An apparatus for placement in a patient, comprising: an electronic device a pressure-transducing substance and a hollow casing, comprising a wall, a portion of which wall defines one or more holes therethrough, inside which casing the electronic device and the pressure-transducing substance are disposed, the casing being configured to facilitate flow of some of the pressure-transducing substance out of the one or more holes when the casing is being filled with the pressure-transducing substance.
