The reservoir contains a piston that divides {TAIL ~ the reservoir} into a water swellable agent chamber and {HEAD ~ an active agent chamber} 
The reservoir contains a piston that divides { TAIL ~ the reservoir } into a water swellable agent chamber and { HEAD ~ an active agent chamber }
OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD