An eye-mountable device (110, 210, 310) comprising: a bio-compatible polymeric material (120, 220); a substrate (130, 230, 705) at least partially embedded within the bio-compatible polymeric material; an antenna (170, 270, 312) disposed on the substrate; an electrochemical sensor disposed on the substrate and including (i) a working electrode (322, 520, 620, 640) having a first side edge and a second side edge, and (ii) a reference electrode (323, 522, 630, 650) situated such that at least a portion of the first and second side edges of the working electrode are adjacent respective sections of the reference electrode; and a controller (150, 610) electrically connected to the electrochemical sensor and the antenna, wherein the controller is configured to: (i) apply a voltage between the working electrode and the reference electrode sufficient to generate an amperometric current related to the concentration of an analyte in a fluid to which the eye-mountable device is exposed; (ii) measure the amperometric current; and (iii) use the antenna to indicate the measured amperometric current; wherein the substrate is a ring-like structure having a mounting surface that extends between an inner edge and an outer edge that are substantially concentric rings; wherein the working electrode and the reference electrode each include a plurality of substantially parallel interdigitated extensions (724, 734) that are approximately locally parallel to the inner and outer edges such that a first one of the working electrode extensions is closer to the inner edge than the outer edge and a second one of the working electrode extensions is closer to the outer edge than the inner edge. a bio-compatible polymeric material (120, 220); a substrate (130, 230, 705) at least partially embedded within the bio-compatible polymeric material; an antenna (170, 270, 312) disposed on the substrate; an electrochemical sensor disposed on the substrate and including (i) a working electrode (322, 520, 620, 640) having a first side edge and a second side edge, and (ii) a reference electrode (323, 522, 630, 650) situated such that at least a portion of the first and second side edges of the working electrode are adjacent respective sections of the reference electrode; and a controller (150, 610) electrically connected to the electrochemical sensor and the antenna, wherein the controller is configured to: (i) apply a voltage between the working electrode and the reference electrode sufficient to generate an amperometric current related to the concentration of an analyte in a fluid to which the eye-mountable device is exposed; (ii) measure the amperometric current; and (iii) use the antenna to indicate the measured amperometric current; wherein the substrate is a ring-like structure having a mounting surface that extends between an inner edge and an outer edge that are substantially concentric rings; wherein the working electrode and the reference electrode each include a plurality of substantially parallel interdigitated extensions (724, 734) that are approximately locally parallel to the inner and outer edges such that a first one of the working electrode extensions is closer to the inner edge than the outer edge and a second one of the working electrode extensions is closer to the outer edge than the inner edge.