Paper skin sensors for environmental monitoring

| Information and Communication Technology

Wearable electronics using paper skin

Wearable and flexible electronics show promise for a variety of applications, such as wireless monitoring of patient health and touch-free computer interfaces.

Now, everyday materials found in the kitchen, such as aluminium foil, sticky note paper, sponges and tape, have been used by a team of electrical engineers from Kaust to develop a low-cost sensor that can detect external stimuli. The team used sticky note paper to detect humidity, sponges and wipes to detect pressure and aluminium foil to detect motion. Colouring a sticky note with an HB pencil allowed the paper to detect acidity levels, and aluminium foil and conductive silver ink were used to detect temperature differences.

The sensor, which is called Paper Skin, is said to perform as well as other artificial skin applications currently being developed, while integrating multiple functions using cost-effective materials.

“Our work has the potential to revolutionize the electronics industry and opens the door to commercialising affordable high-performance sensing devices,” stated Muhammad Mustafa Hussain, Kaust associate professor of electrical engineering from the University’s Integrated Nanotechnology Lab. “Previous efforts in this direction used sophisticated materials or processes. Chemically functionalised inkjet printed or vacuum technology-processed papers – albeit cheap – have shown limited functionality.”

Several challenges must be overcome before a fully autonomous, flexible and multifunctional sensory platform becomes commercially achievable, explained Hussain. Wireless interaction with the paper skin needs to be developed.

Reliability tests also need to be conducted to assess how long the sensor can last and how good its performance is under severe bending conditions. “The next stage will be to optimise the sensor’s integration on this platform for applications in medical monitoring systems. The flexible and conformal sensory platform will enable simultaneous real-time monitoring of body vital signs, such as heart rate, blood pressure, breathing patterns and movement,” Hussain said.

Producing tiny cracks in electrodes – a big boost for nano-electronics

The next generation of electronics, as well as ultra-sensitive medical diagnostics, could depend on near atomic scale cracks — or nanogaps — in electrodes. Researchers at Sweden’s KTH Royal Institute of Technology have developed a method that could pave the way for mass production of nanogap arrays with individually-defined gap widths.

Valentin Dubois, a researcher at KTH’s Department of Micro and Nanosystems, says the new method for creating nanogaps that are only a few atom layers wide improves established ways to achieve gaps in conductive materials — in this case, titanium nitride (TiN).

“Using our method, we do not need to pattern the material directly to define the nanogaps,” Dubois says. “Instead, they arise automatically once certain criteria are met. What we need to do is create a pattern around the area where the gaps should be. This pattern in the material structure is substantially larger than the gaps, and thus simple to create.”

Dubois says is is possible to determine at the outset what the parameters of the nanogaps will be, from 100nm down to below 2nm (less than 10 atom layers) wide.

Nanogaps could enable new types of microprocessors and make a whole range of biosensors possible. In medical diagnostics, for example, nanogaps can improve the detection of molecules that are markers for diseases.

Microwave field imaging using diamond and vapour cells

Researchers from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel have now independently developed two new methods for imaging microwave fields. Both methods exploit the change in spin states induced by an applied microwave field.

Microwaves are indispensable for wireless communication in laptops and mobile phones, where microwave circuits are used to transmit and decode information. A newly emerging field of use in medical diagnostics stems from the fact that cancer cells, for example, absorb microwaves differently from the way healthy tissue does.

Until now, however, there have been almost no quick and easy methods to obtain accurate images of microwave fields.

Traditionally, electromagnetic fields have been imaged using miniaturised antennae. However, these require elaborate calibration and can perturb the fields they are supposed to measure. Instead of antennae, the groups led by Professor Philipp Treutlein and the Georg-H.-Endress Professor Patrick Maletinsky at the University of Basel use the intrinsic angular momentum (spin) of atoms and individual electrons to image microwave fields, specifically, the spin of an electron or atom changes in the presence of a microwave field, with the number of rotations dependent on the strength of the microwave field. As the spins are microscopically small, measuring the change in spin barely affects the microwave field that is to be analysed.

Treutlein’s group images the microwave fields using a thin glass cell filled with rubidium vapour. If a microwave field is applied in the vicinity of this glass cell, it causes a change in the spin state of all the rubidium atoms in the measuring cell.

Maletinsky’s team measures the spin change of individual electrons in a nitrogen vacancy centre in diamond in order to obtain an image of the microwaves’ magnetic field. For this purpose, the researchers initially produce a tiny tip made of monocrystalline diamond. This diamond is modified so that some carbon atoms in the crystal lattice are replaced with nitrogen atoms and a vacant site is located immediately adjacent to these (nitrogen vacancy centres).  This tip is then incorporated into a specially developed microscope and moved into the direct vicinity of a microwave field.

Coating cancels acoustic scattering from odd-shaped objects

Scattering occurs when an object has material properties different than those of the medium surrounding it, such as air or water, and its mode is characterised by the way waves bounce off it. By applying a coating with the appropriate material properties, electromagnetic scattering modes can be cancelled – a process known as “scattering cancellation.” This has long been used to systematically cancel the dominant scattering modes of electromagnetic waves off objects.

Now, researchers from the US Naval Research Laboratory and the University of Texas at Austin has applied the concept to acoustic waves. The work provides fundamental new tools to control acoustic scattering and should improve the ability to make acoustic measurements in the laboratory.

It turns out that this applies equally well to other types of waves, such as acoustic waves. In fact, the principles of scattering are not limited to electromagnetic waves but are a fundamental feature of how any type of wave interacts with its environment.

“Scientists have spent many years studying mathematical solutions to discover how waves scatter from simple objects, such as spheres or cylinders. In most cases, they’ve attempted to solve the ‘forward problem’ to determine what the scattered field will look like for a particular object,”

“For scattering cancellation, a scattered field of zero is desirable, so the team set out to explore the ‘inverse problem’ of determining which coating properties could provide this result,” says Matthew Guild, a National Research Council postdoctoral research associate at the US Naval Research Laboratory. “It’s actually a bit tricky because there are so many possible solutions — most, however, aren’t practical.”

The team at the Naval Research Laboratory previously focussed on a particular set of solutions involving polydimethylsiloxane (PDMS) – a silicon-based organic polymer – to make coated objects “feel” as if they have the same properties as water.

“The key significance of this work is the formulation of a more general approach using acoustic scattering cancellation for complex, odd-shaped objects,” said Guild. Previously, the approach was considered only for relatively simple shapes such as solid spheres and cylinders.

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About Andy Pye

Andy Pye is a graduate of Cambridge University and has had a high profile career in the technical press as well as being a pioneer in web publishing.

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