Researchers in Dr. Akhilesh K. Gaharwar’s lab in the Department of Biomedical Engineering have developed a “lotus effect” by incorporating atomic defects in nanomaterials, which could have widespread applications in the biomedical field including biosensing, lab-on-a-chip, blood-repellent, anti-fouling and self-cleaning applications.

Superhydrophobic materials are used extensively for self-cleaning characteristic of devices. However, current materials require alteration to the chemistry or topography of the surface to work. This limits the use of superhydrophobic materials.

“Designing hydrophobic surfaces and controlling the wetting behavior has long been of great interest, as it plays crucial role in accomplishing self-cleaning ability,” Gaharwar said. “However, there are limited biocompatible approach to control the wetting behavior of the surface as desired in several biomedical and biotechnological applications.”

The Texas A&M design adopts a ‘nanoflower-like’ assembly of two-dimensional (2D) atomic layers to protect the surface from wetting. The team recently released a study published in Chemical Communications. 2D nanomaterials are an ultrathin class of nanomaterials and have received considerable attention in research. Gaharwar’s lab used 2D molybdenum disulfide (MoS2), a new class of 2D nanomaterials that has shown enormous potential in nanoelectronics, optical sensors, renewable energy sources, catalysis and lubrication, but has not been investigated for biomedical applications. This innovative approach demonstrates applications of this unique class of materials to the biomedical industry.

“These 2D nanomaterials with their hexagonal packed layer repel water adherence, however, a missing atom from the top layer can allow easy access to water molecules by the next layer of atoms underneath making it transit from hydrophobic to hydrophilic,” said lead author of the study, Dr. Manish Jaiswal, a senior research associate in Gaharwar’s lab.

This innovative technique opens many doors for expanded applications in several scientific and technological areas. The superhydrophobic coating can be easily applied over various substrates such as glass, tissue paper, rubber or silica using the solvent evaporation method. These superhydrophobic coatings have wide-spread applications, not only in developing self-cleaning surfaces in nanoelectronics devices, but also for biomedical applications. Specifically, the study demonstrated that blood and cell culture media containing proteins do not adhere to the surface, which is very promising. In addition, the team is currently exploring the potential applications of controlled hydrophobicity in stem cell fate.

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Based on focused electron beam deposition, the technique allows structures to be fabricated from gas-phase precursors at rates approaching what could be expected in the liquid phase — all without raising the temperature of substrates. That could lead to manufacturing of the nanometer-scale structures at rates that could make them practical for use in magnetic memory, high-frequency antennas, quantum communication devices, spintronics and atomic-scale resonators.

“We are controlling matter on the atomic scale to bring about new modes of additive manufacturing,” said Andrei Fedorov, a professor in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “This new science could bring about additive manufacturing applications that might otherwise be impossible. The resulting new technology will open up new dimensions for additive manufacturing at the atomic scale.”

The work grew out of frustration with trying to create small structures using the electron beams, which can be just a few nanometers in diameter. The research was supported by the U.S. Department of Energy’s Office of Science, and was reported May 28 in the journal Physical Chemistry Chemical Physics.

“When we went to the lab to use nanofabrication with focused electron beams, which are the size of a few nanometers, we could not grow structures that were just a few nanometers. They grew to be 50 or 100 nanometers,” Fedorov explained. “And it also took a long time to produce the structures, which meant that, without improvements, we’d never be able to produce them at high volume.”

Fedorov and collaborators Matthew Henry and Songkil Kim realized the reactions producing the structures were slow, and tied to the thermodynamic state of the substrate on which they are being grown. They decided to add some energy to the process to speed things up — as much as a hundred times faster.

The result was the invention of a micro-capillary injector just a few micrometers in diameter that could introduce tiny jets of gaseous molecules into the deposition chamber to activate the precursors for the nanometer-scale structures. Partly because the jet is entering a vacuum chamber, the gas accelerates to supersonic speeds. Energy from the supersonic jet excites the precursor molecules that are adsorbed to the substrate.

