Imagine a future in which you could 3D-print an entire robot or stretchy, electronic medical device with the press of a button eliminating hours spent tediously assembling parts by hand. That possibility may be closer than ever thanks to a recent advancement in 3D-printing technology led by engineers at the University of Colorado Boulder.
In a new study, published in the journal Additive Manufacturing, the team laid out a strategy for using currently-available printers to create materials that meld solid and liquid components - which is a tricky feat if you don't want the robot to collapse. And the ultimate objective is to fabricate a complete system like a robot using this process.
3D printers have long been the province of hobbyists and researchers working in labs. They're pretty good at making plastic dinosaurs or individual parts for machines, such as gears or joints. But the researcher believe that they can do a lot more. Specifically, by mixing solids and liquids, 3D printers could churn out devices that are more flexible, dynamic and potentially more useful.
The results could include wearable electronic devices with wires made of liquid contained within solid substrates, or even models that mimic the squishiness of real human organs. The researchers compare this advancement to traditional printers that print in color, rather than just black-and-white. That is, color printers combine a small number of primary colors to create a rich range of images.
The same is true with materials. If you have a printer that can use multiple kinds of materials, you can combine them in new ways and create a much broader range of mechanical properties. To understand those properties, it helps to compare 3D inkjet printers to the 2-D inkjet printers typically found in offices. 2D printers create an image by laying down liquid inks on paper in the form of thousands of flat pixels.
Inkjet 3D printers, in contrast, use a printhead to drop tiny beads of fluid, called "voxels" (a mash-up of the word "volume" and "pixel"), one on top of the other. Very soon after those droplets are deposited, they are exposed to a bright, ultraviolet light. And the curable liquids convert into solids in a second or less. But there are certain cases in which you might want those liquids to stay liquid.
Some engineers, for example, use liquids or waxes to create tiny channels within their solid materials, which they then empty out at a later point. It's a bit like how water can carves out an underground cavern. Engineers have come up with ways to make those kinds of empty spaces in 3D-printed parts, but it usually takes a lot of time and effort to clear them and the channels also have to stay relatively simple.
The researcher decided to find a way around those limitations. To do so, they first designed a series of computer simulations that probed the physics of printing different kinds of materials next to each other. One of the big problems is how to keep droplets of solid materials from mixing into the liquid materials, especially when the droplets of solid material are printed directly on top of the liquid droplets.
The study found that the surface tension of a liquid can be used to support solid material, but it is helpful to pick a liquid material that is more dense than the solid material; that's the same physics that allows oil to float on top of water. Next, the researchers experimented with a real 3D printer in the lab. They loaded the printer up with a curable polymer, (which would become solid), and with a standard cleaning solution (which would remain liquid).
Using this approach, they were able to 3D-print twisting loops of liquid and a complex network of channels not unlike the branching pathways in a human lung. Both of these structures would have been nearly impossible to make using existing methods.