This invention relates to a nebuliser and in particular but not exclusively to a hand held, self powered, nebuliser for medicinal liquids for use in the delivery of therapeutic agents by inhalation.

It is known from GB2265845 to provide a nebuliser in which fluid in a nebulising chamber is nebulised by a vibrating piezoelectric transducer. Air carrying the nebulised fluid is drawn from the chamber via an outlet duct by inhalation or by fan assisted air flow.

A disadvantage of such nebulisers is that the nebulising process continues during exhalation, thereby subjecting the fluid in the chamber to unnecessary transducer vibration which results in unnecessary heating of the fluid due to the dissipation of energy in the transducer and fluid The nature of the medicinal fluid may consequently be degraded

A further disadvantage is that exhaled air may pass through the nebulising chamber resulting in loss of airborne fluid .

A further disadvantage is that the transducer is actuated by means of a drive circuit which typically comprises a transformer to amplify an oscillating drive signal from a relatively low level to an amplitude of about 50 volts. Such transformers are relatively expensive and also suffer the disadvantage of affecting the resonance characteristics of the transducer by virtue of the leakage inductance of the transformer forming an oscillating circuit with the capacitance of the transducer

According to the present invention there is disclosed a nebuliser comprising a housing defining a nebulising chamber receiving in use a fluid to be nebulised, the housing defining an intake duct for the intake of ambient air

and an outlet duct for the inhalation of air and nebulised fluid in use, the intake duct and outlet duct respectively communicating with the chamber, a transducer operable to nebulise fluid in the chamber, a drive circuit operable to drive the transducer, a breath sensor operable to provide an actuating signal in response to inhalation via the chamber and a control circuit operable to turn on the drive circuit in response to the actuating signal.

The transducer may thereby be energised only when necessary during inhalation in order to avoid excessive heating of the fluid in the chamber and to avoid unnecessary wastage of fluid.

Preferably the breath sensor comprises a sensor element and a sensor circuit operable to heat the sensor element and to detect the cooling effect of air flow over the sensor element.

Conveniently the sensor element comprises a thermistor.

The sensor element is advantageously located in the intake duct.

The housing may comprise a base portion and a detachable portion releasably connectable to the base portion and co-operating with the base portion to define the chamber whereby the transducer, the drive circuit and the control circuit may be located in the base portion and the outlet duct may be defined by the detachable portion.

Such an arrangement allows the detachable portion to be removed for cleansing by immersion in a cleansing solution without causing damage to the transducer and circuits. Frequent cleansing of the outlet duct, which includes a mouthpiece, is generally advisable.

The sensor element may be located on an external surface of the base portion, the detachable portion comprising an intake member co-operating with the external surface to define the intake duct therebetween. The intake member is preferably of U-shaped cross section.

This arrangement allows the sensor element to form part of the base portion and at the same time to be exposed to air within the intake duct.

Conveniently the intake member is movably mounted relative to a main body of the detachable portion so as to be moveable between an open position in which an intake aperture is defined and a closed position in which the intake aperture is closed. The intake aperture may thereby be closed when the nebuliser is not in use in order to prevent the ingress of contaminants into the intake duct.

The housing may comprise an outlet portion defining the outlet duct, the outlet portion being pivotally mounted on the main body of the detachable portion The outlet portion may also be telescopically extensible, thereby providing convenience in use.

Preferably an inlet valve is provided to allow one way flow of air in the intake duct in a direction towards the chamber Exhaled air entering the mouthpiece is thereby prevented from passing through the chamber since no exit is provided via the intake duct.

Preferably an outlet valve communicates between the outlet duct and ambient air to allow one way flow of air in a direction out of the outlet duct, thereby responding to positive pressure in the outlet duct to allow exhaled air entering the mouthpiece to be exhausted without passing through the chamber.

