Design of an Implantable Micropump

The implantable programmable micropump is an interesting solution to treat chronic diseases such as diabetes with regular micro-injections of medicine. However, current applications of micropumps are limited by their rather expensive cost. The challenge is therefore to develop a low cost alternative by reducing the number of parts and by simplifying the assembly. As the pump and its tank will be placed under the skin in order to increase comfort, such a system should be small and reliable. In this paper, we present the micropump we developed within the framework of the 4M-μ pump interuniversity project (Methods and Means for the Miniaturization of Machines). INTRODUCTION The explosion of new technologies and particularly recent innovations in the micromechanical and medical areas open up new paths and opportunities to relieve patients’ illnesses. Recent studies have shown that there is a steadily growing market for Microsystems, and in particular for drug delivery systems. According to [5] the drug delivery market is estimated at US $20 billion and is segmented into four categories: oral (53%), inhalation (27%), transdermal (10%) and implanted (8%). The implanted market is growing rapidly. Recently Richard Park [6] reported that the FDA had granted marketing clearance to the first device for diabetics that integrated an insulin Medtronic pump and a Becton dose calculator. These systems constitute a new step in diabetes management which automatically measures the blood sugar concentration then transmits the insulin dosing to the pump. Implanted micro pumps can also be used for the control of refractory cancer pain [7]. An implanted pump permits to reduce the dose and thus to minimize toxicity and "opium" sideeffects. However a study performed by ALCIMED [8] clearly showed the lack of medical implanted programmable pump devices that can be used for specific cancer pain treatment. The only programmable pump available on the market is the SynchroMed® from Medtronic based on US patent 6485464 and following. Other uncertainties upon the use of micro-pumps are lack in medical knowledge in pump implantation and maintenance implanted pump. In the design of the new implanted pump presented in this paper, we try to focus on the cost and in particular on the assembly cost by reducing the number of parts. PUMP SPECIFICATIONS The implanted pump should have a streamlined, flat, small and lightweight shape. A flat ellipsoid for example, affords minimal constraints and maximal comfort to the patient. Adaptable medication flow with flow rates around 0.3 ml per hour and injection unit around 0.2μl covers the demands of the patient and the medical profession. A three-day to three month period between two refilling processes affords sufficient mobility to the patient while three years is the minimum battery life time. Sterilizable biomaterials compatible with body temperature (between 37°C and 42°C), EN-10993 class VI norm [10] and medication are chosen. A negative pressure reservoir and watertightness of the active pump guarantee the safety of the device. CONCEPTUAL DESIGN In a micro-system, many functions need to be fundamentally reconsidered. Scale laws make some physical principles useless for microsystems, while other principles, although without interest in macrosystems, may be extremely useful for miniaturized systems. Pump functions have thus to be carefully analysed during the conceptual design. This analysis is reported in [2]. For example, the hinge function requires particular attention: • classical bearings such as ball bearings, sliding bearings and other pivots may be difficult to realize at the micro scale. As it is very difficult to manufacture small parts with good tolerances, the guiding precision may be insufficient for a particular application. • Assembly of small components may become very difficult, there is therefore a need for a device composed of a minimal number of components • In micromachines, friction may become very important compared to other forces and torques. • In some applications, e.g. in medical devices, cleanliness exigencies practically prohibit the use of greasy lubricants. Consequently, we chose a notch hinge described and analysed in [1]. DESIGN The micropump schematic view is illustrated on Figure 1. The rotating piston is actuated by an electromagnet. A circular notch hinge is used as piston bearing and guiding system. Two globe valves (one inside the piston and one inside the casing) are used to control fluid displacement during piston rotation. It is important to mention that the piston and electromagnet core are made of magnetic stainless steel whereas the casing is made of titanium alloy (EN-TiAl6V4). Figure 1 Pump principle. The micro-pump working principle is illustrated on Figure 2. At the rest position (Figure 2.a.), the piston pushes the input valve ball onto its seat thereby ensuring input valve tightness. This piston thrust is due to the elastic return force of the circular notch. The electromagnet is then powered on to trigger piston rotation (Figure 2.b.). The piston valve ball is pressed onto its seat and a depression occurs in admission chamber. This depression causes the input valve ball to leave its seat and fluid to fill the admission chamber through the input channel. At the other side of the piston, fluid is constrained to leave the ejection chamber through the output channel. This pumping phase ends when the piston reaches its extreme position (Figure 2.c.). The electromagnet is then powered off and, thanks to the elastic return force of the circular notch hinge, the piston returns to its rest position (Figure 2.d.). The overpressure which occurs in the admission chamber causes the input valve ball to be pressed onto its seat. Fluid is transferred from the admission chamber to the ejection chamber through the piston valve. The piston valve ball is no longer maintained on its seat and fluid is free to flow through the piston valve. This pumping phase continues until the piston reaches its rest position (Figure 2.a.). The full pumping cycle is then ready to start over again. Figure 2. Pump principle