The Eppendorf FemtoJet provides the force or pressure necessary for the injection of minute quantities of liquid (DNA or dsRNA) into
C. elegans. We have ours hooked up to our Zeiss Axiovert 200 Inverted Microscope. Connecting the FemtoJet to our microscope was not difficult. We inject a minute amount of DNA or dsRNA (for RNAi) into the syncytial germ line in the distal gonad arm of
C. elegans in order to generate transgenic worms or induce RNAi, respectively. This unit is simply designed with a LCD display; six keypads (clean, auto/manual, count, inject, menu, and standby) underneath the display; and three regulators: for injection pressure (pi), injection time (ti) and compensation pressure (pc), located below the keypads. The pressure tube is connected to the device via a bayonet joint; the connection tube is in the bottom front of the instrument. The unit also comes with a pressure tube, capillary holder with grip head for capillary diameters of 1.0 mm and adaptors for Femtotips, a hand control for triggering injection, Femtotip, Microlader, and Main cable.
The display conveniently shows injection mode (auto or manual), the number of injections performed, the most recently set or relevant parameter, and compensation pressure. It shows the parameters during injection and parameter-setting, and guides the operator through the different functions. The ‘Inject’ keypad, when pressed, performs automatic or manual injection (by holding down the inject key). If no injections are done, compensation pressure is applied permanently. The ‘Clean’ keypad applies maximum pressure to clean the capillary to the tube outlet. This keypad is used to flush out blocked capillaries by applying the maximum available pressure to the outlet with a maximum of 7000 hPa. It also removes any air bubbles that might be present in the capillary. The ‘Auto/Manu’ keypad switches between manual and automatic time-controlled injection. The ‘Count’ keypad is for resetting the injection counter to zero. The ‘Menu’ keypad switches to the menu control. The ‘Standby’ keypad puts the unit on standby mode without de-airing. However, if the ‘Standby’ key is pressed longer, it clears the pressure accumulator with de-airing. This should be done prior to longer periods of non-use or standstill. Just pressing the keypad again will reactivate the unit.
The variable regulator is used to set the parameters for injection pressure, injection time and compensation pressure. A permanent compensation pressure is applied counter to the forces of capillary suction to ensure that no liquid flows into the injection capillary. We find that a compensation pressure between 30-50 hPa is appropriate with our injection capillaries. The injection pressure is applied during the injection process, which is generally higher than the compensation pressure. The injection pressure can range anywhere from 50 to 500 hPa. With our Femtotips, we find an injection pressure between 150 and 200 hPa to be appropriate. The volume injected is dependent on the injection pressure and the injection time. The injection time gives the period for which injection pressure is maintained, with a range between 0.3 and 1.5 seconds.
With this unit, once the ‘Inject’ key is pressed, the injection pressure builds up immediately, so it is present during the injection movement. At the time of injection, the unit switches to compensation pressure. Another great feature of this unit is that one can perform manual or automatic injection. In the manual mode, the injection is performed for as long as the ‘Inject’ key is held down. In the auto operating mode, the built-in timer facilitates a reproducible injection time. The injection runs automatically for the duration of the set time. Also, there is a self-test, which is run automatically before use to check the compressor run time and to check for leaks in the compressor, pressure accumulator, and in the area of working pressure. Another key feature of this unit is that if there is a malfunction, there will be warning signal which states the problem (e.g. supply pressure is not reached, compensation pressure cannot be maintained). Also, there are error messages displayed if certain operating conditions are not reached or the unit records a deviation from a specified condition (e.g. leaks in pressure accumulator or compressor; leak through valve; if there is something wrong with the supply pressure, injection pressure, or compensation pressure). The manual that comes with the unit has possible explanations and solutions for all types of warning or error messages.