This invention provides a method of increasing trafficking of endothelial progenitor cells to ischemia-damaged tissue in a subject comprising administering to the subject an amount of interleukin-8 effective to attract endothelial progenitor cells to the ischemia-damaged tissue. At least one given optical patternsignal is generated on the vehicle, which is characteristic for the operating state of the charging device. This delivery system has the ability to control the release of one or more agent of interest within a fluid environment following zero-order kinetics.