The controlled release of drugs offers several advantages such as increased efficacy, safety, compliance and convenience. Therefore, controlled drug delivery is often preferred above daily administration. A drug delivery system can be designed to deliver one or more drugs at a specified rate, for a specific period and even at a desired location.
Polymers are often applied in drug delivery systems because of their suitable properties. They can either be degradable or non-degradable. The choice between a degradable or non-degradable polymer depends on the specific needs and demands of the drug delivery system. A degradable polymer is eliminated from the body by means of biochemical or physicochemical mechanisms. A non-degradable polymer is preferred above a biodegradable polymer if there is a specific need to remove the drug delivery device from the body at any desired time.
Two examples of non-degradable biocompatible polymers that are frequently used in drug delivery systems are polyurethane (PU) and polyethylene vinyl acetate (EVA). While polyurethane can be used for more hydrophilic drugs, polyethylene vinyl acetate is generally used for hydrophobic drugs. Both polyurethane and polyethylene vinyl acetate can be obtained in grades that exhibit different diffusivity properties.
Two basic types of controlled drug delivery systems can be distinguished: matrix and reservoir systems.
In a matrix design, the drug is dispersed or dissolved in a polymeric matrix. The release rate of the drug is normally not constant and decreases in rapidity over time. In a reservoir system, the drug is dissolved or dispersed in a polymeric core that is enveloped by a rate-limiting membrane. The presence of a membrane normally provides for a more constant release rate. In principle, almost zero order release is possible. Using hot melt extrusion, it is also possible to manufacture multilayer delivery systems that can be designed to deliver one or more drugs at a desired rate and time. Multilayer designs can also be designed in such a way that no more drug is used than is needed. These designs are therefore very beneficial regarding the costs of goods of the delivery system.