Seaweed has for millennia been a rich source of materials useful for humanity. It was arguably the basis of the very first chemical industries, yielding iodine for medical uses in prehistory; its use in agriculture as a fertiliser has often underpinned the economies of island nations with scarce cultivatable land; and, of course, many species are themselves edible. In more recent years, seaweeds and other marine plants have been investigated by industrial chemists as sources of oil is and other materials. Alginate, a natural hydrogel, has found applications in the medical sector, but because they tend to be mechanically fragile and unstable in certain solutions, their uses are somewhat limited.
Engineers at Brown University in Rhode Island have now developed a way of reinforcing alginate by incorporating the atomically-thick layered material graphene oxide into its structure. This produces a material that can be 3D printed into structures that are stiffer and more fracture resistant than alginate alone. Furthermore, changes in the chemical environment can make the composite even stiffer or softer, allowing the structures to respond their surroundings in real time. Despite this change in behaviour, the composite retains some of the useful properties of alginate.
Research leader Thomas Valentin and his colleagues describe their work in the journal Carbon. The structures were formed using stereolithography, in which a computer-aided design system traces an ultraviolet laser across the surface of a photo active polymer solution. In this case, the solution was made from sodium alginate salt mixed with graphene oxide. This took advantage of one of the properties of alginate polymers: the individual strands linked together using electrostatic ionic bonds, which respond to external stimuli.
The team believes that reinforcing alginate with graphene strengthens the material because it changes the way that cracks propagate through its structure. "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," said Ian Wong, senior author of the paper.
In previous research, Brown engineers have shown that ionic cross-linking can be used to make alginate materials that dissolve when treated with substances that can remove ions from their internal structure. In this case, the ion-removing substance didn’t dissolve the gel, but made it swell up and become much softer; replacing the ions restored the material’s stiffness, which could be tuned over a factor of 500 by changing the ionic environment. This could be useful in medical research, Valentin suggests. "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," he said. That could be useful in studying how cancer cells or immune cells migrate through different organs throughout the body.
The printed material was stiff enough for the researchers to manufacture structures with overhanging parts, which would be impossible with alginate alone. On testing, they found that the material retained the ability of alginate to repel oils, which helps give seaweed its characteristic texture and allows it to absorb nutrients from seawater. This opens up the possibility of using the composite material as an antifouling coating for structures that could be used in the sea. "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," Wong said. The extra stiffness afforded by the graphene would make such materials or coatings far more durable than alginate alone.