the application refers to hypodermic needles for the parenteral administration of liquids . the needles incorporate at their sharp - end an obstruction of rapidly water - soluble material that dissolves within living tissues once the needle is injected . these needles ensure that the end user can only operate the syringe , or injection device , once the needle has been injected . the use of these hypodermic needles avoids the possibility of the user expelling any amount of liquid prior injection , ensuring the administration of the complete dose . these hypodermic needles are of special interest in preloaded injection devices and in the administration of small volumes of liquid were ejection of part of the dose by the user may result in the administration of only a fraction of the intended dose .