Water droplets can be used as microvessels for biological assays thanks to a new surfactant developed by scientists in Europe and the US.

The water droplets remain stable and do not fuse, making them ideal for bioassays

Fluorocarbon oils are the ideal medium to carry water droplets for biological assays because gases that are needed by biological cells can be dissolved in these oils, but the oils prevent cross-contamination of biological material between the droplets. They are also ideal for use in microfluidic devices, as the oils do not cause the material forming the channels within the device to swell up.

However until now, all commercially available surfactants for water droplets in fluorocarbon oils have either not stabilised the droplets for long enough or have interacted with the content of the droplet. Now, a group of researchers from Germany, Italy and the US have come up with a new class of surfactants without these problems.

Christian Holtze at BASF Aktiengesellschaft, Ludwigshafen, Germany, and colleagues made surfactants with a fluorocarbon tail and a polyethylene glycol head. By testing different variations of these surfactants they found they could create an emulsion that is stable for weeks. This gives the level of stability needed for biological assays, which can vary in length between hours and days. Holtze also looked at how the emulsion fared under conditions typical of microfluidic devices. They found that even after being highly compressed, for example, the emulsion stayed stable.

Next Holtze trapped a piece of enzyme-coding DNA in the water droplets together with all the molecules needed to make the enzyme. He found that a fluorescent product formed in the droplets, indicating that the new surfactants are biocompatible. The reaction had been done before using ionic surfactants without success. Holtze says he believes that the biocompatibility of the new surfactants could be because they are non-ionic. He also found that his emulsions can be used to contain biological cells.

Holtze says that these surfactants could pave the way for 'unprecedented speed and control for high-throughput analysis ranging from in vitro biochemistry to single cell studies'.

Madelaine Chapman