U.S. patent application number 10/240770 was filed with the patent office on 2004-11-11 for valve assembly for metered dose dispensers.
Invention is credited to Groeger, Joseph.
Application Number | 20040222244 10/240770 |
Document ID | / |
Family ID | 9889855 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040222244 |
Kind Code |
A1 |
Groeger, Joseph |
November 11, 2004 |
Valve assembly for metered dose dispensers
Abstract
A valve assembly for a metered dose dispenser is described. A
metering chamber and a pressurised storage reservoir have a
boundary wall that includes a first vent. An opposing wall of the
metering chamber includes a second vent. Each vent includes a seal.
A valve stem is movable via the second vent between depressed
positions in which the inside of the metering chamber is in
communication with the outside via the valve stem and extended
positions in which it is not. Movement of the valve stem through
its extended positions to its depressed positions closes the first
vent before the stem reaches the threshold between its extended and
depressed positions. The limits of the metering chamber are
accurately defined by the main body of the valve assembly. The
first and second seals are positioned between the outer surface of
the insert and the inner surfaces of the cavity. A compression
spring that surrounds the valve stem acts between the outside of
the metering chamber and the outside of the valve stem to bias the
valve stem to its extended positions. Accordingly, product from the
metering chamber does not come into contact with the spring as it
is dispensed via the valve stem. A ferrule attaches the valve
assembly to the storage reservoir and includes an opening for
passage of the valve stem. The ferrule encapsulates the compression
spring and helps reduce side-streaming.
Inventors: |
Groeger, Joseph; (Storrs,
CT) |
Correspondence
Address: |
ROTHWELL, FIGG, ERNST & MANBECK, P.C.
1425 K STREET, N.W.
SUITE 800
WASHINGTON
DC
20005
US
|
Family ID: |
9889855 |
Appl. No.: |
10/240770 |
Filed: |
December 19, 2002 |
PCT Filed: |
April 17, 2001 |
PCT NO: |
PCT/GB01/01692 |
Current U.S.
Class: |
222/402.1 |
Current CPC
Class: |
B65D 83/425 20130101;
B65D 83/54 20130101 |
Class at
Publication: |
222/402.1 |
International
Class: |
B65D 083/14 |
Claims
1. A valve assembly for a metered dose dispenser comprising: a
metering chamber having a wall that is adapted to form a common
boundary between the metering chamber and a pressurised storage
reservoir to which it is to be attached and in which the boundary
wall includes a first vent that allows the metering chamber to
communicate with the storage reservoir and an opposing wall of the
metering chamber includes a second vent, the first and second vents
including first and second seals respectively; a valve stem that is
movable via the second vent, in sealing contact with the second
seal, between depressed positions in which the inside of the
metering chamber is in communication with the outside via the valve
stem and extended positions in which the inside of the metering
chamber is isolated from the outside; in which movement of the
valve stem through its extended positions to its depressed
positions causes sealing contact with the first seal, thus closing
the first vent, before the stem reaches the threshold between its
extended and depressed positions; and the limits of the metering
chamber are defined by a hollow insert that lies between the first
and second seals, both seals being positioned outside the metering
chamber.
2. A valve assembly according to claim 1 in which the proportion of
the surface area of the metering chamber provided by the seals is
no greater than 10%.
3. A valve assembly according to claim 2 in which the proportion of
the surface area of the metering chamber provided by the seals is
no greater than 5%.
4. A valve assembly according to claim 3 in which the proportion of
the surface area of the metering chamber provided by the seals is
2% or less.
5. A valve assembly according to any one of claims 1-4 in which the
insert is metallic.
6. A valve assembly according to claim 5 in which the insert is
formed from stainless steel.
7. A valve assembly according to any one of claims 1-6 comprising a
main body having a cavity within which the insert is positioned and
in which the first and second seals are positioned between the
outer surface of the insert and the inner surfaces of the
cavity.
8. A valve assembly for a metered dose dispenser comprising: a
metering chamber having a wall that is adapted to form a common
boundary between the metering chamber and a pressurised storage
reservoir to which it is to be attached and in which the boundary
wall includes a vent that allows the metering chamber to
communicate with the storage reservoir; and a valve stem that is
movable between depressed positions in which the inside of the
metering chamber is in communication with the outside to allow
product to be dispensed from the metering chamber via the valve
stem and extended positions in which the inside of the metering
chamber is isolated from the outside; in which movement of the
valve stem through its extended positions to its depressed
positions closes the vent before the stem reaches the threshold
between its extended and depressed positions; and a spring is
provided acting between the outside of the metering chamber and the
outside of the valve stem to bias the valve stem to its extended
positions, such that product from the metering chamber does not
come into contact with the spring as it is dispensed via the valve
stem.
