The technical field of the invention is that of prefilled and disposable needleless syringes functioning with a gas generator and used for intradermal, subcutaneous and intramuscular injections of liquid active principle for therapeutic use in human or veterinary medicine.
For the injection devices according to the invention, a liquid active principle consists of a more or less viscous liquid, or a mixture of liquid, or a gel. The active principle can be a solid dissolved in a suitable solvent for injection. It can also be represented by a powdered solid in more or less concentrated suspension in a suitable liquid. The particle size of the principle must be compatible with the diameter of the conduits in order to avoid blockages.
The needleless syringes according to the invention have the particular feature of being able to inject a variable amount of liquid active principle by simple modification of their reservoir, all of their other characteristics being integrally retained.
In this way, it is possible to create, both easily and at lower cost, off-the-shelf needleless syringes which are provided with their correct dose of liquid active principle without having to modify their geometry or adapt the gas generator to the dose of liquid active principle to be injected.
Needleless syringes designed to inject variable amounts of liquid active principle already exist and have been the subject of a number of patents.
Mention may be made of the U.S. Pat. No. 4,913,699, for example, which describes a needleless injector intended to contain a variable amount of liquid active principle. The pressure of the liquid leaving the injector is regulated by the difference in cross section between the thrust plunger situated upstream and a plunger stopper of smaller cross section which is accommodated in the terminal injection channel. The patent WO 00/10630 also relates to a needleless injector having, inter alia, the particular feature of expelling a variable amount of liquid active principle by reducing the travel of the thrust plunger. Specifically, the reservoir of active principle, of standard size, is systematically filled to the maximum of its capacity, and the plunger, whose travel. can be reduced by a system of pressure-release vents, will ensure injection of only part of the active principle enclosed in the reservoir. After the injection, some liquid active principle remains in the bottom of the reservoir in cases where the travel of the plunger has been reduced.
Finally, mention may be made of the patent application Ser. No. 95/03844 which relates to a needleless injector which also has the characteristic of being able to inject a variable amount of active principle by reducing the travel of the thrust plunger. In this case, the screwing of a button causes the displacement of the plunger at the same time as the compression of a spring. When the plunger is at the required position, triggering of the injector thus frees said plunger, which is propelled by the released spring.
The needleless syringes according to the invention permit injection of a variable amount of liquid active principle by simple readjustment of the available volume in the reservoir. This is because, whatever the amount to be injected, the reservoir takes up a constant space within the syringe and it is only its internal volume which is modified, without having to redefine the characteristics of the gas generator or of the thrust plunger in each case, and without having to redimension the different compartments of the syringe. The needleless syringes according to the invention thus contain the exact amount of active principle to be injected and fully retain their efficacy, without anything having to be modified, and without any active principle being left at the bottom of the reservoir after injection.
The subject of the present invention is a needleless syringe comprising a gas generator, a reservoir formed by a tube which is closed off by an upstream plunger stopper and a downstream stopper between which the liquid active principle is contained, and an injection device, characterized in that said syringe is able to inject a variable amount of active principle by means of the fact that, through displacement of a downstream plunger stopper in the tube, the volume included between the two plunger stoppers is adapted to the amount of active principle to be injected.
This is because the needleless syringes according to the invention are dimensioned in such a way as to maintain their injection capacity despite a dimensional modification of their reservoir. Throughout the remainder of the patent, the assembly formed by the two plunger stoppers and the active liquid principle will be called the “column of liquid”.
Advantageously, the downstream plunger stopper is at first located entirely within the tube, without protruding from it.
