In the University of Oxford, Gabriel Villar has created a 3-D printer with a difference. While most such printers create three-dimensional objects by laying down metals or plastics in thin layers, this one prints in watery droplets. And rather than making dolls or artworks or replica dinosaur skulls, it fashions the droplets into something a bit like living tissue.
Each of your cells, whether it’s a neuron or muscle cell, is basically a ball of liquid encased by a membrane. The membrane is made from fat-like molecules called lipids, which line up next to one another to create two layers. And that’s exactly what Villar’s 3-D printer makes—balls of liquid encased by a double-layer of lipids.
Other scientists have already created 3-D printers that spit out human cells in the shape of living tissues, and some have even created facsimiles of entire organs. But Hagan Bayley, who led Villar’s study, thinks that there’s value in creating tissues that look and behave like living ones, but that don’t actually contain any cells. They would probably be cheaper and without any genetic material, you don’t have to worry about controlling growth or division.
The team’s printer has two nozzles that exude incredibly small droplets, each one just 65 picolitres—65 billionths of a millilitre—in volume. The nozzles “print” the drops into oil at the rate of one per second, laying them down with extreme precision. As each drop settles, it picks up a layer of lipids from the surrounding oil, and the layers of neighbouring drops unite to create a double-layered membrane, just like in our cells.
The printer can lay a drop every second, and create shapes made of up to 35,000 of them. In the diagram below, they’re being printed into a drop of oil, sitting on a metal frame and suspended in some water. It all looks precarious but the drops hold their shape, even after the remaining oil is drawn away. They have the consistency of brains, fat or other soft tissues, and they’ll last for weeks. And by colouring them with dyes, Villar could produce pretty, shiny baubles.
But aesthetics are just the start. The tissues can also do things… like carry currents. Villar loaded some of them with ion channels—little pores that sit in membranes and let charged molecules flow through. In the photo below, the channel-loaded droplets are the green ones, cutting a right-angled path across their translucent neighbours. When Villar touched electrodes to either end of the path, ions flowed through the channels and he registered a current. This mimics some of the properties of a neuron, allowing one end of the tissue to communicate with the other side very quickly and along a fixed path.
Villar could also create tissues that can fold and contract, by printing drops with different salt concentrations. Once they are printed, the saltier drops soak up more water and swell to a greater size. And if two sheets of drops are printed next to each other—one heavily salted and one lightly so—they will automatically coil into a tube. Villar even printed a four-petal flower than folded into a hollow sphere.
These tissues are the start of something a bit like muscle—that is, if the team can reverse the process and get them to unfold. They also hint at potential applications. A hollow sphere is very hard to print, but if you can get one to fold on its own, you have something that could be very useful in medicine. A sphere, for example, could hold a drug. Better still, two spheres could hold molecules that react together to form a drug, but are too unstable to put in a pill. Load these molecules into different drops and inject them separately into a patient, and you could brew a drug on the spot, where it’s needed.
These applications are far-off, as is the team’s long-term goal of creating tissues that fuse these droplets with actual living cells. Perhaps, with more development, they could even be used to support or replace failing organs.
Reference: Villar, Graham & Bayley. 2013. A Tissue-Like Printed Material. Science http://dx.doi.org/10.1126/science.1229495