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Science Translational Medicine  26 Jul 2017:
Vol. 9, Issue 400, eaao0975
DOI: 10.1126/scitranslmed.aao0975

Abstract

Nanomesh fabrication creates breathable, stretchable, comfortable, inflammation-free, on-skin conductors without residual polymeric support for wearable biosensors.

The microelectronics revolution was made possible by planar fabrication techniques that allowed transistors and wires to be densely integrated into the rigid surface of a silicon wafer. To enable wearable flexible electronics for ambulatory monitoring of health and disease, the silicon substrate was dramatically thinned and embedded in an elastomeric polymer to tolerate large mechanical deformation while retaining functionality. Now, Miyamoto and colleagues introduce a new fabrication strategy that allows nanomesh conductors to be fabricated directly on the skin without residual polymeric support.

The approach begins with electrospinning water-soluble polyvinyl alcohol (PVA) nanofibers (~300 to 500 μm) into an intertwined mesh sheet. A thin layer of electrically conductive gold (~100 nm) is then evaporated through a shadow mask (stencil) to conformally coat the nanofibers in the desired wire pattern. After applying the nanofiber mesh to the skin, a brief water spray dissolves the polymer, leaving behind the conductive coating, which collapses into a nanomesh conductor that can traverse the steep curvatures and fine ridges of the finger-tip without occluding pores in the skin. In contrast to elastomer-supported flexible electronics, the nanomesh conductors were breathable and provided no measurable impediments to water vapor transport. Nevertheless, they exhibited predictable electrical properties, with resistance proportional to length and inversely proportional to width as well as strain-dependent conductance for mechanical sensing (15% to 40% strain increase resulted in 4-fold conductance decline). Repeated flexion-extension testing on a human finger demonstrated excellent cycle-to-cycle reproducibility with no mechanical failures, although resistance gradually increased by 2.7-fold over 10,000 cycles. To evaluate biological effects and patient comfort, 1 cm2 skin patches of nanomesh were compared with parylene and silicone in 20 healthy human volunteers. After one week, nanomesh conductors received superior subjective comfort scores and showed no evidence of contact dermatitis although one subject discontinued the study due to a gold allergy. To demonstrate sensing applications, the nanomesh on-skin electrodes were connected with a fingerless glove with backend electronics to successfully measure low frequency touch, temperature, and pressure signals as well as the high frequency electromyograms.

The applications demonstrated in this work leveraged the breathable, inflammation-free, on-skin nanomesh conductors at the sensing surface, but still required conventional electronics for signal conditioning and transmission. Future work will likely expand the repertoire of polymer-free biosensors and explore strategies for seamlessly integrating them with silicon-based flexible electronics to provide long-term, high functionality, comfy wirewear.

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