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Published on September 16th, 2016 | By: April Gocha, PhD


Spark plasma sintering welds graphene into 3-D structures to replace bone

Published on September 16th, 2016 | By: April Gocha, PhD

[Image above] Credit: Lindsey Turner; Flickr CC BY 2.0



Graphene has a lot of game—the 2-D material has received considerable attention recently for its potential applications in everything from microphones to speakers to OLEDs to next-gen consumer electronics.


We’ve even seen graphene pair up with other materials, including alumina and glass, to try to reach its full potential.


But what about bone?


Graphene may be a wonder material, but its potential biomedical applications are often overlooked in favor of its electronic options—despite the fact that carbon materials are incredibly biocompatible.


Researchers at Rice University (Houston, Texas) aren’t missing out on graphene’s skeletal potential, however.


“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,” says Rice postdoctoral research associate Chandra Sekhar Tiwary, co-lead author of a new paper detailing graphene’s potential as a bone replacement, in a Rice University press release. “Four things are important: its mechanical properties, density, porosity, and biocompatibility.”


Using spark plasma sintering of graphene flakes, the researchers fabricated 3-D porous solids from that they say will make an excellent bone replacement material because it ticks all of those important boxes.



A focused ion beam microscope image shows 3-D graphene layers welded together in a block. Credit: Ajayan Group; Rice University


Adjusting the voltage used for spark plasma sintering allowed the scientists to control density of the resulting 3-D graphene, and the researchers found that local sintering temperatures of 300ºC hit a sweet spot.


The team worked out those conditions to fabricate sintered graphene structures that, despite having high yield strength and stiffness, were ~50% porous—half as dense as graphite and a quarter as dense as titanium.


But why do graphene flakes form a 3-D structure instead of falling flat? According to the researchers, oxygen molecules on graphene oxide prevent the flakes from closely bonding to one another—which is the case with pure graphene.


Watch this simulation to see the difference in how graphene and graphene oxide flakes stack up during sintering.


Credit: Rice University; YouTube


But even at such a light density, the 3-D graphene material has a compressive strength of 40 megapascals, which the scientists say makes it strong enough to serve as a bone implant. Picoindentation experiments further showed that two- to five-layer bonded graphene materials could stand up to 70 micronewtons.


And the sintered graphene structures “compare favorably with the mechanical properties and biocompatibility of titanium, a standard bone-replacement,” according to the Rice release.



A pellet of 3-D reduced graphene oxide shows the potential to replace titanium as a material for bone implants. Credit: Rice University


“The nice thing about 2-D materials is that they give you a lot of surface area to connect,” Tiwary says in the release. “With graphene, you just need to overcome a small activation barrier to make very strong welds.”


The paper, published in Advanced Materials, is “3D porous graphene by low-temperature plasma welding for bone implants” (DOI: 10.1002/adma.201603146).


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