Colorized electron microscope image of a micro-modulator made of gold. In the slit in the center of the picture, light is converted into plasmon polaritons, modulated, and then re-converted into light pulses (credit: Haffner et al. Nature Photonics)
Researchers at ETH Zurich have developed a modulator that is a 100 times smaller than conventional modulators, so it can now be integrated into electronic circuits. Transmitting large amounts of data via the Internet requires high-performance electro-optic modulators — devices that convert electrical signals (used in computers and cell phones) into light signals (used in fiber-optic cables).
Today, huge amounts of data are sent incredibly fast through fiber-optic cables as light pulses. For that purpose they first have to be converted from electrical signals, which are used by computers and telephones, into optical signals. Today’s electro-optic modulators are more complicated and large, compared with electronic devices that can be as small as a few micrometers.
The plasmon trick
To build the smallest possible modulator they first need to focus a light beam whose intensity they want to modulate into a very small volume. The laws of optics, however, dictate that such a volume cannot be smaller than the wavelength of the light itself. Modern telecommunications use near-infrared laser light with a wavelength of 1500 nanometers (1.5 micrometers), which sets the lower limit for the size of a modulator.
To beat that limit and to make the device even smaller, the light is first turned into surface-plasmon-polaritons. Plasmon-polaritons are a combination of electromagnetic fields and electrons that propagate along a surface of a metal strip. At the end of the strip they are converted back to light once again. The advantage of this detour is that plasmon-polaritons can be confined in a much smaller space than the light they originated from.
The modulator is much smaller than conventional devices so it consumes very little energy — only a few thousandth of a Watt at a data transmission rate of 70 Gigabits per second. This corresponds to about 100th of the energy consumption of commercial models. And that means more data can be transmitted at higher speeds. The device is also cheaper to produce.
The research is described in a paper in the journal Nature Photonics.
Abstract of All-plasmonic Mach–Zehnder modulator enabling optical high-speed communication at the microscale
Optical modulators encode electrical signals to the optical domain and thus constitute a key element in high-capacity communication links. Ideally, they should feature operation at the highest speed with the least power consumption on the smallest footprint, and at low cost. Unfortunately, current technologies fall short of these criteria. Recently, plasmonics has emerged as a solution offering compact and fast devices. Yet, practical implementations have turned out to be rather elusive. Here, we introduce a 70 GHz all-plasmonic Mach–Zehnder modulator that fits into a silicon waveguide of 10 μm length. This dramatic reduction in size by more than two orders of magnitude compared with photonic Mach–Zehnder modulators results in a low energy consumption of 25 fJ per bit up to the highest speeds. The technology suggests a cheap co-integration with electronics.
This rendering depicts a new “plasmonic oxide material” that could make possible devices for optical communications that are at least 10 times faster than conventional technologies (credit: Purdue University/Nathaniel Kinsey)
Researchers at Purdue University have created a new “plasmonic oxide material” that could make possible modulator devices for optical communications (fiber optics, used for the Internet and cable television) that are at least 10 times faster than conventional technologies.
The optical material, made of aluminum-doped zinc oxide (AZO) also requires less power than other “all-optical” semiconductor devices. That is essential for the faster operation, which would otherwise generate excessive heat with the increase transmission speed.
The material has been shown to work in the near-infrared range of the spectrum, which is used in optical communications, and it is compatible with the CMOS semiconductor manufacturing process used to construct integrated circuits.
Faster optical transistors replace silicon
The researchers have proposed creating an “all-optical plasmonic modulator using CMOS-compatible materials,” or an optical transistor, which allows for the speedup compared to systems that use silicon chips.
A cycle takes about 350 femtoseconds to complete in the new AZO films, which is roughly 5,000 times faster than crystalline silicon.
The researchers “doped” zinc oxide with aluminum (thus the AZO), meaning the zinc oxide is impregnated with aluminum atoms to alter the material’s optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths.
The AZO also makes it possible to “tune” the optical properties of metamaterials.
The ongoing research is funded by the Air Force Office of Scientific Research, a Marie Curie Outgoing International Fellowship, the National Science Foundation, and the Office of Naval Research.
Abstract of Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths
A nanoplasmonic resonator (NPR) consists of a thin silicon dioxide layer sandwiched between metallic nanodisks. NPRs can enhance surface-enhanced Raman spectroscopic (SERS) signals by a factor of 60 billion to detect target molecules with high sensitivity. (credit: Cheng Sun et al./ ACS Nano)
Imagine being able to test your food in your kitchen to quickly determine if it carried any deadly microbes. Technology now being commercialized by Optokey may soon make that possible.
