Q

where Cs is a constant, relating pressure and quality to droplet size, Qe the mass fraction of vapor phase in the expansion chamber, m a constant relating quality of flow to droplet

Stem

Diaphragm

Metering chamber

Mounting cup

Spring

Metering tank

Tank seal

inhaler. (B) Metering valve.

Aluminum canister i

Suspended

Metering chamber

Actuator

Actuator Metering orifice vaive

Figure 4 (A) Propellant-driven metered-

dose

Stem

Diaphragm

Metering chamber

Mounting cup

Spring

Metering tank

Tank seal

inhaler. (B) Metering valve.

size, n a constant relating pressure to droplet size, Pe and PA are the pressure downstream of discharge orifice and atmospheric pressure, respectively.

A theoretical approach was also taken to predicting the droplet formation from a pMDI, and this was followed by experimental validation studies (48,49).

The container components include a canister, valve, and actuator. Each of these components should be considered in the preparation of a pMDI product. The majority of products marketed are in plastic-coated glass or aluminum containers of various sizes.

Figure 4A shows the assembled components of a pMDI in operation. The therapeutic usefulness of this device stems from the accurate metering of small doses of drug, which is achieved by a small but complex metering valve. The components of a metering valve are shown in Figure 4B.

The performance should be evaluated in terms of drug and component physical and chemical compatibilities. Particle size and emitted dose determinations are required. Through-life performance should be evaluated as this is a multidosing reservoir system. The influence of temperature and humidity on stability and performance of the product should also be considered.

Filling may be conducted at low temperature or high pressure and requires specialized equipment. Low-temperature filling is carried out at a temperature substantially lower than the boiling point of the propellant to allow manipulation at room temperature in an open vessel. Pressure filling is conducted in a sealed system from which the propellant is dispensed at its equilibrium vapor pressure at room temperature through the valve of the container (50).

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