Researchers at Brown’s School of Engineering uncovered a surprising weakness in flexible electronics and a way to prevent it.
The team, led by Professor of Engineering Nitin Padture, found that the plastic layers at the base of flexible devices like foldable phones and wearable sensors can develop microscopic cracks when bent repeatedly. Their findings, published in npj Flexible Electronics, could help engineers design longer-lasting bendable technology.
The researchers were initially studying perovskite solar cells — a type of ultra-thin, flexible solar technology — when they noticed something unexpected. Using a focused ion beam to examine samples that had faced repeated bending, postdoctoral researcher and first author Anush Ranka found that the soft plastic base layer beneath the device’s brittle, ceramic-like coating was cracking.
This observation challenged the assumption held since the 1970s that the soft plastic layer is tough enough to deform but not tear, and that it was likely not the root of material failure.
“People are measuring the toughness of the plastic as 10,000 times tougher than a ceramic coating of a film,” Padture said, adding that this assumption was predicted by models from the 1970s.
Associate Professor of Engineering Haneesh Kesari, a co-author on the study, added that before Padture’s work, he had not seen “any experimental evidence that this is exactly how cracks in materials … behave.”
The team was aided by the expertise of Kesari, a theorist who studies how materials deform, as they modeled the mechanical stresses that occur when flexible devices bend. Their analysis showed that a stiffness mismatch between the brittle top layer and the softer plastic base amplifies strain on the plastic layer, also called the substrate, and creates cracks
“Without that understanding, you would not have been able to figure out a way to mitigate the cracking,” Padture said.
To fix this problem, the researchers inserted a thin interlayer — a flexible film that sits between the two materials. This extra layer absorbs the stress and stops the cracks from spreading to the bottom polymer layer. In lab tests, devices with the interlayer were able to maintain stable electrical performance following more than 10,000 bending cycles — a property not previously seen without the interlayer.
“This kind of synergy between theory and experiment doesn’t happen every day,” Kesari said. “We identified the problem, proved why it happens and found a way to prevent it.
This discovery has far-reaching implications for flexible electronics, a growing field that includes foldable smartphones, wearable biomedical sensors and bendable solar panels.
“This is a small material fix with a big real-world payoff,” Ranka said in an email to The Herald. “Anyone who uses flexible devices, now or in the future, could benefit from a longer-lasting product.”
The more a device can change to “shape to its natural environment, the more functionality you can pull out from it,” Kesari said.
Wearable inertial brain sensors, which are used to improve the treatment of traumatic brain injury, is one possible application of the new technology. Engineers may even be able to integrate electronics into gloves or even skin, allowing observers to detect “what’s happening in the head,” Kesari said.
By preventing substrate layer cracks, the flexible interlayer method could make these technologies last longer and perform more reliably.
This study was conducted in collaboration with researchers from Yale and the University of Rome Tor Vergata. The team has since filed a patent for their interlayer design and hopes that the discovery will guide the next generation of flexible electronic materials.