9. A valve assembly according to claim 8 in which the spring is a
compression spring.
10. A valve assembly according to claim 9 in which the spring is a
helical compression spring that surrounds the valve stem.
11. A valve assembly according to claim 9 or claim 10 further
including a ferrule for attaching the valve assembly to the storage
reservoir, the ferrule including an opening for passage of the
valve stem and being adapted, with the walls of the metering
chamber, to encapsulate the compression spring.
12. A valve assembly according to claim 11 in which the spring is a
conical compression spring and in which movement of the valve stem
through its extended positions to its depressed positions causes
the coils of the spring to nest within one another as the spring is
compressed.
13. A valve assembly for a metered dose dispenser substantially as
described herein with reference to any one of FIGS. 2-4 of the
accompanying drawings.
14. A metered dose dispenser comprising a pressurised storage
reservoir and a valve assembly according to any preceding claim
attached to it so that the common boundary wall is a common
boundary of the metering chamber and the storage reservoir.
15. A valve assembly for a metered dose dispenser comprising: a
metering chamber having a wall that is adapted to form a common
boundary between the metering chamber and a pressurised storage
reservoir to which it is to be attached and in which the boundary
wall includes a first vent that allows the metering chamber to
communicate with the storage reservoir and an opposing wall of the
metering chamber includes a second vent, the first and second vents
including first and second seals respectively; a valve stern that
is movable via the second vent, in sealing contact with the second
seal, between depressed positions in which the inside of the
metering chamber is in communication with the outside via the valve
stem and extended positions in which the inside of the metering
chamber is isolated from the outside; in which movement of the
valve stem through its extended positions to its depressed
positions causes sealing contact with the first seal, thus closing
the first vent, before the stem reaches the threshold between its
extended and depressed positions; and the valve stem includes a
friction-reducing coating applied to at least those parts of it
that contact the first and/or second seals.
Description
BACKGROUND TO THE INVENTION
[0001] The present invention relates to a metered dose dispenser.
Metered dose dispensers find many practical applications. One
application that is very tightly regulated, placing many
restrictions on the design, construction and manufacture of the
metered dose dispenser, is the metered dose inhaler. In this
application, the metered dose dispenser is typically used to
dispense a measured dose of a pharmaceutically active substance
from a pressurised aerosol canister into an airway through which
inhalation takes place.
[0002] A valve assembly for a typical metered dose inhaler is
illustrated in FIG. 1. It is designed for attachment to a
pressurised canister within which is contained a drug product and a
propellant. The valve assembly constitutes a metered dose
dispenser, or metering valve, the purpose of which is to allow
doses of the drug product, of a controlled size, to be dispensed
into the inhalation airway via a metering chamber 10. The boundary
wall 12 between the metering chamber 10 and the pressurised
canister, forming the lower wall of the metering chamber 10,
includes a first vent 14 that allows the metering chamber 10 to
communicate with the pressurised canister. A second vent 16 is
provided in the opposite, upper wall 18 of the metering chamber 10
and both vents 14, 16 are occupied by a valve stem 20 which bears
against a circular elastomeric seal in each 22, 24. The valve stem
20 is in the form of a tube that is closed at its lower, inner-most
end 26 and open at its upper, outermost end 28 and includes a side
hole 30 communicating between the inside and the outside of the
valve stem 20 and an indented channel 32 towards its lower end 26
that interrupts its normally circular cross-section.
[0003] The valve stem 20 is normally in an elevated or extended
position, in which the channel 32 is located within the first vent
14 and allows the canister to communicate with the metering chamber
10. This primes the metering chamber 10. As the valve stem 20 is
depressed against the bias of a spring 34, firstly the channel 32
moves below the seal 22 in the first vent 14, isolating the
canister from the metering chamber 10. This is the situation as
illustrated in FIG. 1. Then the side hole 30 moves below the seal
24 in the second vent 16, allowing the metering chamber 10 to
communicate with the outside via the side hole 30 and the open end
28 of the valve stem 20. This discharges the pressurised metering
chamber 10 to the outside via the side hole 30 and the open end 28
of the valve stem 20. Typically, the open end 28 of the valve stem
20 is located in a rebate in an inhaler body that acts as a bearing
surface for depression of the valve stem 20 and also provides a
conduit for the drug to be dispensed into the inhalation
airway.
