Graphene reveals yet another extraordinary property

Dr Nair shows his one micron thick graphene oxide film research sample (Photo: University of Manchester)

Ever since University of Manchester scientists Andre Geim and Konstantin Novoselov first isolated flakes of graphene in 2004 using that most high-tech pieces of equipment - adhesive tape - the one-atom sheet of carbon has continued to astound researchers with its remarkable properties. Now Professor Sir Andre Geim, (he's now not only a Nobel Prize winner but also a Knight Bachelor), has led a team that has added superpermeability with respect to water to graphene's ever lengthening list of extraordinary characteristics.

Graphene has already proven to be the thinnest known material in the universe, strongest material ever measured, the best-known conductor of heat and electricity, and the stiffest known material, while also the most ductile. But it seems the two-dimensional lattice of carbon atoms just can't stop showing off.

Stacking membranes of a chemical derivative of graphene called graphene oxide, which is a graphene sheet randomly covered with other molecules such as hydroxyl groups OH-, scientists at the University of Manchester created laminates that were hundreds of times thinner than a human hair but remained strong, flexible and were easy to handle.

When the team sealed a metal container using this film, they say that even the most sensitive equipment was unable to detect air or any other gas, including helium, leaking through. The team then tried the same thing with water and, to their surprise, found that it evaporated and diffused through the graphene-oxide membranes as if they weren't even there. The evaporation rate was the same whether the container was sealed or completely open.

"Graphene oxide sheets arrange in such a way that between them there is room for exactly one layer of water molecules. They arrange themselves in one molecule thick sheets of ice which slide along the graphene surface with practically no friction, explains Dr Rahul Nair, who was leading the experimental work. "If another atom or molecule tries the same trick, it finds that graphene capillaries either shrink in low humidity or get clogged with water molecules."

Professor Geim added, "Helium gas is hard to stop. It slowly leaks even through a millimetre -thick window glass but our ultra-thin films completely block it. At the same time, water evaporates through them unimpeded. Materials cannot behave any stranger. You cannot help wondering what else graphene has in store for us."

Although graphene's superpermeability to water makes it suitable for situations where water needs to be removed from a mixture without removing the other ingredients, the researchers don't offer ideas for any immediate applications that could take advantage of this property. However, they did seal a bottle of vodka with the membranes and found that the distilled solution did indeed become stronger over time. But they don't foresee graphene being used in distilleries.

However, Professor Geim adds, "the properties are so unusual that it is hard to imagine that they cannot find some use in the design of filtration, separation or barrier membranes and for selective removal of water."

The University of Manchester team's paper, "Unimpeded Permeation of Water Through Helium-Leak-Tight Graphene-Based Membranes," appears in the journal Science.

Skin-mounted electronics that can be applied and worn like a temporary tattoo

Wearable electronics generally take the form of clothing embedded with electronics or miniature electronic devices that can be worn close to the body for purposes such as medical monitoring and communications. Now engineers have developed a device that places electronic components onto an ultra-thin skin-like patch that can be mounted directly onto the skin, paving the way for skin-mounted electronics that could be used for sensing, medical diagnostics, communications and human-machine interfaces.

While the researchers have previously developed thin, flexible silicon electronics on silk substrates that almost completely dissolve when implanted inside the body, the new patches are initially mounted on a thin sheet of water-soluble plastic and are then laminated onto the skin with water, just like applying a temporary tattoo. The researchers say the electronic components could also be applied directly to a temporary tattoo itself to conceal the electronics. Because the circuits are fabricated as tiny, squiggled wires, their wavy, snakelike shape allows then to bend, twist, wrinkle and stretch with the mechanical properties of skin, while maintaining functionality.

The researchers say skin-mounted electronics could be used to replace conventional EEG and EMG sensors for the monitoring of nerve and muscle activity without the need for conductive gel, tape, skin-penetrating pins or bulky wires. They are also more comfortable and less cumbersome than traditional electrodes and give the wearers complete freedom of movement so they can be worn in a natural environment during normal activity.

"If we want to understand brain function in a natural environment, that's completely incompatible with EEG studies in a laboratory," said University of Illinois (U of I) electrical and computer engineering professor Todd Coleman, who co-led the multi-disciplinary team responsible for developing the device. "The best way to do this is to record neural signals in natural settings, with devices that are invisible to the user."

Additionally, skin-mounted electronics could be used to let patients with muscular or neurological disorders to communicate or interface with computers or, when the sensors are applied to the skin of the throat, they can distinguish muscle movement for simple speech. The researchers have already used the electronic patches to control a video game to demonstrate the potential for human-computer interfacing.

