This invention relates to an implantable infusion system.  In particular, it relates to an improvement in an infusate accumulator used with a valve system in such a system operating at positive pressure to dispense medication in accordance with different specified flow rates. 
Implantable infusion pumps utilizing an accumulator positioned between a pair of valves which alternately open and close are disclosed in U.S. Pat.  Nos.  4,838,887 and 5,049,141, commonly assigned.  The disclosure of the '887 patent is expressly incorporated herein by reference.  In such a system, as illustrated in FIG.  1, a drug reservoir 10 is refillable by means of a septum 12.  The reservoir comprises a sealed housing 14 containing a bellows element 16 having a chamber 18 which comprises the drug reservoir.  The bellows 16 separates the housing 14 into a second zone 20 which is normally filled with a two-phase fluid.  The fluid, normally Freon, vaporizes and compresses the bellows 16 thus providing a release pressure to the reservoir 18 through the outlet leading to the infusion site.  During the refill process chamber 18 is loaded with medication via the septum 12.  The two-phase fluid is then pressurized condensing a portion of the vapor and returning it to the liquid phase. 
Such systems also employ an outlet filter 24 and a side port 27 for direct bolus injections.  The reservoir and fluid delivery techniques are well established in constant flow systems.  To provide for increased accuracy in delivery dosage, the use of an accumulator 30 has been proposed for use in a number of applications. 
These systems employ a metering assembly comprising two normally closed valves 26 and 28.  Interposed between the two valves is an accumulator 30.  The valves 26 and 28 are controlled electronically by the module 32 which may be programmed utilizing an external programmer 34. 
In operation of such a system, a constant positive pressure flow from the reservoir 10 to the inlet valve 26 occurs.  Valve 26 is opened while the outlet valve 28 is closed.  This loads the accumulator 30 with a predetermined amount of medication.  The valve 26 is then closed and outlet valve 28 opened so that the contents of the accumulator 30 are delivered to the catheter 36 for delivery to the infusate site.  The rate of switching of the valves 26 and 28 thus determines the frequency of pumping through the system and therefore determines the delivery rate of medication through the catheter 36.  For each cycle a constant amount is delivered, the accumulator volume. 
In the system illustrated in FIG.  1, the accumulator 30 has two alternative forms as illustrated in FIGS.  2 and 3.  For purposes of convenience, FIGS.  2 and 3 utilize the same numbering as in the '887 patent.  As illustrated in FIG.  2, the inlet is represented by numeral 58 wherein fluid enters the accumulator from the inlet valve 26.  The accumulator comprises a diaphragm 90, a backing plate 92, an end cap 94, and fill tube 96 and a spacer plate 98.  The diaphragm 90 deflects in response to fluid entry of the chamber 102.  The backing plate 92 acts as a mechanical stop to limit motion of the diaphragm.  Similarly, the spacer plate 98 is used to limit diaphragm motion during discharge, that is, the passage of fluid through the outlet 59. 
The use of the spacer plate 98 as a mechanical stop creates a problem when fragile materials such as insulin are used.  The material is locally compressed and crushed by the mechanical action of the diaphragm contacting the stop.  This breaks down the fluid.  Since the internal volumes are quite small, this in turn can result in residue formation inside the accumulator. 
The fill tube 96 is used to supply an inert gas to the chamber 104.  The purpose of chamber 104 is to provide a region which is charged at a pressure lower than that of the infusate pressure in drug reservoir 18 so that accumulator chamber 102 fills when inlet valve 26 is opened, but higher in pressure than that of the catheter 36 to allow the diaphragm 90 to deflect back to the spacer plate 98 position as the chamber 102 empties when the outlet valve 28 is opened.  Additional details of this accumulator may be found in U.S. Pat.  No.  4,838,887. 
FIG.  3 illustrates an alternative configuration.  In this alternative arrangement, the backing plate 92 comprises three elements which electrically isolate the center of the plate from the diaphragm 90.  A supply of inert gas is still supplied via the feed fill tube 96.  A lead 110 is attached to a flange forming a portion of the end cap assembly.  A ceramic cup 113, which is lined with metal 111, provides a conductive path between a stop 114 and the lead 110.  The diaphragm 90 is used as a moving switch contact.  This provides a signal indicating that the accumulator is full, that is, the diaphragm in an upward position contacting the stop 114.  This electrical signal is used for diagnostic determinations of the system such as leaks in the valve.  Again, more complete details as to the use of the modified accumulator of FIG.  3 are found in the '887 patent. 
A problem with these types of accumulators is the formation of precipitate on the spacer plate.  Over time this build-up deteriorates the flow accuracy in the system and in an extreme case causes blockage.  U.S. Pat.  No.  5,049,141 addresses this problem by minimizing the contact points with the diaphragm.  This solution reduces stress points on the fluid within the accumulator.  There are however still points of contact and therefore potential damage to the material.