“Our system can do chemistry, biology, biochemistry, molecular biology, clinical diagnosis, and chemical analysis,” said Optokey president and co-founder Fanqing Frank Chen, a scientist at Berkeley Lab who was co-author of an ACS Nano paper on the research. The system can be implemented “very cheaply, without much human intervention,” he said.
SERS is a highly sensitive analytical tool used for “molecular fingerprinting,” but the results have not easily reproducible. Chen and colleagues developed a solution to this problem using what they called “nanoplasmonic resonators,” which measures the interaction of photons with an activated surface using nanostructures to do chemical and biological sensing. The method produces measurements much more reliably.
“At Optokey we’re able to mass produce this nanoplasmonic resonator on a wafer scale,” Chen said. “We took something from the R&D realm and turned it into something industrial-strength.”
The miniaturized sensors use a microfluidic control system for “lab on a chip” automated liquid sampling. “We’re leveraging knowledge acquired from high-tech semiconductor manufacturing methods to get the cost, the volume, and the accuracy in the chip,” said VP of Manufacturing Robert Chebi, a veteran of the microelectronic industry who previously worked at Lam Research and Applied Materials. “We’re also leveraging all the knowledge in lasers and optics for this specific Raman-based method.”
A biochemical nose
Chebi calls Optokey’s product a “biochemical nose,” or an advanced nanophotonic automated system, with sensitivity to the level of a single molecule, far superior to sensors on the market today, he claims. “Today’s detection and diagnosis methods are far from perfect … Also, our system can provide information in minutes, or even on a continuous basis, versus other methods where it could take hours or even days, if samples have to be sent to another lab.”
The potential applications include food safety, environmental monitoring (of both liquids and gases), medical diagnosis, and chemical analysis. Optokey’s customers include a major European company interested in food safety, a Chinese petrochemical company interested in detecting impurities in its products, and a German company interested in point-of-care diagnosis.
“The product we’re envisioning is something that is compact and automated but also connected, and it can go into schools, restaurants, factories, hospitals, ambulances, airports, and even battlefields,” Chen said. Next, they plan to introduce it in the smart home, where a nanophotonic sensor could be built to scan for pollutants not just in food but also in air and water.
Key discovery: nanoplasmonic resonators
Ultimately, Chen and his Berkeley Lab group developed about 20 patents involving hybrid bionanomaterials. The key discovery that led to the formation of Optokey was the development of nanoplasmonic resonators to dramatically improve the signal and reliability of Raman spectroscopy. The method was initially used in the research lab to quickly and accurately detect a biomarker for prostate cancer, which has a high rate of false positives using conventional diagnostic tools.
“There was 10 years of research that went into this, funded by NIH, DARPA, the federal government, private foundations,” said Chen. “Berkeley Lab has a really good culture of multidisciplinary research, excellent engineering, and very strong basic science. Plus it has strong support for startups.”
Abstract of Time-Resolved Single-Step Protease Activity Quantification Using Nanoplasmonic Resonator Sensors
Protease activity measurement has broad application in drug screening, diagnosis and disease staging, and molecular profiling. However, conventional immunopeptidemetric assays (IMPA) exhibit low fluorescence signal-to-noise ratios, preventing reliable measurements at lower concentrations in the clinically important picomolar to nanomolar range. Here, we demonstrated a highly sensitive measurement of protease activity using a nanoplasmonic resonator (NPR). NPRs enhance Raman signals by 6.1 × 1010 times in a highly reproducible manner, enabling fast detection of proteolytically active prostate-specific antigen (paPSA) activities in real-time, at a sensitivity level of 6 pM (0.2 ng/mL) with a dynamic range of 3 orders of magnitude. Experiments on extracellular fluid (ECF) from the paPSA-positive cells demonstrate specific detection in a complex biofluid background. This method offers a fast, sensitive, accurate, and one-step approach to detect the proteases’ activities in very small sample volumes.
Hair-like boron nitride nanotubes intersect a sheet of graphene (top) to create a high-speed digital switch (credit: Michigan Tech, Yoke Khin Yap)
By themselves, graphene is too conductive while boron nitride nanotubes are too insulating, but combining them could create a workable digital switch — which can be used for controlling electrons in computers and other electronic devices.
To create this serendipitous super-hybrid, Yoke Khin Yap, a professor of physics at Michigan Technological University, and his team exfoliated (peeled off) graphene(from graphite) and modified the material’s surface with tiny pinholes, then grew the boron nitride nanotubes up and through the pinholes — like a plant randomly poking up through a crack in a concrete pavement. That formed a “band gap” mismatch, which created “a potential barrier that stops electrons,” he said.
In other words, a switch.
