United States. Researchers at Brown University have demonstrated a way to use graphene oxide (GO) to add some spine to hydrogel materials made of alginate, a natural material derived from algae that is currently used in a variety of biomedical applications.
In a paper published in the journal Carbon, the researchers describe a 3D printing method for creating intricate and durable alginate-GO structures that are much stiffer and more resistant to fractures than alginate alone.
"A limiting factor in the use of alginate hydrogels is that they are very fragile: they tend to crumble under mechanical load or in low-salt solutions," said Thomas Valentin, Ph.D., a student in the Brown School of Engineering who led the work. "What we show is that by including nano-sheets of graphene oxide, we can make these structures much more robust."
Research shows that the material can also become stiffer or softer in response to different chemical treatments, meaning it could be used to make "smart" materials that can react to their environment in real time. In addition, alginate-GO retains alginate's ability to repel oils, giving the new material potential as a tough 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 track patterns across the surface of a photoactive polymer solution. The light causes the polymers to bind together, forming solid 3D structures of the solution. The crawling process is repeated until an entire object is constructed layer by layer from the bottom up. In this case, the polymer solution was manufactured using sodium alginate mixed with sheets of graphene oxide, a carbon-based material that forms nano-sheets one atom thick that are stronger pound for pound than steel.
An advantage of the technique is that sodium alginate polymers bind through ionic bonds. The bonds are strong enough to hold the material together, but they can break with certain chemical treatments. That gives the material the ability to respond dynamically to external stimuli. Previously, Brown's researchers demonstrated that this "ion cross-linking" can be used to create alginate materials that degrade in demand and dissolve quickly when treated with a chemical that removes ions from the material's internal structure.
For this new study, the researchers wanted to see how graphene oxide could change the mechanical properties of alginate structures. They showed that alginate-GO could be made twice as rigid as alginate alone, and much more resistant to failure through cracking.
"The addition of graphene oxide stabilizes the alginate hydrogel with hydrogen bonds," said Ian Y. Wong, assistant professor of engineering at Brown and senior author of the paper. "We think the fracture resistance is due to cracks that have to be deflected around the intermingled graphene sheets rather than being able to break up the just but homogeneous alginate."
The additional stiffness allowed the researchers to print structures that had protruding parts, which would have been impossible using only alginate. In addition, increased stiffness did not prevent alginate-GO from also responding to external stimuli, as alginate alone can. 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 when the ions were restored by bathing in ionic salts. The experiments showed that the stiffness of the materials could be adjusted by a factor of 500 by varying their external ionic environment.
That ability to change its stiffness could make alginate-GO useful in a variety of applications, the researchers say, including coatings.
Because alginate-GO retains the powerful oil-repellent properties of pure alginate, the new material could make an excellent coating to prevent oil and other particles from accumulating on surfaces. In a series of experiments, the researchers showed that an alginate-GO coating could prevent the oil from fouling the glass surface in highly saline conditions. That could make alginate-GO hydrogels useful for coatings and structures used in marine environments, according to the researchers.
"These composite materials could be used as a sensor in the ocean that can continue to take readings during an oil spill, or as an antifouling coating that helps keep ships' hulls clean," Wong said. The additional stiffness provided by graphene would make such materials or coatings much more durable than alginate alone.
The researchers plan to continue experimenting with the new material, looking for ways to optimize its production and continue to optimize its properties.
Additional co-authors of the study include Alexander K. Landauer, Luke C. Morales, Eric M. DuBois, Shashank Shukla, Muchun Liu and Lauren H. Stephens of Brown University, as well as Christian Franck of the University of Wisconsin, and Po-Yen Chen of the National University of Singapore.
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