Power/Performance Bits: Oct. 26

Printing circuits on irregular shapes
Researchers at Pennsylvania State University propose a way to print biodegradable circuits on irregular, complex shapes.

“We are trying to enable direct fabrication of circuits on freeform, 3-D geometries,” said Huanyu “Larry” Cheng, professor in Penn State’s Department of Engineering Science and Mechanics (ESM). “Printing on complicated objects can allow a future Internet of Things where circuits can connect various objects around us, whether they be smart home sensors, robots performing complex tasks together, or devices placed on the human body.”

The printing method uses a thin film covered with an ink made from zinc nanoparticles. This thin film was attached to a mask overlay on the target surface. The researchers then pulsed a high-energy xenon light through the film. Within milliseconds, energy from this light excited the particles enough to transfer them to the new surface through the stencil.

In tests of the method, the researchers printed on a glass beaker and seashells. The transferred zinc formed a conductive electronic circuit that could be adapted for use as a sensor or antenna.

Researchers demonstrated a new printing method using pulsed light to transfer an electronic circuit to a seashell, as depicted in this illustration. (Credit: Jennifer McCann/Penn State)

The team said that the printing process is faster and more cost-efficient than other techniques, as it doesn’t require expensive equipment or vacuum pressurization time. The circuits are also biodegradable, noted Cheng. “Our electronics upgrade every two years or so, and this creates a huge amount of electronic waste. When we look at the future, if our electronics are green enough to be flushed down the toilet, their use will be much better for the environment.”

Cheng added that the biodegradability also allows devices to be securely destroyed. “If your device is only encrypted with software, it can always be cracked and there’s a potential leak for information. This biodegradable device can be physically destroyed so that data can’t be recovered; it presents a unique opportunity that can’t be addressed by traditional silicon devices.”

They also developed a way to make them last longer by submerging the printed zinc circuits into solutions containing copper or silver. They plan to investigate ways to make the process suitable for large-scale manufacturing and optimize it for printing on skin.

Sweat-powered battery
Researchers from Nanyang Technological University Singapore developed a stretchable battery that is powered by perspiration.

The battery is constructed of printed silver flake electrodes that generate electricity in the presence of sweat. When the silver flakes come into contact with sweat, its chloride ions and acidity cause the flakes to clump together, increasing their ability to conduct electricity. This chemical reaction also causes an electric current to flow between the electrodes.

Resistance is further lowered when the battery is stretched. The researchers also noted that it can withstand strain from daily activities, including repeated exposure to stress and sweat.

The battery, which is flat and measures 2 cm by 2cm, is designed to be affixed to a flexible and sweat absorbent textile that is stretchable and attachable to wearable devices, like watches, wrist bands, or arm straps.

“By capitalizing on a ubiquitous product, perspiration, we could be looking at a more environmentally friendly way of powering wearable devices that does not rely on conventional batteries. It is a near-guaranteed source of energy produced by our bodies. We expect the battery to be capable of powering all sorts of wearable devices,” said Lee Pooi See, professor and Dean of NTU Graduate College.

The stretchable textile is absorbent and capable of retaining moisture, so the battery can remain powered when the rate of perspiration is inconsistent.

The battery contains no toxic components, added Lyu Jian, a research fellow from NTU’s School of Materials Science and Engineering. “Conventional batteries are cheaper and more common than ever, but they are often built using unsustainable materials which are harmful to the environment. They are also potentially harmful in wearable devices, where a broken battery could spill toxic fluids onto human skin. Our device could provide a real opportunity to do away with those toxic materials entirely.”

In tests, an individual wearing the battery around their wrist and cycling on a stationary bicycle for 30 minutes was able to generate a voltage of 4.2 V and output power of 3.9 mW, which was sufficient to power a commercial temperature sensor device and send the data continuously to a smartphone via Bluetooth.

The researchers plan to continue investigating the battery, including its interaction with different components of perspiration and the impact of factors such as body heat.

Photovoltaics indoors
Researchers at the National Institute of Standards and Technology (NIST) suggest that using photovoltaics to capture indoor artificial light is a plausible way to power small sensors.

“People in the field have assumed it’s possible to power IoT devices with PV modules in the long term, but we haven’t really seen the data to support that before now, so this is kind of a first step to say that we can pull it off,” said Andrew Shore, a NIST mechanical engineer.

The team tested small, modular PV devices made of different materials: gallium indium phosphide (GaInP), gallium arsenide (GaAs), and silicon.

The researchers placed the centimeters-wide modules underneath a white LED, housed inside an opaque black box to block out external light sources.  The LED produced light at a fixed intensity of 1000 lux, comparable to light levels in a well-lit room, for the duration of the experiments. For the silicon and GaAs PV modules, soaking in indoor light proved less efficient than sunshine, but the GaInP module performed far better under the LED than sunlight. Both the GaInP and GaAs modules significantly outpaced silicon indoors, converting 23.1% and 14.1% of the LED light into electrical power, respectively, compared with silicon’s 9.3% power conversion efficiency.

They took the silicon device, which was the lowest-performing but also cheapest, and connected it to an example IoT device, in this case a temperature sensor. When the silicon PV device was placed under an LED, it was able to feed temperature readings wirelessly to a computer nearby, powered by the silicon module alone. After two hours, they switched off the light in the black box and the sensor continued to run, its battery depleting at half the rate it took to charge.

“Even with a less efficient mini module, we found that we could still supply more power than the wireless sensor consumed,” Shore said, suggesting that solar modules designed for outdoor use could be applied to indoor applications, particularly in commercial applications where lights are in continuous use.

The researchers plan to look into how indoor PV would perform in typical residential scenarios.