A syringe of the two-compartment type embodying the structure described above and wherein a bypass is provided through which the liquid can be forced to flow into the compartment containing an agent to be dispersed in that liquid, is described in Europatent publication No. 85 101 508.
In this syringe, one plunger part forms a partition between the two compartments while the other plunger part is attached to the stem and, as the stem is advanced, the second plunger part displaces the liquid and the first plunger part past the bypass until the two plunger parts abut one another, whereupon the plunger formed by these parts is displaced by the stem to eject the composition from the needle.
The syringe described in the Europatent publication is a so-called disposable syringe which is sterilized and prefilled with the injectable substance in the two-component state.
Such two-compartment syringes have been found to be especially effective when the injectable substance is to be in the form of a mixture in a liquid phase or a solution of a component in the liquid which has only a limited effective life in the liquid composition and thus must be fabricated as shortly before use as is possible.
The two components are thus provided in separate compartments and remain stable in these compartments for long periods of time.
Since one compartment is provided axially behind the other and the two substances are only brought into contact with each other immediately prior to injection, it is necessary to provide a means for permitting the liquid from the compartment most distal from the needle to mix with the solid substance, for example, in the compartment proximal to the needle.
This is achieved by providing the bypass immediately ahead of the plunger member which delimits the liquid compartment at its side closest to the needle.
Consequently, as the plunger stem is activated, i.e. displaced into the cylinder or barrel of the syringe, the plunger part most distal from the needle will drive the body of liquid ahead of it and the other plunger part toward the needle until communication is established between the compartments between the plunger parts and the compartment containing the solid component ahead of the plunger parts and between the latter and the needle.
The bypass thus serves to allow transfer of the liquid substance from the upstream compartment to the downstream compartment for mixing with the solid substance when the plunger part separating the two compartments is shifted toward the needle-end of the barrel to an extent sufficient to allow the bypass to communicate between the compartments around this plunger part.
The drawback of this type of syringe, however, is that with an excessive pressure upon the stem or rod of the piston, the liquid can be transferred through the bypass with an excessive velocity so that the substance within the compartment at the downstream side of the bypass will be forced from the needle passage out of the syringe in an uncontrolled manner. This can lead to undesired contamination of the user of the syringe as well as of the patient and may be detrimental to the patient if the administration of a precise quantity of a medicament is essential.