A needle cannula, a method of producing a needle cannula and use of a needle cannula
The Technical field of the invention:
The invention relates to an elongated tubular needle cannula for injecting a fluid drug into a human body.
The invention furthermore relates to a method of producing such a needle cannula.
Finally the invention relates to the use of such needle cannula in a disposable syringe and in an injection needle assembly.
Description of related art:
A catheter for a spinal anaesthesia procedure is disclosed in US 5.002.535. This know catheter has an outside diameter at the skin piercing distal end which is smaller than the outside diameter at the opposite proximal end in order to provide a strong catheter which only makes a small hole in the dura mater when the catheter is removed thereby preventing leakage of cerebrospinal fluid from the spinal cord. The various diameters of the disclosed catheter is shown in the following table:

Due to the fact that the inside diameter of the skin piercing end is reduced compared to the inside diameter of the opposite end, the passage of fluid through the lumen is somewhat obstructed, and a relatively high pressure is necessary to force the fluid which can be feed into the large diameter at the opposite end through the reduced diameter of the skin piercing end.
When injecting a drug into a human being, either as intramuscular injection or as subcuta-neously injection, the injection needle used has a needle cannula with a substantially smaller diameter than the diameter of the disclosed catheter. Injection needles available today all have a needle cannula with both a uniform outside diameter and a uniform outside diameter,

due to the fact that a needle cannula Is drawn from a tube. Such a prior art needle cannula is illustrated in figure 1.
Some drugs, such as insulin are self-administered, and the typical diabetes person will require subcutaneous injections of insulin several times during the course of the day. Recent studies have indicated that people who inject themselves experience less pain when using a thin needle i.e. a needle cannula having a little outside diameter. In order to reduce the discomfort of having to inject oneself several times a day, injection needles with a very thin needle cannula are preferred among people suffering from diabetes.
The outside diameter of a needle cannula is indicated by a "G" followed by a gauge number, which gauge number increases with thinner needles. At the present, the most commonly used Injection needles among people suffering from diabetes are G30 or G31. Thus the outside diameter of a G 30 is approximately 0,3 millimetres and of a G 31 approximately 0,26 millimetres
Injection needles for insulin delivery pens are disclosed in US 5.462.535. These known injection needles comprises a very thin G 30 needle cannula firmly fastened in a hub for removable mounting the injection needle onto one of the many insulin delivery pens available today.
Description of the invention:
Injection needles having a very thin needle cannula do however present several problems.
Thin injection needles will undergo unintentional deflection when penetrating the tissue of the human body thereby building up a momentum at the junction where the needle cannula is fastened in the hub, which can lead to breakage of the needle cannula at the fastening point.
The injection rate of a thin needle cannula will be very slow, while known injection needles have a uniform inside diameter of the lumen, which decreases when the outside diameter decreases. Injection needles are usually made according to the ISO 9626 standard for Dimensions of tubing. According to this standard a G 30 needle has a minimum inside diameter of 0,133 millimetres whereas a G 31 needle cannula has a minimum inside diameter of

only 0,114 millimetres, thus making the injection rate slower when decreasing the inside diameter of the lumen.
When decreasing the inside diameter of the lumen, the pressure needed to force the fluid drug through the lumen is increased. This again means that people injecting themselves has to press harder on the injection button on the insulin delivery device, which will cause excess pressure in the barrel or cartridge of the syringe.
Due to the reduced lumen of the thinner injection needles, clogging of insulin inside the lumen might occur.
It is henceforth an object of the present invention to provide a thin needle cannula having a reduced sl