A system, comprising: an implantable medical device housing defining an interior and a passage extending from an exterior of the implantable medical device housing to the interior; a one-piece ferrule (108") that is biocompatible, the ferrule conformed to and joined to the passage, the ferrule extending from the exterior to the interior of the implantable medical device housing, the ferrule including: an exterior flange configured to abut an exterior surface of the housing, the exterior flange defining an exit opening; a tunnel extending from the exterior flange, the tunnel defining a tunnel interior extending away from the exit opening and terminating at an inlet opening of the ferrule; at least one interior tab extending away from an interior of the tunnel, the at least one interior tab configured to exert a bias against the housing while the exterior flange abuts the housing, wherein the tunnel includes a notch along its length, away from the opening, with the at least one interior tab extending away from the notch, wherein the at least one interior tab is bent away from material defining the tunnel; an electrically insulative insert disposed at least partially inside the tunnel; and a hermetic seal disposed between the electrically insulative insert and the tunnel. an implantable medical device housing defining an interior and a passage extending from an exterior of the implantable medical device housing to the interior; a one-piece ferrule (108") that is biocompatible, the ferrule conformed to and joined to the passage, the ferrule extending from the exterior to the interior of the implantable medical device housing, the ferrule including: an exterior flange configured to abut an exterior surface of the housing, the exterior flange defining an exit opening; a tunnel extending from the exterior flange, the tunnel defining a tunnel interior extending away from the exit opening and terminating at an inlet opening of the ferrule; at least one interior tab extending away from an interior of the tunnel, the at least one interior tab configured to exert a bias against the housing while the exterior flange abuts the housing, wherein the tunnel includes a notch along its length, away from the opening, with the at least one interior tab extending away from the notch, wherein the at least one interior tab is bent away from material defining the tunnel; an electrically insulative insert disposed at least partially inside the tunnel; and a hermetic seal disposed between the electrically insulative insert and the tunnel. an exterior flange configured to abut an exterior surface of the housing, the exterior flange defining an exit opening; a tunnel extending from the exterior flange, the tunnel defining a tunnel interior extending away from the exit opening and terminating at an inlet opening of the ferrule; at least one interior tab extending away from an interior of the tunnel, the at least one interior tab configured to exert a bias against the housing while the exterior flange abuts the housing, wherein the tunnel includes a notch along its length, away from the opening, with the at least one interior tab extending away from the notch, wherein the at least one interior tab is bent away from material defining the tunnel; an electrically insulative insert disposed at least partially inside the tunnel; and a hermetic seal disposed between the electrically insulative insert and the tunnel.