Nov 22 2018
Researchers at Brown University have demonstrated a new method that involves using graphene oxide, or GO, to fortify alginate-based hydrogel materials. Alginate is a natural material obtained from seaweed and is presently being used in many different biomedical applications.
Brown University researchers have created a hybrid material out of seaweed-derived alginate and the nanomaterial graphene oxide. The 3D printing technique used to make the material enables the creation of intricate structures, including the one above, which mimics that atomic lattice a graphene. (Image credit: Wong Lab/Brown University)
In the study reported in the journal Carbon, the team demonstrated how a 3D printing technique can be used for developing complex and long-lasting alginate-GO structures that are more resistant to fracture and far stiffer than 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,” stated Thomas Valentin, a PhD student in Brown’s School of Engineering who headed the study. “What we showed is by including graphene oxide nanosheets, we can make these structures much more robust.”
The study showed that the novel material also has the potential to become either softer or stiffer in response to varied chemical treatments. This means that this material can possibly be used for making “smart” materials that have the ability to respond to their surroundings in real time. The alginate-GO structure also retains the ability of alginate to repel oils, thus imparting the new material potential as a robust antifouling coating.
Stereolithography is the 3D printing technique used for making the materials. This method involves using an ultraviolet laser, which is regulated by a computer-aided design system, to trace patterns over the surface of a photoactive polymer solution. Here, the light from the laser makes the polymers to join together and form solid 3D structures from the solution.
The tracing process is performed continuously until a whole object is created layer-by-layer from the scratch. The polymer solution, in this example, was made by combining sodium alginate with GO sheets; GO is a carbon-based material that forms nanosheets of one-atom thickness, which are stronger pound-for-pound in comparison to steel.
The technique offers one benefit—the sodium alginate polymers bonds via ionic bonds, which are sufficiently strong to hold the material together. However, some chemical treatments can break down these bonds, giving the material the potential to react dynamically to external stimuli. Earlier, the researchers at Brown University demonstrated that this “ionic crosslinking” can be used for producing alginate materials that degrade when required and quickly dissolve upon treating with a chemical that removes ions from the internal structure of the material.
For this latest research, the investigators sought to see how GO might alter the mechanical characteristics of alginate structures. They demonstrated that it is possible to make alginate-GO as twice as rigid as alginate alone, and much more impervious to failure through cracking.
The addition of graphene oxide stabilizes the alginate hydrogel with hydrogen bonding. 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.
Ian Y. Wong, Senior Author of the Paper and Assistant Professor of Engineering, Brown University.
The additional stiffness allowed the team to print structures that contained overhanging components, which otherwise would have not been feasible with the help of alginate alone. In addition, the increased stiffness also caused the alginate-GO to react to external stimuli, similar to alginate alone.
The investigators further demonstrated that when the materials are bathed in a chemical that removes its ions, they swell up and turn out to be relatively softer. Upon restoring the ions by bathing in ionic salts, the materials were able to regain their stiffness. Experiments revealed that the stiffness of the materials can possibly be adjusted over a factor of 500 by changing their external ionic environment.
According to the researchers, alginate-GO’s ability to modify its stiffness could make it useful for a wide range of applications, including dynamic cell cultures.
You could imagine a scenario where you can image living cells in a stiff environment and then immediately change to a softer environment to see how the same cells might respond.
Thomas Valentin, PhD Student, School of Engineering, Brown University.
That may prove handy for analyzing how immune or cancer cells travel through different organs across the body.
Since alginate-GO is able to retain the robust oil-repellant characteristics of pure alginate, the novel material may serve as a superior coating product to prevent oil and other grime from accumulating on surfaces. In a set of experiments, the team also demonstrated that an alginate-GO coating might help prevent oil from fouling the glass surface in extremely saline conditions. That may make alginate-GO hydrogels useful for structures and coatings applied in marine settings, says the research team.
These composite materials could be used as a sensor in the ocean that can keep taking readings during an oil spill, or as an antifouling coating that helps to keep ship hulls clean.
Ian Y. Wong, Senior Author of the Paper and Assistant Professor of Engineering, Brown University.
Through the added stiffness provided by the graphene, such type of coatings or materials can become much more durable when compared to alginate alone.
The team now intends to explore this novel material, seeking ways to simplify its production and continue to improve 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. The research was supported by the U.S. Department of Education’s GAANN Training Grant in Applications and Implications of Nanotechnology, (P200A150037), the National Science Foundation (DGE-1058262) and a Brown University Hibbitt Postdoctoral Fellowship.