Implantable infusion system with a neutral pressure medication container

An implantable infusion system comprising:

• a housing adapted for implantation in a patient;
• a neutral pressure medication supply container disposed in aid housing for containing liquid medication to be administered to said patient, at least a portion of said medication container being flexible so that said container has a volume which changes dependent on an amount of medication contained in aid medication container;
• a catheter connected to an exterior of said housing and adapted for implantation in the body of said patient with a tip of said catheter disposed at a medication delivery site;
• pumping means, disposed in aid housing and connecting an interior of said medication container to said catheter for pumping medication from said medication container to said catheter;
• and
• means for defining a closed space in which said medication container is disposed, said means for defining a closed space having a volume at least partially contained in said housing, and having a flexible wall portion means exposed to an exterior of said housing, for causing the volume of said closed space to change by a corresponding amount when the volume of said medication container changes for maintaining a pressure acting on said supply container substantially constant.