A new 3D-printing ink being developed at Northwestern University could soon make it possible to build objects which are made of graphene for 60 percent of their volume and 75 percent of their weight. This unprecedentedly high graphene composition means that the oft-praised electric and mechanical properties of graphene might soon find their way into all kinds of macroscopic 3D-printed creations, with important consequences for the electronics and biomedical fields (among many others).

While we've already seen 3D printers that can create objects out of carbon fiber, the ability to print objects made mainly out of graphene could raise the bar even higher for material scientists and hobbyists alike. Previous graphene-based inks could only print objects in two dimensions and, with a graphene content below 20 percent, were unable to preserve the useful properties of the material. More recently, researchers have laid the foundations for building three-dimensional objects out of graphene, though this did not extend to 3D printing.

Now, however, researcher Ramille Shah and team have built a 3D-printing ink that is composed of graphene for 60 percent of volume and 75 percent of weight. What's more, the secondary component of the ink (used as a binder) is a biocompatible, biodegradable and hyperelastic polyester (PLG) that, according to the scientists involved, makes the objects printed with this ink highly flexible and safe for biomedical applications.

According to Shah and colleagues, the secret to packing such a high percentage of graphene into their ink was to embed it in the form of microscopic flakes. In its initial state, the ink is highly viscous and the flakes are oriented in random directions. However, as the ink is extruded, the flakes realign with the direction of flow, creating a single filament that maintains the much-celebrated properties of graphene even as it's being expelled from tips as small as 100 micrometers and at speeds of 40 mm/s (around half the speed of most modern 3D printers) to produce highly uniform and multilayered structures.

According to the researchers, their ink retains an electrical conductivity greater than 800 S/m, an order of magnitude more than previous 3D-printed carbon-based materials, while also being mechanically flexible and robust, biocompatible, neurogenically bioactive, biodegradable, and surgically friendly.

The stretchiness of the material can be controlled by the percentage of binder used. Shah and team found that, while their standard formulation can "only" be stretched by 81 percent in one direction before failing, a formulation with 20 percent graphene can stretch longitudinally by as much as 210 percent (at some cost in terms of electromechanical properties).

The ability to tune the elasticity of the material at will could prove important in manufacturing biomedical polymers that can, for instance, match the stretchiness of the spine safely and without breaking. The 3-D printed graphene scaffolds could also play a role in tissue engineering and regenerative medicine – when the scientists populated one of the graphene-ink scaffolds with stem cells, they found that the cells didn't just survive, but also divided, proliferated, and morphed into neuron-like cells.

Shah and team say that structures made from their ink are flexible and strong enough to be easily sutured to existing tissues, paving the way for these materials to be used for biodegradable sensors and medical implants. But the applications of a highly-conductive graphene ink could be just as exciting in building high-performance electronics.

The study appears in the latest issue of the scientific journal ACS Nano.

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