Power/Performance Bits: Feb. 15

3D printed piezoelectrics
Researchers at University of Notre Dame and Purdue University developed a hybrid 3D printer that combines multi-material aerosol jet printing and extrusion printing, integrating both functional and structural materials into a single printing platform. They used it to create an all-printed piezoelectric wearable device.

The stretchable piezoelectric sensors conform to human skin and are made up of integrated tellurium nanowire piezoelectric materials, silver nanowire electrodes, and silicone films. The devices printed by the team were then attached to a human wrist, accurately detecting hand gestures, and to an individual’s neck, detecting the heartbeat. Neither device used an external power source.

The researchers said that printing piezoelectric devices is challenging because it often requires high electric fields for poling and high sintering temperatures. This adds to the time and cost of the printing process and can be detrimental to surrounding materials during sensor integration.

“The biggest advantage of our new hybrid printing method is the ability to integrate a wide range of functional and structural materials in one platform,” said Yanliang Zhang, associate professor of aerospace and mechanical engineering at the University of Notre Dame. “This streamlines the processes, reducing the time and energy needed to fabricate a device, while ensuring the performance of printed devices.”

Zhang said that the design relies on nanostructured materials with piezoelectric properties, which eliminate the need for poling or sintering, and the highly stretchable silver nanowire electrodes, which are important for wearable devices attached to bodies in motion. “We’re excited to see the wide range of opportunities that will open up for printed electronics and wearable devices because of this very versatile printing process,” said Zhang.

Stretchy perovskite LEDs
Researchers from Washington University in St. Louis and Florida State University propose a way to make flexible, stretchable displays that can be fabricated with inkjet printing.

The team aimed to combine the best properties of organic and inorganic LEDs. Organic LEDs (OLEDs) are cheap and flexible. “You can bend or stretch them — but they have relatively low performance and short lifetime,” said Chuan Wang, assistant professor in the Department of Electrical & Systems Engineering at WUSTL. “Inorganic LEDs such as microLEDs are high performing, super bright and very reliable, but not flexible and very expensive. What we have made is an organic-inorganic compound. It has the best of both worlds.”

They used organometal halide perovskite, a crystalline material usually produced using spin coating. However, that process results in a lot of waste. “Because it comes in a liquid form,” Wang said, “we imagined we could use an inkjet printer” in place of spin coating. The process is also faster, cutting fabrication time from more than five hours to less than 25 minutes.

It also allows for printing onto a variety of substrates that aren’t compatible with spin coating, such as rubber. “Imagine having a device that starts out the size of a cell phone but can be stretched to the size of a tablet,” Wang said.

However, perovskites aren’t flexible. To overcome this, the team embedded the inorganic perovskite crystals into an organic, polymer matrix made of polymer binders, making the perovskite LEDs (PeLEDs) elastic and stretchable. A polymer buffer was used to separate the perovskite layer from the electrodes and prevent them from mixing when printed.

The university has a pending patent on the technology and fabrication method.

Screen-printable electronic ink
Researchers at King Abdullah University of Science and Technology (KAUST) combined screen-printable composite and metallic inks to make foldable electronics that are cheaper to manufacture at industrial scales.

The composite ink is composed of ceramic particles dispersed in the polymer acrylonitrile-butadiene-styrene (ABS). The team used this ink to generate extremely flexible, large-area dielectric substrates with tunable lateral dimensions, thickness, and permittivity. They screen-printed the ink on to glass and, after drying, peeled off the substrates from the support. The substrates presented a minimum thickness of a few microns that could be increased through successive printing passes. They also exhibited a low dielectric loss at 28 gigahertz, which is suitable for 5G antennas.

The researchers screen-printed a silver nanowire-based ink on the dielectric substrates to build conductive patterns. The patterned films maintained high and stable electrical performance when rolled or folded into half, thanks to the polymer binder present in the ink. It also retained performance when incorporated into a four-layer circuit consisting of alternating metal-patterned and dielectric layers, suggesting that the screen-printable inks could be used in multilayer structures, such as multilayer printed circuit boards and automotive radars.

For proof of concept, the researchers screen-printed a flexible quasi-Yagi antenna on a dielectric substrate to show that the device performed well in the millimeter-wave band when bent or folded. “Our approach will be beneficial for novel 5G antennas and accelerate the implementation of 5G,” said Weiwei Li, a postdoc at KAUST.

Li said that both inks are compatible with roll-to-roll processing. “We expect fabrication costs to be extremely low, to the extent that the devices will become disposable,” said Atif Shamim, an associate professor of electrical and computer engineering at KAUST.