According to a further aspect of the present invention there is disclosed a nebuliser comprising a housing defining a nebulising chamber, a transducer operable to nebulise fluid in the chamber and a drive circuit connected to the transducer and operable to apply an alternating voltage across terminals of the transducer at a controlled operating frequency, wherein the drive circuit comprises a bridge circuit having four arms each comprising a respective switching device, two input poles of the bridge circuit being connected to a supply of DC voltage and two output poles of the bridge circuit being connected to the transducer terminals, and wherein the drive circuit is operable to actuate alternate conjugate pairs of the switching devices.

Such a drive circuit has the advantage of avoiding the need to use a transformer between an oscillating circuit and the transducer

Conveniently the switching devices comprise field effect transistors.

Optionally, an inductor may be connected in series with the transducer in order to boost the peak voltage applied to the transducer terminals.

The transducer preferably comprises a piezoelectric vibrator.

The nebuliser preferably comprises a control circuit operable to determine the operating frequency of the drive circuit and current sensing means operable to input to the control circuit a measure of the current through the transducer, whereby the control circuit is further operable to control the operating frequency so as to minimise the transducer current.

The transducer may thereby be operated at an anti-resonance frequency corresponding to a maximum impedance, thereby minimising the amount of heat generated in the transducer.

The nebuliser preferably comprises a low voltage DC power source and a voltage boost circuit operable to derive the supply of DC voltage therefrom at a value of between 40 and 60 volts.

The control circuit is typically operable to provide an operating frequency in the range 1 .5 to 2.5Mhz.

Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings of which;

FIGURE 1 is a schematic sectional side elevation of a nebuliser in accordance with the present invention; FIGURE 2 is a schematic diagram showing the operation of inlet and outlet valves in the nebuliser of Figure 1 during inhalation;

FIGURE 3 is a further schematic drawing showing the operation of inlet and outlet valves of Figure 2 during exhalation;

FIGURE 4 is a schematic circuit diagram of a control circuit and drive circuit of the nebuliser of preceding Figures;

FIGURE 5 is a schematic oblique view of an alternative nebuliser in accordance with the present invention;

FIGURE 6 is a schematic oblique view of the nebuliser of Figure 5 in a stowed configuration; FIGURE 7 is a schematic oblique view of a further alternative nebuliser in accordance with the present invention;

FIGURE 8 is a schematic oblique view of a further alternative nebuliser in a partially exploded configuration; and

FIGURE 9 is a schematic diagram showing the operation of the transducer of Figure 4

FIGURE 1 shows schematically a nebuliser 1 having a housing 2 formed of plastics material and shaped so as to be hand holdable and portable. The nebuliser 1 is self powered by means of batteries 3 located within the housing 2 and is operated by means of a switch 4 mounted externally on the housing. The housing 2 comprises a base portion 5 and a detachable portion 6 which is releasably connectable to the base portion by co-operating push fit connector formations 7 to enable the detachable portion to be cleaned by immersion in a cleansing solution.

The housing 2 defines a well 8 which in the normal upright orientation of the nebuliser 1 as shown in Figure 1 is capable of holding a quantity of liquid 9 which is required to be nebulised and such that a piezoelectric transducer 10 mounted at the bottom of the well is covered by liquid.

The detachable portion 6 co-operates with the well 8 to define a nebulising chamber 1 1 in which air above the level of the liquid 9 receives a fine mist of nebulised liquid in use when the transducer 10 is vibrationally excited at an ultrasonic frequency.

The detachable portion 6 has an outlet portion 1 2 which is generally tubular and defines an outlet duct 1 3 communicating between the chamber 1 1 and a mouthpiece 14 through which air from the chamber may be inhaled by a user.

The detachable portion 6 also defines an elongate intake member 1 5 which is generally U-shaped in cross section and overlays an external surface 16 of the base portion 5 to define therebetween an intake duct 1 7. An upper portion 18 of the intake member 1 5 overlays a wall 19 of the detachable portion 6, the wall defining an intake port 20 communicating between the intake duct 17 and the chamber 1 1 A one way inlet valve 21 , shown schematically as a simple flap valve, is deployed in the intake port so as to

allow air into the chamber 1 1 via the intake port but to prevent the out flow of air therefrom.

