Modern medicine has proved that hypodermic syringes must be disposable and must be treated as medical trash after use to avoid secondary cross infection during the process of in-putting or out-taking medium for people or animals. Presently, the most commonly used disposable syringe consists of a hollow barrel, a needle, and a plunger inside the barrel. The biggest problem of this kind of syringe is that the needle of it is still exposed after use. The sharp needle makes the medical trash inconvenient for transportation or destruction Medical staffs are often hurt by the needles, which unavoidably bring secondary cross infection. Especially, the syringes used for patients with serious infectious diseases (such as AIDS, hepatitis and SARS) are great medical hidden dangers which cannot be ignored. Practices have proved that needle stick injuries caused by unsafe needles are much more than that caused by other accidents.
In recent years, as it was found that disposable syringes were taken back and used by unscrupulous persons, an auto-disposal syringe which would automatically become destroyed or locked and could not be reused, was invented at the end of last century. However, currently some syringes have serious shortcomings. Nurses have to withdraw the needle by hand after injection, which makes the injection process complicated and brings inconvenience for doctors and nurses. There is another kind of syringe, of which the needle is retracted into the cylinder under the action of spring after injection. However, the needle is retracted too fast in a sudden and blood will burst out under the body pressure from the needle hole on the body, which brings secondary cross infection.