Power/Performance Bits: Jan. 9

Eel-inspired power
Researchers at the University of Michigan, the University of Fribourg, and the University of California-San Diego developed soft power cells with the potential to power implanted medical devices. Made of hydrogel and salt, the soft cells form the first potentially biocompatible artificial electric organ that generates more than 100 volts at a low current, the team says, enough to power a small medical device like a pacemaker.

The team was inspired by electric eels, although the device doesn’t provide nearly as much electricity generation. “The eel polarizes and depolarizes thousands of cells instantaneously to put out these high voltages,” said Max Shtein, U-M associate professor of materials science and engineering.

Electric eels use a phenomenon called transmembrane transport. Specialized electrical organs contain thousands of alternating compartments, each with an excess of either potassium or sodium ions. The compartments are separated by selective membranes that, in the eel’s resting state, keep the two ions separate. When the eel needs to create a jolt of electricity, the membranes allow the ions to flow together.

The researchers built a similar system, though instead of sodium and potassium, they used sodium and chloride dissolved in water-based hydrogel. They printed thousands of tiny droplets of the salty gel on a plastic sheet, alternating them with hydrogel droplets of pure water. The alternating droplets are similar to the eel’s compartments.

To mimic the eel’s selective membrane, they used a second sheet of alternating droplets, this one made of charge-selective hydrogel. Each droplet allows either positively charged sodium or negatively charged chloride to pass, excluding the other.

Charge-selective (yellow and green) and freshwater and saline (blue and red) hydrogels, printed on a sheet that has been laser-cut in a Miura fold pattern. (Source: Biophysics group, Adolple Merkle Institute)

To generate power, the two sheets are pressed together, connecting saline and freshwater droplets across the charge-selective droplets in series. As the salty and fresh solutions mix, the charge-selective droplets move the sodium and chloride ions in opposing directions, producing an electric current.

To combine all the cells simultaneously in the right order, the researchers made use of Miura folds, an origami technique used to pack solar panels into satellites. They alternated all four droplet types in a precise pattern on a flat sheet that had been laser-scored in a Miura pattern. When pressure was applied, the sheet quickly folded together, stacking the cells in the right positions to generate electricity.

The team is working to improve the device’s efficiency.

Standardizing perovskite measurements
Researchers at EPFL proposed a way to standardize the measurements of perovskite solar cell stability. While perovskite solar cells have shown promising efficiencies, increasing their stability is key to make the technology commercially viable.

While research efforts are continuously made to improve perovskite stability, the team argues that a lack of measurement standards means results coming in from different laboratories and companies cannot be easily compared to each other.

To address this, the researchers investigated the effects of different environmental factors on the ageing of perovskite solar cells, looking at the impact of illumination (sunlight-level light), temperature, atmospheric, electrical load, and testing a systematic series of combinations of these.

“We designed and built a dedicated system to carry out this study,” said Konrad Domanksi of EPFL. “It is state-of-the-art for measuring stability of solar cells – we can vary light intensity over samples and control temperature, atmosphere etc. We load the samples, program the experiments, and the data is plotted automatically.”

The study showed how certain behaviors specific to perovskite solar cells can distort the results of experiments. For example, when the cells are left in the dark, they can recover some of the losses caused by illumination and “start fresh in the morning.” As solar cells naturally undergo day-night cycles, this has implications on how to define that a solar cell degrades in the first place.

“The work can lay the foundations for standardizing perovskite solar cell ageing,” said Wolfgang Tress of EPFL. “The field can use it to develop objective and comparable stability metrics, just like stabilized power is now used as a standard tool for assessing power-conversion efficiency in perovskite solar cells. More importantly, systematically isolating specific degradation factors will help us better understand degradation of perovskite solar cells and improve their lifetimes.”

Printing metal, flexible circuits
Researchers from North Carolina State University developed a technique, using multiple metals and substrates, for directly printing metal circuits to create flexible, stretchable electronics at a lower cost.

“Flexible electronics hold promise for use in many fields, but there are significant manufacturing costs involved – which poses a challenge in making them practical for commercial use,” said Jingyan Dong, associate professor at NC State. “Our approach should reduce cost and offer an efficient means of producing circuits with high resolution, making them viable for integrating into commercial devices.”

The technique uses electrohydrodynamic printing technology, which is used in many manufacturing processes that use functional inks. But instead of ink, the team used molten metal alloys with melting points as low as 60 degrees Celsius. The researchers have demonstrated their technique using three different alloys, printing on four different substrates: one glass, one paper and two stretchable polymers.

A prototype using the new technique for printing flexible, stretchable circuits. (Source: Jingyan Dong, North Carolina State University)

The researchers tested the resilience of the circuits on a polymer substrate and found that the circuit’s conductivity was unaffected even after being bent 1,000 times. The circuits were still electrically stable even when stretched to 70% of tensile strain.

The low melting point also meant it was also possible to ‘heal’ damaged circuits by heating the broken area to around 70 degrees Celsius.

The researchers demonstrated the functionality of the printing technique by creating a high-density touch sensor, fitting a 400-pixel array into one square centimeter.

The team is open to working with industry to implement the technique in manufacturing wearable sensors or other electronic devices.