An implantable intravascular device 101 suitable for deployment inside a human or animal, the device comprising a body of crystalline material 110 and a membrane of crystalline material 100 fixed to the body by a hermetic bond 116； and an assembly of components 102a, 102b carried on the crystalline material and arranged for responding to electrical signals. The body and the membrane at least partially encapsulate the assembly. A cap 120 may be secured to the body, the cap comprising a series of channels including a connection channel (124: Figure 2) which is used to secure an antenna to the device. The cap may include fixture channels (122a, 122b: Figure 2) for fixing the device to a strut in the channel. The hermetic bond may comprise a metal interlayer. The cap and the body may comprise the same material. The channels may be used alongside an anchor (150: Figure 5) for anchoring the device inside a human or animal. A cavity 112 in the body may be isolated from the environment exterior to the implantable device 101.
