3-D printing development assists in soft robots creation.
In order to print in three dimensions, a specialized robot, called a 3-D printer, must first slice the desired object into 2-D slices. This plan is then used as the machine’s blueprint as it begins to lay down layers upon layers of material, usually some type of plastic. Using this basic format, 3-D printing has been used to create many things from shoes to houses.
One aspect of this technology which Oregon State University researchers are working to progress is the printing of liquids. More specifically, graduate students in the laboratory of Yigit Manguc, a part of the Collaborative Robotics and Intelligent Systems Institute, are 3-D printing liquid silicone for various projects.
Steph Walker, a fourth-year, dual Ph.D. candidate in Materials Science and Robotics, began the project in 2016. In the laboratory, she primarily works on the actual materials being printed, primarily silicone polymers.
“Much of soft robotics still relies on manual molding methods to create their robots and actuators which can introduce defects due to gluing issues, weak seams and alignment problems,” Walker said via email. “By introducing silicone 3-D printing methods to soft robotics, and to the production of soft devices in general, we can create soft 3-D prints which are customizable and complex, difficult to mold.”
Once integrated into the field, 3-D printing soft robots could enable researchers to create their robots using only one machine or system, Walker said via email. The use of 3-D printing technology would also reduce the amount of time required to create the soft robots in comparison to the relatively lengthy molding process.
“Instructions of how to build our printer system will also be fully open-source, after publication, using off-the-shelf silicones that anyone can buy and use, unlike some processes which would require chemical lab facilities,” Walker said via email.
Three of these 3-D silicone printers were built and modified by Osman Dogan Yirmibesoglu, a third-year Dual Ph.D. candidate in Robotics and Mechanical Engineering, allowing it to functionally 3-D print the viscous silicone material used.
“None of the 3-D printers out there currently are 100 percent capable of 3D printing soft, functional robots made out of 2 part platinum cure silicone materials,” Yirmibesoglu said. “So our goal was to fill that gap that can print soft, functional robots with complex geometries.”
One of the most important innovations of this project was determining which additives will make silicone material suitable for 3-D printing. The best material compositions for 3D printing are those with high viscosity, (which can hold their shapes after deposition) Yirmibesoglu said. These such materials require more force to push through the tubing in the printer and resist movement more than other liquids.
Two key fixtures that Yirmibesoglu and John Morrow (former lab member) added were the mixture system used in the head of the 3-D printer and the heater system used to cure the printed silicone.
Yirmibesoglu and the other project members designed the print head to combine the two parts of the silicone material. They colored each portion with different dyes so that when two materials are mixed the team could tell how well the material had been mixed before the time of printing. For example, they used yellow and red to create an orange-colored final product.
“At that moment, when mixed silicone is poured onto a heated bed, it starts to cure,” Yirmibesoglu said. “Curing means it becomes solidified by staying soft and flexible. And this curing rate is really important for successful 3-D prints.”
A common issue found in the earlier days of the project was that after a certain height, the heated plate could no longer cure the upper layers of the printed silicone, Yirmibesoglu said. To fix this, Yirmibesoglu crafted two heaters which were attached on either side of the mixer head to blow warm air on the newly printed sections gently.
“In general, in conventional 3-D FDM type printers, heating systems are used to melt the filament,” Yirmibesoglu said. “But in our case, heating systems are blowing hot air on top of the printed material to cure it. A very different approach.”
Other difficulties may result from using silicone; for example, its high compressibility and temperature sensitivity can result in longer periods of time before an optimal level is reached, as well as a change in flow. Additionally, the fact that it’s a liquid means it cannot be retracted into the printer’s head to move to the next layer like it does in solid 3-D printing. The liquid is continually oozing out which can cause bulges and errors in the pattern of the soft robot if not accounted for, said Gabriel Kulp, a second-year physics major, working on the software side of the project.
“It’s like drawing a picture without being able to lift the pen,” Kulp said. “When you’re printing with silicone you can’t lift the pen and the software that other people have already written to convert a 3-D object into a set of paths, or 2-D slices, that a normal 3-D printer follows—that software is not designed to work without lifting the pen.”
This hurdle was overcome by configuring the available 3-D printing software to suit the needs of the project, Kulp said. In contrast to the traditional, molding method of creating soft robots, this technology allows for much more flexibility.
With the help of 3-D silicone printing, researchers can 3-D print soft robots with increased complexity and achieve more interconnected channels. Currently used molding techniques to create soft robots could not accomplish the same results, Yirmibesoglu said.
“With this machine, because you are printing layer by layer, you will be able to create more complex soft robots, in the near future,” Yirmibesoglu said. “And also, right now, we don’t use any support materials, when we introduce support materials, 3-D printing capacity of our printers will achieve even more complex shapes. ”
Some potential applications for soft robotics could range anywhere from medical devices to wearable technology to industrial settings, Walker said via email.
“Some fun projects off the top of my head would be soft fitness watches, customizable footwear, soft exoskeletons, orthotics, toys, bio- inspired submarine vessels and more,” Walker said via email. “The more you think of what soft, stretchy, customizable products can do, the crazier the ideas become.”
One application of interest to the mLab soft robotics research laboratory is soft actuators which could be used in a variety of settings. For example, mlab members designed a soft actuator to be fitted on a small submarine which allowed researchers to grasp corals without harming them, Yirmibesoglu said.
Another application of soft robotics researchers are exploring is the motion of soft snakes, Kulp said. Currently, most soft robots move around via a system of legs, however Callie Branyan is studying the way soft robots can move without legs, like snakes. As with the initial 3-D printing project, Kulp assists with the software side of Branyan’s work.
“I took inspiration from music software,” Kulp said. “Instead of a note, it’s the valve being opened. Instead of the volume of the note, it’s how much the valve is opened. Instead of the place on the staff, it’s which valve is opened. So the software takes an input file that can be easily handwritten and is human-readable and human-editable that defines the motion.”
The advantage of this system is that instead of writing and modifying a complex, mathematical model of the motion of the soft snake, the researcher can easily write a more simple definition for each snake’s movement, Kulp said.
“Imagine writing a song where you have to write an equation that describes how the pitch and the volume of the notes change,” Kulp said. “So this method allows for faster iteration; she can try more things faster.”
Other potential paths for this project include the use of soft robots under radiation environments, the development of soft electronics and more, Yirmibesoglu said.
“The future directions are going to be like multi-material 3-D printing, giant soft robots, 3-D printing more complex soft robots so that we can see what is our limit, how far can we go,” Yirmibesoglu said.
