pH-Responsive Polymer Microspheres: Rapid Release of Encapsulated Material within the Range of Intracellular pH Financial support was provided by the NSF (Cooperative Agreement No. ECC9843342 to the MIT Biotechnology Process Engineering Center), the NIH (GM26698), and the Department of the Army (Cooperative Agreement DAMD 17-99-2-9-001 to the Center for Innovative Minimally Invasive Therapy). D.M.L. wishes to thank the NIH for a Postdoctoral Fellowship (NRSA Fellowship No. 1 F32 GM20227-01). Scanning electron microscopy and confocal microscopy images were acquired by William Fowle at the Northeastern University Center for Electron Microscopy. Dr. David Putnam, David Ting, and Tommy Thomas are thanked for helpful discussions.