The chemical structures of graphene (gray) and boron nitride nanotubes (pink and purple) can be used to create a digital switch at the point where the two materials come in contact (credit: Michigan Tech, Yoke Khin Yap)
High switching speed
The band gap mismatch results from the materials’ structure: graphene’s flat sheet conducts electricity quickly, and the atomic structure in the nanotubes halts electric currents. This disparity creates a barrier, caused by the difference in electron movement as currents move next to and past the hair-like boron nitride nanotubes. These points of contact between the materials, called heterojunctions, are what make the digital on/off switch possible.
Yap and his research team have also shown that because the materials are respectively so effective at conducting or stopping electricity, the resulting switching ratio is high. So how fast the materials can turn on and off is several orders of magnitude greater than current graphene switches. And this speed could eventually quicken the pace of electronics and computing.
Yap says this study is a continuation of past research into making transistors without semiconductors. The problem with semiconductors like silicon is that they can only get so small, and they give off a lot of heat; the use of graphene and nanotubes bypasses those problems. In addition, the graphene and boron nitride nanotubes have the same atomic arrangement pattern, or lattice matching. With their aligned atoms, the graphene-nanotube digital switches could avoid the issues of electron scattering.
“You want to control the direction of the electrons,” Yap explains, comparing the challenge to a pinball machine that traps, slows down and redirects electrons. “This is difficult in high speed environments, and the electron scattering reduces the number and speed of electrons.”
The journal Scientific Reports recently published their work in an open-access paper.
Abstract of Switching Behaviors of Graphene-Boron Nitride Nanotube Heterojunctions
High electron mobility of graphene has enabled their application in high-frequency analogue devices but their gapless nature has hindered their use in digital switches. In contrast, the structural analogous, h-BN sheets and BN nanotubes (BNNTs) are wide band gap insulators. Here we show that the growth of electrically insulating BNNTs on graphene can enable the use of graphene as effective digital switches. These graphene-BNNT heterojunctions were characterized at room temperature by four-probe scanning tunneling microscopy (4-probe STM) under real-time monitoring of scanning electron microscopy (SEM). A switching ratio as high as 105 at a turn-on voltage as low as 0.5 V were recorded. Simulation by density functional theory (DFT) suggests that mismatch of the density of states (DOS) is responsible for these novel switching behaviors.
Researchers from Berkeley Lab and Columbia University have created the world’s highest-performance single-molecule diode, using a combination of gold electrodes (yellow) and a “TDO” molecule (purple, with molecular structure on the left) in propylene carbonate, an ionic solution (light blue). The circuit symbols on the right represent a battery and an ammeter (A) to measure current flow. (credit: Brian Capozzi et al./Nature Nanotechnology)
A team of researchers from Berkeley Lab and Columbia University has created “the world’s highest-performance single-molecule diode,” using a combination of gold electrodes and an ionic solution.
The diode’s rectification ratio (ratio of forward to reverse current at fixed voltage) is in excess of 200, “a record for single-molecule devices,” says Jeff Neaton, Director of the Molecular Foundry, a senior faculty scientist with Berkeley Lab’s Materials Sciences Division and the Department of Physics at the University of California Berkeley and a member of the Kavli Energy Nanoscience Institute at Berkeley (Kavli ENSI).
Ultimate electronic miniaturization
Single-molecule devices represent the ultimate limit in electronic miniaturization, the researchers say. In 1974, molecular electronics pioneers Mark Ratner and Arieh Aviram theorized that an asymmetric molecule could act as a diode, or rectifier (a one-way conductor of electric current). Diodes have a number of uses in electronic devices.
Since then, development of functional single-molecule electronic devices has been a major pursuit, with diodes — one of the most widely used electronic components — at the top of the list.
A p–n junction (credit: Wikimedia Commons)
A typical current diode consists of a silicon p-n junction between a pair of electrodes (anode and cathode) that serves as the “valve” of an electrical circuit, directing the flow of current by allowing it to pass through in only one “forward” direction. The asymmetry of a p-n junction presents the electrons with an “on/off” transport environment (p–n junctions are elementary “building blocks” of most semiconductor electronic devices such as transistors, solar cells, LEDs, and integrated circuits).
Scientists have previously fashioned single-molecule diodes either through chemical synthesis of special asymmetric molecules (analogous to a p-n junction) or using symmetric molecules with different metals as the two electrodes. However, the resulting asymmetric junctions yielded low rectification ratios and low forward current. Neaton and his colleagues at Columbia University have now discovered a way to address both deficiencies.*
The Berkeley Lab-Columbia University team believes their new approach to a single-molecule diode provides a general route for tuning nonlinear nanoscale-device phenomena that could be applied to systems beyond single-molecule junctions and two-terminal devices, such as ionic liquid gating and two-dimensional materials.