[0004] Such metered dose inhalers have to undergo type approval or
product authorisation before they can be put on the market in the
US or the EU. In the US, product authorisation is granted by the
Food and Drug Administration ("FDA"). Hitherto, the propellant used
in aerosol canister for inhalers has been a chlorofluorocarbon
("CFC"). These are inert gases that liquefy under relatively low
pressure, allowing a constant vapour pressure within the canister
to be maintained throughout the life of the canister CFC
propellants in many aerosol devices, such as domestic aerosols,
deodorants, polishes, etc., have been or are shortly to be banned
both in the US and the EU. It is expected that the same will happen
with metered dose inhaler canisters at some point in the near
future and accordingly the industry has for some time been
searching for an acceptable alternative to CFC propellants.
[0005] Various alternatives to CFCs have been proposed, such as
other fluorinated hydrocarbons, but it has been found that these
alternative propellants have their own problems. For example, the
drug product may be more inclined to agglomerate, particularly if
it is stored in the canister as an emulsion or suspension, or to
stick to the surfaces of the inhaler with which it makes contact,
such as the inside of the canister. The latter problem has been
addressed with some measure of success by coating the inside of the
canister with PTFE or other non-stick compounds. The former problem
has been addressed by including in the formulation of drug and
propellant a surfactant of an appropriate kind. The surfactant can
help to keep the droplets of an emulsion stable, to prevent
droplets of the drug from adhering to the walls of the canister or
to one another and can also act as a lubricant for the valve
stem.
[0006] However, it is now suggested that the US FDA will in due
course discourage the use of surfactants in drug formulations for
metered dose inhalers. This brings to the foreground the
deficiencies in conventional dose metering systems that are caused
or exacerbated by the switch from CFC propellants. These
deficiencies include inconsistent dose sizes, a rate of leakage of
product from the canister, usually via a path different from the
dispensing path, that is higher than is desirable and friction
between the moving parts of the inhaler causing inconsistent
dosing, or poor perception of quality on the part of the user.
Other deficiencies include the migration or extraction of additives
from the materials used in the manufacture of the elastomeric seals
into the drug in storage or as it is dispensed. The embodiments of
the present invention, as illustrated in and described with
reference to FIGS. 2-4, are designed to address a number of these
deficiencies. Different aspects of the present invention, as set
out below, address different deficiencies.
SUMMARY OF THE INVENTION
[0007] Lack of dose uniformity can have a number of causes, but the
most common and perhaps the most significant is that the limits of
the metering chamber, and hence its volume, are defined in
substantial part by the seals that lie between the walls of the
metering chamber and the valve stem. These steals are often tightly
crimped into position and highly stressed to ensure that the
integrity of the seal with the moving valve stem is good when the
device is new. However, as the device ages, the stressed
elastomeric compounds of the seal tend to creep and this changes
their shape. Not only is the integrity of the seal adversely
affected, allowing an increased leakage rate, but also the changing
shape of the seal causes a change in the volume of the metering
chamber, making for inconsistent dose sizes as the device ages.
Further changes in shape of the seal may be occasioned by swelling
of the seal material as it absorbs the medicament formulation over
time. FIG. 1 illustrates a valve assembly of just this kind, in
which the top of the metering chamber is defined by the seal 24 in
the second vent 16.
[0008] Another valve assembly of this kind is the subject of
international patent application no. PCT/GB88/00197.
[0009] Again, the top of the metering chamber is defined by the
seal in the upper, second vent. Although the valve stem as
illustrated in this document includes a relatively large sealing
flange that covers a substantial portion of the upper seal when the
valve stem is in its extended position, the size of the metering
chamber is only defined once the stem has been partially depressed
to a position at which its lower end makes sealing contact with the
lower, first vent. At that position, the stem flange is clear of
the upper seal and the upper boundary of the metering chamber is
therefore defined by the seal.
[0010] The present invention is designed to deal with this problem.
