The importance of monitoring the condition of skin is increasing as its relevance to health is becoming more well understood. Inappropriate humidity levels can cause atopic dermatitis or hair loss. However, conventional film substrates used in electronic skin monitoring devices cause accumulation of sweat or gas between the device and biological tissue, leading to negative effects in long-term humidity measurements. Thus, real-time measurements of skin humidity over long periods are difficult using conventional film devices. Here, a breathable nanomesh humidity sensor that can monitor skin humidity for a long time is developed by using biocompatible materials such as gold, poly(vinyl alcohol), and Parylene C. The sensor presents excellent gas and sweat permeability and precisely detects the humidity level of an object for a long time. This study demonstrates the successful real-time detection of the humidity level from human skin and also detects the relative humidity of a plant surface over a prolonged period. This sensor is expected to have wide applicability for cultivating delicate plants as well as to reveal correlations between skin humidity and disease for biomedical applications.

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Nanofiber-based electronic devices have attracted considerable interest owing to their conformal integration on complicated surfaces, flexibility, and sweat permeability. However, building complicated electronics on nanomesh structure has not been successful because of their inferior mechanical properties and processability. This limits their practical application. To achieve systemlevel device applications, organic field-effect transistors are one of the key components to be integrated with various sensors. Herein, a successful method for fabricating a biocompatible, ultrathin (≈1.5 µm), lightweight (1.85 g m -2 ), and mechanically durable all-nanofiber-based organic transistor is reported that can be in conformal contact with curved skin. Furthermore, it is the first development with a substrate-less nanomesh organic field effect transistor. The devices exhibit satisfactory electrical performance, including an on/off value of 3.02 × 10 4 ± 0.9 × 10 4 , saturation mobility of 0.05 ± 0.02 cm 2 V − 1 s − 1 , subthreshold slope of 1.7 ± 0.2 V dec -1 , and threshold voltage of −6 ± 0.5 V. The mechanism of crack initiation is analyzed, via simulation, to understand the deformation of the nanomesh transistors. Furthermore, active matrix integrated tactile sensors entirely on the nanomeshes is successfully demonstrated, indicating their potential applicability in the field of biomedical electronics.

Conformal integration of an epidermal device with the skin, as well as sweat and air permeability, are crucial to reduce stress on biological tissues. Nanofiber-based porous mesh structures (breathable devices) are commonly utilized to prevent skin problems. Noble metals are normally deposited on nanomesh substrates to form breathable electrodes. However, these are expensive and require high-vacuum processes involving time-consuming multistep procedures. Organic materials are suitable alternatives that can be simply processed in solution. We report a simple, cost-effective, mechanically biocompatible, and breathable organic epidermal electrode for biometric devices. Poly­(3,4-ethylenedioxythiophene):poly­(styrene sulfonate) (PEDOT:PSS) is sprayed on a nanofiber-mesh structure, treated using only heat and water to enhance its biocompatibility and conductivity, and used as the electrode. The treatment is accomplished using an autoclave, simultaneously reducing the electrical resistance and sterilizing the electrode for practical use. This research can lead to affordable and biocompatible epidermal electrodes with improved suitability for various biomedical applications.

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