A side wall 22 of the outlet portion 1 2 defines a vent 23 in which is deployed an outlet valve 24 shown schematically as a simple flap valve. The outlet valve 24 allows exhaled air entering the outlet duct to be vented to atmosphere and prevents air from entering the outlet duct via the vent 23 during inhalation.

A sensor element 25 constituted by a thermistor is mounted on the external surface 1 6 so as to be exposed to air within the intake duct 1 7, the sensor element being connected to a sensor circuit 26 as indicated schematically in Figure 9 which is operable to produce an actuating signal 30 representative of the sensed temperature of air in the intake duct.

The actuating signal 30 of the sensor circuit 26 is input to a control circuit 27 which responds to a transient decrease in sensed temperature by actuating a drive circuit 28 which operates to apply a high frequency drive signal to the transducer 10. The transducer 10 responds by dissipating vibrational energy into the liquid 9 with resulting nebulisation.

The control circuit 27 turns off the drive circuit 28 after a predetermined period which in the preferred embodiment is 0.3 seconds. The predetermined period is selected such that nebulisation occurs during an initial phase of the breathing cycle of the user and specifically during the first half of an inhalation period. This ensures that nebulisation begins with the commencement of inhalation, as a result of the cooling affect of the inflow of air through the intake duct 17, and nebulisation ceases before inhalation is completed, thereby avoiding unnecessary nebulisation of liquid since droplets made airborne during the last part of the intake of breath would have a low

probability of being delivered to the intended site of deposition within the lungs of the user because of the finite time taken to travel to this site.

During exhalation, air flow through the intake duct 1 7 ceases because the intake port 20 is closed by operation of the inlet valve 21 .

The operation of the valves 21 and 24 is illustrated schematically in Figures 2 and 3.

In Figure 2 the inhalation of breath is shown to result in air flowing into the intake duct 1 7, through the intake port 20 via the inlet valve 21 , through the chamber 1 1 and through the mouthpiece 14 via the outlet duct 1 3.

During inhalation, the vent 23 is closed by operation of the outlet valve 24 which is closed by negative pressure within the outlet duct 1 3.

Nebulised droplets 29 are made airborne within the chamber by the vibration of the transducer 10 and are carried into the user's lungs via the mouthpiece 14 by the airflow.

At the end of the predetermined period, actuation of the transducer 10 ceases and inhaled air continues to flow through the outlet duct 1 3, carrying with it the remnants of nebulised droplets from the chamber 1 1 . Inhalation then ceases and exhalation commences, thereby creating positive pressure within the outlet duct 1 3 and opening the valve 24 At the same time, positive pressure within the chamber 1 1 causes the inlet valve 21 to close, thereby preventing the flow of air over the sensor element 25 Consequently actuation of the transducer 10 is prevented since the control circuit 27 does not receive a temperature change signal

This cycle of operation repeats with every inhalation of the user.

In Figure 4, the drive circuit 28 is illustrated in greater detail and includes a bridge circuit 31 of four FET (Field Effect Transistors) 32a, 32b, 32c and 32d constituting four arms of the bridge circuit. Two input poles 33a, 33b are connected to a DC supply voltage supplied by a voltage boost circuit 34 which generates a δOvolt DC output from a low voltage provided by the batteries 3.

Output poles 35a, 35b of the bridge circuit 31 are connected to the terminals of the piezoelectric transducer 10, an inductor 36 being connected in series with the transducer.

The FETs 32a to 32d are connected to the outputs of an integrated circuit 37 operable to alternately switch the FETs in conjugate pairs 35a, 35b and 32b, 32d so as to oscillate the transducer 10 at a frequency of approximately 2MHz.

