CREDIT: University Wong / Brown
Brown University researchers have shown a way to use graphite oxide (GO) to add some spines to hydrogel materials made from alginate, a natural material derived from seaweed, currently used in various biomedical applications. In a report published in the magazine carbon, the researchers describe a 3-D printing method for making complex and durable alginate-GO structures that are much harder and more resistant to fractures than alginate alone.
"A limiting factor in the use of alginate hydrogels is that they are very fragile – they break down under mechanical stress or in low-salt solutions," said Thomas Valentin, a PhD student at Brown's School of Engineering, who runs work. "What we have shown is by incorporating graphene oxide nanoparticles, we can make these structures much healthier."
The material can become harder or softer in response to different chemical treatments, which means it can be used to create "smart" materials that are able to react to the environment in real time, the study shows. In addition, GO alginate retains the ability of alginate to repel oils by giving new material as a healthy antifouling coating.
The 3-D printing method used to make the materials is known as stereolithography. The technique uses an ultraviolet laser controlled by a computer-designed system for tracking models on the surface of a photoactive polymer solution. Light causes the polymers to bind to form solid 3-D structures from the solution. The tracking process is repeated as long as the entire object is laid layer by layer from bottom to top. In this case, the polymer solution is made using sodium alginate mixed with graphite oxide sheet, carbon material that forms nanoparticles with a thickness of one atom, which are stronger pounds per kilogram of steel.
One advantage of the art is that the sodium alginate polymers are bonded through ionic bonds. Connections are strong enough to hold the material together, but they can be broken by certain chemical treatments. This gives the material the ability to react dynamically to external stimuli. Previously, Brown researchers have shown that this "ionic cross-linking" can be used to create alginate materials that break down on demand quickly dissolve when treated with a chemical that ejects ions from the inner structure of the material.
For this new study, the researchers wanted to see how graphene oxide can alter the mechanical properties of alginate structures. They showed that alginate-GO can be made twice as harder than alginate and much more resistant to failure through cracks.
"The addition of graphite oxide stabilizes the alginate hydrogel with a hydrogen bonding," said Jan W. Wong, an engineering professor at Brown and senior author of the paper. "We think the fracture resistance is due to cracks that have to circumvent the interface graphite sheets instead of breaking even though homogeneous alginates."
Additional stiffness allowed researchers to print structures with overhangs that would be impossible only with alginate. In addition, increased stiffness does not prevent alginate GO from reacting to external stimuli such as alginate alone. Researchers have shown that by bathing materials in a chemical that removes its ions, the materials swell and become softer. The materials regained their firmness when the ions were restored by bathing in ionic salts. Experiments show that the hardness of the materials can be adjusted by a factor of 500, changing their external ionic environment.
This ability to change its hardness can make alginate-GO useful in various applications, researchers say, including dynamic cell cultures.
"You can imagine a scenario in which you can recreate living cells in a hard environment and immediately change into a softer environment to see how the same cells can react," Valentine said. This could be useful in studying how cancer cells or immune cells migrate through different organs in the body.
And since GO alginate retains the powerful properties that suppress pure alginate oil, the new material can make excellent coverage to keep oil and other surface contamination. In a series of experiments, researchers have shown that alginate-GO coating can prevent oil from contaminating the surface of the glass under severe physiological conditions. This could make the alginates-GO hydrogels useful for coatings and structures used in marine conditions, the researchers say.
"These composite materials can be used as an ocean sensor that can continue to read during oil spill or as an antifouling coating that helps keep clean hulls," Wong said. The additional stiffness provided by graphene would make such materials or coatings far more durable than alginate.
Researchers plan to continue experimenting with the new material by looking for ways to rationalize their production and continue to optimize their properties.
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