Pressure injections of fluid in the nanoliter range via micropipettes

The relationship between pressure, ejection duration and volume ejected was experimentally determined in vitro for micropipettes with different external tip diameters. The relationship between ejection duration and ejected volume is linear in the steady state (i.e. with ejection durations of 1 s or longer) and at sufficiently high pressures (above about 100 kPa) and for pipettes with a sufficiently high hydrodynamic conductance (larger than 1 pl s-1 kPa-1 at 230 kPa). In this range, flows were found with low Reynolds numbers (smaller than 10), which is consistent with laminar flows. For all but the largest micropipettes, the relationship between pressure and ejected volume is alinear: the pipettes' apparent hydrodynamic conductances increase with increasing pressure. Micropipettes with apparent hydrodynamic conductances between 0.04 and 1400 pl s-1 kPa-1 (at 230 kPa) were tested. Duration-pressure combinations could be defined where the duration-volume relationship was either linear or monotonic. such duration-pressure combinations were different for pipettes with different apparent hydrodynamic conductances. A quick method is described to measure the pipette's apparent hydrodynamic conductance at the pressure used, corrected for the fluid's viscosity. Measurement of this conductance permits predictable injections of known volumes of fluid in the range of 100 pl to 1 microliter with a precision of 10-20%.