Note: Descriptions are shown in the official language in which they were submitted.
WO 94119035 6 ~ PCTlSE94100117
1
Improvements in pistons for infection cartridges
The present invention refers to improvements in
pistons for displacing a liquid inside a cylindrical
chamber, especially in injection cartridges. More Spe-
cifically, the invention refers to an improved shape of
pistons to be used in injection cartridges, which gives
an improved accuracy in the metering and dosing of
preparations from said cartridges.
Pistons to be used in injection cartridges are usual-
ly made of rubber or a similar plastic material having
resilient properties. In the manufacture of such
pistons, one or both of their end surfaces are made
planar and at a right angle to the longitudinal axis of
the piston. Also, the planar end surfaces are often
provided with a number of small projections to prevent
said surfaces from sticking together in the manufacture
and handling of the pistons.
When such a piston is inserted into the barrel of an
injection cartridge, it will be deformed and con-
stricted. This is because the internal diameter of the
barrel must be smaller than the external diameter of
the piston to assure a good seal between the piston and
the internal wall of the barrel. When the piston is
thus deformed elastically, its previously planar end
surfaces will now assume a concave configuration, with
the center of the end surface lying deeper than the
peripheral portions of said surface.
The non-planar configuration of the end surface of
the piston will cause a decrease in the metering accu-
racy when the piston is displaced by a certain distance
to meter out a determined amount of a liquid prepara-
tion.
The degree of concavity is usually of the magnitude
of about 0.1 mm. The displacement of the piston in the
metering out of a dose of liquid is often about 1 mm.
Thus, it will be seen that the concavity of the piston
may cause a metering error of up to 10 percent. An
WO 94/19035 PCT/SE94/00117
2
error of this magnitude is usually not acceptable.
This problem is aggravated in injection cartridges of
the dual-chamber type. As is known, these cartridges
comprise a front chamber, which usually contains the
solid component of the preparation to be injected, and
a rear chamber, which usually contains the liquid
component of said preparation. The two chambers are
separated by a movable front piston, and the rear end
of the rear chamber is sealed by a movable rear piston.
When the two components are to be mixed, the rear
piston is moved forward, and the pressure generated by
this movement is transmitted through the essentially
incompressible liquid to the front piston, which in its
turn is moved forward, This forward movement of the
front piston opens a bypass connection, such that the
liquid in the rear chamber is made to flow over into
the front chamber by the movement of the rear piston
and mix with the solid component to form the desired
injectable preparation. When all of the liquid compo-
nent has been urged over into the front chamber, the
front surface of the rear piston will abut the rear
surface of the front piston, and the two pistons will
now act together as one single piston in metering out
the injectable preparation formed in the front chamber.
However, when the two end surfaces of the two pistons
meet, they will not abut each other completely, as they
are not planar. Due to the deformation, the concave
surfaces formed will at first only abut each other
along their peripheral portions, and this causes a
spring effect, which makes the movement of the two
combined pistons inaccurate. As the front piston cannot
be displaced relative to the barrel without friction,
and as the resting friction is greater than the fric-
tion during movement, the transmittal of the metering
movement, which may be about 1 mm, will not be con-
tinuous. This means that the positioning of the front
piston will not be exact, which will make the metering
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inaccurate.
This is especially apparent with pistons having a
small mass, which are displaced with a low velocity against
a considerable friction resistance. The movement of the
piston rod will then not be transmitted to the front surface
of the front piston without deviations.
The above spring effect will also cause the radial
clamping force exerted on the barrel wall by the pistons to
increase when the pistons are further deformed during the
transmittal of the axial force from the piston :rod. Thus,
the friction force increases when the pistons are to be
displaced. This makes it still more difficult to obtain the
required accuracy in the metering of the injectable
preparation.
Through the present invention, the problems
mentioned above are largely eliminated.
In a broad aspect, the invention provides a liquid
displacement arrangement comprising a cylindrical chamber
and at least one piston of a resilient material arranged in
the chamber, said piston having a) a circumferential part,
and b) at least one end surface, and characterised in that
the end surface is deformation corrected in i) t:hat the end
surface has a convex shape at unconstricted piston diameter
state, and in ii) that the degree of end surface convexity
in the unconstricted state is adapted to give an essentially
planar end surface in a constricted state to a :smaller
piston diameter within said chamber.
