Combining liquids, solids could lead to faster, more flexible 3D creations
University of Colorado at Boulder
Imagine
a future in which you could 3D-print an entire robot or stretchy, electronic
medical device with the press of a button -- no tedious hours spent assembling
parts by hand.
A network of capillaries 3D-printed using a
newly developed technique.
(Credit: Hayes et al. 2022, Advanced Materials)
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, the team lays out a strategy for using currently-available
printers to create materials that meld solid and liquid components -- a tricky
feat if you don't want your robot to collapse.
"I
think there's a future where we could, for example, fabricate a complete system
like a robot using this process," said Robert MacCurdy, senior author of the
study and assistant professor in the Paul M. Rady Department of Mechanical
Engineering.
MacCurdy,
along with doctoral students Brandon Hayes and Travis Hainsworth, published
their results April 14 in the journal Additive Manufacturing.
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 MacCurdy believes that they can do a lot
more: By mixing solids and liquids, 3D printers could churn out devices that
are more flexible, dynamic and potentially more useful. They 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 engineer compares the advancement to traditional printers that print in color, not just black-and-white.
"Color
printers combine a small number of primary colors to create a rich range of
images," MaCurdy said. "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."
Empty
space
To
understand those properties, it helps to compare 3D printers to the normal
printers in your office. Paper printers create an image by laying down liquid
inks in thousands of flat pixels. Inkjet 3D printers, in contrast, use a
printhead to drop tiny beads of fluid, called "voxels" (a mash-up of
"volume" and "pixel"), one on top of the other.
"Very
soon after those droplets are deposited, they are exposed to a bright,
ultraviolet light," MacCurdy said. "The curable liquids convert into
solids within a second or less."
But,
he added, there are many 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 drips of water can carve 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 clean them. The channels also
have to stay relatively simple.
MacCurdy
and his colleagues decided to find a way around those limitations -- better
understanding the conditions that would allow engineers to print solid and
liquid materials at the same time.
Liquid
courage
The
researchers 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, MacCurdy said is: How can you keep your droplets of solid
materials from mixing into the liquid materials, even when the droplets of
solid material are printed directly on top of the liquid droplets?
"We
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 -- the same physics that allow oil to float on top of
water," Hayes said.
Next,
the researchers experimented with a real 3D printer in the lab. They loaded the
printer up with a curable polymer, or plastic (the solid), and with a standard
cleaning solution (the liquid). Their creations were impressive: The group was
able to 3D-print twisting loops of liquid and a complex network of channels not
unlike the branching pathways in a human lung.
"Both
structures would have been nearly impossible to make through previous
approaches," Hainsworth said.
MacCurdy
also recently joined a team of researchers from CU Boulder and the CU Anschutz
Medical Campus who are developing ways to 3D-print realistic models of human
tissue. Doctors could use these models to practice for procedures and make
diagnoses. The project will employ MacCurdy's liquid-solid approach among other
tools.
"We
hope that our results will make multimaterial inkjet 3D printing using liquids
and solids more accessible to researchers and enthusiasts around the
world," he said.
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