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Gallium nitride shows promise for biomedical implants
Scanning electron microscope image of cell growth on gallium nitride that has been coated with peptides. Credit: Albena Ivanisevic, North Carolina State University.
We live in good times, medically speaking. Creaky, painful knees and hips can be replaced. Arteries can be held open with stents. A variety of implants hold us together, stabilize our heartbeats, relieve our pain and keep us going for a good long time.
Semiconductor materials are of interest for biomedical implants for applications like electrodes for neurostimulation therapy and sensors. However, the interior of the body is a harsh environment-corrosive, reactive and sensitive to toxins.
A new paper published by a North Carolina State University and Purdue research team reports that gallium nitride, a semiconductor, shows strong promise as a biocompatible and non-toxic candidate material for biomedical implants.
In the press release, co-author and professor at NC State Albena Ivanisevic says, “The first finding is that GaN, unlike other semiconductor materials that have been considered for biomedical implants, is not toxic.” Mass spectrometry was used to show that GaN is very stable in in-vitro environments, releasing so little gallium as to be considered non-toxic. The chemical stability is critical because gallium oxides are toxic to humans.
The team also tested the biocompatibility of GaN by coating specimens with peptides—the building blocks of proteins—and comparing cell interactions with coated and uncoated samples.
The functionality of the material is important, Ivanisevic says, “because we want materials that give us some control over cell behavior, for example, being able to make cells adhere to a material or to avoid it.”
The uncoated GaN did not interfere with cell growth, but the peptide-coated GaN bonded with the cells. Per the abstract: “Peptide terminated GaN promoted greater cell spreading and extension of neurites.” Ivanisevic says the result also suggests, “we may also be able to coat GaN with peptides that would help prevent cell growth—and keep the implant ‘clean.’”
All of this means that, according to the abstract, “peptide-modified GaN is a biocompatible and non-toxic material that can be used to probe chemical and electrical stimuli associated with neural interfaces.”
Full details are available in “Gallium Nitride is Biocompatible and Non-toxic Before and After Functionalization with Peptides,” written by Scott A. Jewett, Matthew S. Makowski, Benjamin Andrews, Michael J. Manfra and Ivanisevic, and published in Acta Biomaterialia, (doi:10.1016/j.actbio.2011.09.038)
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Mechanism for electric field effects during sintering
The image on the left is 3Y-TZP sintered at 1,500°C. The image on the right is of 3Y-TZP, which had a 60 Hertz AC electric field applied to it followed by sintering at 1,250°C. Credit: Hans Conrad, NC State University.
Last year we reported on a number of papers that were published in 2010 on electric field assisted sintering, including a few by Hans Conrad at North Carolina State University. According to press releases, the application of AC or DC fields during sintering was effective in reducing processing temperatures necessary for elimination of porosity, while simultaneously reducing grain growth. The effect is athermal, i.e., essentially independent of heating rate and sintering temperature.
Two mechanisms for the effect have been proposed to explain the suppression of grain growth. One possibility is the grain boundary energy (the driving force) is reduced because of an interaction of the electric field with the space charge. The other is that there may be Joule heating at the grain boundaries.
A Rapid Communication (doi: 10:111/j.1551-2916.2011.04823.x) in the August 2011 Journal of The American Ceramic Society by Hans Conrad presents an analytical approach to testing the feasibility of the grain boundary energy reduction mechanism.
Yttria-stabilized zirconia (3Y-TZP) samples were sintered under electric field at temperatures between 800°C and 1500°C. (Process parameters are detailed in referenced papers). Grain size was measured using SEM.
Starting with the equation for grain growth, which relates change in grain size over a time interval to an activation energy term and to the driving force. Field-adjusted values are substituted in for the variables. The mathematics eventually leads to a value for the space charge potential, which the paper states “is in reasonable accord with predictions, calculations, and measurements of the potential in oxides.”
The authors conclude that the “agreement of the calculated values of the space charge potential and the grain boundary energy with theoretical considerations and actual measurements provides support [for the theory] that the mechanism for the retardation of grain growth” is related to an electric field-induced reduction of the grain boundary energy through an interaction between the applied field and the space charge.
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Materials stories that may also be of interest
Schematic showing the architecture of the sensor developed at Princeton. Credit Stephen Y. Chou; Princeton.
Check ‘em out:
Princeton engineers make breakthrough in ultrasensitive Raman-based sensor
Princeton researchers have invented an extremely sensitive sensor that opens up new ways to detect a wide range of substances, from tell-tale signs of cancer to hidden explosives. The sensor, which is the most sensitive of its kind to date, relies on a completely new architecture and fabrication technique developed by the Princeton researchers. It’s operation is based on surface-enhanced Raman scattering.
Organic solar cells, light-emitting diodes, and thin-film transistors could be enhanced by polymers that mimic the properties of traditional inorganic semiconductors.
DOE and HUD launch energy efficiency loan program
“PowerSaver “loans, backed by the Federal Housing Administration, will be available from 18 lenders in certain regions of the country to provide homeowners up to $25,000 to make energy-efficient improvements, including door and window replacement. The two-year pilot program was just kicked off by the Department of Housing and Urban Development and the Department of Energy.
Chicago’s Willis Tower to become a vertical solar farm
The iconic Willis Tower (formally the Sears Tower) is set to become a massive solar electric plant with the installation of a pilot solar electric glass project. They will replace the windows on the south side of the 56th floor with a new type of photovoltaic glass developed by Pythagoras Solar which preserves daylighting and views while reducing heat gain and producing the same energy as a conventional solar panel. The project could grow to 2 MW in size.