In another aspect, the invention provides in a
liquid displacement arrangement comprising a barrel having
at least a section which has an essentially cylindrical
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shape and at least one piston of resilient material wherein
said at least one piston has: a) a substantially
cylindrical mantle surface or circumferential ridges, and b)
at least one end surface substantially transversal to the
cylindrical mantle surface or circumferential ridges, and
the barrel surrounding the piston having a barr~=1 interior
surface in contact with the mantle surface or
circumferential ridges, the improvement comprising that the
piston end surface is deformation corrected, i) in that said
end surface has a convex shape under unstressed conditions
for the piston resilient material, and ii) in that the
degree of end surface convexity under unstressed conditions
is adapted to give an essentially planar end surface shape
under conditions of mantle surface constriction or
circumferential ridges constriction, and that the barrel
internal diameter and the piston external diameter are
adapted to provide said conditions of constriction.
In another aspect, the invention provides an
injection cartridge of a dual-chamber type, comprising a
front piston and a rear piston arranged inside a barrel,
said barrel having at least a section which has an
essentially cylindrical shape and at least one of said front
piston and said rear piston being of resilient material
wherein said at least one of said front piston and said rear
piston has: a) a substantially cylindrical mantle surface
or circumferential ridges, and b) at least one end surface
substantially transversal to the cylindrical mantle surface
or circumferential ridges, and the barrel surrounding the
piston having a barrel interior surface in contact with the
piston mantle surface or circumferential ridges, the
improvement comprising that the piston end surface is
deformation corrected, i) in that said end surface has a
convex shape under unstressed conditions for the piston
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resilient material, and ii) in that the degree of end
surface convexity under unstressed conditions is adapted to
give an essentially planar end surface shape under
conditions of mantle surface constriction or circumferential
ridges constriction, and that the barrel internal diameter
and the piston external diameter are adapted to provide said
conditions of constriction.
In another aspect, the invention provides in a
piston of a resilient material, for use in displacing a
liquid inside a cylindrical chamber wherein, the piston has
an external diameter adapted for causing mantel surface
constriction when placed inside said cylindrical chamber and
having: a) a substantially cylindrical mantle surface or
circumferential ridges, and b) at least one end surface
which is substantially transversal to the cylindrical mantle
surface or circumferential ridges, the improvement
comprising that the piston end surface is deformation
corrected, i) in that said end surface has a convex shape
under unstressed conditions for the piston resilient
material; and ii) in that the degree of end surface
convexity under unstressed conditions is adapted to give an
essentially planar end surface shape under conditions of
mantle surface or circumferential ridges constriction.
According to an embodiment of the invention, a
piston of a resilient material is provided to be used in an
injection cartridge of the dual-chamber type, wherein a
front piston and a rear piston are arranged in a. cartridge
barrel, and at least one of the end surfaces of said piston
has a convex shape in its unconstricted state, such that
when said piston has been placed in said barrel of the
cartridge, said end surface of the piston is essentially
planar.
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In a preferred embodiment of the invention, both
the front and the rear pistons in the cartridge have at
least one end surface with a convex shape, such that the two
end surfaces which are to abut each other in the cartridge
are essentially planar.
The invention is further described by means of the
following specification and the appended drawings. In the
drawings, Figure 1 shows a piston according to the prior art
before it has been placed in the barrel of an injection
cartridge. Figure 2 shows the same piston after it has been
arranged in said barrel. Figure 3 shows the barrel of an
injection cartridge where two
WO 94/19035 PCT/SE94/00117
pistons abut each other with their end surfaces. Figure
4 shows a piston according to the invention before
being inserted into the barrel of an injection
cartridge, and Figure 5 shows the same piston after
having been inserted into said barrel. Figure 6 shows
y
the barrel of an injection cartridge where two pistons
in accordance with the invention abut each other with
their end surfaces. In the figures, like features have
the same reference numbers.
Figure 1 shows a side view of a piston of the prior
art before being inserted into the barrel of an injec-
tion cartridge. The piston 1 has a front face surface
2, which is planar, and a rear face surface 3, which
may be flat, convex or concave and may be provided with
means (not shown) for attaching a piston rod. To
achieve a better sealing effect and a decreased
friction, the piston is usually provided with circum-
ferential ridges 4, 5 and 6. Furthermore, the front
planar surface of the piston may be provided with small
projections 7, to prevent the planar surfaces from
sticking together in the manufacture and handling of
the pistons.
