3D Printing Flexible Devices Sans Mechanical Joints

Andrew Corselli

For engineers working on soft robotics or wearable devices, keeping things light is a constant challenge: Heavier materials require more energy to move around, and — in the case of wearables or prostheses — cause discomfort. Elastomers are synthetic polymers that can be manufactured with a range of mechanical properties, from stiff to stretchy, making them a popular material for such applications. But manufacturing elastomers that can be shaped into complex 3D structures that go from rigid to rubbery has been unfeasible until now.

“Elastomers are usually cast so that their composition cannot be changed in all three dimensions over short length scales. To overcome this problem, we developed DNGEs: 3D-printable double network granular elastomers that can vary their mechanical properties to an unprecedented degree,” said Esther Amstad, head of the Soft Materials Laboratory in EPFL’s School of Engineering.

Eva Baur, a PhD student in Amstad’s lab, used DNGEs to print a prototype “finger,” complete with rigid “bones” surrounded by flexible “flesh.” The finger was printed to deform in a pre-defined way, demonstrating the technology’s potential to manufacture devices that are sufficiently supple to bend and stretch, while remaining firm enough to manipulate objects.

With these advantages, the researchers believe that DNGEs could facilitate the design of soft actuators, sensors, and wearables free of heavy, bulky mechanical joints.

The key to the DNGEs’ versatility lies in engineering two elastomeric networks. First, elastomer microparticles are produced from oil-in-water emulsion drops. These microparticles are placed in a precursor solution, where they absorb elastomer compounds and swell up. The swollen microparticles are then used to make a 3D printable ink, which is loaded into a bioprinter to create a desired structure. The precursor is polymerized within the 3D-printed structure, creating a second elastomeric network that rigidifies the entire object.

While the composition of the first network determines the structure’s stiffness, the second determines its fracture toughness, meaning that the two networks can be fine-tuned independently to achieve a combination of stiffness, toughness, and fatigue resistance. The use of elastomers over hydrogels — the material used in state-of-the-art approaches — has the added advantage of creating structures that are water-free, making them more stable over time. To top it off, DNGEs can be printed using commercially available 3D printers.

“The beauty of our approach is that anyone with a standard bioprinter can use it,” said Amstad.

Here is an exclusive Tech Briefs interview — edited for length and clarity — with Amstad and Baur.

Tech Briefs: What was the biggest technical challenge you faced while developing A) the DNGEs and B) the prototype finger?

Amstad: We had a basic idea of how it could work because we used a similar strategy for the processing of hydrogels. But hydrogels swell a lot, so we can load them with a lot of reactive agents. The elastomers swell a lot less. So, the biggest challenge was to load enough agents into the vascular particles to enable their crosslinking afterwards. Basically, you can view the particles as sponges.

Baur: The elastomers don't naturally swell a lot, so I had to find a way to make them swollen in the second precursor solution. And then for the prototype finger, I would use different types of materials and print them together. The printer we use is not always easy; sometimes you have to play with it a bit.

Amstad: I think the first one is really a material/chemistry challenge. The second one's much more a technical challenge.

Tech Briefs: Can you explain in simple terms how DNGEs work? Specifically, how the two networks can be fine-tuned independently to achieve a combination of stiffness, toughness, and fatigue resistance.

Baur: You have the microparticles — the granular parts that we usually use. We made them very stiff, so we have a network that is very dense, that has a lot of crosslinks between them. On the other side, we connect these stiff microparticles with a very stretchable second network, which will allow our material to be tougher and more fracture resistant. Whereas the microparticles will still give it stiffness, so that it will be load-bearing at the end.

Tech Briefs: The article I read says, “The Soft Materials Lab is already working on the next steps toward developing such applications by integrating active elements — such as responsive materials and electrical connections — into DNGE structures.” My question is: How is that coming along? Do you have any updates you can share on that front?

Amstad: I think we are making good progress on the electrical conductivity part. With regards to the sensing part, the activation is a bit more challenging, we are making steps but it's a bit slower than what I expected.

Tech Briefs: What are your next steps?

Amstad: We really would like to functionalize it in a way that you can use it for robotics first, but the dream is still to eventually use it in biomedical fields. Next to getting the activation right — meaning controlling the speed with which we can activate, the extent at which it can be activated, and how it activates the motion — if you want to go into biomedicine, you then also have to go through all the biocompatibility issues and that will take a lot more time. So, we'll plan to first use it ex vivo where you have fewer regulations in motion guiding applications, but then with time go through biocompatibility studies and eventually go in vivo.

Tech Briefs: The article also says, “With these advantages, the researchers believe that DNGEs could facilitate the design of soft actuators, sensors, and wearables free of heavy, bulky mechanical joints.” How soon could we see DNGEs implemented on such a scale?

Baur: Right now, we're still on the lab scale. To do it on a much bigger scale would take some time, and it would depend on whether we could find some funds or some people who would be interested in implementing it at this large scale. But it's something we could do. If you locally change the mechanical properties, you can almost have no more mechanical joints. For time scale: it depends.

Amstad: Scaling this up takes time, but there is nothing I see that could be a showstopper in the upscaling. There is no synthesis needed that can be performed only on a milligram scale or anything like that. In principle, we can also make the particle through mechanical fragmentation, which you can do in kilograms or tons if you go to the right facilities. So, it's a question of adjusting, but there is nothing that fundamentally limits us, at least that we are aware of at this point.

Tech Briefs: Do you have any advice for engineers or researchers aiming to bring their ideas to fruition?

Baur: To me it was always about teamwork. In our labs, we all work on things, not similar things, but things that look a bit the same. So, I could always count on my lab mates and ask them questions when I was stuck. Also, never give up because at some point it may work.

Amstad: I think you need to be convinced and have the will to overcome difficulties and hurdles. And also have an open mind to finding solutions that you might not have thought of initially.

Baur: Especially with the elastomers. We read papers and spoke to people who said it would not work because elastomers are not known for the properties we needed, but actually we were able to use them. So, always try, and even if at first you think it's not possible but then you succeed, it's very gratifying.

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