|AIS 2 uses air pressure to force the injection sample into cells. The new Eppendorf FemtoJet® is best suited for this task. It provides instantaneous pressure changes, control of both compensation pressure and of injection pressure, and control of injection time. The pressures can be adjusted precisely to the needs of any cells.
For automation the FemtoJet® features a true RS 232 interface which is used to remotely control the device from the AIS 2 program. Parameters can be changed quickly from both the computer and the device itself.
|Pi: injection pressure
Pc: compensation pressure
ti: injection time
|In 1982 Wilhelm Ansorge published a microinjection device featuring three different pressure levels (Ansorge: Improved system for capillary microinjection into living cells, Exp. Cell Res. 140, 31-37, 1982)
The FemtoJet was build according to this principle providing the pressures Pc, Pi, and Pclean.
Pc is the compensation pressure which stands at the pressure outlet when idle. It prevents backflow of culture medium into the capillary due to capillary forces. Pc can be adjusted to exactly match the capillary force, so no flow takes place inside the capillary. However, in standard microinjection experiments the compensation pressure is set higher than the capillary force, thus providing an outflow of microinjection sample. Flow keeps the sample DNA or protein in movement, preventing attachment to the glass walls of the capillary to some extent. This leads to reduced clogging and thus the capillary can be used to inject more cells. Typical pressure values range from 15 to 50 hPa. Pc can also be used for a constant injection pressure. In this case there is no pressure change at injection. The AIS 2 program supports both modes.
Pi is the pressure level which is activated after the injection is triggered. It is terminated by a timer after injection time ti. The required value depends on pressure, capillary tip orifice, injection time, and, to some extend, on injection sample viscosity. It typically ranges between 80 and 400 hPa, although much higher values might be required occasionally.
Pclean represents the maximum pressure that can be produced by the FemtoJet®. It is activated if something blocks the capillary tip. In many cases this blows the capillary free and gets the sample flowing again, so injection can continue without replacing the capillary.