referring to fig1 a and 1b , an implantable system or device 10 made in accordance with the preferred embodiment may be implanted below the skin of a patient . the implantable device 10 has a port 14 into which a hypodermic needle can be inserted through the skin to inject a quantity of microcapsules 31 containing a medication or drug 29 . catheter 1 is positioned to deliver the agent to specific target sites 30 in a patient . device 10 may take the form of the like - numbered device shown in u . s . pat . no . 4 , 692 , 147 ( duggan ), assigned to medtronic , inc ., minneapolis , minn . commercially available as the synchromed ® infusion pump , which is incorporated by reference . as further shown in fig2 drug 29 is maintained in capsule 31 by capsule wall 32 . as shown in fig2 capsule wall 32 can be broken by ultrasonic waves 40 emitted from an ultrasonic sound emitter 41 . broken capsule walls 32 ′ are illustrated in fig2 . once drug 29 is freed from capsule 31 , it dissolves in a carrier fluid 50 in mixing tank reservoir 51 . in a preferred embodiment , a filter 52 is placed at the outlet 53 of the reservoir 51 . filter 52 will allow drug 29 ions in the fluid 50 to exit the reservoir 51 , but not permit the opened capsule 31 material to exit reservoir 51 . carrier fluid 50 is supplied to reservoir 51 through entry way 54 . carrier fluid 50 can be any suitable fluid , including bodily fluids . in one preferred embodiment , the reservoir 51 can be emptied of capsule material , e . g ., by accessing the reservoir 51 with a hypodermic needle through port 14 . the empty capsule pieces will be small enough to pass through the hypodermic needle and removed from reservoir 51 . in another preferred embodiment , electromechanical pump 60 will pump the mixture of drug 29 and fluid 50 to catheter 1 , where catheter 1 then conveys the mixture through proximal end 13 and lumen 34 of catheter 1 , and through openings 35 at distal end 12 of catheter 1 , to the target site 30 within the patient . device 10 is capable of changing the drug delivery of drug 29 based on reading from a sensor 100 measuring conditions at a target site 30 within the patient . alternatively , device 10 can be programmed for drug delivery and / or drug delivery by device 10 can be changed from outside the patient via a telemetry unit 101 . by way of example , as shown in fig2 device 10 can have an electrical control circuit 91 which controls ultrasonic sound emitter 41 via sound emitter control pathway 95 and the ultrasonic sound waves 40 therefrom . those skilled in the art will recognize that electrical control circuit 91 can also control the flow of carrier fluid 50 to reservoir 51 via control carrier fluid pathway 93 and controlling carrier fluid metering device 43 . those skilled in the art will also recognize that electrical control circuit 91 can also control pump 60 via pump control pathway 94 . thus , electrical control circuit 91 can be used to control the pumping of the mixture of dissolved drug 29 and carrier fluid 50 to patient site 30 as desired . it is contemplated that the above device and method for drug delivery will be able to permit drug delivery for about a one year period . in this embodiment , enough encapsulated drug would be stored in device 10 and last for the expected time period . at the end of that time period the implantable device 10 can be replenished via port 14 or explanted as desired . as shown in fig3 in another preferred embodiment , encapsulated drug 29 may be stored in a premixing vessel 71 , and outside of reservoir 51 . drug 29 can be metered from premixing vessel 71 into reservoir 51 as needed via any suitable metering device 44 . if more accurate drug infusion is required , a drug concentration sensor 90 can be placed in reservoir 51 . sensor 90 can send sensor signals via signal pathway 92 to an electrical control circuit 91 in device 10 . the control circuit 91 controls drug metering device 44 via drug control signal pathway 45 so that drug metering device 44 only meters drug 29 into the reservoir 51 when the concentration of the drug 29 within reservoir 51 falls to a preset limit . the sensor 90 can also measure the concentration of drug 29 and electrical circuit 91 can control fluid metering device 43 via fluid control signal pathway 93 to precisely infuse into reservoir 51 the amount of carrier fluid 50 that is required to deliver a specified amount of drug 29 to the patient . in fig3 encapsulated drug 29 can be provided to premixing vessel 71 through port 15 . in this preferred embodiment port 14 is used only to remove broken capsules 32 ′. by using the foregoing techniques , numerous drug delivery applications can be achieved to treat numerous conditions , including motor disorders , with a controlled degree of accuracy previously unattainable . further , in accordance with the present invention , reagents , which are used to produce a substance that can be measured by conventional sensors , can be replenished . a preferred embodiment is illustrated in fig4 . for example , an oxidase 400 can be contained within microcapsules 404 , which are in turn contained in sensor reservoir 401 . in this embodiment , a glucose containing fluid 402 from a target site is supplied to the sensor reservoir 401 , wherein the glucose containing fluid 402 reacts with the oxidase 400 to produce oxygen . sensor 403 can measure the oxygen produced , and since the amount of oxygen is directly proportional to the amount of glucose in the glucose containing fluid 402 , the amount of glucose can be determined . oxidase 400 can be supplied to sensor reservoir 401 in any suitable manner . as the oxidase in the sensor is consumed , additional oxidase can be freed from the microcapsules allowing the sensor to continue operation . as shown in fig4 for example , microcapsules 404 containing oxidase 400 can be supplied by inserting a hypodermic needle ( not shown ) through the skin of the patient and through port 405 to supply oxidase 400 to the sensor reservoir 401 . further , as oxidase 400 is consumed in the reaction with glucose , it can be replenished as may be desired by inserting a hypodermic needle ( not shown ) through the skin of the patient and through port 405 to supply additional oxidase 400 to the sensor reservoir 401 . glucose containing fluid 402 from the target site can be supplied in any suitable manner . for example , the sensor 403 can be placed at a target site so that the glucose containing fluid 402 can flow through a semi - permeable membrane 406 and come into contact with the microcapsules 404 and thus oxidase 400 when freed from the microcapsules 404 . as an alternative , glucose containing fluid 402 can be provided from the target site via a pump ( not shown ) to the sensor reservoir 401 . those skilled in the art will also recognize that drug delivery in accordance with the present invention can be achieved by measuring the physiological conditions at the patient target site 30 . for example , the measurement of hyperexcited cells can be detected with a sensor 100 as shown in fig2 and 3 , or sensor 403 as shown in fig4 . further , sensor 100 can send a signal to electrical control circuit 91 , which as shown in fig3 as an example , controls the mixing of drug 29 and carrier fluid 50 . the sensor 403 shown in fig4 can also be used to send a signal to an electrical control circuit 91 , which in turn can regulate drug delivery from an implantable drug delivery device , including those shown in fig2 and 3 . those skilled in the art will recognize that the capsules can be broken in any suitable manner , including : ultrasonic waves , mechanical crushing or grinding ; chemically dissolving or splitting ; applying an electrical current to potentiate a chemical reaction ; heating ; or applying pressure ( e . g . hydrostatic pressure ). those skilled in that art will recognize that the preferred embodiments may be altered or amended without departing from the true spirit and scope of the invention , as defined in the accompanying claims .