Nanomesh on-skin electronics

In many clinical diagnostics, skin serves as a measurement window for quantitative assessments of physiological health. Prominent examples are in biopotential recordings that yield insights into cardiovascular activity, skeletal muscle behaviour and brain function through electrocardiograms, electromyograms and electroencephalograms, respectively. Signals in the latter two cases can also serve as the basis of systems for human–machine control interfaces. Standard skin electrodes for such recordings rely on bulky, pasteon conductive pads with hard-wired connections to separate data acquisition electronics. Although suitable for use in hospital and laboratory settings, such approaches are incompatible with continuous, long-term monitoring at home, in a way that does not disrupt routine daily activities. Now, writing in Nature Nanotechnology, Miyamoto et al.1 describe a multilayered, open-mesh network of hollow metallic nanofilaments constructed in a manner that allows their direct integration with the soft surface of the skin. Ultrathin electrical interfaces formed in this manner enable nearly unimpeded transepidermal water loss and air permeation, without mechanical constraint or dermatological irritation, even for chronic operation in electrophysiological recordings and other forms of electronic sensing at the skin interface. The results represent valuable contributions to the broader field of skinintegrated electronic technologies2,3, as next-generation wearables with capabilities that greatly exceed those of traditional wristband-mounted devices. The authors fabricate their nanomesh constructs by first electrospinning a watersoluble polymer, polyvinylalcohol (PVA), to form a spaghetti-like, multilayer entanglement of fibres with diameters of 300–500 nm. Coating this nanostructure with a thin layer of gold, transferring the resulting film to the surface of the skin, and then removing the PVA by gently washing it away with water completes the process. The resulting conductive, skin-integrated gold nanomesh consists of an interconnected network of hollow, metallic half-cylinders with open spaces in between, where adhesion to the skin is hypothesized to involve an ultrathin (a few tens of nanometres) residual layer of PVA at the interface. The sparse area coverage and the thin geometry combine to yield exceptionally low bending stiffness, such that capillary effects associated with the drying of water following dissolution of the PVA are sufficient to pull the structure into perfect, conformal contact with the textured surface of the skin. Figure 1 shows micrographs associated with experiments on human skin and skin replicas. By comparison to previously reported microscale, two-dimensional metallic mesh structures deterministically formed by lithographic processes and transferred to the skin using elastomeric stamps4, the nanoscale networks introduced here offer improved mechanics, enhanced air/water permeability and superior ability to conform to curved surfaces. Detailed dermatological studies reveal a complete absence of redness, irritation or other adverse reactions of the skin. Certain aspects of skin compatibility follow from compliant mechanical properties associated with the nanomesh architectures and the hollow half-cylinder geometries of the constituent filaments, as demonstrated by measurements of conductivity in stretched and unstretched states. Data suggest optimal deformability for samples that are first transformed, by controlled buckling mechanics5,6, into ‘wavy’ shapes. Here, Figure 1 | Skin-interfaced nanomesh electronics. a, Image of patterned conductive traces formed from gold nanomesh structures and configured along the length of a finger, extending to its tip. b, Magnified image of similar traces on the fingertip, illustrating their ability to conform to the fingerprint ridge patterns. Scale bar, 1 mm. c, Scanning electron micrograph of the mesh architecture. Scale bar, 5 μm. d, A schematic illustrating the hollow, half-cylindrical gold nanofilament constituents of the mesh. Reproduced from ref. 1, Macmillan Publishers Ltd. a b