In a study that was led by Rice University scientists, solid materials were created by welding flakes of graphene together. The team believes that the resultant material may be suitable for bone implants.
Pulickel Ajayan, a materials scientist at the Rice lab and fellow scientists from India, Texas and Brazil, welded flakes of graphene oxide into porous solids by using spark plasma sintering. The result compared well with titanium when looking at both the biocompatibility and mechanical properties. Titanium is currently being used as a standard for bone replacement procedures.
According to the researchers, graphene should be easier to process than specialty metals as the technique they used allows them to use graphite molds to create highly complex shapes in a very short time. Chandra Sekhar Tiwary, a Rice postdoctoral research associate, together with Dibyendu Chakravarty of the International Advanced Research Center for Powder Metallurgy and New Materials in Hyderabad, India, are the co-lead authors of the paper. They note that the four properties of graphene critical for this type of application are its biocompatibility, mechanical properties, density and porosity.
In industry, spark plasma sintering is used to make composite parts, normally with ceramics. Tiwary explains that the process welds the flakes together instantly by using a high pulse current. Instead of using high temperatures or pressure, only high voltage is required. The graphene material created by the team has 40 mega Pascals compressive strength with a density only a quarter that of titanium metal and half that of graphite. It is also nearly 50 percent porous. These qualities make it highly suitable for bone implants. It won’t disintegrate in water due to the strength of the bonds between sheets.
The density of the material is controlled by varying the voltage that delivers the localized blast of heat needed to create the nanoscale welds. By experimenting with sintering temperatures between 200 and 400 degrees Celsius, the researchers determined that the samples manufactured at 300 degrees C were of the best quality. In spite of the high temperatures used for the actual weld, the environment remained at a comfortable room temperature. Tiwary notes that two-dimensional materials have a lot of surface area that can be connected to. Graphene specifically has a very small activation barrier that needs to be overcome to make very strong welds.
Thin sheets of between two and five layers of bonded graphene was stressed repeatedly to test the load-bearing capacity. Colleagues at Hysitron in Minnesota assisted with this process. A picoindenter attached to a scanning electron microscope was used for measuring and the sheets were found to be stable up to 70 micro Newtons.
To show its biocompatibility, another team at the University of Texas MD Anderson Cancer Center successfully cultured cells on the material. During this process, researchers discovered that the sintering process could reduce graphene oxide flakes to pure bilayer graphene. The result is a material that is more stable and stronger than graphene oxide or graphene monolayers.
Ajayan cites this as an example of using unconventional materials in conventional technologies. He does however caution that 3-D solids with appropriate density and strength instead of 2-D graphene layers ultimately need to become available before the technology can be used commercially. The biggest challenge in achieving this is to create strong interfaces and junctions between nanoscale building blocks. He believes that strong 3-D solids can be created by joining graphene sheets with spark plasma sintering.
The full study was published in the journal Advanced Materials.