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Aug 07, 2023

Curvy electronics come a step closer to mass production with inspiration from candy wrappers

Imagine a baseball with a curved sensor on its leather hide that could tell pitchers every detail of their performance. Imagine a contact lens embedded with a flexible microchip that could read

Imagine a baseball with a curved sensor on its leather hide that could tell pitchers every detail of their performance. Imagine a contact lens embedded with a flexible microchip that could read glucose levels in the eye, so that diabetics wouldn’t have to prick their fingers.

These are just a couple of the applications that engineers are dreaming up for curved, flexible electronics. But up until now, there hasn’t been a viable method of mass production. That may change, thanks to a recent paper in Science Advances, which details an elegant method of wrapping traditionally two-dimensional electronics, mass-produced on flat, planar sheets, around curved surfaces.

“This is just like wrapping a candy sweet,” says co-senior author Xue Feng, a professor of engineering at Tsinghua University, in Beijing, China. The wrapper is a 2D plane, he explains, but it can deform to hug a round candy.

Most wearable electronics today—for instance, the processing chips in an Apple Watch—are embedded in flat silicon wafers (see Wearable tech meets tattoo art in a bid to revolutionize both). The body of the watch is flat. Early on in this recent work, co-senior author Ying Chen, an electrical engineer at the Institute of Flexible Electronics Technology of Tsinghua University at Zhejiang, tried to bend and flex silicon chips to wrap a curved surface. “But silicon is brittle and rigid and hard to deform,” she says. “It’s easy to fracture.”

Chen began looking for alternative solutions and came across a mathematics paper from 2009. It presented theoretical solutions for how to most efficiently wrap a round object in a flat sheet, avoiding the wrinkles that would waste material. Based on the equations in that paper, Chen and Feng used geometric modeling to design the most efficient shape for a 2D sheet that would easily wrap around a curve. Rather than a rectangle, like a piece of paper wrapping around a balloon, they found that the most efficient shape is similar to a pressed flower with petals that can close around a curved object. Instead of brittle silicon, they used stretchy elastomer, semiconducting materials, and metals.

The next question was how to physically wrap the petals around a curved object, so that they’d fall in exactly the same position for thousands of identical objects manufactured on a production line. “Alignment is important,” Chen says, because it may ultimately influence the product’s function.

At first, she was gently bending the petals onto round objects by hand, then peeling away a backing and using UV light to cure the electronics onto the curved surface. To automate the process, she and Feng designed a gentle press, in which the petal-shaped sheet and round object are both placed inside a thin tube, about a centimeter across, the width of a test tube. A balloon then inflates inside the tube, applying gentle and even pressure to the petals, so that they wrap onto the curved object.

Petals wrap around this sphere much more efficiently than would a rectangular sheet. Image credit: Xingye Chen, Xue Feng, Ying Chen

One of the key challenges in trying to manufacture curved sensors “is actually applying them to the object we care about,” says Tyler Ray, a professor of mechanical engineering at the University of Hawaiʻi at Mānoa. These authors devised a “clever, elegant method,” he says, of manufacturing sensors using traditional 2D methods and then adhering them to three-dimensional objects. Ray notes, however, that there are other ways to accomplish this task—for instance, using 3D printing methods, rather than the transfer printing suggested here. But the beauty of this method, he emphasizes, is that it relies on well-understood and well-controlled manufacturing techniques, while 3D printing would be more time-intensive and expensive per unit.

Biomedical engineer Philipp Gutruf says he thinks the scalable approach and design guidelines provided by this paper are a valuable addition to the literature. He has seen the general approach of wrapping 2D templates around 3D objects demonstrated in other papers—for example, wrapping the heart in a wireless device for control and monitoring in mice. But the scalable approach here is new, and the design guidelines for spherical shapes could be very useful, notes Gutruf, who’s at the University of Arizona in Tucson. Whether this work is applicable to other complex shapes, the sort required for biomedical devices, remains to be seen.

Proofing out the concept via mass production for industry is the next step, Feng says. He imagines ultrathin, curved sensors in smartphones and cameras. His group is currently working on curved sensors in contact lenses to detect blood glucose for diabetics. They hope to have results to share later this year.

For now, Feng says, this latest work offers “a simple and robust method to transfer curved chips to any surface.”

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