Implantable continuous-flow pumps

An implantable pump comprising:

• a rigid housing;
• within the rigid housing, a drug reservoir having a first expandable membrane and a second expandable membrane affixed to an equatorial rib, the first and second expandable membranes defining a drug chamber between the first and second expandable membranes for containing a liquid medicament, the equatorial rib including a portion extending radially inward between the first and second expandable membranes and having beveled interior surfaces, the equatorial rib and the first and second expandable membranes being configured such that the first and second expandable membranes contact the beveled interior surfaces when the drug chamber is depleted, or being depleted, of the liquid medicament, but do not contact the beveled interior surfaces when the drug chamber is filled with the liquid medicament;
• a propellant chamber occupying an interior space of the rigid housing, the propellant chamber containing the drug reservoir and a propellant therein, the propellant exerting a substantially constant pressure on the drug reservoir;
• a cannula in fluid communication with the drug reservoir;
• and
• a flow restrictor between the drug reservoir and the cannula.