This invention relates to a pump for delivering small volumes of liquids, and is particularly but not exclusively applicable to medical uses. 5 It is known to provide medication, for instance in the relief of post-operative pain, by infusing a liquid by means of a pump having a very low delivery rate. The common technique in present use is based on 10 a standard syringe driven by some form of linear actuator or stepping motor. Such known apparatus is relatively expensive and cumbersome, but does have the advantage that the syringe may be replaced as required by a fresh sterile syringe. 15 An object of the present invention is to provide a pump suitable for these uses which is relatively simple and inexpensive, which allows the parts in contact with the liquid to be interchangeable and disp¬ osable, and which can readily be made in a small, compact 20 form.

The invention accordingly provides a pump comprising a body, an electrical actuator secured to the body, electric drive means for driving the actuator at predetermined intervals and a pumping module removably 25 positioned in the body, the pumping module comprising one-way inlet and outlet valves communicating with a chamber part of whose periphery is formed by a flexible diaphragm which when the pumping module is positioned as aforesaid, is located adjacent the actuator to be 30 driven thereby. _, Said actuator is preferably a solenoid.

Preferably, the pumping module is formed to n provide a positive end stop to the movement of the diaphragm when driven by the solenoid. 35 Preferably also, the diaphragm is of stepped configuration.

The pump may include a reservoir, suitably in the form of a flexible bag contained within said body.

The pump may also include an outlet conduit, e.g. in the form of an injection set, and the reservoir, pumping module and outlet conduit are preferably removable and replaceable as a unit.

Embodiments of the invention will now be described, by way of example, with reference to the drawings, in which: Fig. 1 is a diagrammatic side view of a pump embodying the invention;

Fig. 2 is an end view corresponding to Fig. 1; Fig. 3 is an exploded perspective view of parts of the pump of Fig. 1; and Fig. 4 illustrates the pumping module of the pump of Fig. 1 in a disassembled condition.

Referring particularly to Figs. 1 and 2, the pump has a body or housing 10 within which are mounted a battery 12, a printed circuit board 14 carrying control and driving circuits, a solenoid 16, a pumping module 18, and a reservoir in the form of a flexible plastics bag 20. The inlet of the pumping module 18 is connected to the reservoir 20 by a plastics tube 22, and its outlet is connected to a subcutaneous injection set 24.

In accordance with the invention, the parts 18,20,22,24 can be readily removed from the pump and replaced by a fresh set» Suitably, a complete set of these parts, with the reservoir 20 full of the appropriate solution, can be supplied as a unit in a sterile package. Thus, the circuit board 14 and solenoid 16 are permanently mounted in and are secured to the housing 10, whereas the pumping module 18 is removably located in the housing in a predetermined relationship to the solenoid 16. With some materials having a limited lifetime, e.g. morphine, it is desirable to fill the reservoir immediately before use. For this purpose.

the reservoir 20 is provided with a septum 25.

The pumping module 18, as seen in Fig. 4, comprises a main body 26 to which a diaphragm 28 is secured to define a pump chamber 30. An inlet valve is formed by a ball 32 biased by a spring 34 against a seat in an apertured plug 36 secured (e.g. by adhesive) in a recess 38 in the body 26. Similarly, an outlet valve is formed by ball 32', spring 34', plug 36' and recess 38 ¹ , the valve seat in this case being provided by the body 26.

The centre of the diaphragm 28 is engaged by the end of the solenoid armature. The control circuit applies pulses to the solenoid 16 at predetermined intervals; each pulse causes the armature to drive the centre of the diaphragm 28 until it butts against the opposte face 40 of the pump chmber 30.

The diaphragm 28 is preferably of a stepped form as shown in Fig. 4. This allows the diaphragm to operate without hysteresis and therefore give a uniform quantity of liquid per stroke. It has been found that a flat diaphragm tends to take a permanent deformation after a period of use. The diaphragm 28 is suitably of an elastic plastics material, and may be secured to the body 26 by ultrasonic welding. Suitable forms of electronic circuit for driving the solenoid 16 will be readily apparent to -hose skilled in the art, and are therefore not described in detail. A master clock frequency is divided down and applied to a power amplifier at the desired intervals. Suitably, the driving frequency can be adjusted as desired before the pump is put into use. The electronic circuit may also include, for example, a battery low warning indicator, and a means (e.g. a flashing LED) to indicate that the pump is operational. It would also be possible to include a facility whereby the solenoid is pulsed rapidly after fitting a new pump and reservoir,

to prime the pump. The electronics may suitably be based on a microprocessor.

An experimental pump in accordance with the invention provides 10]i£/stroke and has been operated at 4 stroke/min. The rate may of course be varied widely. Modifications of the above embodiments may of course be made within the scope of the invention. For example, the reservoir may be separated from the pump itself and may take a form other than a flexible bag. The face of the pumping module body opposite the diaphragm may be recessed to give a larger pump chamber or for use with a different shape of diaphragm. The stepped portions of the diaphragm may be inter¬ connected by curved or re-entrant portions. Valves other than ball balves may be used, for example reed valves. The diaphragm could be arranged to be both pushed and pulled by the actuator, thus removing reliance on the elasticity of the diaphragm; for example a short rod could be secured to the diaphragm and provided with interfitting means (such as a tongue and slot connection) for coupling with the solenoid armature.