Research Bits: Jan. 9

Making stretchy semiconductors

Researchers from Pennsylvania State University, University of Houston, Purdue University, and Texas Heart Institute developed a new method to make soft, stretchable transistors easier and cheaper to manufacture.

The lateral phase separation induced micromesh (LPSM) process involves mixing a semiconductor and an elastomer and spin coating the liquid mixture precursors to fabricate rubbery semiconductor thin films. The spin-coated film automatically triggers a mechanism called lateral phase separation, which generates micromesh structures within seconds.

Resembling a basket weave, these micromesh structures allow the material to stretch. “The LPSM films used to create the stretchy semiconductors promise simultaneous efficient charge transport and mechanical stretchability,” said Cunjiang Yu, associate professor of engineering science and mechanics and associate professor of biomedical engineering and of materials science and engineering at Penn State.

The LPSM method was used to create both p-type and n-type semiconductors. Yu said the team used these to create soft electronic devices such as transistors, inverters, and photodetectors that can stretch to a large extent while maintaining functionality. In addition, the researchers created a rubbery bioelectronic device called an epicardial patch and implanted it in a rodent.

“As the rat’s heart expanded and contracted with its heartbeat, the entirely rubber-based epicardial patch also moved with it,” Yu said. “We recorded multiple channels of electrophysiology readings simultaneously with the patch. Recording at multiple sites of the heart is important to identify cardiac problems such as arrhythmia.”

Going forward, researchers hope to further optimize the LPSM process and to investigate the detailed properties of the semiconductor materials, according to Yu. They also plan to employ the LPSM semiconducting thin film in various high-performance integrated electronics and functional systems.

Embroidering power-generating yarns

Researchers at North Carolina State University, Ocean University of China, South China University of Technology, and Hong Kong Polytechnic University developed power-generating yarns onto fabric that was used to embed a self-powered, numerical touchpad and movement sensors into clothing.

“Our technique uses embroidery, which is pretty simple – you can stitch our yarns directly on the fabric,” said Rong Yin, assistant professor of textile engineering, chemistry and science at North Carolina State University. “During fabric production, you don’t need to consider anything about the wearable devices. You can integrate the power-generating yarns after the clothing item has been made. This is a low-cost method for making wearable electronics using commercially available products.”

Yu Chen, graduate student at NC State, demonstrates embroidery techniques. (Credit: NC State)

To make the yarns durable enough to withstand the tension and bending of the embroidery stitching process, they used five commercially available copper wires, which had a thin polyurethane coating, together. Then, they stitched them onto cotton fabric with PTFE.

The power is generated using the triboelectric effect. “In our design, you have two layers – one is your conductive, polyurethane-coated copper wires, and the other is PTFE, and they have a gap between them,” Yin said. “When the two non-conductive materials come into contact with each other, one material will lose some electrons, and some will get some electrons. When you link them together, there will be a current.”

Researchers tested their yarns as motion sensors by embroidering them with the PTFE fabric on denim. They placed the embroidery patches on the palm, under the arm, at the elbow and at the knee to track electrical signals generated as a person moves. They also attached fabric with their embroidery on the insole of a shoe to test its use as a pedometer, finding their electrical signals varied depending on whether the person was walking, running or jumping.

“The electrical properties of our prototypes were comparable to other designs that relied on the same power generation mechanism. You can embroider our yarns onto clothes, and when you move, it generates an electrical signal, and those signals can be used as a sensor,” Yin said. “When we put the embroidery in a shoe, if you are running, it generates a higher voltage than if you were just walking. When we stitched numbers onto fabric, and press them, it generates a different voltage for each number. It could be used as an interface.”

After hand washing and rinsing the embroidery with detergent, and drying it in an oven, they found no difference or a slight increase in voltage. They also found that there was no significant change in electrical output performance of their designs after 10,000 rubbing cycles in an abrasion test.

“The next step is to integrate these sensors into a wearable system,” Yin said.