The researchers, led by John A. Rogers, the Lee J. Flory-Founder professor of engineering at the U of I, have demonstrated their concept by mounting a variety of electronic components, including sensors, LEDs, transistors, radio frequency capacitors, wireless antennas, and conductive coils and solar cells for power, on a thin, rubbery substrate.

"The blurring of electronics and biology is really the key point here," said Yonggang Huang, a professor at Northwestern University who, with his group, collaborated with Rogers to create the patches. "All established forms of electronics are hard, rigid. Biology is soft, elastic. It's two different worlds. This is a way to truly integrate them."

Because the researchers used simple adaptations of techniques used in the semiconductor industry, they say the patches are easily scalable and manufacturable. Rogers has co-founded a company, mc10, which is already working to commercialize certain versions of the technology.

The researchers are also working to integrate the various devices mounted on the platform so that they work together as a system, instead of as individually functioning devices. They are also working to add Wi-Fi capabilities.

"We think this could be an important conceptual advance in wearable electronics, to achieve something that is almost unnoticeable to the wearer," says Coleman. "The technology can connect you to the physical world and the cyberworld in a very natural way that feels very comfortable."

Monitor blood pressure while scrolling and clicking with the MDMouse

Monitoring blood pressure at home is recommended by the American Heart Association for the estimated 74.5 million American adults suffering from hypertension. CalHealth has created a blood pressure monitor that's housed in a computer mouse. After a user pushes a finger into the cuff monitor, the device sends readings to software on a PC for analysis, or to send on to doctors via email.
CalHealth's MDMouse is a fully functional USB optical mouse with a sphygmomanometer payload. The blood pressure meter extends on a rotating arm out of the body of the mouse. The user inserts a finger, and an air pump expands an air bag inside the tube around the digit. A pressure sensor stops the pump when it detects that the right amount of pressure has been applied and the user sets the monitoring to start via the computer software.
The pressure on the finger is initially increased beyond cutoff and then slowly decreased until arterial vessel pulsation is detected. CalHealth says that "the corresponding cuff pressure at this point will be substantially equal to systolic blood pressure which is the pressure when the heart is pumping."
The decrease of pressure continues until the device no longer registers arterial pulsation where, according to the company, "the pressure of the cuff at this point will be substantially equal to diastolic blood pressure."
The readings are then interpreted by the software and displayed for the user. The software can also store data from previous tests and present the user with graphs for onward email transmission to medical personnel.
There's a release valve to let the air out after each test, so there's no fear of the experience turning into some Jigsaw nightmare where the device starts to menacingly crush the trapped finger.
However, there has been some doubt cast on the accuracy of finger-based monitors. The American Heart Association recommends an automatic, cuff-style, upper-arm monitor: "Wrist and finger monitors are not recommended because they yield less reliable readings."

Any home monitoring device should be checked for accuracy by medical practitioners.

Electronic explosive-detecting sensor out-sniffs sniffer dogs

The recent Yemeni bomb threat has only highlighted the need for quick, accurate ways of detecting explosives. With their excellent sense of smell and the ability to discern individual scents, even when they’re combined or masked by other odors, this task is usually given to man’s best friend. But training these animals can be expensive and good sniffer dogs can be hard to find. Scientists have now developed an electronic sensor they say is more sensitive and more reliable at detecting explosives than any sniffer dog.
The new sensor, developed by scientists at Tel Aviv University, is able to detect multiple kinds of explosives and is especially effective at detecting TNT – an explosive that currently requires equipment that is high cost, has lengthy decoding times, is large and needs expert analysis to be detected.
"There is a need for a small, inexpensive, handheld instrument capable of detecting explosives quickly, reliably and efficiently," says lead researcher Prof. Fernando Patolsky of Tel Aviv University's Raymond and Beverly Sackler School of Chemistry.
The device is made from an array of silicon nanowires, coated with a compound that binds to explosives to form a nanotransistor. To enhance the device’s sensitivity, the scientists developed each one with 200 individual sensors that work together to detect different kinds of explosives with what the scientists say is an unprecedented degree of reliability, efficiency and speed.
In addition to being portable, the device is also capable of detecting explosives at a distance. This means it can be mounted on a wall, with no need to bring it into contact with the item being checked. Also, unlike other explosives sensors, the device provides a definitive identification of the explosive that it has detected. Its developers say that, to date, the device has not produced a single detection error.