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Physical layer secret key generation (SKG) technology can provide lightweight encryption for resource-constrained Internet of Things (IoT) devices. However, many IoT devices are stationary, and the environment is usually static, which leads to ultralow/zero secret key rates. To circumvent this problem, a novel SKG scheme based on reconfigurable intelligent surface (RIS) is proposed for multiple-input single-output (MISO) communications. First, the phase shifts at the RIS are tuned randomly in each frame, which makes time-varying reflection channels superimposed on the original channels. Correspondingly, the key capacity of all nodes within the cell coverage area can be remarkably improved. Compared with the existing methods, the scheme is loosely coupled with the original system. Besides, the closed-form expressions of key capacity taking into account eavesdropping are derived for the case studies of independent and correlated channels. Based on these derivations, we can conclude that sharing the same phase shifts randomness will not lead to key information leakage. More importantly, compared with user-introduced randomness schemes, the proposed scheme can maintain good performance even the number of eavesdropping nodes increases. Furthermore, the impacts of critical parameters on key performance are analyzed, respectively, which offers guidelines for system design. Finally, simulation results verify the theoretical analysis.

Thermoplastic polyurethane (TPU) has been widely implemented in many fields due to its good elasticity. Microcellular foaming not only can achieve weight reduction, but also can enhance the elasticity of TPU products. However, the enhancement of elasticity by foaming is limited, while the introduction of corrugated structures can significantly improve the elasticity of TPU foams. Considering the difficulties to produce TPU parts with corrugated structures for conventional processes, fused filament fabrication (FFF) combined with microcellular foaming was developed to fabricate TPU foams with high elasticity. An FFF 3D printer was used to prepare solid TPU samples with corrugated structures, and microcellular foaming was used to fabricate foamed TPU samples. As expected, the corrugated structures have been successfully and precisely preserved in the obtained TPU foams, and further, the TPU foams have expansion ratio of more than 7 and cell density of higher than 1012cells/cm3. Thanks to the corrugated structures and refined cellular morphology, the 7.09 expanded TPU foams exhibited outstanding compressive properties with an energy loss coefficient of 28% and a maximum stress of higher than 0.25 MPa. It is further found that introducing the corrugated structures or enlarging cell sizes could improve the elasticity and buffering performance of TPU foams.