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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.
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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.
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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.
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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.
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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.
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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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