A skull made of 3DG, a mechanically robust and flexible 3D-printed material. Image: Northwestern University
Because of its exceptional electronic, mechanical, thermal, and biocompatible properties, considerable research has been focused on graphene and how it can be used in high-speed electronics, electrochemical sensors, drug delivery systems, stem cell differentiation, neuro tissue engineering and regeneration, and many other high-tech applications.
From a 3D printing perspective, graphene has been previously incorporated into 3D-printed materials, but most of these constructs contain less than 20 volume percent of graphene, greatly inhibiting its many celebrated properties and its range of applications.
Now researchers at Northwestern University, led by Ramille Shah, assistant professor of materials science and engineering at McCormick School of Engineering, have developed a novel graphene-based ink that can be used to print large, robust 3D structures that contain 60 percent to 70 percent graphene and exhibit unique mechanical and biological properties. This discovery is the first to show that high-volume-fraction graphene composite constructs can be formed from easily extrudable liquid ink.
An Innovative Formulation
Adding higher volumes of graphene flakes to the ink mix typically results in printed structures that are too brittle and fragile. At 60 percent to 70 percent graphene, however, Shah’s formulation preserves the material's unique properties, including its electrical conductivity. It’s also flexible and robust enough to 3D-print robust macroscopic structures.
The secret of Shah’s success is a special mix of solvents and a biodegradable elastomer in the ink.
“The ink is comprised of graphene powder, a biocompatible and biodegradable elastomer, and a mixture of solvents,” says Shah. “When combined in the proper ratios, we can make 3D-printable graphene ink that results in majority graphene 3D-printed structures.”
False-colored electron micrograph of the top few layers of a single, 40-layer scaffold. Image: Northwestern University
Initially a liquid suspension, once the ink is extruded from a nozzle it rapidly solidifies into a self-supporting structure. The ink is also tailored in such a way that it remains wet enough after extrusion to seamlessly merge with previously deposited 3D-printed layers and adjacent material. “In this way,” she says, “small [<1 mm] or large [many centimeter] objects comprised of single layer, or many hundreds or even thousands of layers, can be fabricated and handled immediately after printing at linear print rates approaching 10 centimeters per second.”
The resulting 3D-printed material—called 3DG—has many unique physical, mechanical, electrical, and biological properties. For example, 3DG material is mechanically robust and flexible while retaining electrical conductivities greater than 800 S/m, an order of magnitude increase over previously reported 3D-printed carbon materials. 3DG sheets can be rolled, stretched, folded, cut, and even sutured to soft tissue, all of which are advantageous properties for surgical implementation. Furthermore, although not as conductive as pristine, single-sheet graphene, “We believe 3DG is the most conductive, non-metallic material to be 3D-printed, which also happens to be biocompatible,” says Shah. “3DG can support stem cell growth and proliferation, as well as induce glial and neurogenic cellular differentiation without the need for added neurogenic factors or external stimuli. It does not elicit a strong immune response when implanted in mice and rapidly vascularizes and integrates with host tissue.”
Shah demonstrated that liquid-based inks can be 3D-printed at ambient temperature via simple extrusion to result in user-defined, high-fidelity structures comprised primarily of graphene. 3DG is very flexible and can be easily printed into small- or large-scale objects.
Applications for this new material abound—with its high electrical conductivity, ability to store energy, and ultra-strong and lightweight structure, graphene has potential for many applications in electronics, energy, the environment, and even medicine.
Shah is especially excited about its use as a biomaterial. “A significant motivating factor behind this work is the need for more innovative biomaterials for nervous tissue regeneration, as well as biomaterials that are translatable—scalable and not so expensive to produce,” she says. “Our 3D-printable graphene inks are relatively easy to produce in a scalable fashion, can be rapidly fabricated into an infinite variety of forms, including patient-specific implants, and are also surgically friendly.”
3DG composites also have the capacity to induce mesenchymal stem cell differentiation toward neurogenic lineages, without the need for any other neurogenic growth factors or external stimuli. “This is truly exciting,” says Shah, “because it indicates that a material alone can induce a strong biological response, which can be leveraged for tissue engineering and regenerative medicine applications.”
Shah is currently planning for functional in vivo applications, which include investigating 3DG's ability to regenerate damaged nervous and cardiac tissue, as well as probing its capacity to be combined with other 3D-printable ink systems developed in her lab for engineering more complex nerve, muscle, and bone tissue composite structures. “We will also be exploring its potential use for non-biomedical applications,” concludes Shah. “Innovations in 3D-printing systems themselves are still needed to be able to easily scale and multi-material print at a commercial manufacturing level.”
Mark Crawford is an independent writer.
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This discovery is the first to show that high-volume-fraction graphene composite constructs can be formed from easily extrudable liquid ink.
Prof. Ramille Shah, Northwestern University