Strong, Stretchy, 3D-Printed Metamaterials

Metamaterials are synthetic materials with microscopic structures that give the overall material exceptional properties. (Image: Courtesy of the researchers)

In metamaterials design, the name of the game has long been “stronger is better.”

Metamaterials are synthetic materials with microscopic structures that give the overall material exceptional properties. A huge focus has been in designing metamaterials that are stronger and stiffer than their conventional counterparts. But there’s a trade-off: The stiffer a material, the less flexible it is.

MIT engineers have now found a way to fabricate a metamaterial that is both strong and stretchy. The base material is typically highly rigid and brittle, but it is printed in precise, intricate patterns that form a structure that is both strong and flexible.

The key to the new material’s dual properties is a combination of stiff microscopic struts and a softer woven architecture. This microscopic “double network,” which is printed using a plexiglass-like polymer, produced a material that could stretch over four times its size without fully breaking. In comparison, the polymer in other forms has little to no stretch and shatters easily once cracked.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Carlos Portela, Robert N. Noyce Career Development Associate Professor at MIT, and First Author James Utama Surjadi, Postdoctoral Associate.

Tech Briefs: What was the biggest technical challenge you faced while developing these metamaterials?

Portela: Manufacturing is really challenging. A lot of these features are quite complex, and we were lucky in the sense that we had the right tools that allow you to do large, unsupported features. At the time, we thought that was the only technique that was going to allow us to do that. But through more experimentation, we now see that there are many other 3D printing techniques that can do these types of morphologies at these scales.

I think one other challenge was, again, involving manufacturing: to print at these sufficiently fast speeds such that we could test large tessellations of these unit cells. Because, otherwise, we're not getting a lot of useful information if we just print few very small samples.

MIT engineers have found a way to fabricate a metamaterial that is both strong and stretchy. The base material is typically highly rigid and brittle, but it is printed in precise, intricate patterns that form a structure that is both strong and flexible. (Image: Courtesy of the researchers)

Tech Briefs: What was the catalyst for this project? How'd the work come about?

Portela: We started back in 2020. There was an original paper about highly compliant architecting materials. So, basically, introducing the idea of woven lattices. And then we realized their materials were interesting, but we thought they're not very useful because they were very compliant — so easy to deform, very easy to break as well. So, we started thinking, and we saw a lot of literature out there on so-called inter-penetrating lattices.

So, what is different in our approach is that instead of just picking two lattices that are already stiff and kind of blending them together, we saw that in other synthetic systems such as double network hydrogels, you have something that's both stiff and compliant. We've been working on these stiff things for decades, but these woven compliant things were relatively new. So, really, our idea at the moment was to just combine both and see if we get the same effective responses as in all of the double network hydrogels.

Tech Briefs: Can you please explain in simple terms how you print them and how everything works?

Surjadi: In typical 3D printing systems, especially resin 3D printers, you start from a liquid photo resist, and use an LCD light projector, however, we use a laser instead.

We focus a laser using an objective onto a single point. The spot at which that laser is focused will solidify the liquid photo resist, and we then scan around this point in 3D dimensional space to create the 3D architecture.

Testing is a bit more complicated because the features of the structures are on the order of, say, two microns, that's about 50 times smaller than the width of your hand. So, we have to use a scanning electron microscope (SEM). We have to put the samples inside the SEM, along with a nanoindenter, to test it.

So, something that's a bit challenging is aligning the samples together inside the SEM because they are really small and you have to get the alignment right in order to grip them and do tension experiments. You cannot change the orientation of your samples inside the SEM. That's basically how we test it.

Tech Briefs: Do you have any set plans for further research, work, etc.?

Portela: Yeah, we have demonstrated the concept with these polymeric materials and, from here, we see a couple of opportunities. One is extending this to more brittle material systems. The real dream will be to be able to do this with glasses, other ceramics, or even metals — things that normally we don't expect to deform a lot before they break. Brittle materials are the perfect candidates for us to try to make into woven-type architectures.

The second aspect is that we're realizing that a lot of the mechanics, the benefits behind the mechanical response, are due to weird entanglements and knotting or contact that take place within these architectures. And, so far, we really didn't go too far in terms of designing for optimal entanglements. We designed something and kept them as a byproduct, but we could take the other approach and say, ‘We know that these types and numbers of entanglements will give us a very tough fabric-like response. So, maybe we can use that to design new types of textiles or fabrics that can be tougher, harder to break.’

These are stiff, but at the same time, compliant materials. So, one of the things we've played around with is trying to see if they can be used as cell scaffolds. If you put certain types of cells on top of a 3D microscopic cube, they'll use it as a substrate for them to proliferate and grow. And they seem to like certain mechanical properties in these scaffolds. That's one of the immediate applications that we're already exploring.