Among other things, there are disclosed apparatuses and methods for medically sealing an opening in a vessel or wall. For example, in medical applications, a delivery tube is provided for insertion through a sheath into a vessel (e.g. a blood vessel). A dome-shaped seal fixed to a filament, an absorbent and/or compressible buffer, and a locking member are provided in the delivery tube. The delivery tube is configured so that it can be inserted into the vessel through a sheath. Tension is maintained on the filament to hold the seal against the tube or sheath. The sheath is pulled out, which pulls out the tube at the same time, leaving the seal over the opening in the vessel wall. The locking member compresses the buffer against the outside of the vessel and with the filament holds the seal in place against the inside of a vessel.