Less is more: Researchers pinpoint graphene’s varying conductivity level
Researchers at North Carolina State University have found one of the first roadblocks to utilizing graphene for fast electronic devices, by showing that its conductivity decreases significantly when more than one layer is present. With the help of the high performance computers at Oak Ridge National Lab, the NC State team discovered both good and bad news about graphene: With a single layer of graphene, the mobility — and therefore conductivity — shown by the researchers’ simulations turned out to be much higher than they had originally thought; the bad news is that the mobility of electrons in bilayer graphene is roughly an order of magnitude lower than in a single graphene sheet.
Jell-O device detects organ failure
Using only aluminum foil, gelatin, a 12-cent LED light, and a few other inexpensive materials, researchers have developed a sensor that can detect pancreatitis quickly and easily. About the size of a matchbox, the sensor relies on a two-step process to diagnose the disease, a sudden inflammation of the pancreas that can lead to severe stomach pain, nausea, fever, shock and, in some cases, death.
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MS&T’11 symposium to highlight career of Jay Narayan
Credit: Narayan, NCSU.
One of the things I am looking forward to at this fall’s MS&T program is a special symposium dedicated to the research contributions of Jagdish (Jay) Narayan, a professor at North Carolina State University. Narayan, a member of ACerS and many other sci-tech organizations, has covered a lot of territory in his research, from vacancies and interstitials in ionic solids to laser thin-film deposition, novel LEDs, high temperature superconductors and diamond-like films.
I always wince a little bit when I get notices about honorary symposia, such as this, because there is a tendency to assume the honoree is either dead or very late in his or her career. However, I had the pleasure of meeting Narayan at a special NSF conference in 2010, and I definitely came away from that encounter being impressed with the number of projects he is working on.
Besides his research, Narayan also has been a leader in his field, serving in oversight capacities at the Oak Ridge National Lab, the Microelectronics Center of North Carolina and the National Science Foundation. He is currently director of the NSF Center for Advanced Materials and Smart Structures at NCSU
To mark this materials allstar’s contributions, the Army Research Lab, ASM International, TMS, the Kopin Corp. and The American Ceramic Society are sponsoring this special symposium to pay tribute to Narayan.
Formally titled the International Symposium on Advances in Nanostructured Materials and Applications/Acta Materialia Gold Medal Symposia these sessions will delve into a wide range of topics:
Electronic materials:
Defects, interfaces, and thin film epitaxy across the misfit scale in dissimilar materials
Synthesis and non-equilibrium processing of functional electronic materials
Self-assembly of nano-structured functional materials
Nano-scale and atomic-scale characterization
Nano-structure-property correlations and modeling
Structural materials:
Control of nano-structures and processing of bulk nanostructured materials
Stability of nano-structured materials and grain size effects
Role of twinning and properties of nano-materials
Control of ductility and fracture toughness
Mechanical properties and related applications
The MS&T’11 website has list of the impressive speakers who have already confirmed their participation.
As far as I know, the window is still open for submitting abstracts for the symposium. The official cutoff for abstracts for all of MS&T’11 is March 15 (although these deadlines tend to be fairly elastic).
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Quantum dots and microneedles: A possible new approach to diagnosing skin disease
Microneedles fabricated with two-photon polymerization:
Credit: Royal Society of Chemistry
I first covered ACerS member Roger Narayan’s work in the field of two-photon polymerization a little more than a year ago in a story for ACerS’ membership magazine, the Bulletin. For several years, Narayan, a professor in the Joint Biomedical Engineering Department that is connected with NC State’s College of Engineering and the University of North Carolina at Chapel Hill, has been examining the use of this rapid prototyping approach using ceramic–polymer hybrid materials to create patient-specific microscale medical prostheses, scaffolds for tissue engineering and microscale medical devices.
One of set of applications he has been working on, in particular, is using two-photon polymerization to create arrays of fine microneedles. (Conceptually, Narayan’s polymerization process is like a 3D ink jet process that builds up structures on the nanoscale.)
Recently, Narayan coauthored a paper on the novel use of microneedles to deliver quantum dots into the skin. “Our findings are significant, in part, because this technology will potentially enable researchers to deliver quantum dots, suspended in solution, to deeper layers of skin. That could be useful for the diagnosis and treatment of skin cancers, among other conditions,” Narayan says in a news release from NCSU.
QDs, sometimes called “artificial atoms,” are semiconductor materials that fall into the category of nanocrystals, and they contain a variable number of electrons that occupy well-defined, discrete quantum states.
This groups is attracted to the use of QDs because of their ability to serve as fluorophores and also work as drug delivery vehicles. QD-based fluorescent probes can be engineered to be superior to organic dye fluorophore by being brighter and having better photostability (can fluoresce after one hour of continuous excitation), signal-to-noise ratio, emission ranges and fluorescent lifetimes. Researchers report they can use their intense fluorescence to track individual molecules.
Sample quantum dot with bio coating. Credit: Histesh R. Patel
At this point, Narayan and the other researchers just are using the microneedles on pig skin and can capture images of the quantum dots entering the skin using multiphoton microscopy. Although this work is still preliminary, these images allow the researchers to verify the basic effectiveness of the microneedles as a delivery mechanism for quantum dots.
The hope is that multiphoton microscopy will have clinical applications using real-time imaging materials such as the quantum dots for faster diagnosis of cancers or other medical problems.
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