It is proposed that the limits of the metering chamber should be
defined by a hollow insert rather than the first and second seals.
Accordingly, a first aspect of the present invention provides a
valve assembly for a metered dose dispenser comprising:
[0011] a metering chamber having a wall that is adapted to form a
common boundary between the metering chamber and a pressurised
storage reservoir to which it is to be attached and in which the
boundary wall includes a first vent that allows the metering
chamber to communicate with the storage reservoir and an opposing
wall of the metering chamber includes a second vent, the first and
second vents including first and second seals respectively;
[0012] a valve stem that is movable via the second vent, in sealing
contact with the second seal, between depressed positions in which
the inside of the metering chamber is in communication with the
outside via the valve stem and extended positions in which the
inside of the metering chamber is isolated from the outside;
[0013] in which movement of the valve stem through its extended
positions to its depressed positions causes sealing contact with
the first seal, thus closing the first vent, before the stem
reaches the threshold between its extended and depressed positions;
and
[0014] the limits of the metering chamber are defined by a hollow
insert that lies between the first and second seals, both seals
being positioned outside the metering chamber.
[0015] Because the hollow insert lies between the two seals, and
defines limits of the metering chamber, the seals lie outside the
metering chamber, that is to say they play no palpable part in
defining the volume of the metering chamber. Therefore, even if the
material from which the seals are made does creep as it ages, this
will not affect the volume of the metering chamber.
[0016] The surface area of the metering chamber is in part provided
by the insert, in part by the stem and in part by the seals, owing
to the necessary clearances between the stem and the insert.
Subject to this limit, the proportion of the surface area provided
by the seals should be as low as possible. In the present
invention, it is preferred that this proportion be no greater than
10%, but for best effect it should be no greater than 5%. As the
lower limit defined by the necessary clearances between the stem
and the insert is approached, a proportion of 2% or less can be
achieved.
[0017] So as to ensure that the volume of the metering cavity, as
defined by the insert, remains constant under different conditions
and throughout the life of the device, it is preferable that the
insert should be structurally rigid and dimensionally stable. It is
also very important that the volume of the metering chamber can be
accurately and consistently defined during the manufacturing
process. This can be achieved with a metallic insert, for example
one formed from stainless steel, since metal can be formed to very
tight tolerances, by conventional deep-drawing or rolling
processes. Alternatively, a suitable engineering plastic could be
used.
[0018] The insert may be a one piece insert. If metallic, it may be
deep-drawn to form sides and base and then rolled to form the top.
Alternatively it may be cast or moulded using a sacrificial core.
These manufacturing techniques are complex or expensive. On the
other hand, the insert may be in two pieces with, for example, a
dish-shaped base and an inverted dish-shaped top. Either could be
made by deep-drawing, or conventional casting or moulding, but
there will be a line of leakage where the two parts meet at the
waist of the insert. This may not necessarily be a problem so long
as the two parts are accurately manufactured and located.
[0019] The amount of stress to which the seals need to be subjected
can be minimised if the main body of the valve assembly has a
cavity within which the insert is positioned and in which the first
and second seals are positioned between the outer surface of the
insert and the inner surfaces of the cavity. This can accurately
locate the seals ensuring a good seal at lower levels of applied
stress.
[0020] These lower levels of stress help to ensure that the
likelihood of the valve stem sticking to the seals, or not
operating as smoothly as would be desired, because of friction is
reduced. A further improvement of the seal-stem boundary
characteristics can be achieved by coating the valve stem with a
friction-reducing coating, for example a coating of bonded
fluoropolymer, silicones or fluoro-silicones. Examples of materials
for use in a suitable application process are aqueous suspensions
of PTFE, PVDF or PFA. Accordingly, the present invention further
provides a valve assembly for a metered dose dispenser
comprising:
[0021] a metering chamber having a wall that is adapted to form a
common boundary between the metering chamber and a pressurised
storage reservoir to which it is to be attached and in which the
boundary wall includes a first vent that allows the metering
chamber to communicate with the storage reservoir and an opposing
wall of the metering chamber includes a second vent, the first and
second vents including first and second seals respectively;
[0022] a valve stem that is movable via the second vent, in sealing
contact with the second seal, between depressed positions in which
the inside of the metering chamber is in communication with the
outside via the valve stem and extended positions in which the
inside of the metering chamber is isolated from the outside;
[0023] in which movement of the valve stem through its extended
positions to its depressed positions causes sealing contact with
the first seal, thus closing the first vent, before the stem
reaches the threshold between its extended and depressed positions;
and
[0024] the valve stem includes a friction-reducing coating applied
to at least those parts of it that contact the first and/or second
seals.