The research is described in Nature Nanotechnology.
* “Electron flow at molecular length-scales is dominated by quantum tunneling,” Neaton explains. “The efficiency of the tunneling process depends intimately on the degree of alignment of the molecule’s discrete energy levels with the electrode’s continuous spectrum. In a molecular rectifier, this alignment is enhanced for positive voltage, leading to an increase in tunneling, and is reduced for negative voltage. At the Molecular Foundry we developed an approach to accurately compute energy-level alignment and tunneling probability in single-molecule junctions. This method allowed myself and Zhenfei Liu to understand the diode behavior quantitatively.”
In collaboration with Columbia University’s Latha Venkataraman and Luis Campos and their respective research groups, Neaton and Liu fabricated a high-performing rectifier from junctions made of symmetric molecules with molecular resonance in nearly perfect alignment with the Fermi electron energy levels of the gold electrodes. Symmetry was broken by a substantial difference in the size of the area on each gold electrode that was exposed to the ionic solution. Owing to the asymmetric electrode area, the ionic solution, and the junction energy level alignment, a positive voltage increases current substantially; a negative voltage suppresses it equally significantly.
“The ionic solution, combined with the asymmetry in electrode areas, allows us to control the junction’s electrostatic environment simply by changing the bias polarity,” Neaton says. “In addition to breaking symmetry, double layers formed by ionic solution also generate dipole differences at the two electrodes, which is the underlying reason behind the asymmetric shift of molecular resonance. The Columbia group’s experiments showed that with the same molecule and electrode setup, a non-ionic solution yields no rectification at all.”
Abstract of Single-molecule diodes with high rectification ratios through environmental control
Molecular electronics aims to miniaturize electronic devices by using subnanometre-scale active components. A single-molecule diode, a circuit element that directs current flow, was first proposed more than 40 years ago and consisted of an asymmetric molecule comprising a donor–bridge–acceptor architecture to mimic a semiconductor p–n junction. Several single-molecule diodes have since been realized in junctions featuring asymmetric molecular backbones, molecule–electrode linkers or electrode materials. Despite these advances, molecular diodes have had limited potential for applications due to their low conductance, low rectification ratios, extreme sensitivity to the junction structure and high operating voltages. Here, we demonstrate a powerful approach to induce current rectification in symmetric single-molecule junctions using two electrodes of the same metal, but breaking symmetry by exposing considerably different electrode areas to an ionic solution. This allows us to control the junction’s electrostatic environment in an asymmetric fashion by simply changing the bias polarity. With this method, we reliably and reproducibly achieve rectification ratios in excess of 200 at voltages as low as 370 mV using a symmetric oligomer of thiophene-1,1-dioxide. By taking advantage of the changes in the junction environment induced by the presence of an ionic solution, this method provides a general route for tuning nonlinear nanoscale device phenomena, which could potentially be applied in systems beyond single-molecule junctions.
Samples of the new hybrid sol-gel material are shown placed on a clear plastic substrate for testing (credit: John Toon, Georgia Tech)
Using a hybrid silica sol-gel material and self-assembled monolayers of a common fatty acid, Georgia Tech researchers have developed a new supercapacitor material that provides electrical-energy storage capacity rivaling some batteries.
Capacitors can provide large amounts of current quickly (high power density), unlike batteries. So if this material can be scaled up from laboratory samples, devices made from it could surpass traditional electrolytic (high-capacity) capacitors for applications in areas where quick-discharge is needed, such as electromagnetic propulsion, electric vehicles, and defibrillators. The new material also has high energy density (ability, like batteries, to store a lot of power).
Schematic representation of new thin-film capacitor using bilayer dielectric formed by self-assembled monolayer (SAM) and sol-gel and electrode layers formed by the gray discs (representing aluminum electrodes) and ITO (indium tin oxide) (not to scale). (credit: Yunsang Kim et al./ Advanced Energy Materials)
The new bilayer dielectric material is composed of a nanoscale self-assembled monolayer (SAM) (insulating) material formed between a sol-gel film and the aluminized mylar film electrodes. The bilayer structure blocks the injection of electrons into the sol-gel material, providing low leakage current, high breakdown strength, and high energy extraction efficiency.
The researchers showed that the capacitor could be rolled and re-rolled several times while maintaining high energy density, demonstrating its flexibility.
The research, supported by the Office of Naval Research and the Air Force Office of Scientific Research, was reported July 14 in the journal Advanced Energy Materials.
