Power/Performance Bits: Sept. 22

Drawing sensors on skin
Researchers from the University of Houston and University of Chicago created an ink pen that can draw multifunctional sensors and circuits directly on skin.

These “drawn-on-skin electronics” aim to provide more precise health data, free of the artifacts that are associated with wearable devices and flexible electronic patches. Caused when the sensor doesn’t move precisely with the skin, these artifacts can lead to incorrect readings of things like heart readings or temperature.

“It is applied like you would use a pen to write on a piece of paper,” said Cunjiang Yu, associate professor of mechanical engineering at the University of Houston. “We prepare several electronic materials and then use pens to dispense them. Coming out, it is liquid. But like ink on paper, it dries very quickly.”

A new form of electronics known as “drawn-on-skin electronics” allows multifunctional sensors and circuits to be drawn on the skin with an ink pen. (Source: University of Houston)

The drawn-on-skin electronics can be customized to collect different types of information, and Yu said it is expected to be especially useful in situations where it’s not possible to access sophisticated equipment.

The drawn-on-skin electronics are comprised of three inks, serving as a conductor, semiconductor and dielectric. “Electronic inks, including conductors, semiconductors, and dielectrics, are drawn on-demand in a freeform manner to develop devices, such as transistors, strain sensors, temperature sensors, heaters, skin hydration sensors, and electrophysiological sensors,” the researchers wrote.

The electronics are able to track muscle signals, heart rate, temperature and skin hydration, among other physical data, he said. The researchers also reported that the drawn-on-skin electronics have demonstrated the ability to accelerate healing of wounds.

Single-crystal perovskite fabrication
Researchers at University of California San Diego, Texas A&M University, Los Alamos National Laboratory, Tsinghua University, and Shenzhen University found a way to fabricate flexible single-crystal perovskite thin films. Compared to the standard polycrystalline perovkites used in solar cells and optical devices, the single-crystal films showed fewer defects, greater efficiency, and enhanced stability.

“Our goal was to overcome the challenges in realizing single-crystal perovskite devices”, said Yusheng Lei, a nanoengineering graduate student at UCSD. “Our method is the first that can precisely control the growth and fabrication of single-crystal devices with high efficiency. The method doesn’t require fancy equipment or techniques–the whole process is based on traditional semiconductor fabrication, further indicating its compatibility with existing industrial procedures.”

“Modern electronics such as your cell phone, computers, and satellites are based on single-crystal thin films of materials such as silicon, gallium nitride, and gallium arsenide,” added Sheng Xu, a professor at UCSD. “Single crystals have less defects, and therefore better electronic transport performance, than polycrystals. These materials have to be in thin films for integration with other components of the device, and that integration process should be scalable, low cost, and ideally compatible with the existing industrial standards. That had been a challenge with perovskites.”

The team was able to use standard lithography processes by adding a polymer protection layer to the perovskites followed by dry etching of the protection layer during fabrication. In the new work, the researchers were able to control the growth of the perovskites at the single crystal level by designing a lithography mask pattern that allows control in both lateral and vertical dimensions.

Using lithography, the team etched a mask pattern on a substrate of hybrid perovskite bulk crystal. The mask pattern controls the growth of the perovskites at the single crystal in both lateral and vertical dimensions.

A single-crystal thin perovskite film during the transfer process. (Source: Yusheng Lei / UC San Diego)

The single-crystal layer is then peeled off the bulk crystal substrate and transferred to an arbitrary substrate while maintaining its form and adhesion to the substrate. A lead-tin mixture with gradually changing composition is applied to the growth solution, creating a continuously graded electronic bandgap of the single-crystal thin film.

The method allowed the researchers to fabricate single-crystal thin films up to 5.5 cm by 5.5 cm squares, while having control over the thickness of the single-crystal perovskite from 600 nanometers to 100 microns.

“Further simplifying the fabrication process and improving the transfer yield are urgent issues we’re working on,” said Xu. “Alternatively, if we can replace the pattern mask with functional carrier transport layers to avoid the transfer step, the whole fabrication yield can be largely improved.”

Lithium-metal electrolyte
Researchers from Lawrence Berkeley National Laboratory and Carnegie Mellon University built a soft, solid electrolyte for lithium-metal batteries that prevents the formation of dendrites.

Dendrites are thin, metallic whiskers that can penetrate a battery’s electrolyte and cause short circuits or fires. Lithium metal batteries are promising for their high capacity, but they are prone to dendrite formation. Solid electrolytes have been investigated as a way to stop dendrites, but without stopping formation in the first place, tiny cracks form and spread on the electrolyte.

Instead, the researchers developed a new class of soft, solid electrolytes that are made from both polymers and ceramics. The new material is able to suppress dendrites in that early nucleation stage, before they can propagate and cause the battery to fail.

“Our dendrite-suppressing technology has exciting implications for the battery industry,” said Brett Helms, a staff scientist in Berkeley Lab’s Molecular Foundry. “With it, battery manufacturers can produce safer lithium metal batteries with both high energy density and a long cycle life.”

Key to the design of these new soft, solid-electrolytes was the use of soft polymers of intrinsic microporosity, or PIMs, whose pores were filled with nanosized ceramic particles. Because the electrolyte remains a flexible, soft, solid material, battery manufacturers will be able to manufacture rolls of lithium foils with the electrolyte as a laminate between the anode and the battery separator. These lithium-electrode sub-assemblies, or LESAs, are drop-in replacements for the conventional graphite anode, allowing battery manufacturers to use their existing assembly lines, Helms said.

The team used X-rays at Berkeley Lab’s Advanced Light Source to create 3D images of the interface between lithium metal and the electrolyte, and to visualize lithium plating and stripping for up to 16 hours at high current. Continuously smooth growth of lithium was observed when the new PIM composite electrolyte was present, while in its absence the interface showed telltale signs of the early stages of dendritic growth.

“In 2017, when the conventional wisdom was that you need a hard electrolyte, we proposed that a new dendrite suppression mechanism is possible with a soft solid electrolyte,” said co-author Venkat Viswanathan, an associate professor of mechanical engineering and faculty fellow at Scott Institute for Energy Innovation at Carnegie Mellon University. “It is amazing to find a material realization of this approach with PIM composites.”

An awardee under the ARPA-E IONICS program, 24M Technologies, has integrated these materials into larger format batteries for both EVs and electric vertical takeoff and landing aircraft.