[0025] Metered dose inhalers conventionally require a spring to
bias the valve stem back to its extended positions once it has been
depressed to dispense a dose of drug product. However, the
conventional positioning of such springs has lest them prone to
attract deposits of the drug product. Uniformity of dosing is
compromised when the active drug is deposited, then later released.
In addition to forming a drug adhesion site, the spring can suffer
corrosion.
[0026] Both factors can lead to poor quality of operation of the
valve stem, which may result in incomplete dosing. It can also
interfere with the free flow of drug product out of the device.
Again, FIG. 1 show just such an arrangement, in which the spring 34
acts between a cup 36, in which the closed end 26 of the valve stem
20 is received, and the base 38 of a perforated cage 40. It is
permanently exposed to drug product within the pressurised
canister.
[0027] The present invention is designed to deal with this problem.
It is proposed that the spring should act between the outside of
the metering chamber and the outside of the valve stem, so that
product from the metering chamber does not come into contact with
it. Accordingly, a second aspect of the present invention provides
a valve assembly for a metered dose dispenser comprising
[0028] a metering chamber having a wall that is adapted to form a
common boundary between the metering chamber and a pressurised
storage reservoir to which it is to be attached and in which the
boundary wall includes a vent that allows the metering chamber to
communicate with the storage reservoir; and
[0029] a valve stem that is movable between depressed positions in
which the inside of the metering chamber is in communication with
the outside to allow product to be dispensed from the metering
chamber via the valve stem and extended positions in which the
inside of the metering chamber is isolated from the outside;
[0030] in which movement of the valve stem through its extended
positions to its depressed positions closes the vent before the
stem reaches the threshold between its extended and depressed
positions; and
[0031] a spring is provided acting between the outside of the
metering chamber and the outside of the valve stem to bias the
valve stem to its extended positions, such that product from the
metering chamber does not come into contact with the spring as it
is dispensed via the valve stem.
[0032] Naturally, since the spring is outside the metering chamber
and outside the valve stem, and the dispensing path is from the
inside of the metering chamber via the inside of the valve stem,
this arrangement leaves the spring free from the deficiencies
identified above.
[0033] A suitable arrangement would involve the use of a helical
compression spring that surrounds the valve stem. This arrangement
has advantages over and above those previously identified, as will
now be elaborated.
[0034] Conventional metered dose inhalers suffer to a greater or
lesser degree from the leakage of product from the canister,
usually via a path different from the dispensing path, that is
higher than is desirable. This may be caused by ineffective seals,
perhaps again attributable to seal material creeping over time. It
may on the other hand be caused by what is known as
"side-streaming." Side streaming takes place when lateral as
opposed to axial forces are applied to the valve stem as it is
depressed. Lateral forces cause the valve stem to lever against one
or both seals, pushing them in opposite directions If the pressure
is sufficient, the integrity of the seal opposite the side at which
it is being pushed by the valve stem can break down, allowing the
escape of drug product. This escape is known as "side-streaming"
for obvious reasons.
[0035] The valve assembly of a typical metered dose inhaler is
attached to the pressurised canister by a ferrule of an appropriate
form. The ferrule includes an opening for passage of the valve stem
and this overlies the second vent at the top of the metering
chamber. However, where a helical compression spring is to be
accommodated, the ferrule must be raised, in the region of the
opening, to form a crown that makes room for the spring. This,
ensures that the ferrule encapsulates the spring, protecting it
from the environment, but also means that there are now three
openings through which the valve stem must pass: the first and
second vents and the opening in the ferrule, all of which are
spaced apart from one another. This substantially reduces the
incidence of side streaming, since the extent to which the valve
stem can lever against the seals is limited by the opening in the
ferrule.
[0036] Accordingly, it is preferred that the valve assembly of the
second aspect of the present invention should include a ferrule for
attaching the valve assembly to the storage reservoir, the ferrule
including an opening for passage of the valve stem and being
adapted, with the walls of the metering chamber, to encapsulate the
helical compression spring.