The inductor 36 serves to enhance the peak frequency of the transducer drive.

The transducer current is sensed by means of resistor 38 connected to a current sensing input of the integrated circuit 37 and the temperature of the transducer is monitored by means of temperature sensor 39, also connected to an input of the integrated circuit 37.

The sensor element 25, together with a further temperature sensor 40 for measuring the temperature of the drive circuit, are also connected to respective inputs of the integrated circuit 37.

The control circuit 27 in the form of a micro-controller is connected to the integrated circuit 37 via a bi-directional databus 31 and receives user inputs via manual switch 4.

The control circuit 27 also has outputs to indicator lights 41 which indicate low battery level and also indicate complete exhaustion of the liquid within the well 8.

A 10MHz oscillator 42 is also connected to the control circuit 27 in order to control operation of the micro-controller.

A diagnostic port 43 is also shown connected to the micro-controller 27.

The control circuit 27 controls the drive frequency at which the integrated circuit 37 switches the FETs 35a to 35d in a manner which minimises the sensed value of the transducer current. The current drawn by the transducer 10 will vary as a function of drive frequency and typically exhibits a peak value coinciding with a resonance condition of the transducer. By feed back control of the transducer operation to minimise current, it is therefore possible to operate the transducer at an anti-resonance frequency at which it exhibits a maximum impedance. This impedance will vary according to whether there is liquid remaining in the well 8, a change in the measured value of the sensed current at anti-resonance being detected by the control circuit 27 as an indication of liquid within the well 8 having been exhausted.

Actuation of the drive circuit 28 may then be automatically discontinued and a warning signal displayed via the indicator lights 41 .

Alternative nebulisers are illustrated with reference to Figures 5 to 8 using corresponding reference numerals to those of preceding Figures for corresponding elements where appropriate.

In Figure 5, a nebuliser 1 has an intake member 1 5 which is slidable longitudinally relative to the detachable portion 6 between a deployed position as shown in Figure 5 and a stowed position as shown in Figure 6. In the stowed position of Figure 6, a mouth 44 constituting an intake aperture of the intake duct 17 is closed by contact between the intake member 1 5 and a shoulder portion 45 of the base portion 5. In the deployed position of Figure 5 however, the mouth 44 is opened since the intake member 15 is longitudinally displaced in a direction away from the shoulder 45, thereby allowing the intake of air.

The switch 4 is located on the shoulder portion 45 so as to be accessible only in the deployed configuration of Figure 5, thereby preventing accidental actuation of the nebuliser 1 in the stowed position of Figure 6 when the nebuliser is stored between use.

The outlet portion 1 2 is pivotally connected to the detachable portion 6 so as to define the outlet duct 1 3 and includes an extension portion 46 which is telescopically extensible and which includes the mouthpiece 14 The use of the pivotal connection and the telescopic extension portion 46 allows the nebuliser 1 to be adjustable in use for the comfort of the user.

The extension portion 46 is stowed in a retracted position within the outlet portion 12 which is pivoted so as to extend in contact with the base portion 5 when stowed as shown in Figure 6.

In the stowed configuration as shown in Figure 6, the mouthpiece 14 is shielded against the ingress of contaminates such as dust during storage of the nebuliser 1 This is important in order to prevent the inhalation of contaminates, particularly when administering asthma relieving drugs.

A further nebuliser 1 shown in Figure 7 has similar operating features to those of the nebuliser of Figures 5 and 6 and similarly includes a pivotal outlet portion 1 2 which is shown in a stowed position. The outlet portion 12 is however on the same side of the housing 2 as the switch 4 and the switch is a permanently accessible rotary switch.

A further alternative nebuliser 1 is shown in Figure 8 in a partly exploded configuration, revealing the well 8 defining the nebulising chamber 1 1 . The outlet portion 1 2 is telescopically extensible by means of a slider 47 and pivotally mounted on the detachable portion 6.