Cylindrical, microscale devices with 820 μm (diameter) × 3 mm (length) were manufactured from medical-grade Delrin acetal resin blocks (DuPont) by micromachining (CNC Micro Machining Center, Cameron). This material was selected for its combination of structural rigidity, machinability, and biocompatibility (compliance to ISO 10993-1 and USP Class IV). Circular reservoirs (3 to 30 per device) were shaped on the outer surface of devices in dimensions ranging from 150 to 350 μm (diameter) × 150 to 250 μm (depth). Adjacent reservoirs were positioned 400 to 750 μm apart to prevent the intersection of compounds in tissue. All drugs were purchased in powder form (Selleckchem) and stored according to the manufacturer’s instructions. Cetuximab labeled with Alexa 488 was given by K. D. Wittrup (Massachusetts Institute of Technology). For doses with specific concentrations, compounds in the appropriate amounts were added to PEG-1000 or PEG-1450 (Alfa Aesar) and vortexed for 5 min above its melting point (37°C). For insoluble drugs, a mixture of drug, PEG, and an organic solvent (ethanol or acetone) was heated to ~45°C until completely dissolved (table S1). The solution was placed on a rotary evaporator (Yamato Scientific) for ~30 to 40 min at below respective vapor pressures to completely evaporate the solvent, leaving a homogeneous mixture of drug and PEG. Pure powders and concentrations in PEG were packed in solid form into device reservoirs using a tapered, metal needle (Electron Microscopy Sciences) until the reservoirs were completely filled. Before implantation, the devices were rotated on adhesive tape to remove excess compounds on the exterior of the device.