An inventive medical device for transcutaneously inserting a cannula into body tissue. The device has a cannula with a wall that at least partially encloses a lumen. The device also has a patch for mounting onto skin of a user. The patch has a base and has a reservoir for storing a medical fluid. The patch also has a spring driven inserter. The inserter has a drive that urges the cannula from a storage position within the patch to an inserted position within the body tissue. The inserter also has a lock configured for securing the drive in a fixed position. The lock is fixedly connected to the drive and has a snap closure. Also disclosed is a medication pump that is fluidly connectable to the cannula. A method of transcutaneously using the medical device to transcutaneously insert a cannula into body tissue is also disclosed.