FIELD OF THE INVENTION
The present invention relates generally to a method and apparatus for the delivery of fluids to a desired location within the body; and more particularly, relates to an implantable drug infusion device for delivery of pharmaceutical agents or other fluids directly from a reservoir within the device to a specific desired location within a patient's body.
BACKGROUND OF THE INVENTION
Implantable drug infusion devices usually are implanted for several years, during which time there is no opportunity to service or repair these devices. Accordingly, both the implanted device and the drug to be infused must remain operative and be stable for a long period.
The current trend is toward extending significantly the intended life of implanted drug infusion devices. Further, new drugs having different physical and chemical properties are continually being developed, the use of which may not be sufficiently compatible or properly interactive with current devices.
Conventional implantable fluid delivery systems for drug infusion typically include a reservoir to store the fluid, and a separate pump or other flow control device to deliver the fluid. This type of implantable drug infusion system is shown generally in FIG. 1. Reservoirs in conventional implanted drug infusion devices undergo little motion, so that the materials which form the reservoir can be selected primarily for compatibility with the drug that is to be delivered. Pumps, on the other hand, typically have many moving parts. The materials from which the pumps are made thus must be selected for specific mechanical properties, such as flexibility, durability, and strength. The materials used may not exhibit all desirable mechanical properties, or may not be compatible with some drugs to be infused. An example is a silicone tube in a peristaltic pump. Silicone is very flexible and durable, but is permeable to many fluids and is not as strong as other elastomers.
With respect to another incongruity of design objectives, because implantable infusion devices preferably are small in size, the devices typically have only limited internal space, so that any materials used in the manufacture of the device must be of the type that can be used to manufacture very small parts. One example of such a material is silicon formed by micromachining. Silicon, however, is difficult to join to other materials.
Along the same lines, although it is desirable to have implantable fluid delivery systems that are small in size, the reduced size of certain conventional devices can be problematic in some applications. For example, pumps or valves with very small passages are not able to effectively deliver certain fluids. Some drugs, such as insulin, often are damaged by shear forces when flowing through such small passages.
Finally, another design incongruity arises with conventional drug infusion devices in which the drug is routed from the reservoir to the pump, and then to the device outlet where it is delivered to a desired location within the body. A certain amount of the drug thus is contained in the pump and fluid passages. This fluid, held in what commonly is referred to as the “dead space,” must be displaced whenever the drug in the reservoir is changed. Delivery systems with relatively more dead space require more unused, unwanted drug to be infused before the drug can be changed.
SUMMARY OF THE INVENTION
The fluid delivery assembly and method of the present invention overcomes the above-noted and other shortcomings of prior drug infusion systems. In accordance with the present invention, a drug or other fluid to be delivered to a specific desired location within the body is stored in a reservoir that is directly displaced by a force to infuse the drug from the device into the patient. Several methods can be used to displace the reservoir, which are described in more detail below. For simplicity the present invention is described herein in terms of three presently preferred embodiments which include, generally, hydraulic displacement, mechanical screw-type displacement, and spring force displacement of the fluid reservoir. However, one of ordinary skill in the art having the benefit of this disclosure will recognize that the invention is not limited strictly to the embodiments described herein. The Figures and other description contained herein is merely illustrative of the present invention. Actual implementation of the present invention in an implantable drug infusion device will undoubtedly vary depending upon the particular circumstances involved with its intended use, particularly with respect to the exact shape, size, specifications, etc. of the device. In any event each embodiment herein described incorporates the teachings of the present invention, and achieves the advantages of (a) eliminating contact and interaction between the drug and, other materials, including those comprising the pump; (b) eliminating small drug passages, for example, through the pump; and (c) of allowing controlled filling and emptying of the drug reservoir.