[0037] Additional benefits can result from the use of a conical
compression spring that is capable of being compressed to the
height of a single coil, as opposed to a helical spring As the
coils of the spring are obliged to nest within one another as the
conical spring is compressed, this can help to stabilise and centre
the valve stem over the second vent, further reducing the
likelihood of side-streaming.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0038] The present invention will now be described by way of
example with reference to the accompanying drawings in which:
[0039] FIG. 1 is a conventional valve assembly for a metered dose
inhaler;
[0040] FIGS. 2-4 show essentially similar metered dose inhalers
according to the present invention, of different nominal dosing
volumes; and
[0041] FIG. 5 shows a metered dose inhaler that uses a conical
spring.
DETAILED DESCRIPTION
[0042] The conventional inhaler of FIG. 1 has already been
described. FIG. 2 shows a metered dose inhaler according to the
present invention having a nominal dose size of 100 .mu.l. The
inhaler consists of a pressurised aluminium, stainless steel, glass
or plastic canister 40 that acts as a reservoir for a drug product,
upon which is mounted a valve assembly that acts as a metered dose
dispenser, or metering valve. An engineering plastic main body 42
of the valve assembly is located over the open end of the canister
40 and includes an annular recess 44 that provides a seat for an
annular sealing ring 46. The main body 42 is secured to the end of
the canister 40 by a ferrule 48 or formable material, such as
stainless steel, aluminium alloy or engineering plastic. The
ferrule 48 shown in FIG. 2 has been partially formed, but requires
a further forming operation to conform to the exterior shape of the
end of the canister 40, so as to secure the main body 42 in
place.
[0043] The main body 42 includes a central, substantially
cylindrical cavity that extends to the top of the main body 42, but
not as far as the base. A hole 50 in the base of the main body 42
is provided to form part of a first vent between the metering
chamber 10 and the canister 40.
[0044] Thus, the base of the main body 42 provides a part of a
boundary wall between the metering chamber 10 and the pressurised
canister 40. The remainder of the boundary wall is provided by a
first seal 22 and an insert 52, which will be described later. Both
the hole 50 and an equivalent hole 54 in the base of the insert 52
are shown fluted. This is to aid rapid pressure filling of the
canister.
[0045] The central, cylindrical cavity in the main body 42 of the
valve assembly contains the first elastomeric seal 22, the insert
52 and a second elastomeric seal 24. Above the second seal 24 is a
thermoplastic bushing 56. The top of the insert 52, the second seal
24 and the bushing 56 constitute the upper wall of the metering
chamber 10. A hole 58 in the bushing 56 forms part of a second
vent. These various components are secured in position by the
ferrule 48.
[0046] It will be appreciated that, in FIG. 2, the volume of the
metering chamber is defined almost wholly by the insert 52 and, of
course, the valve stem 20. The insert 52 is a bare clearance fit
within the cavity in the main body 42, so is accurately positioned
within it. Therefore, there is very little likelihood of the volume
of the metering chamber 10 changing over time or being affected by
rough handling. The seals effectively lie outside the metering
chamber and play no palpable part in defining the volume of the
metering chamber. Therefore, even if the material from which the
seals are made does creep as it ages, this will not affect the
volume of the metering chamber.
[0047] The insert 52 provides about 80% of the surface area of the
metering chamber. About 18% of the surface area is provided by the
stem 20 and about 2% by the seals 22, 24.
[0048] In FIG. 2, the insert is shown as a one piece insert. It is
formed from stainless steel, deep-drawn to form the sides and base
and then rolled to form the top. Stainless steel is chosen because
of its excellent rigidity and dimensional stability as compared
with other materials, its strength and workability. It is very
important that the volume of the metering chamber can be accurately
and consistently defined during the manufacturing process. This can
be achieved with stainless steel, since it is easy to work to very
fight tolerances, by conventional drawing or rolling processes
[0049] Stainless steel is the preferred material, but other
materials can be used to ensure that the volume of the metering
cavity remains constant under different conditions and throughout
the life of the device. For example, a suitable engineering
plastics could be used instead of stainless steel.
