New ultra-thin 'skin' for flexible electronics promises advancements for medical devices and prosthetics
(Nanowerk Spotlight) Electronic devices intricately mounted onto human skin have long captivated scientific imagination, tantalizing specialists by promising a seamless fusion of man and machine. Such skin-hugging circuitry could unlock medical sensors of unprecedented acuity or bionic limbs wired to the nervous system. But despite decades of diligent research, inadequate materials have obstructed progress towards this visionary goal.
Now a team of Chinese materials scientists report a breakthrough in fabricating tissue-thin films ideal for mounting flexible electronics. Publishing in Advanced Materials ("Aramid Nanodielectrics for Ultraconformal Transparent Electronic Skins"), researchers from Nankai University detail their tailored “aramid nanodielectric” polymer layers - dubbed ANDs - which uniquely combine nanoscale thickness and wing-like flexibility with hardy resiliency to thermal and chemical stresses that would ruin more fragile films.
At just 100 nanometers thick – thousands of times thinner than a human hair – the AND films combine high strength with easy adherence to skin. They are also transparent, breathable, and can withstand temperatures exceeding 300 °C.
Demonstration of ultraconformable electronics based on ANDs. a) The hierarchical structure of aramid fibers. b) Schematic illustrations of the fabrication and self-delamination process of the devices. c) Photograph of the devices on ANDs fabricated on a 3-inch silicon wafer. d) Photograph showing the self-delamination process of the ANDs. e,f) Photographs of the ultraconformal electronics based on ANDs transferred onto e) human skin and f) a leaf. (Reprinted with permission by Wiley-VCH Verlag)
“For emerging applications like bioelectronics, conformal contact with the local surface allows precision signal acquisition and comfort,” explains senior author Dr. Jian Zhu, materials scientist at Nankai University. “Our AND films can enable skin-like electronics to intimately adapt to the intricate topological texture of human skin.”
The vision of electronics seamlessly fusing with human physiology has fueled ardent research for decades. Also dubbed ‘epidermal electronics’, the concept burst into popular awareness in 2011 when biomedical engineer John Rogers at the University of Illinois unveiled “epidermal temporary transfer tattoo” devices that stick directly onto skin, sensing motion and heart rate. But such early designs faced limitations like potential skin irritation during long-term use.
The crux challenge has been devising adequately thin yet durable materials that can support electronic components while flexing imperceptibly on skin. Elastomers provide stretchability but minimal breathability. Polymer films like parylene excel as insulators but prove cumbersome to handle at nanoscale. Other options like polyimide require complex chemical processing using unstable sacrificial layers.
The Nankai researchers pursued an unconventional approach, deriving their nanodielectric from aramid polymer, the same hardy material used in Kevlar bulletproof vests. They term the films ANDs.
“Our aramid nanodielectric holds great promise as dielectrics for future skin-like electronics”, says Zhu.
The team exfoliates aramid into nanofibers just 10 nanometers wide, spin coating them into films of tailored thickness. The remarkably strong intermolecular bonding within the polymer chains enhances adhesion and heat resistance while keeping layers wafer-thin. ANDs can also be transparent and permeable to water vapor, making them ideal substrates for transdermal biosensing patches.
“AND films are mechanically strong, yet compliant, smooth and transparent” says Zhu. “They are also chemically inert, thermally stable up to 300 –C, and self-detachable from processing substrates without etching solvents.”
To demonstrate real-world viability, the researchers built two very different devices using AND films. One is sweat-immune electrodes for precision monitoring of electrophysiological signals like electromyography and electrocardiography. The other is flexible field effect transistor microprocessors thin enough to wrap around human hairs without performance loss.
The skin-mounted silver nanowire electrodes with AND films successfully recorded muscle and cardiac activity even when test subjects sweated profusely or showered. Conventional gel electrodes failed under such conditions as sweat degrades conductivity. The researchers attribute superior performance to AND breathability preventing sweat buildup beneath detectors. Quantitative measurements showed the 100 nm AND films to have a water vapor transmission rate exceeding 90% of open air exposure.
Meanwhile, the hair-hugging field effect transistors, with AND films serving simultaneously as gate dielectric and substrate, showed excellent electronic properties like low-voltage operation down to just 4V, mobilities of 40 cm2/Vs, and on/off ratios exceeding 100,000. And they endured extreme bending to circumferences smaller than a human hair without apparent damage or performance decline.
“The ultraflexible transistors can function properly when wrapped around human hair without any degradation in performance,” notes Zhu.
Zhu asserts that most existing skin electronics rely on nonstandard nanofabrication techniques too bespoke for large-scale manufacturing, essential for mainstream medical adoption. But the amenability of AND films to conventional lithographic workflows overcomes this obstacle.
“Our wafer-scale approach enables mass production and integration with modern electronics fabrication methods,” he explains.
Though concurring more work remains before patients benefit from the advancements promised by flexible electronics, Zhu strikes a tone of tempered optimism.
“We foresee aramid nanodielectrics transforming next generation miniaturized medical devices and bionics,” he says. “Bringing this technology from the realm of science fiction towards clinical reality could profoundly impact human health.