Soft robots, with their compliant bodies, minimal environmental disturbance, and ability to withstand ambient pressures, offer promising solutions for deep-sea exploration. However, a common challenge of stiffening in soft materials impairs their effective actuation in harsh conditions. In this work, we integrated a liquid dielectric plasticizer within an electrohydraulic soft robot, serving dual critical functions as a softening agent to maintain the softness of the polymer shell and an electrohydraulic fluid for efficient actuation. In addition, by using the surrounding seawater as alternating electrodes, we prevented charge retention in dielectric layers, enabling sustained actuation performance. Field tests at depths of ~1360, 3176, and ~4071 meters confirmed the robot’s ability to sense the environment, navigate complex trajectories, and withstand unsteady disturbances. Our work offers a generalized and straightforward framework for developing soft materials tailored for deep-sea applications, paving the way for soft robots to execute real-world missions.

In this study, an Al₂O₃-40TiO₂ coating was deposited on 20MnNiMo steel via atmospheric plasma spraying. The corrosion behavior of the coating was investigated in both artificial seawater and a simulated environment with hydrogen sulfide and high pressure. Additionally, ion dissolution experiments were conducted to evaluate the coating’s bio-friendliness. In artificial seawater, the corrosion rate (based on the corrosion current) of the Al₂O₃-40TiO₂ coating initially decreased before increasing. It was speculated that the blocking of corrosion products in the defect channels was helpful in delaying the progress of corrosion in the early stage. The coating had a corrosion current on the order of 10⁻⁶ A/cm² in artificial seawater, suggesting good protection in conventional seawater environments. In the simulated environment, the corrosion rate (based on the weight loss) of the Al₂O₃-40TiO₂ coating showed a continuously declining trend. It was deduced that, unlike corrosion products in artificial seawater, the corrosion products in the simulated environment (e.g., metal sulfide) might be more chemically stable, leading to a longer blocking effect. Therefore, a minimal corrosion rate of 0.0030 mm/a was obtained after the coating was immersed for 30 days. The amount of dissolved coated elements was negligible and there were only small amounts of dissolved non-coated elements such as Ni and Mo. The developed coating can be considered to be highly biofriendly if the non-coated area of the specimen is well sealed.

TC4 ELI alloy is widely used in the marine, medicine, and aviation fields. The failure performance of TC4 ELI alloy is significantly different from that of other metal materials, such as steels. In this paper, a modified Mohr–Coulomb criterion is calibrated based on several kinds of specimens under different stress states and a 3D geometric representation of a modified Mohr–Coulomb fracture locus for TC4 ELI is obtained based on these parameters. The effectiveness of the modified M-C criterion is studied by a ring-stiffened cylinder made of TC4 ELI. The ultimate strength of the cylinder obtained in the simulation with the modified M-C criterion is close to that obtained in an external pressure experiment, which shows that the modified M-C criterion is suitable for predicting failure in pressure hulls made of titanium alloy used in the deep-sea field.

It is crucial to accurately characterize the static and dynamic properties of seals for the design of deep-sea equipment. This study investigated the movement resistance of O-rings under high pressure (up to 30 MPa) and low temperature (2 °C). Numerical simulation models were set up to investigate the effects of material properties and seal structure dimensions on the reciprocating friction and installation force of O-rings. Furthermore, a novel resistance testing rig was developed to facilitate the measurement of reciprocating friction and installation force of O-rings under high pressure and low temperature. The results of this study indicate that nitrile rubber (NBR) has a reduced sensitivity to temperature variations, and the hardening of the material due to the low temperatures encountered in deep-sea environments does not substantially increase its movement resistance. Conversely, fluororubber (FKM) exhibits superior static sealing reliability. We optimized the sealing structure of the pressure-maintaining trapping instrument (PMTI) and successfully conducted field tests at the South China Sea. The results of this study may serve as a valuable reference for the sealing design of deep-sea equipment.

In this study, the Mo/C276 coating was deposited on Q690E steel by high velocity oxy-fuel spraying (HVOF), and the coating was treated with an organosilicon sealer. Further, the corrosion behaviors of the coating in the simulated deep-sea cold spring environment with hydrogen sulfide and high pressure were studied. The results show that when the oxygen flow rate is 220 NL/min, the coating has the lowest porosity of 1.71% and excellent mechanical properties. Combined with the micromorphology and elemental analysis of the coating, it was assumed that the Fe element generated by the corrosion of the Q690E substrate migrates to the surface of the coating. The corrosion tests in the simulated deep-sea cold showed that before the failure of the coating in the edge and corner areas, the corrosion rate of the coating was less than 0.002 mm/a, which could meet the long-term use requirements in the real cold spring environment.