Provided is an extended use self-contained, wearable medical device. The device is preferably configured with an infusion deployment mechanism for variably inserting and retracting an infusion needle to different depths, or completely retracting the infusion needle from the infusion site and then re-inserting the infusion needle after a predetermined period of time, throughout an infusion cycle for extending the viability of the infusion site. Another embodiment comprises dual needle deployment mechanisms which may also variably insert and retract the infusion needles. A flow sensor is preferably provided for detecting the stoppage of flow through the infusion cannula and signaling the needle deployment mechanism to attempt infusion at a different depth or to deploy a second infusion needle. A re-fillable reservoir assembly is preferably provided for supplying a drug over the extended use of the device. Another embodiment comprises a partially reusable and partially disposable medical device implementing the above features.