App note from NXP Semiconductors shows I2C-bus galvanic isolation solution by splitting the bi-directional bus to unidirectional logic signals and passing them through opto-isolator couplers. Link here (PDF)
Many systems that can benefit from the advantages of I2C-bus control are not allowed to share any common reference signal (ground) potential. To prevent any chance of electrocution, AC mains control and medical patient monitoring equipment require a safety isolation barrier, to kilovolt levels, between the system components. Regulations require isolation from telephone lines and Standards require isolation from Ethernet wiring, especially when power is also supplied by the Ethernet wiring (PoE). Techniques enabling reliable I2C communication while addressing these different requirements all require first splitting the normally bidirectional SDA/SCL signals into two uni-directional components and then applying conventional techniques to provide the necessary tolerance to difference in local ground potentials, or to provide total galvanic isolation.