A body-mountable device may be formed of a polymeric material. A molding form can define a cavity and polymeric material can be formed using the mold to create a device shaped in accordance with the cavity. Electronics disposed on a substrate can be encapsulated within the polymeric material by forming a first layer of polymeric material, positioning the substrate on the first layer, and then forming a second layer of polymeric material over the substrate. A channel can expose a sensor disposed on the substrate. The channel may be formed while molding the second layer of polymeric material over the substrate. The molding form can include a protrusion that extends toward the sensor. A sacrificial sealant material can be applied to the sensor or the protrusion to create a seal between the protrusion and the sensor. The polymeric material forms around the sealed protrusion to create the channel.