Electronics
Grayscale optical images (left) and simulation results (right) showing how different confi gurations affected the level of buckling, marked by the colours and the R/ (resistance vs resistivity) value.
sensors, convincing the medical community of the benefits. But while none of these barriers are easily surmountable, doing so is dependent on something much simpler – putting the devices on the body. Nanshu Lu, a professor in the Department of Aerospace Engineering and Engineering Mechanics at the University of Texas at Austin, explains why this seemingly simple conundrum is anything but simple: “Conventional electronics are based on silicon wafers, which are flat and rigid. But the human body has exactly the opposite mechanical properties. It’s very curvilinear and soft.
“The limitation of fl exible devices is that they are not able to conform to 3D curvilinear surfaces. They are not able to wrap around a spherical surface like the eyeball or an arbitrary surface like the heart.”
Nanshu Lu
This means there’s an intrinsic mismatch between the two.” To better understand the limitations of flexible electronics, Lu points to the most recognisable example – foldable smart phones. “They are based on flexible LEDs and circuits, but the limitation of flexible devices is that they are not able to conform to 3D curvilinear surfaces,” she says. In the smartphone example, flexible electronics are able to wrap around a cylinder because they’re able to bend. But, Lu adds, “They are not able to wrap around a spherical surface like the eyeball or an arbitrary surface like the heart.”
Flexible versus stretchable It’s worth mentioning that researchers have demonstrated bioelectronic capabilities using
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materials able to adhere to the surfaces of the human body. But these, as Lu explains, are examples of “stretchable” not “flexible” electronics. “Flexible electronics, although well industrialised and very mature in terms of materials and fabrication, are still not fully ready for body conformable electronics,” she says. “On the other hand, stretchable electronics are able to fully conform to 3D curvilinear and arbitrary surfaces, because they can stretch in addition to bending, but their industrialisation is still ongoing and there’s a lot of challenges in fabricating them at scale.” Another key variable pertains to exactly what you can do with each of these categories of device. Stretchable electronics open up possibilities like muscular strain sensing, for instance, but to engineer such a device requires an appropriate amount of space between the components. “There are some special applications where you need a very high density of electronics over a very limited area,” explains Lu. In this case, components need to be tightly packed together, which means adding stretchable interconnects isn’t an option. Lu and her collaborator, Ying Li, an associate professor of mechanical engineering at the University of Wisconsin–Madison, joined forces to answer the question of whether it was possible to design a device with all the benefits of a flexible electronic, but could conform to 3D curvilinear surfaces on the body. There are examples in the literature of attempts to do this, Lu says, “but those are from empirical inspirations,” with examples including the truncated icosahedron design of a soccer ball and the petals of a flower. Both having a background in mechanics and the use of mathematical modelling tools, Lu and Li wanted a more objective solution. To provide a further impetus, in a separate project, Lu and her colleague Dae-Hyeong Kim from Seoul National University designed an ultrathin artificial retina to treat damaged retinas, but were grappling with how to fit it around the eye. “We need thousands of photodetectors over a 1cm diameter spherical cap,” she says. “In that case, we need to have very densely packed active electronics. Flexible electronics for a high-density artificial retina is the most viable approach, but a retina is curvilinear and flexible electronics manufactured on wafers are planar.” Lu compares the ultra-thin material to a piece of paper to explain how attempting to wrap a flexible sheet around a curvilinear surface like the eyeball results in “buckling”, seen as wrinkling in the paper example. This, she says, is due to the extra material, which can be cut away to improve conformability. “But without a model guiding us, we don’t know how
Medical Device Developments /
www.nsmedicaldevices.com
Conformability of fl exible sheets on spherical surfaces DOI:10.1126/
sciadv.adf2709
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