The need for controlled release of drugs is obvious. Conventional drug delivery methods such as subcutaneous injection and oral ingestion have many problems, including side effects due to non-site-specific delivery of the drug, drug concentrations being too high or too low, patient compliance due to strict drug intake schedule, and so forth. An ideal drug delivery system is one that releases the drug to a specific location at a constant rate and at a desirable concentration of the drug (Figure 2.1).