Scientists manipulate water droplets to create biochemical reactors

A team of researchers at Ritsumeikan University, Japan, have presented a novel technique that allows scientists to individually select water droplets for contact in droplet-array sandwiching. The study could lead to fruitful combinations of droplet-handling techniques and pave the way for advanced cell culturing and drug screening.

Currently, scientists use droplet-array sandwiching in which tiny water droplets are laid out on two parallel flat surfaces opposite each other. Each top droplet makes contact with the opposite bottom droplet, exchanging chemicals and transferring particles or even cells. These droplets act as small reaction chambers or cell cultures and fulfil the role of liquid-handling tools.

However, there is no individual control of droplets. Once the top surface is lowered, each droplet on the bottom surface necessarily makes contact with one on the top surface. As such, the team investigated whether they could control the height of individual droplets on the bottom surface to make some stand taller than others, bringing both surfaces close together such that only those droplets make contact with their counterparts while sparing the rest. 

The researchers had previously attempted to use electricity to control the “wettability” of the dielectric material in the area below each droplet. This approach, known as “electrowetting-on-dielectric (EWOD)”, alters the balance of forces that holds a water droplet together when resting on a surface. By applying an electric voltage under the droplet, it is possible to spread it out slightly, increasing its area and reducing its height. However, this process was not easily reversible as droplets would not spontaneously recover their original height once the voltage was turned off.

The team then developed an EWOD electrode with a hydrophilic-hydrophobic pattern. When the electrode is turned on, the previously described process makes the water droplet on top of it spread out and become shorter. When the electrode is turned off, the outer hydrophobic part of the electrode repels the droplet while the inner hydrophilic part attracts it, restoring the original shape and height of the droplet.

The researchers showcased their method by laying out multiple EWOD electrodes on the bottom surface of a droplet-array sandwiching platform. By applying voltage to selected electrodes, they could choose which pairs of droplets came into contact when the top platform was lowered.

“Our approach can be used to electrically set up individual contacts between droplets, allowing us to effortlessly control the concentration of chemicals in these droplets or even transfer living cells from one to another,” explained Professor Satoshi Konishi who led the study.

“We envision that lab-on-chip technology using droplets will replace conventional manual operations using tools such as pipettes, thereby improving the efficiency of drug screening. In turn, this will accelerate the process of drug discovery,” Konishi concluded. Culturing cells in hanging water droplets will also make cell-based evaluation of drugs and chemicals cheaper and faster, representing a valuable tool for biochemistry and cell biology, Konishi added.

The study was published in Scientific Reports.