A needle injector device for connection in fluid- flow communication with the inlet or outlet port of a reservoir, the needle injector comprising: a housing structure; a moveable needle supported by the housing structure for movement between a retracted position and an extended position; a needle carriage supported for movement by the housing structure with the moveable needle, between the retracted position and the extended position, the needle carriage having a needle passage through which the moveable needle extends when the moveable needle and needle carriage are in the retracted position and moving toward the extended position; a hollow needle or cannula attached to the needle carriage and moveable with the needle carriage between the retracted position and the extended position, the hollow needle or cannula having a central passage through which the moveable needle extends when the moveable needle, needle carriage and hollow needle or cannula are in the retracted position and moving toward the extended position; the moveable needle comprising a needle head that is arranged adjacent the interior surface of the wall, when the moveable needle is in the retracted position; a locking structure (60, 62) for locking the needle carriage and hollow needle or cannula in the extended position, upon the needle carriage and hollow needle or cannula being moved to the extended position; a bias member operatively coupled to the moveable needle, for withdrawing the moveable needle from at least a portion of the hollow needle or cannula, once the needle carriage and hollow needle or cannula are locked in the extended position by the locking structure (60, 62); the housing structure including a wall having an opening through which an external plunger may be inserted for imparting a force on the needle head of the moveable needle in a direction for moving the moveable needle from a retracted position to the extended position, the wall having an interior surface; the needle injector device further comprising a seal disposed between the needle head of the moveable needle and the interior surface of the wall of the housing structure, to provide a seal between the moveable needle and the interior surface of the wall when the moveable needle is in the retracted position. a housing structure; a moveable needle supported by the housing structure for movement between a retracted position and an extended position; a needle carriage supported for movement by the housing structure with the moveable needle, between the retracted position and the extended position, the needle carriage having a needle passage through which the moveable needle extends when the moveable needle and needle carriage are in the retracted position and moving toward the extended position; a hollow needle or cannula attached to the needle carriage and moveable with the needle carriage between the retracted position and the extended position, the hollow needle or cannula having a central passage through which the moveable needle extends when the moveable needle, needle carriage and hollow needle or cannula are in the retracted position and moving toward the extended position; the moveable needle comprising a needle head that is arranged adjacent the interior surface of the wall, when the moveable needle is in the retracted position; a locking structure (60, 62) for locking the needle carriage and hollow needle or cannula in the extended position, upon the needle carriage and hollow needle or cannula being moved to the extended position; a bias member operatively coupled to the moveable needle, for withdrawing the moveable needle from at least a portion of the hollow needle or cannula, once the needle carriage and hollow needle or cannula are locked in the extended position by the locking structure (60, 62); the housing structure including a wall having an opening through which an external plunger may be inserted for imparting a force on the needle head of the moveable needle in a direction for moving the moveable needle from a retracted position to the extended position, the wall having an interior surface; the needle injector device further comprising a seal disposed between the needle head of the moveable needle and the interior surface of the wall of the housing structure, to provide a seal between the moveable needle and the interior surface of the wall when the moveable needle is in the retracted position.