Hypodermic syringe needles are sterile, single-use devices used for injecting medicinal substances hypodermically or for draining fluids. Hypodermic needles consist of a sharpened, apyrogenic stainless steel cannula, which allows painless penetration into the dermis and underlying layers. The needle is straight, hollow in length, and can be attached to a syringe either by simple interlocking (so-called Luer Slip graft) or by a screwing thread (Luer Lock). Needles vary in size and caliber, with the tip cut obliquely, in a flute beak shape, to achieve a sharp profile capable of piercing tissue with minimal trauma. The needles are usually covered with a plastic needle cover to prevent accidental puncture of the operator.