Nano-infused ceramic could report on its own health
February 5th, 2019: Rice University
A ceramic that becomes more electrically conductive under elastic strain and less conductive under plastic strain could lead to a new generation of sensors embedded into structures like buildings, bridges and aircraft able to monitor their own health.
The electrical disparity fostered by the two types of strain was not obvious until Rice University's Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering and of materials science and nanoengineering, and his colleagues modeled a novel two-dimensional compound, graphene-boron-nitride (GBN).
Under elastic strain, the internal structure of a material stretched like a rubber band does not change. But the same material under plastic strain -- caused in this case by stretching it far enough beyond elasticity to deform -- distorts its crystalline lattice. GBN, it turns out, shows different electrical properties in each case, making it a worthy candidate as a structural sensor.
Shahsavari had already determined that hexagonal-boron nitride -- aka white graphene -- can improve the properties of ceramics. He and his colleagues have now discovered that adding graphene makes them even stronger and more versatile, along with their surprising electrical properties.
The magic lies in the ability of two-dimensional, carbon-based graphene and white graphene to bond with each other in a variety of ways, depending on their relative concentrations. Though graphene and white graphene naturally avoid water, causing them to clump, the combined nanosheets easily disperse in a slurry during the ceramic's manufacture.
The resulting ceramics, according to the authors' theoretical models, would become tunable semiconductors with enhanced elasticity, strength and ductility.
The research led by Shahsavari and Asghar Habibnejad Korayem, an assistant professor of structural engineering at Iran University of Science and Technology and a research fellow at Monash University in Melbourne, Australia, appears in the American Chemical Society journal Applied Materials and Interfaces.
Ceramics with networked nanosheets of graphene and white graphene would have the unique ability to alter their electrical properties when strained, according to a researcher at Rice University. The surprising ability could lead to new types of structural sensors. Credit: Rouzbeh Shahsavari/Rice University
Graphene is a well-studied form of carbon known for its lack of a band gap -- the region an electron has to leap to make a material conductive. With no band gap, graphene is a metallic conductor. White graphene, with its wide band gap, is an insulator. So the greater the ratio of graphene in the 2D compound, the more conductive the material will be.
Mixed into the ceramic in a high enough concentration, the 2D compound dubbed GBN would form a network as conductive as the amount of carbon in the matrix allows. That gives the overall composite a tunable band gap that could lend itself to a variety of electrical applications.
"Fusing 2D materials like graphene and boron nitride in ceramics and cements enables new compositions and properties we can't achieve with either graphene or boron nitride by themselves," Shahsavari said.
The team used density functional theory calculations to model variations of the 2D compound mixed with tobermorite, a calcium silicate hydrate material commonly used as cement for concrete. They determined the oxygen-boron bonds formed in the ceramic would turn it into a p-type semiconductor.
Tobermorite by itself has a large band gap of about 4.5 electron volts, but the researchers calculated that when mixed with GBN nanosheets of equal parts graphene and white graphene, that gap would shrink to 0.624 electron volts.
When strained in the elastic regime, the ceramic's band gap dropped, making the material more conductive, but when stretched beyond elasticity -- that is, in the plastic regime -- it became less conductive. That switch, the researchers said, makes it a promising material for self-sensing and structural health monitoring applications.
The researchers suggested other 2D sheets with molybdenum disulfide, niobium diselenide or layered double hydroxides may provide similar opportunities for the bottom-up design of tunable, multifunctional composites. "This would provide a fundamental platform for cement and concrete reinforcement at their smallest possible dimension," Shahsavari said.
Ehsan Hosseini, Mohammad Zakertabrizi, Asghar Habibnejad Korayem, Rouzbeh Shahsavari. Tunable, Multifunctional Ceramic Composites via Intercalation of Fused Graphene Boron Nitride Nanosheets. ACS Applied Materials & Interfaces, 2019; DOI: 10.1021/acsami.8b19409
See-through fish aid scientists in autism-related breakthrough
February 6th, 2019
University of Miami researchers have discovered a clue in the humble zebrafish's digestive tract that, one day, could help people on the autism spectrum alleviate one of the most common yet least studied symptoms of their disorder: gastrointestinal distress.
By replicating a mutation in zebrafish that causes a rare, autism-related genetic condition known as Phelan-McDermid Syndrome in humans, the researchers found a drastic reduction in the number of cells that produce serotonin in the mutant zebrafish's gut. Perhaps better known as the body's natural mood stabiliser, serotonin also initiates the contractions that move food through the digestive system.
