Many polymer-based drug delivery systems have progressed to the clinical stage of testing but so far only a small number have reached commercial production. Genetic engineering is continuing to struggle to create dependable, commercially successful polymeric systems.Often a series of limitations have to be overcome before a commercial solution is found. Typical obstacles that have to be overcome include biocompatibility, biodegradability, response time to stimuli, burst release, optimum release rate simulation and variable sensitivity to the changing metabolic state of patients. All these factors pose major formulation challenges for polymer scientists.