Brown University researchers have shown a way to use graphene oxide (GO) to add some backbone to hydrogel materials made from alginate, a natural material derived from seaweed that’s currently used in a variety of biomedical applications. In a paper published in the journal Carbon, the researchers describe a 3-D printing method for making intricate and durable alginate-GO structures that are far stiffer and more fracture resistant that alginate alone.
“One limiting factor in the use of alginate hydrogels is that they’re very fragile — they tend to fall apart under mechanical load or in low salt solutions,” said Thomas Valentin, a Ph.D. student in Brown’s School of Engineering who led the work. “What we showed is by including graphene oxide nanosheets, we can make these structures much more robust.”
The material is also capable of becoming stiffer or softer in response to different chemical treatments, meaning it could be used to make “smart” materials that are able to react to their surroundings in real time, the research shows. In addition, alginate-GO retains alginate’s ability to repel oils, giving the new material potential as a sturdy 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-aided design system to trace patterns across the surface of a photoactive polymer solution. The light causes the polymers to link together, forming solid 3-D structures from the solution. The tracing process is repeated until an entire object is built layer-by-layer from the bottom up. In this case the polymer solution was made using sodium alginate mixed with sheets of graphene oxide, a carbon-based material that forms one-atom-thick nanosheets that are stronger pound-for-pound than steel.
One advantage to the technique is that the sodium alginate polymers link through ionic bonds. The bonds are strong enough to hold the material together, but they can be broken by certain chemical treatments. That gives the material the ability to respond dynamically to external stimuli. Previously, the Brown researchers showed that this “ionic crosslinking” can be used to create alginate materials that degrade on demand, rapidly dissolving when treated with a chemical that sweeps away ions from the material’s internal structure.
For this new study, the researchers wanted to see how graphene oxide might change mechanical properties of alginate structures. They showed that alginate-GO could be made twice as stiff as alginate alone, and far more resistant to failure through cracking.
“The addition of graphene oxide stabilizes the alginate hydrogel with hydrogen bonding,” said Ian Y. Wong, an assistant professor of engineering at Brown and the paper’s senior author. “We think the fracture resistance is due to cracks having to detour around the interspersed graphene sheets rather than being able to break right though homogeneous alginate.”
thumbnail courtesy of brown.edu