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Drug delivery systems have unusual materials requirements which derive mainly from their therapeutic role: to administer drugs over prolonged periods of time at rates that are independent of patient-to-patient variables. The chemical nature of the surfaces of such devices may stimulate biorejection processes which can be enhanced or suppressed by the simultaneous presence of the drug that is being administered. Selection of materials for such systems is further complicated by the need for compatibility with the drug contained within the system. A review of selected drug delivery systems is presented. This leads to a definition of the technologies required to develop successfully such systems as well as to categorize the classes of drug delivery systems available to the therapist. A summary of the applications of drug delivery systems will also be presented. There are five major challenges to the biomaterials scientist: (1) how to minimize the influence on delivery rate of the transient biological response that accompanies implantation of any object; (2) how to select a composition, size, shape, and flexibility that optimizes biocompatibility; (3) how to make an intravascular delivery system that will retain long-term functionality; (4) how to make a percutaneous lead for those delivery systems that cannot be implanted but which must retain functionality for extended periods; and (5) how to make biosensors of adequate compatibility and stability to use with the ultimate drug delivery system-a system that operates with feedback control.

An analysis of oxygen absorption by blood flowing through a smoll oxygen-permeable fiber in steady state laminar flow is presented. The rigidity and geometry of the fibers eliminate unpredictable shunting and distention, permitting a more detailed analysis o f blood-membrane factors than has previously been undertaken. The mathematical analysis treats the blood as a homogeneous, non-Newtonian fluid with a reversible nonlinear oxygen sink (erythrocytes). The differential equations are solved numerically and the results of the parametric analys'is are presented. The parameters that have a major influence on residence time necessary to obtain a specified oxygen content are the Grashof number, the concentration of hemoglohin in Comparison of the experimental results to the model indicate that mixing due to the heterogeneous nature of blood is minimal and that the major limitation in oxygen absorption i s the blood film. Means of reducing this resistance are discussed.

The energy stored in an extended elastomeric reservoir can be usefully employed in the controlled delivery of drug solutions at constant rates of flow through a fixed resistance. The absolute level and constancy of pressure presented to the upstream end of the flow restrictor can be related, respectively, to the geometry and composition of the elastomeric reservoir. (The reservoir is to contain, in its distended form, the volume of drug to be delivered to the human venous system.) It has been established that the defining criteria for minimizing static pressure loss in the reservoir and for maintaining constancy of pressure through most of the delivery interval are stress decay and hysteresis, as measured in uniaxial stress-strain testing. The elastomeric composition most successfully meeting these requirements has been found to be non-reinforced isoprene rubber, crosslinked with a peroxide.