system for delivery of a biologically active substance into an environment . first and second chambers are separated by a moveable partition . the first chamber includes a polymer gel network which undergoes a volume change in response to an environmental condition such as ph . the first compartment includes a screen or membrane for confining the polymer gel network while allowing communication with fluid in an environment . the second compartment contains a biologically active compound or drug which is delivered to the environment through an orifice in the second compartment . upon the occurrence of the triggering environmental condition , the polymer gel network undergoes a volume change which moves the moveable partition expelling the drug through the orifice . drug delivery is initiated and continues only when the appropriate environmental condition or trigger is met .