Power/Performance Bits: July 10

Wearable heart monitoring; cheaper graphene; photovoltaic nanotubes.


Wearable heart monitoring
Researchers at the University of Texas at Austin developed a lightweight, stretchy heart monitoring patch that can be worn externally. Along with being easy to wear, the graphene-based ‘e-tattoo’ is more accurate than existing electrocardiograph machines, according to the team.

The e-tattoo measures cardiac health using both electrocardiograph and seismocardiograph readings simultaneously. Electrocardiography (ECG) records the rates of electrical activity produced each time the heart beats, while seismocardiography (SCG) is a measurement technique using chest vibrations associated with heartbeats.

The team noted that ECG readings alone are not accurate enough for determining heart health, but they provide additional information when combined with SCG signal recordings. Like a form of quality control, the SCG indicates the accuracy of the ECG readings.

“We can get much greater insight into heart health by the synchronous collection of data from both sources,” said Nanshu Lu, an associate professor in the departments of Aerospace Engineering and Engineering Mechanics and Biomedical Engineering at UT Austin.

The stretchable e-tattoo for heart monitoring. (Source: Cockrell School of Engineering, The University of Texas at Austin)

While flexible ECG patches have been developed, previous SCG sensors were made from rigid materials.

The e-tattoo is made of a piezoelectric polymer called polyvinylidene fluoride, which is capable of generating its own electric charge in response to mechanical stress. The device also includes 3D digital image correlation technology that is used to map chest vibrations in order to identify the best location on the chest to place the e-tattoo.

The stretchy and light nature of the patch allows it to be placed over the heart for extended periods with little or no discomfort, and it can provide days of constant heart monitoring. The researchers are working on improvements to data collection and storage for the device, as well as ways to power the e-tattoo wirelessly for longer periods. They are also developed a smartphone app to store and show data.

Cheaper graphene
Researchers from RMIT University and National Institute of Technology Warangal propose a much cheaper and more sustainable way to produce graphene: eucalyptus bark.

The new method could reduce the cost of production from $USD100 per gram to just $USD0.5 per gram, said Suresh Bhargava, a Distinguished Professor at RMIT. “Eucalyptus bark extract has never been used to synthesize graphene sheets before and we are thrilled to find that it not only works, it’s in fact a superior method, both in terms of safety and overall cost.”

The strong, flexible, highly conductive material shows promise for a range of applications including bio-sensors, solar panels, and flexible electronics. Typically, graphene oxide is synthesized into graphene using a chemical reduction method that uses reducing agents harmful to both humans and the environment.

Instead, the researchers turned to a Eucalyptus polyphenol solution, obtained from an extract of ground Eucalyptus bark. The polyphenol compounds were capable of reducing exfoliated graphene oxide to soluble graphene under reflux conditions in an aqueous medium.

The team tested the graphene produced by this method as a supercapacitor and found it matched the quality and performance characteristics of traditionally-produced graphene without the toxic reagents. Along with being a much cheaper process given the abundance of eucalyptus trees in Australia, the researchers note that not using hazardous reagents means the graphene produced could potentially be used for biocompatible applications.

Photovoltaic nanotubes
Researchers at the University of Tokyo, Max Planck Institute for Solid State Research, HIT-Holon Institute of Technology, and Weizmann Institute of Science developed a new form of photovoltaic material that behaves much differently than standard photovoltaics.

The material uses nanotubes made up of rolled-up sheets of tungsten disulfide (WS2). The sheets do not induce a current in the presence of light unless rolled into tubes. This is an emergent behavior, one not intrinsic to the material until it’s modified.

Unlike most photovoltaics, the WS2 nanotubes do not rely on a p-n junction to generate current. Instead, when exposed to light, they generate a current throughout their entire structure or bulk. Called the bulk photovoltaic effect (BPVE), it’s thanks to the asymmetry of WS2 nanotubes, which allows the current to flow. Other symmetrical nanotubes, such as carbon nanotubes, don’t exhibit BPVE despite being great electrical conductors.

The team says size constraints on the device mean that the most promising applications are high-fidelity optical sensors and infrared imaging chips.

“Our research shows an entire order of magnitude improvement in efficiency of BPVE compared to its presence in other materials,” said Yoshihiro Iwasa, a professor at University of Tokyo. “But despite this huge gain, our WS2 nanotube cannot yet compare to the generating potential of p-n junction materials. This is because the device is nanoscopic and will be difficult to make larger. But it is possible and I hope chemists are inspired to take on that challenge.”

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