a system for controlled release both in vivo and in vitro of entrapped substances , either at a constant rate over a period of time or in discrete pulses , is disclosed . biologically active substances , such as drugs , hormones , enzymes , genetic material , antigens including viruses , vaccines , or inorganic material such as dyes and nutrients , are entrapped in liposomes which are protected from the biological environment by encapsulation within semi - permeable microcapsules or a permeable polymeric matrix . release of the entrapped substance into the surrounding environment is governed by the permeability of both the liposome and surrounding matrix to the substance . permeability of the liposome is engineered by modifying the composition and method for making the liposomes , thereby producing liposomes which are sensitive to a specific stimuli such as temperature , ph , or light ; or by including a phospholipase within some or all of the liposomes or the surrounding matrix ; or by destabilizing the liposome to break down over a period of time ; or by any combination of these features .