Most people use ink-jet printers to print documents and pictures -- a technique that involves squeezing tiny droplets of coloured ink from a nozzle. However, the technology is increasingly being used to create small volumes of liquid for a range of applications in electronics and medicine. For example, it has been used to create 2D and 3D patterns of living cells by squeezing a solution containing the cells from a needle using piezoelectric crystals. However, the method is limited by the diameter of the needle, which controls the size of the cell droplet. As a result, it cannot produce droplets smaller than about 100 microns, which means that "small" biological structures with fine features are difficult to make.

The new technique developed by the London researchers overcomes this problem. Known as "electrohydrodynamic jetting", it involves passing a liquid suspension of live human cells through a stainless-steel needle with a diameter of 500 microns at a controlled flow rate. A voltage of up to 30kV is applied between the needle and an electrode, which charges the liquid. After leaving the needle, the external electric field turns the liquid into a jet that becomes unstable and disperses into a myriad of droplets.

The advantage of this method compared to conventional ink-jet technology is that it can create droplets as small as just a few microns across from needles with diameters as large as hundreds of microns. Until now, however, researchers were unsure if the high voltages required for this technique would damage living cells. Jayasinghe and co-workers have demonstrated that cells can be processed at electric fields as high as 30 kilovolts without being harmed.

The technique may have huge potential for patterning predetermined 2D and 3D biological architectures, such as tissues and organs, at the micron and nanometre scales says Jayasinghe -- a feat currently impossible using other jet-based methods.