the throughput from these systems, there is a drive to increase the number of wells in the plates, which results in a decrease in the volume of each well. Today, plates are being developed with as many as 1536 wells.  


Microplate Stirrer Units


The most common microplate liquid handling operations involve adding or removing liquid from a well. The ways to achieve these operations are, at least conceptually, obvious. This, however, is not the case for stirring the contents of a microplate well-particularly in the smaller wells of the newer high-capacity plates. This problem has been tackled by V&P Scientific, Inc. (San Diego, CA), which employs magnetic stirring bars and disks to stir the contents of microplates. Unlike the spinning action of conventional magnetic stirring, the motion of the microplate bars and disks in the Alligator microplate tumble stirrer (V&P Scientific, Figure 1) is a tumbling action. The unit causes stirrers to tumble end over end inside each well. (This action is similar to the way an alligator tumbles its prey.) The unit will stir most common types of microplates up to 1536 wells. It will also stir deep well plates, V and U bottom microplates, PCR plates, microcentrifuge tubes, and test tubes and bottles. The unit will fit into most incubators. Two production models are offered: The first (model VP 709) simultaneously stirs two layers of nine microplates, a total of 18 plates of 1728 samples in 96-well plates, or 6912 samples in 384-well plates; the second (model 709A) has the same footprint but uses more powerful magnets to permit stirring of four layers of nine microplates. The number of samples that can be handled in the unit is proportionately larger. Tumble speeds are variable from 3 to 65 revolutions per min.


Stirrer Dispensers


Dispenser systems are offered to permit easy insertion of stirrers into the wells (Figure 2). Operation of the dispensers is simple. The stirrers are poured into the appropriate size dispenser for the microplate, and the dispenser is shaken until each of the holes is filled with a stirrer. The excess stirrers are poured off, the dispenser is positioned over the microplate, and the bars are dropped into the wells. A microplate can be filled with stirrers in less than 10 sec. The dispenser units may be sterilized or cleaned with solvent or bleach.




Some microplate applications that require or can be sped up with stirring include stirring microbial genomic libraries to increase product yield, stimulation of growth of aerobic organisms, resuspension of settled microorganisms, breaking up filamentous organisms, dissolution of solid compounds, mixing two or more compounds, making oil and water emulsions, facilitation of serial dilutions, and scraping tissue culture monolayers to release DNA. For stirring operations that are carried out over long periods of time, a humidity box is available as an enclosure for the stirring unit to prevent evaporation of solvent.




Stirrers are offered in a variety of sizes to match the various well sizes (Figure 3) and come as both stainless steel and Parylene (di para xylene) coated. Stainless steel stirrers are highly corrosion resistant-withstanding attack by common corrosive reagents and solvents. They are also nontoxic to microorganisms. The stirrers are inexpensive and may be discarded after use if the user does not wish to clean them. The Parylene-coated stirrers are recommended for combinatorial chemistry where the iron in stainless steel may interfere with the reaction. The Parylene coated stirrers are resistant to strong acids, bases, and organic solvents and are nontoxic to tissues.




Apparatus have been described for stirring the contents of microplates. The stirring is carried out by inserting a small magnetic stir bar in each well and causing it to tumble in a magnetic field. The ability to stir the contents of microplate wells presents the possibility of miniaturizing new types of operations on large scale and also making some of the current operations more efficient.


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