A device suitable for being implanted in a human or animal body, said device being equipped for signal delivery or signal acquisition within said body, the device further being suitable for fixation in hard tissue, on hard tissue, in hard tissue replacement material or on hard tissue replacement material: wherein the device comprises at least one of a plug portion to be inserted in an opening reaching through a hard tissue layer, from an accessible to a non-accessible side thereof, of the hard tissue or replacement material and a cover portion to be positioned over a mouth of such an opening in the hard tissue or replacement material, the plug or cover portion comprising a complete ring of a material having thermoplastic properties and being liquefiable through application of vibrational energy to the ring or to device portions adjacent to the ring, said ring being situated around a cross section of the plug portion or on a tissue facing surface of the cover portion, and wherein the device is equipped for transmission of the vibrational energy to the ring or to the device portions adjacent to the ring for the purpose of liquefying at least part of the material having thermoplastic properties and to bring the material into contact with the hard tissue to form on re-solidification a complete sealing ring around the opening, characterised in the device being equipped for signal delivery or acquisition on the non-accessible side of the hard tissue layer, and comprising a signal delivery or acquisition portion arranged on a distal side of the plug or cover portion (P, C) or comprising a container with a perforated, fenestrated or otherwise permeable wall portion on a distal side of the plug or cover portion (P, C) and a non-permeable wall portion on the proximal side of the plug or cover portion (P, C), wherein the signal delivery or acquisition portion is arranged or arrangeable in the container. wherein the device comprises at least one of a plug portion to be inserted in an opening reaching through a hard tissue layer, from an accessible to a non-accessible side thereof, of the hard tissue or replacement material and a cover portion to be positioned over a mouth of such an opening in the hard tissue or replacement material, the plug or cover portion comprising a complete ring of a material having thermoplastic properties and being liquefiable through application of vibrational energy to the ring or to device portions adjacent to the ring, said ring being situated around a cross section of the plug portion or on a tissue facing surface of the cover portion, and wherein the device is equipped for transmission of the vibrational energy to the ring or to the device portions adjacent to the ring for the purpose of liquefying at least part of the material having thermoplastic properties and to bring the material into contact with the hard tissue to form on re-solidification a complete sealing ring around the opening, characterised in the device being equipped for signal delivery or acquisition on the non-accessible side of the hard tissue layer, and comprising a signal delivery or acquisition portion arranged on a distal side of the plug or cover portion (P, C) or comprising a container with a perforated, fenestrated or otherwise permeable wall portion on a distal side of the plug or cover portion (P, C) and a non-permeable wall portion on the proximal side of the plug or cover portion (P, C), wherein the signal delivery or acquisition portion is arranged or arrangeable in the container.