In general, the needleless syringes according to the invention are intended to inject liquid active principle clearly through the skin without loss of said principle on the skin as a result of insufficient speed. The gas generator is thus able, in a first phase, to impart a very high speed to the liquid active principle so as to guarantee the latter an immediate power of penetration through the skin and, in a second phase, to maintain, as a result of almost constant pressure, a sufficient speed of the liquid, so as to guarantee that the latter passes through the skin throughout the duration of injection once perforation has been obtained. The release of the gases by the generator will create a pressurized space between said generator and the upstream plunger stopper due to the displacement of said plunger stopper. This pressurized space, the volume of which increases up to the end of the injection, corresponds to a gas expansion chamber. Said expansion chamber preferably does not exist when the syringe is not functioning. However, it is possible to conceive a needleless syringe according to the invention with a free space already existing between the gas generator and the thrust plunger stopper. Typically, the variation in pressure over time delivered by the gas generator in the space situated between said generator and the upstream plunger stopper causes, in a first stage, the appearance of an instantaneous and very intense pressure peak, then, in a second stage, the establishment of an almost constant pressure which is always greater than the threshold pressure of injection. The term “gas generator” is a generic term designating, without particular distinction, a source of energy which, when it is activated by the user, is able to produce gases in the syringe.
The gas generator is preferably a pyrotechnic gas generator comprising a pyrotechnic charge and an initiation system.
The initiation system advantageously involves a percussion device and a primer. It is also possible to use an initiation system based on a piezoelectric crystal or a roughened area formed by two friction surfaces whose displacement creates an inflammation zone. Compared to a reserve of pressurized gas, a pyrotechnic charge has the advantage of taking up little space and avoids the syringe being permanently subjected to a high internal pressure, including in the storage phase. The pyrotechnic charge is preferably formed by the mixture of a first powder and a second powder, the first powder having a dynamic vivacity greater than that of the second powder. The first powder advantageously has a dynamic vivacity of greater than 8 (MPa.s)−1, and the second, powder advantageously has a dynamic vivacity of less than 16 (MPa.s)−1.
One way of obtaining the pressure profile permitting clean injection of the liquid active principle is in fact to use a pyrotechnic charge consisting of the mixture of a fast-burning powder and a slow-burning powder. A powder is described as “fast-burning” if it has a high dynamic vivacity, and as “slow-burning” if it has a low dynamic vivacity. Thus, the fast-burning powder makes it possible to instantaneously create a very intense pressure peak, while the slow-burning powder, for the rest of the injection, ensures a pressure level which is substantially constant and sufficiently high to allow the liquid active principle to pass through the skin once perforation has been obtained. This type of powder mixture is entirely suitable for the needleless syringes according to the invention. The dynamic vivacity values given above are determined by the formula:
where P is the instantaneous pressure corresponding to the state of advance z.
Pmax is the maximum pressure reached.
dP/dt is the derivative of the pressure over time
They correspond to half-combustion values, that is to say for z=0.5 in a manometric chamber under the following conditions:
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| ||chamber volume || 27.8 cm3, |
| ||charging density ||0.036 g/cm3 |
| ||powder mass || 1 g. |
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The powder referred to as slow-burning will always have a dynamic vivacity less than that of the fast-burning powder. The two powders are advantageously mixed loose, that is to say that the two powders are in the state of particles which are mixed randomly, with no particular order, the resulting powder matching the shape of the container in which it is situated, while at the same time forming interstices between the particles. However, it can also be envisaged that at least one of the two powders is present in an ordered or specific manner, for example in the form of a bundle of strands, or in the form of a single particle of considerable size, or even in agglomerated form.
The volume of the reservoir between the two plunger stoppers can preferably vary from 0.1 ml to 2 ml depending on the injection requirements.
The upstream plunger stopper is advantageously situated, in a fixed manner, at one of the two ends of the tube. At least one plunger is preferably lodged between the gas generator and the upstream plunger stopper. The upstream plunger stopper is preferably in contact with the plunger.
The tube is advantageously made of glass and has a thickness of between 0.5 mm and 4 mm and preferably between 1.5 mm and 2.5 mm. The glass tube must be sufficiently thick to withstand a very high internal pressure, without suffering irreversible damage such as fissuring or even bursting open.
