Enhancing and manipulating the mechanical properties of graphene oxide (GO)-based structures are challenging because the GO assembly is easily delaminated. We develop nacre-like bionanofilms whose in-plane mechanical properties can be manipulated through water vapor annealing without influencing their mechanical properties in the thickness direction. These bionanofilms are prepared from GO, silk fibroin (SF), and cellulose nanocrystals (CNCs) via a spin-assisted layer-by-layer assembly. The postannealing mechanical properties of the films are determined with atomic force microscopy (AFM) bending and nanoindentation, and it is confirmed that the mechanical properties of the bionanofilms are altered only in the in-plane direction. While AFM bending shows Young’s moduli of 26.9, 36.3, 24.3, and 41.4 GPa for 15, 15 annealed, 30, and 30 annealed GO/SF/CNC trilayers, nanoindentation shows reduced moduli of 19.5 ± 2.6 and 19.5 ± 2.5 GPa before and after annealing, respectively. The unaltered mechanical properties of the bionanofilms along the thickness direction after annealing can be attributed to the CNC frame in the SF matrix acting as a support against stress in the thickness direction, while annealing reorganizes the bionanofilm structure. The tunability of the bionanofilms’ mechanical properties in only one direction through structure manipulation can lead to various applications, such as e-skin, wearable sensors, and human–machine interaction devices.

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.

In the munitions industry, there have been considerable efforts spent to develop low-cost, simply fabricated, easily wearable, and biocompatible bulletproof armors. Recently, long fiber-reinforced composites and shear thickening fluids (STFs) were inceptively utilized to improve bulletproof performance with solid or fabric materials. In this study, Hanji, a cornstarch suspension, Korean traditional long fiber paper, and a well-known STF, respectively, were examined for bulletproof applications to evaluate their own effects on bulletproof performance; tests were carried out in the field and finite element analysis (FEA) was performed to evaluate the behavior of materials regarding with perforated clay areas from in-field tests. It was found that both Hanji and STF influenced the bullet penetration by two factors, namely the momentum of bullet and stress propagation. The cornstarch suspension, rather than Hanji, showed outstanding performance in decreasing the linear velocity of the bullet and minimized the stress propagation to the protecting object. Thus, although STF performed a key role in bulletproof performance, Hanji also proved to be a suitable material as an exterior covering for absorbing the initial impact stress and maintaining the durability and stability of the armor itself.

No abstract

Skin temperature and skin humidity are used for monitoring physiological processes, such as respiration. Despite advances in wearable temperature and humidity sensors, the fabrication of a durable and sensitive sensor for practical uses continues to pose a challenge. Here, we developed a durable, sensitive, and wearable temperature and humidity sensor. A reduced graphene oxide (rGO)/silk fibroin (SF) sensor was fabricated by employing a layer-by-layer technique and thermal reduction treatment. Compared with rGO, the elastic bending modulus of rGO/SF could be increased by up to 232%. Furthermore, an evaluation of the performance of an rGO/SF sensor showed that it had outstanding robustness: it could withstand repeatedly applied temperature and humidity loads and repeated bending. The developed rGO/SF sensor is promising for practical applications in healthcare and biomedical monitoring.