The first preferred method of displacing the reservoir, given as an example, is hydraulic. See FIGS. 2 and 3. In this configuration, three separate reservoir compartments are used along with a separate pump. The pump does not contact the drug, but recirculates a working fluid inside the device. The working fluid can be selected to be non-corrosive, or even to enhance the operation of the pump. Again, the choice of working fluid depends upon the circumstances involved in a particular application. However, examples of working fluids include sterile water, sterile saline, silicone oil, or a lubricant.
In operation, the pump withdraws the working fluid from the bottom reservoir and pumps it into an intermediate reservoir under the drug reservoir. The drug and intermediate reservoirs are separated by, for example, a diaphragm (See FIG. 2) or a piston (See FIG. 3). Of particular importance is that the drug and intermediate reservoirs be sealed, so that no fluid may pass from one into the other.
As the intermediate reservoir fills, and the drug reservoir is displaced, the drug is forced out of the device. The pump is programmable to any pattern or rate utilized in an implantable device. The drug infusion will follow the rate or pattern programmed. Examples of program rates and patterns include those commonly used with the SynchroMed™ Programmable Pump, available commercially from Medtronic, Inc., Minneapolis, Minn.
A means of maintaining constant pressure in the device, such as a volatile fluid, is required under the internal drug reservoir. To refill the drug reservoir, a syringe is introduced into the reservoir via a hypodermic needle. The drug exit path is closed and the pump is then reversed to draw the drug from the syringe into the reservoir. This method also eliminates the possibility of forcing the drug into the reservoir with the syringe and damaging the device or over-infusing the drug.
In the second preferred embodiment, the internal drug reservoir is directly displaced mechanically, either by a motor and lead screw (See FIG. 4) or a motor and worm gear (See FIG. 5).
In the first configuration of the second preferred embodiment, the motor rotates and displaces a nut on the lead screw. The nut is attached directly to the reservoir base and directly displaces it as it moves. This system is less susceptible to changes in pressure inside the device during operation. However, a volatile fluid still may be used to ensure that a relatively constant internal pressure is maintained. Again, the motor may be programmed to any pattern or rate, such as those used in commercially available implantable devices like the SynchroMed™, and the motor is reversed to refill the drug reservoir. This first configuration is most suited for a donut-type bellows to ensure that no contaminant reaches the interior of the drug reservoir.
In the second configuration of the second preferred embodiment of the present invention, a motor rotates a worm gear that displaces a rack coupled to the bottom portion of the drug reservoir. A flexible side bellows allows the drug reservoir base to be displaced upwards as the worm gear turns, forcing drug from the reservoir to the desired delivery site. Again, the motor may be programmed so that drug is delivered at a desired rate or pattern, and may be reversed to facilitate the refilling of the drug reservoir. The side bellows are hermetically sealed to keep the drug free of possible contamination from unwanted sources, and to protect any adverse impact which might result from their contacting the drugs.
The third embodiment of the present invention utilizes the spring force of the reservoir bellows to displace the drug reservoir. The bellows is annealed, so that in its relaxed, natural state it is completely collapsed. In this configuration (See FIG. 6), the bellows is expanded by a device such as a motor which turns a flywheel or pulley to wind one end of a cable about the pulley or flywheel, the other end of the cable being connected to the bottom of the drug reservoir. Once the reservoir is filled, the pulley/cable is relaxed to allow the bellows to return to its natural, collapsed state. Again, the motor may be programmed to any pattern or rate to ensure that the displaced drug is delivered according to the desired schedule, and the motor may be programmed to allow for controlled refilling.
As stated above, and although the present invention described herein in terms of three separate embodiments, there are certain advantages which all three embodiments share over prior devices. For example, again, in each embodiment the drug contacts only one material. Preferably, the material used is titanium or some other inert material suitable for use in a wide variety of medical applications. Moreover, the active mechanism in each embodiment, a pump or motor, is completely sealed off from the drug to be delivered. Further, there are no small passages for the drug to travel through, and the amount of dead space is minimized. Finally, the active mechanism is able to empty and refill the reservoir, eliminating the need for medical personnel to do so. This minimizes the possibility of overfilling.