Researchers have created networks of water droplets that mimic some properties of cells in biological tissues. Using a three-dimensional printer, a team at the University of Oxford, UK, assembled tiny water droplets into a jelly-like material that can flex like a muscle and transmit electric signals like chains of neurons. The work is published today in Science¹.

These networks, which can contain up to 35,000 droplets, could one day become a scaffold for making synthetic tissues or provide a model for organ functions, says co-author Gabriel Villar of Cambridge Consultants, a technology-transfer company in Cambridge, UK. “We want to see just how far we can push the mimicry of living tissue,” he says.

The network relies on each water droplet having a lipid coating, which forms when the droplets are in a finely-tuned mix of oil and a pure lipid (see video, top).

Velcro action

The lipid molecules have a water-loving head, which sticks to the droplet's surface, and a water-fearing tail, which pokes out into the oily solution. When two lipid-coated droplets come together, each with its carpet of water-fearing tails, they stick to each other like Velcro, forming a lipid bilayer, similar to those in cell membranes. The bilayer creates a structural and functional connection between droplets.

Although previous studies^{2, 3} have shown that lipid-coated droplets can form such connections, their watery composition and spherical shape made them tricky to assemble. “I already made a raft of droplets that stuck together,” says biomedical engineer David Needham of the University of Southern Denmark in Odense, who was not involved in the study. “But to print them is really an achievement.”

To pull off the tricky feat, Villar, who at the time was a graduate student in Hagan Bayley's lab at the University of Oxford, created a printer that squirts water droplets from a glass nozzle into a 5 millimetre-deep container full of an oil–lipid mixture. As the droplets sank to the bottom, they gathered their lipid coating (see video, below).

A motorized platform then moved the container ever so slightly so that the next droplet fell either beside or on top of the previous one, eventually creating droplet networks shaped as spheres, cubes and even castles and flowers.

Villar then added a second nozzle so that two kinds of droplets could be squirted at once. To make the networks flex, he printed layers of salty droplets next to low-salt droplets. Because water can permeate the bilayer, the salty droplets swelled with water from their neighbours, causing the whole structure to curve. To create a pathway for electrical current, Villar printed droplets containing a toxin that bores holes in lipid bilayers, through which a current can pass.

Bioengineer Karen Burg of Clemson University in Clemson, South Carolina, says that the technology is still too rudimentary to work in clinical settings or to be used for modelling actual organs. “You can argue long and hard about how complex something has to be to give you good information.”

“If their vision really is to make tissues, then I think they’ve still got a long way to go,” says Needham. “But I think they’ve gone about it in the right way.”