Better energy density than thin-film lithium ion batteries
In their structures, the researchers demonstrated maximum extractable energy densities up to 40 joules per cubic centimeter, an energy extraction efficiency of 72 percent at a field strength of 830 volts per micron, and a power density of 520 watts per cubic centimeter.
The performance exceeds that of conventional electrolytic capacitors and thin-film lithium ion batteries, although it doesn’t match the lithium ion battery formats commonly used in electronic devices and vehicles.
The next step will be to scale up the materials to see if the attractive properties transfer to larger devices. If that is successful, the researchers expect to commercialize the material through a startup company or SBIR project.
The U.S. Naval Research Laboratory was also involved in the project.
Abstract of Bilayer Structure with Ultrahigh Energy/Power Density Using Hybrid Sol–Gel Dielectric and Charge-Blocking Monolayer
A hybrid sol–gel dielectric bilayer structure yields a maximum energy density of 40 J cm−3 with high extraction efficiency. The silica sol–gel dielectric is coated by an alkylphosphonic acid monolayer, as a charge-blocking layer. The dense monolayer suppresses charge injection and electrical conduction, leading to high energy extraction efficiency, which exhibits nearly linear dielectric behavior suitable for high energy density applications.
Exfoliation setup. Inset: graphite electrode during exfoliation (credit: Mario Hofmann/Nanotechnology)
Taiwanese researchers reported today (July 30) in the journal Nanotechnology that they have developed a simple electrochemical approach that allows for defects to intentionally be created in graphene, altering its electrical and mechanical properties and making the material more useful for electronic devices and drug delivery, for example.
Current graphene synthesis techniques, such as chemical vapor deposition and reduction of graphene oxide, can only produce graphene with a narrow range of characteristics, limiting the usefulness of produced graphene, the researchers say.
The researchers used a technique called electrochemical synthesis to exfoliate, or peel off graphite flakes into graphene layers. By varying the pulsed voltage, they could change the resulting graphene’s thickness, flake area, and number of defects, altering the properties of graphene.
They also found they need to use a solvent for intercalation (adding a fluid or material between layers) as the necessary first step.
To monitor the evolution of the graphene in the solvent they found that simply tracking the solution’s transparency with an LED and photodiode could give them quantitative information on the efficiency and onset of exfoliation.
They next plan to study the effects of adjusting the pulse durations throughout the exfoliation process to improve the amount of exfoliated graphene and to introduce more complex pulse shapes to selectively produce certain types of graphene defects.
Abstract of Controlling the properties of graphene produced by electrochemical exfoliation
The synthesis of graphene with controllable electronic and mechanical characteristics is of significant importance for its application in various fields ranging from drug delivery to energy storage. Electrochemical exfoliation of graphite has yielded graphene with widely varying behavior and could be a suitable approach. Currently, however the limited understanding of the exfoliation process obstructs targeted modification of graphene properties. We here investigate the process of electrochemical exfoliation and the impact of its parameters on the produced graphene. Using in situ optical and electrical measurements we determine that solvent intercalation is the required first step and the degree of intercalation controls the thickness of the exfoliated graphene. Electrochemical decomposition of water into gas bubbles causes the expansion of graphite and controls the functionalization and lateral size of the exfoliated graphene. Both process steps proceed at different time scales and can be individually addressed through application of pulsed voltages. The potential of the presented approach was demonstrated by improving the performance of graphene-based transparent conductors by 30 times.
Range of voluntary movement prior to receiving stimulation compared to movement after receiving stimulation, physical conditioning, and the drug buspirone. The subject’s legs are supported so that they can move without resistance from gravity. The electrodes on the legs are used for recording muscle activity. (credit: Edgerton Lab/UCLA)
In a study conducted at UCLA, five men who had been completely paralyzed were able to move their legs in a rhythmic motion thanks to a new, noninvasive neuromodulation and pharmacological procedure that stimulates the spinal cord.
The researchers believe this to be the first time voluntary leg movements have ever been relearned in completely paralyzed patients without surgery. The results are reported in an open-access paper in the Journal of Neurotrauma.
“These findings tell us we have to look at spinal cord injury in a new way,” said V. Reggie Edgerton, senior author of the research and a UCLA distinguished professor of integrative biology and physiology, neurobiology and neurosurgery.
Edgerton said although it likely will be years before the new approaches are widely available, he now believes that it is possible to significantly improve quality of life for patients with severe spinal cord injuries, and to help them recover multiple body functions.
Earlier this year, a the researchers demonstrated that they could induce involuntary stepping movements in healthy, uninjured people using noninvasive stimulation. The finding led Edgerton to believe the same approach could be effective for people with complete paralysis.
Reawakening neural connections with electrical charges and a drug
In the new research, five men were given one 45-minute training session per week for 18 weeks. For four weeks, the men were also given twice daily doses of buspirone, a drug often used to treat anxiety disorders, as part of the treatment.
