A body-mountable device may include a first polymer layer, a second polymer layer, and a structure that includes a sensor between the first and second polymer layers. Forming the body-mountable device may involve positioning the structure on the first polymer layer and then forming, in a molding piece, the second polymer layer over the structure positioned on the first polymer layer. The molding piece includes a surface that supports the second polymer layer as the second polymer layer is being formed and a protrusion that extends from the surface to the sensor through the second polymer layer as the second polymer layer is being formed. The body-mountable device that is removed from the molding piece has a channel to the sensor formed by the protrusion.