The two plunger stoppers are advantageously made of a deformable material. They are in particular obtained by molding of elastomers which are compatible with the liquid active principle over a long duration. These elastomers can for example be chlorobutyl or bromobutyl. The injection device preferably comprises at least two peripheral injection conduits which are situated outside a hollow component serving as receptacle for the downstream plunger stopper, the depth of said component permitting clearance of the inlets of the peripheral conduits when said plunger stopper comes into contact with the bottom of said hollow part.
In terms of its function, the column of liquid moves until the downstream plunger stopper occupies the hollow component. Once blocked in said component, the plunger stopper deforms slightly so as to clear the inlet of the lateral injection conduits and allow the active principle to be expelled. The length of the tube is preferably constant. Specifically, the needleless syringes according to the invention are intended to receive a reservoir of standard size dictated by the length of the tube, the variations of its internal volume situated between the two plunger stoppers being without consequence on its external dimensions. The plunger is advantageously lodged in a hollow body with the same internal diameter as that of the tube and situated in its continuation, said hollow body and said tube being held in an envelope.
The envelope is preferably made of plastic and exerts a slight pressure on the tube so as to increase its resistance to shearing. In this way, the tube is able to withstand greater internal pressures. According to one alternative embodiment of the invention, the reservoir comprising the tube and the two plunger stoppers constitutes a component which is autonomous of said syringe. In other words, the reservoir can be introduced into the syringe or withdrawn at any moment. Likewise, it can be filled separately, with the exact dose of active principle, then introduced into the syringe.
A free space advantageously exists between the downstream plunger stopper and the bottom of the hollow component. Thus, irrespective of the amount of active principle to be injected, and therefore irrespective of the position of the downstream plunger stopper in the tube, the column of liquid will have to undergo a substantial displacement before the injection proper commences. The smaller the amount of product to be injected, the closer is the position of the downstream plunger stopper to that of the upstream plunger stopper, and the longer is the travel of the column of liquid before injection. The presence of a free space between the downstream plunger stopper and the bottom of the receptacle is particularly suitable for the needleless syringes according to the invention. Assuming that the development of the pressure over the course of time in the expansion chamber is adapted to a clean injection of liquid active principle without any losses, it is imperative, for the small doses of active principle to be injected, that the free space between the downstream plunger stopper and the bottom of the receptacle is sufficiently large to allow the column of liquid to be sufficiently accelerated before the start of the injection. This acceleration is made necessary to compensate for the decrease in pressure in the expansion chamber due to the considerable increase of its volume, induced by the displacement of the column of liquid. In other words, for the small amounts of active principle to be injected, the gases emitted by the gas generator will allow the column of liquid to move with an increasing speed throughout its travel. Hence, the greater the displacement of the column, the greater the energy of its impact on the bottom of the receptacle because of the increasing speed, this despite the significant drop in pressure in the expansion chamber.
By way of example, the rise in pressure in the liquid for a reservoir of 0.5 ml occurs at an earlier stage than for a reservoir of 0.2 ml. This is explained by the fact that, for a quantity of 0.5 ml, the displacement travel is smaller than for a quantity of 0.2 ml. The equal intensities of the maximum pressure peaks for the two configurations corresponding to 0.2 ml and to 0.5 ml is justified by the fact that the kinetic energy of impact of the column of liquid compensates for a decrease in the intrinsic thrust of the liquid, on account of a drop in the gas pressure due to an increase of the volume of the space created downstream of the pyrotechnic charge.
The needleless syringes according to the invention make it possible to inject amounts of liquid active principle by simple adaptation of their reservoir, without having to modify their geometry or their size, or even the gas generator, and without having to add other components. They do not therefore require any redimensioning and no supplementary machining, which are sources of increased costs.
In addition, they remain the same size since the reservoir, irrespective of the amount of liquid active principle to be injected, retains its external dimensions.
Finally, adapting the reservoir remains a simple and precise operation, making it possible to have needleless syringes on hand carrying the exact amount of liquid active principle to be injected.