A mountable device includes a bio-compatible structure embedded in a polymer that defines at least one mounting surface. The bio-compatible structure has a first side defined by a first layer of bio-compatible material, a second side defined by a second layer of bio-compatible material, an electronic component, and a conductive pattern that defines sensor electrodes. A portion of the second layer of bio-compatible material is removed by etching to create at least one opening in the second side in which the sensor electrodes are exposed. The etching further removes a portion of the first layer of bio-compatible material so as to create at least one opening in the first side that is connected to the at least opening in the second side. With this arrangement of openings, analytes can reach the sensor electrodes from either the first side or the second side of the bio-compatible structure.