Gear-based metamaterials supersede abilities of adaptive chemical or electro-responsive materials

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. There are many scenarios where you really need a structure to be soft and pliable for one application and then rigid for another, or where a variable environment demands robust material properties. One example is a robot leg that needs to bend to allow the robot to move and then become stiff to bear a load but must also operate in extreme environments. 

PETER GUMBSCH: A lot more of this functionality can really be integrated into the material but that requires different thinking. The change of the properties in the built machine is something which is not in any of our current design codes. 

LAURA LEAY: That was Professor Peter Gumbsch, who is affiliated with both the Karlsruhe Institute of Technology and the Fraunhofer Institute for Mechanics of Materials in Germany. His team has been working on a new solution to the challenge of adaptable materials. Using off-the-shelf techniques such as 3D printing and lithography, a lattice of tiny mechanical gears that can be as small as 1 mm in diameter was produced. The team demonstrated that no assembly was required; these metamaterials can be produced in a single go using integrated manufacturing. Each gear is analogous to an atom but there’s a key difference between these metamaterials and classical ways of thinking about the coupling between atoms: you can’t change the properties of the bonds between atoms but you can change the properties of the coupling between the gears. 

PETER GUMBSCH: We followed two directions. The first one was: we directly built a lattice out of gears and used the anisotropy built into each individual gear and the fact that we can adjust their relative orientation. This allows access to, for example, adjusting compressive strength. 

LAURA LEAY: This initial concept required a frame to hold the gears together which meant the mechanical properties of the metamaterial were limited by the properties of the frame. To remove this effect the team then built a metamaterial using planetary gears which did not require a frame. The team also used finite element analysis to confirm the performance of these physical systems. These gear-based metamaterials offer several advantages over adaptive chemical or electro-responsive materials: their response time is much quicker – of the order of seconds rather than minutes – and there is much greater variability in their mechanical properties. 

PETER GUMBSCH: With our gears, in terms of speed, we are limited by inertia so we can go really fast depending on the motor we put on it. With the effective moduli, we can go into the ten gigapascal regime. None of the thermally responsive shape-morphing materials can really get to such robust structural properties.  

LAURA LEAY: The applications for these metamaterials are manifold and right now the main limitation is in the 3D printing technology. But to see adaptable, tunable metamaterials used to build something will require a radical shift in thinking: instead of choosing a material for a given application and compromising on material properties, in the future engineers might be thinking of how they can use an adaptable metamaterial to build shape-morphing structures that can withstand any environment. This work was published in a recent issue of Nature Materials. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.