Made of Sterner Stuff (ODTmag)
Graphene—the two-dimensional allotrope of carbon taking the form of a two-dimensional, atomic scale hexagonal lattice in which one atom forms each vertex—was discovered by University of Manchester scientists in 2004. Or, rather, one should say it was rediscovered, as it had unintentionally been produced in small amounts with pencils or other graphite uses for centuries. The University of Manchester researchers were the first to create and document graphene flakes one atom thick, and because of this, graphene was awarded its 2D status. Two hundred times stronger than steel, immensely light, and an excellent conductor, it is being hailed as the next “wonder material” for biomedical applications. And once manufacturers can discover a method to produce it beyond haphazard flakes, graphene could usher in new classes of orthopedic technologies.
For example, in September 2016, the Rice University lab of materials scientist Pulickel Ajayan, in concert with colleagues in Texas, Brazil, and India, were able to weld flakes of graphene oxide in porous solids using spark plasma sintering. The researchers discovered the 3D graphene material compared favorably with titanium—a standard bone-replacement material—in both mechanical properties and biocompatibility.
“We started thinking about this for bone implants because graphene is one of the most intriguing materials with many possibilities and it’s generally biocompatible,” commented Rice postdoctoral research associate Chandra Sekhar Tiwary, co-lead author of the paper on the discovery in Advanced Materials, with Dibyendu Chakravarty of the International Advanced Research Center for Powder Metallurgy and New Materials in Hyderabad, India. “Four things are important: its mechanical properties, density, porosity, and biocompatibility.”
The researchers are confident this method will facilitate forming highly complex shapes out of graphene in minutes using graphite molds. They later discovered the sintering process is capable of reducing graphene oxide flakes into pure bilayer graphene, making it much more stable than graphene monolayers.
“This example demonstrates the possible use of unconventional materials in conventional technologies,” Ajayan said. “But these transitions can only be made if materials such as 2D graphene layers can be scalably made into 3D solids with appropriate density and strength.”
Rice University lab was back to its orthopedic graphene hijinks later in September, this time under the tutelage of chemist James Tour. Tour’s lab has spent over a decade experimenting with graphene nanoribbons, resulting in a material dubbed “Texas PEG” that could knit damaged or even severed spinal cords. Texas PEG is a combination of graphene nanoribbons customized for medical use with polyethylene glycol (PEG, hence the name), a biocompatible polymer gel used in surgeries and pharmaceutical products. The PEG chains functionalize the edges of the biocompatible nanoribbons, and when further mixed with PEG, form an electrically active network that could help reconnect the severed ends of a spinal cord.
Tour said only about 1 percent of the Texas PEG material consists of nanoribbons, but that’s more than enough to form a conductive scaffold to reconnect the cord. The material was successful in restoring function in a procedure performed at Konkuk University in South Korea. Motor and sensory signals bridged the gap via Texas PEG a day after complete spinal transection, and almost perfect motor control recovery was gained in two weeks.
Material Witnesses
Graphene is certainly a trendy material to work with in the research stages, but has a long way to go before becoming a viable orthopedic material. The market dictates that orthopedic devices are made to last a long time and resist wear, and current studies on graphene are inconclusive in this regard. Although graphene has demonstrated favorable biocompatibility—which is of the utmost importance for musculoskeletal implants—an innovative material must undergo a lengthy regulatory and trialing process before consideration in a Class III orthopedic device.