Researchers placed electrodes at strategic points on the skin, at the lower back and near the tailbone and then administered a unique pattern of noninvasive, painless transcutaneous (through the skin) electrical currents*. The electrical charges caused no discomfort to the patients, who were lying down.
“The fact that they regained voluntary control so quickly must mean that they had neural connections that were dormant, which we reawakened,” said Edgerton, who for nearly 40 years has conducted research on how the neural networks in the spinal cord regain control of standing, stepping and voluntary control of movements after paralysis. “It was remarkable.”
* The researchers used monopolar rectangular pulsed stimuli (30 Hz at T11 and 5 Hz at Co1 with 1 ms duration for each pulse) filled with a carrier frequency of 10 kHz and at an intensity ranging from 80 to 180 mA .
Edgerton Lab/UCLA | Non-invasive Neuromodulation to regain voluntary movements after paralysis
Edgerton said most experts, including himself, had assumed that people who were completely paralyzed would no longer have had neural connections across the area of the spinal cord injury.
The researchers do not know yet whether patients who are completely paralyzed can be trained to fully bear their weight and walk. But he and colleagues have now published data on nine people who have regained voluntary control of their legs —four with epidural implants and five in the latest study.
“Many people thought just a few years ago we might be able to achieve these results in perhaps one out of 100 subjects, but now we have nine of nine,” Edgerton said. “I think it’s a big deal, and when the subjects see their legs moving for the first time after paralysis, they say it’s a big deal.”
The men in the newest study ranged in age from 19 to 56; their injuries were suffered during athletic activities or, in one case, in an auto accident. All have been completely paralyzed for at least two years. Their identities are not being released.
The research was funded by the National Institutes of Health’s National Institute of Biomedical Imaging and Bioengineering (grants U01EB15521 and R01EB007615), the Christopher and Dana Reeve Foundation, the Walkabout Foundation and the Russian Scientific Fund.
“These encouraging results provide continued evidence that spinal cord injury may no longer mean a life-long sentence of paralysis and support the need for more research,” said Dr. Roderic Pettigrew, director of the National Institute of Biomedical Imaging and Bioengineering. “The potential to offer a life-changing therapy to patients without requiring surgery would be a major advance; it could greatly expand the number of individuals who might benefit from spinal stimulation. It’s a wonderful example of the power that comes from combining advances in basic biological research with technological innovation.”
Edgerton estimates that cost to patients of the new approach could be one-tenth the cost of treatment using the surgical epidural stimulator (which is also still experimental) — and, because no surgery is required, it would likely be more easily available to more patients.
The study’s co-authors were Gerasimenko, who conceived the new approach and is director of the laboratory of movement physiology at Russia’s Pavlov Institute and a researcher in the UCLA department of integrative biology and physiology, as well as Daniel Lu, associate professor of neurosurgery, researchers Morteza Modaber, Roland Roy and Dimitry Sayenko, research technician Sharon Zdunowski, research scientist Parag Gad, laboratory coordinator Erika Morikawa and research assistant Piia Haakana, all of UCLA; and Adam Ferguson, assistant professor of neurological surgery at UC San Francisco.
Edgerton and his research team also plan to study people who have severe, but not complete, paralysis. “They’re likely to improve even more,” he said.
The scientists can only work with a small number of patients, due to limited resources, but Edgerton is optimistic that the research can benefit many others. Almost 6 million Americans live with paralysis, including nearly 1.3 million with spinal cord injuries.
“A person can have hope, based on these results,” Edgerton said. “In my opinion, they should have hope.”
Abstract of Noninvasive Reactivation of Motor Descending Control after Paralysis
The present prognosis for the recovery of voluntary control of movement in patients diagnosed as motor complete is generally poor. Herein we introduce a novel and noninvasive stimulation strategy of painless transcutaneous electrical enabling motor control and a pharmacological enabling motor control strategy to neuromodulate the physiological state of the spinal cord. This neuromodulation enabled the spinal locomotor networks of individuals with motor complete paralysis for 2-6 years (AIS B) to be reengaged and trained. We showed that locomotor-like stepping could be induced without voluntary effort within a single test session using electrical stimulation and training. We also observed significant facilitation of voluntary influence on the stepping movements in the presence of stimulation over a four-week period in each subject. Using these strategies we transformed brain-spinal neuronal networks from a dormant to a functional state sufficiently to enable recovery of voluntary movement in 5/5 subjects. Pharmacological intervention combined with stimulation and training resulted in further improvement in voluntary motor control of stepping-like movements in all subjects. We also observed on-command selective activation of the gastrocnemius and soleus muscles when attempting to plantarflex. At the end of 18 weeks of weekly interventions the mean changes in the amplitude of voluntarily controlled movement without stimulation was as high as occurred when combined with electrical stimulation. Additionally, spinally evoked motor potentials were readily modulated in the presence of voluntary effort, providing electrophysiological evidence of the re-establishment of functional connectivity among neural networks between the brain and the spinal cord.
