While human tissues and organs are mostly soft, wet and bioactive; machines are commonly hard, dry and biologically inert. Bridging human-machine interfaces is of imminent importance in addressing grand societal challenges in health, security, sustainability, education, and joy of living. However, interfacing human and machines is extremely challenging due to their fundamentally contradictory properties. At MIT SAMs Lab, we propose to harness ¿hydrogel technology¿ to form long-term, high-efficacy, compatible and seamless interfaces between humans and machines. On one side, hydrogels with similar mechanical and physiological properties as tissues and organs can naturally integrate with human body over the long term, greatly alleviating the foreign body response and mechanical mismatches. On the other side, the hydrogels with intrinsic or integrated electrodes, optical fibers, sensors, actuators, and circuits can effectively bridge external machines and human bodies via electrical, optical, chemical, and mechanical interactions. In this talk, I will first discuss the mechanisms to design extreme properties for hydrogels, including tough, resilient, adhesive, strong and anti-fatigue, for long-term robust human-machine interfaces. Then I will discuss a set of novel hydrogel devices that interface with the human body, including i). hydrogel neural probes capable of electro-opto-fluidic interrogation of single neurons in mice over the life time; ii). ingestible hydrogel pills capable of continuously monitoring core-body physiological conditions over a month; and iii). untethered fast and forceful hydrogel robots controlled by magnetic fields for minimal invasive operations. I will conclude the talk by proposing a systematic approach to design next-generation human-machine interfaces based on hydrogel technology.