A drug delivery device includes a catheter； a retention cap attached to an end of the catheter, having an operably openable surface； a flexible membrane having a rim securely connected to an inner surface of the catheter to define a first volume and a second volume therein, such that the second volume is substantially the same as a desired drug volume； and a pressurizable member coupled to the catheter. In use, the desired drug volume is preloaded into the second volume of the catheter that in turn is slided into a guidance device, when the guidance device is placed in a target, the pressurizable member applies a pressure into the first volume to exert a force upon the flexible membrane to operably squeeze the second volume, which in turn pressurizes the operably openable surface of the retention cap to open, thereby unloading the desired drug volume into the target.
