Bubble pressure measurement of micropipet tip outer diameter

The bubble pressure of a micropipet tip is the minimum transmural pressure necessary to expel bubbles into a liquid in which the tip is submerged. Bubble pressure measurement is a simple, accurate and non-destructive technique for determining the tip outer diameter of micropipets used in microinjection, intracellular recording and patch-clamp experiments. For micropipets pulled from thin-wall glass capillary tubing, with tip outer diameters between 0.1 and 1.5 micrograms, observed bubble pressures are close to those predicted by considering surface tension at the pipet tip. Also, as predicted, neither glass composition nor shank geometry influences the relationship between bubble pressure and tip outer diameter. Tip outer diameter for a given bubble pressure is larger for micropipets pulled from thick-wall glass capillary tubing than it is for pipets pulled from thin-wall glass. However, for a given wall thickness, the outer diameter vs bubble pressure behavior is sufficiently reproducible to allow accurate size determination from bubble pressure measurements. The size of micropipet tips modified by fire-polishing or beveling can also be estimated by bubble pressure measurement, although with less accuracy. Thus, bubble pressure measurement can be a useful technique for following the progress of micropipet tip modification.