Kyungjin Kim stretches a strip of the novel conductive elastomer film. Photo: Kyungjin Kim.
Kyungjin Kim stretches a strip of the novel conductive elastomer film. Photo: Kyungjin Kim.

Semiconductors are moving away from rigid substrates that are cut or formed into thin discs or wafers to more flexible plastic material and even paper thanks to new material and fabrication discoveries. The trend toward more flexible substrates has led to the fabrication of numerous devices, from light-emitting diodes to solar cells to transistors.

Now, researchers at Georgia Institute of Technology (Georgia Tech) have created a photodetector that acts like a second skin layer and can be stretched by up to 200% without significantly losing its electric current. Developed over three years by researchers from both the mechanical and computing engineering labs at Georgia Tech, the photodetector is made from a synthetic polymer and an elastomer that absorbs light to produce an electrical current.

Photodetectors today are used as wearables for health monitoring, such as rigid fingertip pulse oximeter reading devices. They convert light signals into electrical ones and are commonly used on wearable electronics. According to the researchers, the new soft, flexible photodetectors could enhance the utility of medical wearable sensors and implantable devices, among other applications. They report their work in a paper in Science Advances.

Given that conventional flexible semiconductors break under a few percentages of strain, the Georgia Tech findings are “an order-of-magnitude improvement”, said Olivier Pierron, professor in the George W. Woodruff School of Mechanical Engineering, whose lab measures the mechanical properties and reliability of flexible electronics under extreme conditions.

“Think of a rubber band or something that's soft and stretchable like human skin yet has similar electrical semiconducting properties of solid or rigid semiconductors,” said Canek Fuentes-Hernandez, formerly in the School of Electrical and Computer Engineering (ECE) at Georgia Tech and now an associate professor in Electrical and Computer Engineering at Northeastern University. “We’ve shown that you can build stretchability into semiconductors that retains the electrical performance needed to detect light levels that are around hundred million times fainter than produced by a light bulb used for indoor illumination.”

Bernard Kippelen, vice provost for international initiatives and an ECE professor, oversaw the work of Youngrak Park, the study’s first author and a PhD candidate in ECE. Following two-and-a-half years of research, Park uncovered the right combination of chemical compounds for producing a super-soft material with the ability to generate and conduct electricity when exposed to light.

Park found the perfect ratio for ensuring all parts of the semiconductor layer maintain high performance in the photodetector. But it was painstaking work to prove the materials’ stretchability, especially given that a single layer was 1000 times thinner than a human hair.

Park relied on Kyungjin Kim, then a Georgia Tech mechanical engineering PhD student, to test the material’s reliability. He continued to provide Kim with larger, thicker samples until one with a thickness of 500nm worked.

“It was still super thin. Under dry conditions, it would just crumble. We had to use a water reservoir to keep its shape,” recalled Kim, now an assistant professor in the University of Connecticut’s Department of Mechanical Engineering.

Elaborating on how difficult it was to measure pure mechanical properties of a photoactive layer, Pierron said: “Electronic devices are very brittle typically, which is okay with conventional devices fabricated on rigid substrates. But as soon as you use soft substrates that becomes an issue.” The water acted like plastic wrap, keeping the thin films in place without crumbling or losing shape and allowing the researchers to stretch the material and measure its mechanical properties.

To test for electrical signals coming out of the device under illumination, electronic terminals had to be embedded in it. Yet those terminals had to be deformable too, or the entire device would become rigid.

“Fabricating stretchable electronic terminals was a major challenge in and of itself,” said ECE PhD graduate Felipe Andres Larrain, who worked closely with Park and focused on the embedded components. He is now an assistant professor at Adolfo Ibáñez University in Chile.

While this breakthrough material has been initially integrated into a photodetector and tested for electrical functionality, more testing and optimization is needed to show the materials’ stretchability under multimodal loads and its shelf stability.

“What's exciting is what these materials and the devices will enable us to develop – namely, the concept of intelligence systems. You have functional surfaces that combine sensors that monitor all kinds of physical properties,” said Samuel Graham, former chair of the Woodruff School of Mechanical Engineering and now Dean of Engineering at the University of Maryland.

“This is a very good example of interdisciplinary research – none of this work would have been possible without the collaboration between electrical and mechanical engineers,” Kippelen said. “In the lab we didn’t have any prior experience with stretchable materials. Figuring out how to measure this took a lot of perseverance, creativity and hard work.”

The researchers are most excited about the potential of the material for enhancing medical wearables. Typically, wristwatches that use rigid biosensors have limitations, since flexing the wrist can completely change the sensor’s measurements. They are subject to 'motion artifact', or degraded image quality, caused when a person moves.

“Moving around can drastically affect the usability of collected data but being able to reposition devices on the body to minimize or eliminate motion artifact is a big deal,” noted Gabriel Cahn, a project manager for Huxley Medical, a biosensor start-up in Atlanta, who recently graduated from Georgia Tech with a doctorate in flexible electronics. “Having electronics that can flex, twist, bend and conform to non-flat surfaces and move with your body will allow you to place these sensors in more advantageous places to collect biometric data. It will be infinitely more useful in helping diagnose or monitor existing medical illnesses.”

The research team also foresees rich applications for the soft and stretchable polymer blend beyond wearables for health monitoring. “The soft device also could be attractive for implantable electronics for bio-electronic applications, since the interfaces comply with the dynamic motion of the soft biological tissues, reducing the foreign body reaction,” said Kim.

“The potential is fantastic,” added Larrain. “In the long-term, you could develop sensors that could enhance or even replace the human eye or be applied to robotic eyes.”

Fuentes also sees the material working in smart agriculture applications, where farmers could attach light sensors on fruits or other produce to monitor growth, disease and to better time harvesting. Kippelen believes rubber-like photodiodes that detect ultralow light levels could find applications in detecting, identifying and characterizing ionizing radiation for nuclear fuel cycle monitoring.

This story is adapted from material from Georgia Tech, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.