Computational Simulation of microbiome-host interactions. (A) A basic gene circuit forms the core of all simulated gene network behavior. (B) Green fluorescent protein (GFP, shown as a green dot) from this circuit is conceptualized to be detected by an onboard miniature, epifluorescent microscope (EFM). (C) A computational simulation of microbiome GFP production based upon an analytical model for the circuit in (A). In a built system, this protein fluorescence signal would be the light detected by the EFM. (D) The conceptualized robot uses onboard electronics to convert the measured light signals into electrical (voltage) signals. (E) Voltage signals meeting specific criteria activate pre-programmed robot motion subroutines. (F) The resulting emergent behavior potentially leads a robot to a carbon fuel depot. Here, robot behavior resulting from a simulation of the circuit in (A) is shown. The robot was programmed with motion subroutines that activate to seek arabinose (synthesized from glucose, orange square) depots following receipt of lactose (cyan triangles). (credit: Keith C. Heyde & Warren C. Ruder/Scientific Reports)
Virginia Tech scientist Warren Ruder, an assistant professor of biological systems engineering, has created an in silico (computer-simulated) model of a biomimetic robot controlled by a bacterial brain.
The study was inspired by real-world experiments where the mating behavior of fruit flies was manipulated using bacteria, and in which mice exhibited signs of lower stress when implanted with probiotics (“healthy” bacteria).
A math model of microbiome-controlled behavior
The deeper motivation for the study was to understand how the microbiome (the bacteria in the human body, thought to number ten times more than human cells) might influence human behavior. For example, some studies show that the gut microbiome influences human eating behavior and dietary choices to favor the survival of the bacteria. (See Do gut bacteria control your mind? for example.)
As explained in an open-access paper published recently in Scientific Reports, Ruder’s study revealed unique decision-making behavior by a bacteria-robot system by coupling and computationally simulating equations that describe three distinct elements: engineered gene circuits in E. coli, microfluid bioreactors, and robot movement.
In the mathematical model, the theoretical robot was equipped with sensors and a miniature microscope to measure the color. The hypothetical robot was designed to read E. coli bacterial gene expression levels (how much protein is created by specific genes), using light sensors in miniature microscopes. The bacteria turned green or red, depending on what they ate.
Bacteria that act like tigers?
Interestingly, the bacteria in the model began to approach a fuel source with “stalk-pause-strike” behavior, characteristic of predators.
Ruder’s modeling study also demonstrates that these sorts of biosynthetic experiments could be done in the future with a minimal amount of funds, opening up the field to a much larger pool of researchers.
Understanding the biochemical sensing between organisms could have far reaching implications in ecology, biology, and robotics, Ruder suggests.
In agriculture, bacteria-robot model systems could enable robust studies that explore the interactions between soil bacteria and livestock. In healthcare, further understanding of bacteria’s role in controlling gut physiology could lead to bacteria-based prescriptions (probiotics) to treat mental and physical illnesses. Ruder also envisions droids that could execute tasks such as deploying bacteria to remediate oil spills.
Bacteria effects on behavior
The findings also add to the ever-growing body of research about bacteria in the human body that are thought to regulate health and mood, and especially the theory that bacteria also affect behavior.
“We hope to help democratize the field of synthetic biology for students and researchers all over the world with this model,” said Ruder. “In the future, rudimentary robots and E. coli that are already commonly used separately in classrooms could be linked with this model to teach students from elementary school through the Ph.D.-level about bacterial relationships with other organisms.”
Ruder plans next to create a real-world version of the experiment, creating mobile robots with bioreactors on board that harbor living colonies of bacteria that direct the robot’s behavior.
The Air Force Office of Scientific Research funded the mathematical modeling of gene circuitry in E. coli, and the Virginia Tech Student Engineers’ Council has provided funding to move these models and resulting mobile robots into the classroom as teaching tools.
Virginia Tech | Scientist shows bacteria could control robots
Abstract of Exploring Host-Microbiome Interactions using an in Silico Model of Biomimetic Robots and Engineered Living Cells
The microbiome’s underlying dynamics play an important role in regulating the behavior and health of its host. In order to explore the details of these interactions, we created anin silico model of a living microbiome, engineered with synthetic biology, that interfaces with a biomimetic, robotic host. By analytically modeling and computationally simulating engineered gene networks in these commensal communities, we reproduced complex behaviors in the host. We observed that robot movements depended upon programmed biochemical network dynamics within the microbiome. These results illustrate the model’s potential utility as a tool for exploring inter-kingdom ecological relationships. These systems could impact fields ranging from synthetic biology and ecology to biophysics and medicine.