“This energetic thermal state allows the electrons from the beam to much more easily break chemical bonds, and as a result, structures grow much faster,” Fedorov said. “All of this amplification, both the molecule transport and the rate of reaction, are exponential, meaning a small change can lead to a dramatic increase in outcome.”

That much has been observed experimentally, but to understand how to control the process and expand its applications, the researchers wanted to create a theory for what they were seeing. They used nano-scale thermometric techniques to measure the temperature of the adsorbed atoms — also known as adatoms — subjected to the jet, and used that information to help understand the basic physics at work.

“Once we have a model, it essentially becomes a design tool,” Fedorov said. “With this understanding and the capabilities we have demonstrated, we can expand them to other fields such as directed self-assembly, epitaxial growth and other areas. This could enable a whole host of new capabilities to use this kind of direct-write nanofabrication.”

Development of the model and understanding of the first-principles physics behind it could also allow other researchers to find new applications.

“With this, you can have almost the same order of magnitude growth rate as you’d have with liquid phase precursors, but still have access to the richness of possible precursors, the ability to manipulate alloying, and all the experience that has been developed over the years with gas phase deposition,” Fedorov said. “This technology will allow us to do things at a scale that is meaningful from a practical standpoint and cost-effective.”

The ability to rapidly produce small, three-dimensional structures could open up a range of new applications.

“If you can adapt additive direct-write techniques, this could bring a lot of unique capabilities for magnetic memory, superconducting materials, quantum devices, 3D electronic circuitry, and many more things,” he said. “These structures are currently very hard to make using conventional methods.”

Beyond using the jets to accelerate deposition of precursor materials already on the substrate, the researchers have also created hybrid jets that contain both high-energy inert gas and precursor gases, which allow not only dramatic acceleration of nanostructure growth but also precisely control the material composition during growth. In future work, the researchers plan to use these hybrid approaches to enable formation of nanostructures with phase and topology that cannot be achieved by any existing nanofabrication techniques.

“This is some of the first research on reminding kids about their multi-faceted selves,” said lead author Sarah Gaither, an assistant professor of psychology and neuroscience at Duke. “Such reminders boost their problem-solving skills and how flexibly they see their social worlds — all from a simple mindset switch.”

Better problem-solving was just one positive finding of the study, Gaither said. After considering their own various identities, children also showed more flexible thinking about race and other social groupings — a behavior that could be valuable in an increasingly diverse society.

The research appears July 2 in the journal Developmental Science.

In a series of experiments, Gaither and her colleagues looked at 196 children, ages 6 and 7. All were native English speakers.

In one experiment, the first group of children was reminded they have various identities, such as son, daughter, reader or helper. A second group of children was reminded of their multiple physical attributes (such as a mouth, arms and legs).

In another experiment, one group of children was again reminded they have various identities. A second set of children received similar prompts — but about other children’s many roles, not their own.

All the children then tackled a series of tasks. Children who were reminded of their various identities demonstrated stronger problem-solving and creative thinking skills. For instance, when shown pictures of a bear gazing at a honey-filled beehive high up in a tree, these children had more creative ideas for how the bear might get the honey, such as flipping over a bowl so that it becomes a stool. In other words, they saw a new use for the bowl.

Children who were reminded of their multiple roles also showed more flexible thinking about social groupings. When asked to categorize different photos of faces, they suggested many ways to do so. For instance, they identified smiling faces vs. unsmiling ones, and old vs. young faces. The other children, meanwhile, primarily grouped people’s faces by race and gender.

Because the results suggest simple ways to promote flexible, inclusive thinking for the young, they could be especially valuable for teachers, Gaither said.

“We have this tendency in our society to only think about ourselves in connection with one important group at a time,” Gaither said. “When we remind kids that they have various identities, they think beyond our society’s default categories, and remember that there are many other groups in addition to race and gender.

“It opens their horizons to be a little more inclusive.”

The research was supported by a University of Chicago Provost’s Postdoctoral Scholarship, by NICHD R01HD070890 and by the Chicago Center for Practical Wisdom.

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