A disposable ambulatory infuser apparatus provides an elastomeric balloon within a telescopic housing. One end of the balloon may be attached to an upper housing portion and the other end of the balloon may be attached to a bottom housing portion. As the balloon is charged with a drug, it may expand primarily radially first and then axially. The balloon may be configured to have the majority of displacement occur in the axial direction, which enables the two housing portions to expand away from each other and shrink toward one another in the axial direction as the balloon is filled with the drug and the drug is subsequently infused in a patient. The balloon may expand axially naturally within the housing to eliminate possible pressure change due to constraints by the housing or clamps, which enables more constant flow rate of the drug being dispensed.
