System Bits: Jan. 9

Smartphone micro-spectrometer; lego-like circuits; plant sensors.

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Microspectrometer smartens up smartphones
Thanks to researchers at TU Eindhoven, smartphones are about to get much smarter to do things like checking how clean the air is, whether food is fresh or a lump is malignant thanks to a spectrometer that is so small it can be incorporated easily and cheaply in a mobile phone.

The little sensor developed at TU Eindhoven is just as precise as the normal tabletop models used in scientific labs, the team reported.

An electron microscope image of the perforated membrane with the crystal cavity in the middle. Detail: the crystal cavity which captures light. Source: TU Eindhoven

Spectrometry is the analysis of visible and invisible light, and has an enormous range of applications. As every material and every tissue has its own ‘footprint’ in terms of light absorption and reflection, it can thus be recognized by spectrometry but the most precise spectrometers are large since they split up the light into different colors (frequencies), which are then measured separately. Just after the light is split, the beams, which have different frequencies, still overlap each other; highly precise measurements can therefore only be made some tens of centimeters after the splitting.

However, the TU researchers developed a sensor that is able to make such precise measurements in an entirely different way using a special ‘photonic crystal cavity’, a ‘trap’ of just a few micrometers into which the light falls and cannot escape, they said. This trap is contained in a membrane, into which the captured light generates a tiny electrical current, and that is measured. The cavity was made such that it is very precise, retaining just a very tiny frequency interval and therefore measuring only light at that frequency.

To be able to measure a larger frequency range, the researchers reported that they placed two of their membranes very closely one above the other. The two membranes influence each other: if the distance between them changes slightly, then the light frequency that the sensor is able to detect shifts too.

The structure of the device. The blue perforated plane is the upper membrane with the photonic crystal cavity in it, which captures light of a very specific frequency. When this happens it generates a current that is measured (A). Source: TU Eindhoven

For this purpose the researchers incorporated a MEMS (a micro-electromechanical system) that allows the distance between the membranes to be varied, and thereby the measured frequency. Ultimately, then, the sensor covers a wavelength range of around thirty nanometers, within which the spectrometer can discern some hundred thousand frequencies, which is exceptionally precise. This is made possible by the fact that the researchers are able to precisely determine the distance between the membranes to just a few tens femtometers (10-15 meters).

Given the breadth of applications, micro-spectrometers are expected to eventually become just as important an element of the smartphone as the camera. For example, to measure CO2, detect smoke, determine what medicine you have, measure the freshness of food, the level of your blood sugar, and so on, the researchers added.

Automated IC assembly?
A team of researchers from King Abdullah University of Science and Technology has devised a strategy for assembly of electronic systems, specifically the flexible structures needed for high-performance devices of the future, using ICs like Lego building blocks, which sounds very similar to the chiplet approach that is gaining traction as of late.

The researchers hope this method of assembly creates completely new options for manufacturing processes of these systems.

The team reminded that existing technologies for building electronic devices, such as computers, smartphones and robots, rely on complex automated manufacturing processes, which involve high-precision equipment to align and package thousands of components that range from a few millimeters upward in size. Once aligned, these components are connected to printed circuit boards using numerous tiny pins.

These manufacturing processes work well for today’s electronic components and supports, which are rigid and hard. However, they do not suit the emerging electronic systems—increasingly multifunctional, high-performance devices that demand greater miniaturization in addition to better accuracy and precision. In particular, bonding and alignment are exceptionally difficult to achieve for flexible integrated circuits and soft substrates needed for devices that are wearable and implantable.

To tackle this problem, the KAUST team developed a modular approach in which building blocks presenting complementary geometries produce electronic systems via lock-and-key-type assembly to eliminate any steps that require bonding or soldering. They carved various shapes displaying different sizes, heights and angles out of the back of fragile, ultra-thin flexible silicon integrated circuits to form so-called male modules. Next, they etched grooves, corresponding to inverse replicas of these shapes, into the flexible substrate to generate hosts for the male modules. 

Plant tattoo sensors
Iowa State University plant scientists have developed a low-cost, easily produced, graphene-based sensor that can be attached to plants to provide new kinds of data to researchers and farmers.

By understanding, for example, how different strains of corn move water from their roots to their lower leaves and then to their upper leaves, they can breed plants that are more efficient in using water.

The tool making these water measurements possible is a tiny graphene sensor that can be taped to plants – researchers have dubbed it a “plant tattoo sensor.” Graphene is a wonder material. It’s a carbon honeycomb just an atom thick, it’s great at conducting electricity and heat, and it’s strong and stable. The graphene-on-tape technology in this study has also been used to produce wearable strain and pressure sensors, including sensors built into a “smart glove” that measures hand movements.

Part of what makes this so useful is that the sensors are cheaper to make but still high performing, the team said.

To do that, the researchers have developed a process for fabricating intricate graphene patterns on tape. The first step is creating indented patterns on the surface of a polymer block, either with a molding process or with 3-D printing. Engineers apply a liquid graphene solution to the block, filling the indented patterns. They use tape to remove the excess graphene. Then they take another strip of tape to pull away the graphene patterns, creating a sensor on the tape.

The process can produce precise patterns as small as 5 millionths of a meter wide – just a twentieth of the diameter of the average human hair. Making the patterns so small increases the sensitivity of the sensors.

In the case of plant studies, the sensors are made with graphene oxide, a material very sensitive to water vapor. The presence of water vapor changes the conductivity of the material, and that can be quantified to accurately measure transpiration (the release of water vapor) from a leaf.

The plant sensors have been successfully tested in lab and pilot field experiments, the researchers said.

Interestingly, the team believes this technology could open a new route for a wide variety of applications, including sensors for biomedical diagnostics, for checking the structural integrity of buildings, for monitoring the environment and, after appropriate modifications, for testing crops for diseases or pesticides.