to be published in Proceedings of the Association for Biology Laboratory Education (ABLE), vol. 26, 2004
III. Stretch Pipets
Purpose of tool:
reliable, and easy-to-use tool is extremely useful for very finely controlled
transfer of small, slow-moving microorganisms from one place to another using a
very small water volume. Stretch pipets are preferable to glass transfer pipets
because stretch pipets are unbreakable, more maneuverable, and can be used with
much finer control of suction and water level than with standard glass pipets
with rubber bulbs. Examples of microorganisms that are easily transferred with
stretch pipets include: large protozoans, rotifers, tardigrades, nematodes, and
small aquatic oligochaetes. Stretch pipets are also very useful for making tiny
adjustments of liquid within well slide chambers (see Figs. 9 & 12). Lastly,
stretch pipets are excellent means for precisely delivering tiny body fragments
of Lumbriculus variegatus (blackworms) to small predators, such as hydra
and planaria, in order to observe predatory attack and feeding behaviors. Also,
the tool can be used for collection and transfer of wet microfossils (see:
Disposable polyethylene pipets (smooth, uncalibrated)[e.g. Fisher Cat.#13-711-7]
New, single-edge razor blade
Creation and use of this tool:
Place the plastic pipet with its tip extending over the edge of the counter. Grip the bulb firmly with one hand and rest this hand on the counter. Use the other hand to firmly grip about 5-6 mm of the tip with the pliers. Then, very slowly, firmly, and steadily pull the tip straight out and away from the pipet until it is stretched about 3-4 cm (Fig. 7). Stretching beyond this point or stretching too rapidly will cause the tip to break away, ruining the effect you are trying to create. When properly done, the stretched portion of the pipet will appear clear with a uniform diameter of about 1-2 mm. Use a new, single-edge razor blade to squarely cut off the flattened tip at a point shown in Fig. 7.
Procedure for stretching plastic pipets.
SPECIAL NOTE: When using stretch pipets, it is highly preferable to grip pipets by the barrel region, just as you would hold a pencil. Then, fluid levels in the pipet are very precisely controlled by gently squeezing or releasing the barrel (Fig. 8), rather than by squeezing the bulb.
Figure 8. Final configuration of stretch pipet. Note the fine tip. By gripping and squeezing the pipet along the barrel, like a pencil, it is possible to draw up and precisely transfer very small volumes of fluids containing one or more microorganisms.