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34 PCB007 MAGAZINE I NOVEMBER 2018 by David L. Chandler, Massachusetts Institute of Technology Tiny robots no larger than a cell could be mass-produced using a new method developed by researchers at the Massachusetts Institute of Technology (MIT). The microscopic devic- es—which the team calls "syncells" (short for synthetic cells)—might eventually be used to search for disease while floating through the bloodstream, or to monitor conditions inside an oil or gas pipeline. To make such tiny devices in large quantities, the team developed a method called autoperfo- ration to control the natural fracturing process of atomically-thin, brittle materials, directing the fracture lines so that they produce minis- cule pockets of a predictable size and shape. Embedded inside these pockets are electronic circuits and materials that can collect, record, and output data. The novel process is described in a paper published in the journal Nature Materials by MIT Professor Michael Strano, postdoc Ping- wei Liu, graduate student Albert Liu, and eight others at MIT. The system uses a two-dimensional (2D) form of carbon called graphene, which forms the outer structure of the tiny syncells. One layer of the material is laid down on a surface, then small dots of a polymer material contain- ing the electronics for the devices are depos- ited by a sophisticated laboratory version of an inkjet printer. Then, a second layer of gra- phene is laid on top. "We discovered that you can use the brittle- ness," says Strano, who is the Carbon P. Dubbs Professor of Chemical Engineering at MIT. "It's counterintuitive. Before this work, if you told me you could fracture a material to control its shape at the nanoscale, I would have been in- credulous." However, the new system does just that. It controls the fracturing process so that rather than generating random shards of material, like the remains of a broken window, it pro- duces pieces of uniform shape and size. "You can impose a strain field to cause the fracture to be guided, and you can use that for con- trolled fabrication," Strano says. When the top layer of graphene is placed over the array of polymer dots, which form round pillar shapes, the places where the graphene drapes over the round edges of the pillars form lines of high strain in the ma- terial. As Liu describes it, "Imagine a table- cloth falling slowly down onto the surface of a circular table. One can very easily visu- alize the developing circular strain toward the table edges, and that's very much anal- ogous to what happens when a flat sheet of graphene folds around these printed polymer pillars." As a result, the fractures are concentrat- ed along those boundaries, Strano says, "And then something pretty amazing happens—the graphene will completely fracture, but the frac- ture will be guided around the periphery of the pillar." The result is a neat, round piece of gra- phene that looks as if it had been cleanly cut out by a microscopic hole punch. Because there are two layers of graphene above and below the polymer pillars, the two resulting disks adhere at their edges to form something like a tiny pita-bread pocket with the polymer sealed inside. "The advantage here is that this is essentially a single step," Strano says, in contrast to many complex cleanroom steps needed by other processes to try to make microscopic robotic devices. The researchers have also shown that other 2D materials in addition to graphene, such as molybdenum disulfide and hexagonal boroni- tride, work just as well. Cell-like Robots Ranging in size from that of a human red blood cell—about 10 micrometers across—up to about 10 times that size, these tiny objects "start to look and behave like a living biolog- ical cell," according to Strano. "In fact, under a microscope, you could probably convince most people that it is a cell," he says. How to Mass Produce Cell-sized Robots