Fig. 1. A cross section of the Chronogesic™ Sufentanil Pain Therapy system is shown. A titanium cylinder, 4 mm in diameter and 4.4 cm in length, is plugged at one end with a semipermeable membrane and at the other with an orifice. A piston separates the “osmotic engine” (which contains a sodium chloride tablet) from the drug reservoir (which contains a concentrated solution of sufentanil). When implanted subcutaneously, the osmotic potential of the salt engine causes tissue water to cross the membrane, increasing the size of the engine, displacing the piston, and forcing sufentanil through the orifice. An excess of sodium chloride in the osmotic engine ensures constant (zero-order) delivery after the initial startup period. Although units in the present study delivered sufentanil for 60 days, future units will deliver sufentanil for more than 90 days.