[0050] The first and second seals 22, 24 are positioned between the
outer surface of the insert 52 and the inner surfaces of the cavity
in the main body (for which purpose the bushing 56 :s regarded as
apart of the main body 42). This accurately locates the seals
ensuring a good seal at levels of applied stress lower than is
conventional. These lower levels of stress help to ensure that the
likelihood of the valve stem sticking to the seals, or not
operating as smoothly as would be desired, because of friction is
reduced.
[0051] Both vents are occupied by a valve stem 20 which bears
against the seals 22, 24. The valve stem 20 is formed from
deep-drawn stainless steel into the form of a tube that is closed
at its lower, innermost end 26 and open at its upper, outermost end
28 and includes a side hole 30 communicating between the inside and
the outside of the valve stem 20 and one or more indented channels
32 towards its lower end 26 that interrupt its normally circular
cross-section. A further improvement of the seal-stem boundary
characteristics can be achieved by coating the valve stem with a
friction-reducing coating, for example a coating of bonded
fluoropolymer, silicones or fluorosilicones. Examples of materials
for use in a suitable application process are aqueous suspensions
of PTFE, PVDF or PFA.
[0052] The valve stem includes a crimped waist 60 that forms an
abutment for one end of a helical compression spring 62, the other
end of which bears against the bushing 56. To accommodate the
spring, the ferrule 48 is formed into a crown 64 through which the
open end 28 of the valve stem 20 passes. The opening in the crown
64 is smaller than the waist 60 of the valve stem, preventing it
from coming free. Accordingly, the ferrule 48 and the bushing 56
encapsulate the spring, protecting it from the environment, In
addition, the crown 64 of the ferrule 48 provides a third openings
through which the valve stem must pass: the first and second vents
and the opening in the ferrule. All these openings are spaced apart
from one another. This substantially reduces the incidence of
side-streaming, since the extent to which the valve stem can lever
against the seals is limited by the opening in the ferrule,
combined with those of the bushing and the lower vent.
[0053] A conical compression spring could be used as opposed to a
helical spring. The coils of a conical spring are obliged to nest
within one another as the conical spring is compressed. If adjacent
coils contact one another, this can help to stabilise and centre
the valve stem 20 over the second vent, further reducing the
likelihood of side-streaming. An inhaler that uses a conical spring
is illustrated in FIG. 5. As can be seen from this figure,
depression of the valve stem will result in adjacent coils of the
spring nesting within one another.
[0054] The valve stem 20 is normally in an elevated or extended
position as shown in FIG. 2, in which the channel 32 is located
within the first vent and allows the canister 40 to communicate
with the metering chamber 10. This primes the metering chamber 10.
As the valve stem 20 is depressed against the bias of the spring
62, firstly the channel 32 moves below the seal 22 in the first
vent, isolating the canister 40 from the metering chamber 10. Then
the side hole 30 moves below the seal 24 in the second vent,
allowing the metering chamber 10 to communicate with the outside
via the side hole 30 and the open end 28 of the valve stem 20. This
discharges the pressurised metering chamber 10 to the outside via
the side hole 30 and the open end 28 of the valve stem 20. Movement
of the valve stem is arrested by complete compression of the spring
62 causing its coils to contact one another.
[0055] FIG. 3 shows an inhaler similar to that of FIG. 2, but is
designed to a nominal dose size of 50 .mu.l. Many of the same
components are used in FIG. 3 as are used in FIG. 2 and the only
real differences are these: the hollow insert 52 is smaller, the
seals 22, 24 are smaller in outer diameter and the bushing 56 is
replaced by a sleeve 66. The Sleeve sits between the outside
diameter of the insert 52 and the inside diameter of the cavity
within the main body 42, ensuring that the smaller insert 52 is
accurately located. The sleeve 66 also provides a seat for the
pouter diameters of the seals 22, 24, retains the upper seal 24 in
place and provides a bearing surface for the lower end of the
spring 62.
[0056] In FIG. 3, the insert 52 provides about 65% of the surface
area of the metering chamber. About 33% of the surface area is
provided by the stem 20 and about 2% by the seals 22, 24.
[0057] FIG. 4 shows an inhaler essentially similar to that of FIG.
3, but with a nominal dose size of 25 .mu.l. It has a yet smaller
insert 52, smaller seals 22, 24 and a thicker sleeve 66. In FIG. 4,
the insert 52 provides about 55% of the surface area of the
metering chamber. About 43% of the surface area is provided by the
stem 20 and about 2% by the seals 22, 24.
* * * * *