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Monitoring the physiological signals of people by using wearable sensors attached to the skin while they go about their daily routine is a practical approach to prevent diseases and monitor people's health status. However, conventional monitoring devices are bulky, their signals are corrupted by the noise generated by external sources, and they are prone to mechanical failure, which hinder their practical use. Herein, a mechanically durable biomaterial-based arterial pulse thin-film sensor is developed for real-time monitoring of cyclic physiological signals. A hierarchical layered material (rGO/SF/CNC) comprising reduced graphene oxide (rGO), silk fibroin (SF), and cellulose nanocrystals (CNCs) is fabricated using the spin-assisted layer-by-layer (SA-LbL) technique and thermal reduction. Thermal reduction is performed to tailor the defects in rGO, which significantly enhances its mechanical robustness and bending properties, as well as in-plane stress sensitivity, owing to a change in conductivity. The thin-film (thickness: 40.9 nm) sensor has a rapid response time (69 ms) and excellent durability exceeding 8000 cycles. Moreover, it is insensitive to strain in the orthogonal direction, which significantly reduces the noise level and dramatically increases its sensitivity. The sensor's suitability for practical use as an arterial pulse sensor for monitoring physiological signals is demonstrated. The developed thin-film flexible stress sensor has the potential to contribute toward the improvement of wearable healthcare monitoring devices and human–machine interaction. [ABSTRACT FROM AUTHOR]

Understanding the mechanical properties of bionanofilms is important in terms of identifying their durability. The primary focus of this study is to examine the effect of water vapor annealed silk fibroin on the indentation modulus and hardness of graphene oxide–silk fibroin (GO–SF) bionanofilms through nanoindentation experiments and finite element analysis (FEA). The GO–SF bionanofilms were fabricated using the layer-by-layer technique. The water vapor annealing process was employed to enhance the interfacial properties between the GO and SF layers, and the mechanical properties of the GO–SF bionanofilms were found to be affected by this process. By employing water vapor annealing, the indentation modulus and hardness of the GO–SF bionanofilms can be improved. Furthermore, the FEA models of the GO–SF bionanofilms were developed to simulate the details of the mechanical behaviors of the GO–SF bionanofilms. The difference in the stress and strain distribution inside the GO–SF bionanofilms before and after annealing was analyzed. In addition, the load-displacement curves that were obtained by the developed FEA model conformed well with the results from the nanoindentation tests. In summary, this study presents the mechanism of improving the indentation modulus and hardness of the GO–SF bionanofilms through the water vapor annealing process, which is established with the FEA simulation models. [ABSTRACT FROM AUTHOR]