Figure 2 shows a sectional view of a piston of the
prior art which has been inserted into the barrel 8 of
an injection cartridge. As the internal diameter of the
barrel 8 is smaller than the diameter of the piston 1,
the piston will be deformed inside the barrel such that
the originally planar front surface 2 will be deformed
into a concave shape. For reasons of clarity, the con-
cavity of the surface 2 is shown exaggerated in the
f figures .
Figure 3 shows a sectional view of a piston 1 of the
prior art, which has been inserted as a rear piston
into the barrel of an injection cartridge. The piston 1
has been moved forward by means of the piston rod 9
while urging the liquid phase over into a front chamber
10 through a bypass connection (not shown). When all
WO 94119035 ~ PCTISE94/00117
~l~~~fi9
the liquid has been urged over into the front chamber
10, the rear piston 1 will abut against a front piston
1'. This front piston 1' may have the same shape as he
rear piston 1, with an originally planar rear surface
5 and circumferential ridges 4',
a front surface 3'
2'
,
,
and 6'. When this front piston 1' has been inserted
5'
,
into the barrel 8 of an injection cartridge, its rear
surface 2' has been deformed from its previously planar
configuration into a concave configuration, which is
shown exaggerated in the figure for the sake of clari-
ty.
It will be seen that when the front surface 2 of the
rear piston 1 abuts against the rear surface 2' of the
the two surfaces will at first only
front piston 1'
,
contact each other along their peripheral portions. To
make the two surfaces abut each other over their
complete areas, a higher force is necessary. Due to the
resilience of the piston material, a spring effect is
created, like that exerted by a cup spring. Because of
this, a forward movement by the piston rod 9 will not
be accurately transmitted to the front piston 1', and
the metering of a liquid in the front chamber 10 will
therefore be inaccurate. As the metering movements of
the front piston may be quite small, on the order of
.
it follows that inaccuracies
about 1 mm or even less,
in said movements may lead to considerable metering
errors. Such errors may have serious consequences for
the patient receiving the metered injection.
Figure 4 shows a side view of a piston in accordance
with the present invention. The piston 11 has a front
surface 12 which has been made slightly convex. For the
sake of clarity, this convexity has been exaggerated in
the figure. In all other respects, the piston 11 is
similar to the piston 1 shown in figure 1, and has a
rear surface 13, which may be planar, concave or
convex, and circumferential ridges 14, 15 and 16.
However, no projections will be necessary on the front
WO 94/19035 PCT/SE94/00117
6
surface 12, as the convex surfaces will not stick to-
gether easily in the manufacture and handling of the
pistons.
Figure 5 shows a partly sectional view of a piston
according to the invention having been inserted into
r
the barrel 18 of an injection cartridge. As the inter-
nal diameter of the barrel 18 is smaller than the dia-
meter of the piston il, the piston will be deformed
after having been placed in the barrel, and this defor-
to mation will make the originally convex front surface 12
assume a planar configuration, as is shown in the
figure 5 .
Figure 6 shows a partly sectional view of an arrange-
ment similar to that in Figure 3, but using two pistons
in accordance with the present invention. The rear
piston 11 has been moved forward in the barrel 18 of
the injection cartridge, displacing all of the liquid
component through a bypass arrangement (not shown) over
into the front chamber 20. The front surface 12 of the
rear piston 11 now abuts the rear surface 12' of the
front piston 11', and it will be seen that as said two
surfaces are planar in accordance with the invention,
they will contact each other over their complete
surfaces, an no spring effect will arise. Therefore, a
forward movement of the piston rod 19 will be accurate-
ly transmitted through the two pistons 11 and 11' into
the same forward movement of the front piston 11', and
the metering out of the mixed preparation in the front
chamber 20 will be accurate even for very small move-
ments.
