Powered by recent advances in electronics and materials technologies, more and more devices start to be used around human body in highly intimate manner than ever. However, conventional engineering materials in such devices suffer from stark dissimilarities in mechanical and chemical properties to biological tissues. In light of the challenge of bridging artificial systems and human body, hydrogel-based human-machine interfaces or so-called “hydrogel machines” have emerged in a wide range of applications including tissue engineering, ingestible devices, smart wound dressing, neural interfaces, and soft robotic implants to name a few. However, more extensive use of hydrogels in the next generation hydrogel machines has faced a significant challenge from the weak and unstable interfacial integration between hydrogels and various engineering and biological materials. Notably, the recent development of tough bonding of hydrogels has enabled unprecedented level of robust hydrogel-based hybrid materials, which address the challenge and shed light toward the new class of hydrogel machines. Here, we will discuss tough bonding of hydrogels for a wide class of materials, with emphasis on mechanistic principles behind tough bonding of hydrogels. We will further highlight our recent tough bonding based advances in novel hydrogel machines including hydrogel coatings for medical devices, epidermal living sensors, hydrogel soft actuators and robots, and hydrogel-based hybrid neural probes.