Published last week in the journal Molecular Autism, the study is the first to show that a mutation of a particular autism-associated gene, SHANK3, causes food to move through the digestive tract at an abnormally slow pace. While the finding applies to zebrafish, a common freshwater aquarium species that shares about 70 percent of its genes with humans, it could explain why many people with an autism spectrum disorder, and particularly those with Phelan-McDermid Syndrome (PMS), often suffer severe and disruptive bouts of constipation, diarrhoea, reflux, and/or cyclical vomiting.
More importantly, it could open the door to treatments that alleviate such unpleasant gastrointestinal, or GI, symptoms.
"The fact that we see reduced motility in the GI tract of fish with shank3 mutations is likely explained by the absence of these serotonin-producing cells, and that suggests that if we could find a way to increase motility we might be able to help with the GI problem," said senior author Julia Dallman, associate professor of biology whose UM lab studies inherited nervous system disorders by modelling them in zebrafish. "One issue in autism right now is that a lot of the drugs given to kids with autism to address their behavioural issues actually make their gut function worse. So by having a model of gut function we can try to find treatments that don't make it worse."
Dallman and two of her graduate students, David M. James and Robert A. Kozol, the first co-authors of the study, began earnestly modelling GI distress in zebrafish after they were invited to the 2016 biennial meeting of the international Phelan-McDermid Syndrome Foundation in Orlando. There they heard firsthand accounts from parents of children born with PMS which, caused by a deletion or mutation of the SHANK3 gene at the end of chromosome 22, is characterised by moderate to severe intellectual disabilities and developmental delays, absent or delayed speech, decreased muscle tone, and epileptic seizures.
Yet, despite all those very daunting challenges, parents often expressed deep frustration over their child's GI issues, which affected their quality of life on a daily basis, often making ordinary routines and milestones, like eating and toilet-training, exceedingly difficult.
"At that time, there hadn't been much published about GI issues in our community, but when you listen to parents they would always talk about how troubling it was," said Geraldine Bliss, past chair of the foundation's research support committee."There was a whole constellation of issues -- sometimes diarrhoea, sometimes constipation, sometimes alternating diarrhoea and constipation, reflux -- all kinds of GI problems, but it hadn't really been characterised or reported in the literature."
Now, the common zebrafish and James' interest in characterising GI issues in autism seem destined to help shed light on the confounding problem.
About an inch long at maturity, the zebrafish is a valuable model for human disease because, aside from having a similar genetic structure to humans, it develops rapidly and, until its second week of life, is almost completely transparent. That's when it begins developing the telltale dark stripes for which it is named.
That early transparency, coupled with an apparent zeal for what juveniles consider yummy food, enabled James and Kozol to record what happens when zebrafish lacking the shank3 gene digested chicken egg yolks, which were infused with tiny, florescent-green tracking beads. The films clearly showed that the mutant fish not only had slower and less frequent gut contractions than normal zebrafish, but that the ingested food particles got stuck in the junction between the stomach and the intestine for prolonged periods of time.
The reason came as a surprise. Initially, the researchers hypothesised the shank3 mutation somehow damaged the fish's enteric nervous system -- the "brain" that controls the gut -- but that was perfectly normal. Instead, the researchers found the shank3 mutants had drastically fewer serotonin-producing cells than their wild-type counterparts.
"It was definitely an 'Aha!' moment," James said. "It opens up a lot, in terms of studying potential mechanisms for how all this dysmotility is occurring. If we can figure that out, we can start talking about what the proper treatments are, and what's going to fix this -- that's the real question."
The answer may lie in the distant future, but this first crucial step in unraveling a biological reason for autism-related GI distress is heartening to members of the PMS foundation, which began as a family support group in 1998, when 20 of the 23 families then known to be affected by a deletion or mutation of chromosome 22 first met.
Today, nearly 200 affected families from around the world attend the foundation's meetings, the last of which was held in Texas last summer. That's when James presented UM's research on intestinal dysmotility in zebrafish, which not only won the conference's research award but gave parents like Gail Fenton hope for a better future for their children.
"Joanna's tummy troubles have been a major source of family stress for 15 years," Fenton wrote from Australia about her 36-year-old daughter. "She is often hunched over because of tummy pain. She cries, even yells, when her tummy is sore. She'll always have Phelan-McDermid Syndrome but her life, and the lives of fellow sufferers with these complex GI issues, would be so much happier without these negatives and we are so very thankful for the research that can lead to any helpful interventions."
David M. James, Robert A. Kozol, Yuji Kajiwara, Adam L. Wahl, Emily C. Storrs, Joseph D. Buxbaum, Mason Klein, Baharak Moshiree, Julia E. Dallman. Intestinal dysmotility in a zebrafish (Danio rerio) shank3a;shank3b mutant model of autism. Molecular Autism, 2019; 10 (1) DOI: 10.1186/s13229-018-0250-4