An injector apparatus comprises an elongate structure having one or more openings positionable near a penetrable barrier of an implantable device so as to receive fluid of the implantable device. The apparatus comprises a needle and a sheath extending over at least a portion of the needle. The elongate structure may comprise a distal tip to penetrate tissue and the penetrable barrier, and a distal opening near the tip to release therapeutic fluid into the implantable chamber. In many embodiments the distal tip, the distal opening, and the plurality of openings are separated from a stop that engages a tissue of the patient and limit penetration depth such that the distal opening and the plurality of openings are located along an axis of the implantable device to increase an efficiency of the exchange.