This focused ion beam microscope image shows 3D graphene layers welded together in a block. Image: Ajayan Group/Rice University.
This focused ion beam microscope image shows 3D graphene layers welded together in a block. Image: Ajayan Group/Rice University.

Flakes of graphene welded together into solid materials may be suitable for use as bone implants, according to a study led by scientists at Rice University.

The Rice lab of materials scientist Pulickel Ajayan, together with colleagues in Texas, Brazil and India, used spark plasma sintering to weld flakes of graphene oxide into porous solids that possess similar mechanical properties and biocompatibility to titanium, a standard bone-replacement material. They report this work in a paper in Advanced Materials.

The researchers believe their technique will give them the ability to create highly complex shapes out of graphene within minutes using graphite molds.

"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," said Rice postdoctoral research associate Chandra Sekhar Tiwary, co-lead author of the paper 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."

According to Tiwary, spark plasma sintering is currently being used in industry to make complex parts, generally with ceramics. "The technique uses a high pulse current that welds the flakes together instantly. You only need high voltage, not high pressure or temperatures," he said.

The material they made is nearly 50% porous, with a density half that of graphite and a quarter that of titanium metal. Nevertheless, the material has enough compressive strength – 40 megapascals – to find use as a bone implant. The strength of the bonds between the sheets also keeps it from disintegrating in water.

The researchers controlled the density of the resultant material by altering the voltage that delivers the highly localized blast of heat that welds the graphene flakes together. In this way, they made graphene solids of various density by raising the sintering temperatures from 200°C to 400°C, finding that samples made at local temperatures of 300°C proved best. "The nice thing about two-dimensional materials is that they give you a lot of surface area to connect. With graphene, you just need to overcome a small activation barrier to make very strong welds," Tiwary said.

With the help of colleagues from the nanomechanical testing company Hysitron, the researchers measured the load-bearing capacity of thin sheets of two- to five-layer bonded graphene. They did this by repeatedly stressing the sheets with a picoindenter attached to a scanning electron microscope, finding that the sheets were stable up to 70 micronewtons.

Colleagues at the University of Texas MD Anderson Cancer Center then successfully cultured cells on the material to show its biocompatibility. As a bonus, the researchers also discovered that the sintering process can reduce graphene oxide flakes to pure bilayer graphene, which is stronger and more stable than graphene monolayers or graphene oxide.

"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. Engineering junctions and strong interfaces between nanoscale building blocks is the biggest challenge in achieving such goals, but in this case, spark plasma sintering seems to be effective in joining graphene sheets to produce strong 3D solids."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.