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PROVIDENCE, R.I. [Brown University] – Researchers at Brown University have shown a way to use graphene oxide (GO) to add an essential element to alginate-based hydrogel materials, a naturally derived material seaweed currently used in various biomedical applications. In an article published in the journal Carbon, the researchers describe a 3D printing method for making complex and durable alginate-GO structures, which are much stiffer and more fracture resistant than alginate alone.
"A limiting factor in the use of alginate hydrogels is that they are very fragile – they tend to disintegrate under the effect of a mechanical load or in solutions. low-salt, "said Thomas Valentin, Ph.D. student at the Brown & s School of Engineering who led the work. "What we have shown, is by including graphene oxide nanosheets, we can make these structures much more robust."
The material is also able to become stiffer or more flexible in response to different chemical treatments, which means that it could be used to make "smart" materials that can react to their environment in real time, according to research . In addition, alginate-GO retains the ability of alginate to repel oils, giving it a new potential as a solid antifouling coating.
The 3D printing method used to make the materials is known as stereolithography. The technique uses an ultraviolet laser controlled by a computer-aided design system to trace patterns on the surface of a photoactive polymer solution. Light causes the binding of polymers, forming solid 3D structures from the solution. The tracing process is repeated until an entire object is built layer by layer from bottom to top. In this case, the polymer solution was prepared using sodium alginate mixed with graphene oxide sheets, a carbon-based material that forms nanowires of a thickness of 1 atom and which are stronger than steel.
One advantage of the technique is that the sodium alginate polymers bind by ionic bonds. The bonds are strong enough to hold the material together, but some chemical treatments can break them. This gives the material the ability to respond dynamically to external stimuli. Previously, Brown's researchers had shown that this "ionic crosslinking" could be used to create alginate materials that degrade on demand and dissolve quickly when they are treated with a chemical that scavenges ions of the internal structure of the material.
For this new study, researchers wanted to see how graphene oxide could alter the mechanical properties of alginate structures. They showed that alginate-GO could be made twice as stiff as alginate alone and much more resistant to cracking failure.
"The addition of graphene oxide stabilizes the hydrogel of alginate with a hydrogen bond," said Ian Y. Wong, assistant professor of engineering at Brown and senior author of the journal. "We believe that the fracture resistance is due to the fact that the cracks must be bypassed around the scattered graphene sheets rather than being able to break with homogeneous alginate although homogeneous."
The extra rigidity allowed the researchers to print structures with overhanging parts, which would have been impossible with alginate alone. In addition, increased stiffness does not prevent alginate-GO from responding to external stimuli such as alginate alone. The researchers showed that by bathing the materials in a chemical that removes their ions, the materials swelled and became much softer. The materials regained their rigidity during the ion restoration thanks to a bath of ionic salts. Experiments have shown that the stiffness of the materials can be adjusted to a factor of 500 by varying their external ionic environment.
This ability to alter its rigidity could make alginate-GO useful in many applications, according to researchers, including dynamic cell cultures.
"You can imagine a scenario in which you can imagine living cells in a rigid environment, then immediately move to a more flexible environment to see how the same cells might respond," said Valentin. This could be useful for studying how cancer cells or immune cells migrate through different organs of the body.
And because alginate-GO retains the powerful oil-repellent properties of pure alginate, the new material could be an excellent coating to prevent oil and other soiling from building up on surfaces. In a series of experiments, the researchers showed that a layer of alginate-GO could prevent the oil from fouling on the surface of the glass under very saline conditions. According to the researchers, alginate-GO hydrogels may be useful for coatings and structures used in the marine environment.
"These composite materials could be used as a sensor in the ocean that can continue to take action during a spill of oil, or as an antifouling coating contributing to the cleanliness of the hulls of ships "said Wong. The added rigidity offered by graphene would make these materials or coatings much more durable than alginate alone.
The researchers plan to continue experimenting with the new material, looking for ways to rationalize production and optimize its properties.
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