In a preferred embodiment of the invention, the rear
surface of the pistons is markedly convex or even more
or less pointed, such that it clearly differs from the
slightly convex front surface of the pistons. Because
of the marked difference between the two surfaces, it
will be easy to orient the pistons correctly by means
of mechanical handling equipment when inserting them
WO 94119035 ,~ 9 PCTISE94/00117
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7
into the barrel of an injection cartridge. At the same
time, no projections will be necessary on the front
surfaces of the pistons, as has been stated in the
foregoing. These projections contribute to the metering
error, but have been considered to be necessary to
prevent the pistons from sticking together with their
planar surfaces. As the front surfaces are (no not
longer) planar when outside of the barrel, there will
be much less risk of the pistons sticking together, and
the projections can thus be dispensed with.
The injection cartridge as such is of a conventional
design and comprises such features as a bypass arrange-
ment, for example a channel in the wall of the barrel,
and a closure at the front end of the cartridge, which
can be pierced by a hollow needle. For reasons of
clarity, and as such features are well-known to those
skilled in the art, these features are not shown in the
figures of the drawing.
When the injection cartridge is to be used for the
metering out and administering of the mixed preparation
in the front chamber, it is usually placed in a holder
device, which may include a metering and dosing
arrangement. Many such devices are well-known to those
skilled in the art, and cartridges including the piston
or pistons of the present invention may be used in them
without any adjustment of their function having to be
made. This is a further advantage of the present in-
vention.
The pistons of the invention may be manufactured from
conventional materials which are commonly used for
pistons in injection cartridges, and no special
materials are necessary. Thus, the person skilled in
the art may select a suitably resilient rubber or
plastic material among those known for this purpose.
The degree of convexity to be given to the surface or
surfaces of the pistons of the invention may be deter-
mined in a number of ways, which are apparent to those
WO 94/19035 , PCT/SE94/00117
~~.~~6~9
s
skilled in the art. The method most close at hand is by
experimentation. It is easy to prepare a number of
pistons having a varying degree of convexity of the end ,
surfaces and subsequently test the pistons by inserting
them in a tube having the same interior diameter as the ,
barrel of the injections cartridge. The test piston
whose end surface becomes planar inside the tube is
then selected.
Another way of determining the desired convexity is
by calculation. In such a calculation, one starts from
the piston in its place in the cylindrical barrel and
having a planar front surface. With knowledge of such
parameters as the dimensions and shape of the piston
and the barrel, and the elastic properties of the mate-
rial used for the piston, one can calculate backwards
to find the desired convexity of the piston before it
is inserted into the barrel. There are computer
programs available for this, using the so-called finite
element method (FEM).
It is important that the internal diameter of the
barrel of the injection cartridge is manufactured with
a high precision and accuracy. Small deviations in the
internal diameter have a profound influence on the
shape of the front surface of the piston when it has
been inserted into the barrel, and thus also on the
metering accuracy.
Except for the convex shape of one or both end sur-
faces, the shape of the pistons of the invention is the
same as that of conventional pistons for use in injec-
tion cartridges. This gives the advantage that the
moulds used for the manufacture of the pistons will
only have to be slightly modified. Also the process for
the manufacture of the pistons will be essentially
unchanged from the conventional process. As the convex
surfaces of the pistons will not have the same tendency
as the conventional flat surfaces to stick together
during the manufacture and handling of the pistons,
PCTISE94100117
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9
there will be no need to provide the surfaces with
small projections.
When necessary, the pistons of the invention are
provided with a suitable attachment for a piston rod.
This attachment may be of any conventional type.
Through the present invention, pistons for use in
injection cartridges are provided which have a number
of advantages in comparison to conventional pistons for
the same use. As the contact surface is planar when the
front and the rear pistons abut each other in an injec-
tion cartridge, the "spring effect" is eliminated. This
leads to an improved accuracy in the metering of the
preparation which is to be administered. Furthermore,
the force necessary to displace the two pistons
together is decreased, which leads to less stringent
requirements of an antifriction treatment of the
intrior wall of the cartridge or the surface of the
piston, such as siliconizing.
In the foregoing specification, the invention has
been described and exemplified with special reference
to the drawing. However, it goes without saying that
other embodiments and variants of the invention are
possible without going outside the scope of the
appended claims. Thus, although the advantages of the
invention are most apparent in the metering of liquid
doses froma dual-chamber injection cartridge, the in-
vention also brings advantages when used in simple
injection syringes and single-chamber cartridges. In
general, the invention can be applied whenever a liquid
is to be precision metered into doses by the displace-
ment of a piston inside a cylindrical or tubular
s
chamber.
