Disclosed herein is a fluid conductivity sensor that can be used to obtain in-vivo measurements of conductivity of biological fluid samples, for example, to determine osmolarity. The conductivity sensor can be disposed on a substrate that is at least partially embedded within a polymeric material of a body-mountable device. The conductivity sensor can include a frame having a trench formed therein that defines a fluid sample cell. First and second electrodes can be formed on sidewalls of the trench, such that the first and second electrodes are on opposite sides of the fluid sample cell. A controller in the body-mountable device can operate the sensor by applying a voltage to the electrodes and measuring a current through a fluid occupying the fluid sample cell. The body-mountable device may indicate the current measurements wirelessly using an antenna.