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