Innovative techniques for reducing solar-cell installation costs by capturing more solar energy per unit area by using hybrid materials have recently been announced by two universities.
Capturing more of the spectrum
Chemists at the University of California, Riverside have found an ingenious way to lower solar cell installation costs by reducing the size of solar collectors (credit: David Monniaux)
The University of California, Riverside strategy for making solar cells more efficient is to use the near-infrared region of the sun’s spectrum, which is not absorbed by current solar cells.
The researchers report in Nano Letters that a hybrid material that combines inorganic materials (cadmium selenide and lead selenide semiconductor nanocrystals) with organic molecules (diphenylanthracene and rubrene) could allow for an increase of solar photovoltaic efficiency by 30 percent or more, according to Christopher Bardeen, a UC Riverside professor of chemistry.
The new material also has wide-ranging applications such as in biological imaging, data storage and organic light-emitting diodes. “The ability to move light energy from one wavelength to [a] more useful region — for example, from red to blue — can impact any technology that involves photons as inputs or outputs,” he said.
The research was supported by grants from the National Science Foundation and the U.S. Army.
Plasmonic nanostructures and metal oxides
Rice researchers selectively filtered high-energy hot electrons from their less-energetic counterparts using a Schottky barrier (left) created with a gold nanowire on a titanium dioxide semiconductor. A second setup (right), which included a thin layer of titanium between the gold and the titanium dioxide, did not filter electrons based on energy level. (credit: B. Zheng/Rice University)
Meanwhile, new research from Rice’s Laboratory for Nanophotonics (LANP) has found a way to boost the efficiency and also reduce the cost of photovoltaic solar cells by using high-efficiency light-gathering plasmonic nanostructures combined with low-cost semiconductors, such as metal oxides.
“We can tune plasmonic structures to capture light across the entire solar spectrum,” claims Rice’s Naomi Halas, co-author of an open-access paper in Nature Communications. “The efficiency of [conventional] semiconductor-based solar cells can never be extended in this way because of the inherent optical properties of the semiconductors.”
The researchers found in an experiment that a solar cell using a “Schottky barrier” device allowed only “hot electrons” (electrons in the metal that have a much higher energy level) to pass from a gold nanowire to the semiconductor, unlike an “Ohmic device,” which let all electrons pass.
Today’s most efficient photovoltaic cells use a combination of semiconductors that are made from rare and expensive elements like gallium and indium, so this finding promises to further reduce the cost of solar cells.
Abstract of Hybrid Molecule–Nanocrystal Photon Upconversion Across the Visible and Near-Infrared
The ability to upconvert two low energy photons into one high energy photon has potential applications in solar energy, biological imaging, and data storage. In this Letter, CdSe and PbSe semiconductor nanocrystals are combined with molecular emitters (diphenylanthracene and rubrene) to upconvert photons in both the visible and the near-infrared spectral regions. Absorption of low energy photons by the nanocrystals is followed by energy transfer to the molecular triplet states, which then undergo triplet–triplet annihilation to create high energy singlet states that emit upconverted light. By using conjugated organic ligands on the CdSe nanocrystals to form an energy cascade, the upconversion process could be enhanced by up to 3 orders of magnitude. The use of different combinations of nanocrystals and emitters shows that this platform has great flexibility in the choice of both excitation and emission wavelengths.
Abstract of Distinguishing between plasmon-induced and photoexcited carriers in a device geometry
The use of surface plasmons, charge density oscillations of conduction electrons of metallic nanostructures, to boost the efficiency of light-harvesting devices through increased light-matter interactions could drastically alter how sunlight is converted into electricity or fuels. These excitations can decay directly into energetic electron–hole pairs, useful for photocurrent generation or photocatalysis. However, the mechanisms behind plasmonic carrier generation remain poorly understood. Here we use nanowire-based hot-carrier devices on a wide-bandgap semiconductor to show that plasmonic carrier generation is proportional to internal field-intensity enhancement and occurs independently of bulk absorption. We also show that plasmon-induced hot electrons have higher energies than carriers generated by direct excitation and that reducing the barrier height allows for the collection of carriers from plasmons and direct photoexcitation. Our results provide a route to increasing the efficiency of plasmonic hot-carrier devices, which could lead to more efficient devices for converting sunlight into usable energy.
