A jet injector is a type of medical injecting syringe device used for a method of drug delivery known as jet injection, in which a narrow, high-pressure stream of liquid penetrates the outermost layer of the skin (stratum corneum) to deliver medication to targeted underlying tissues of the epidermis or dermis ("cutaneous" injection, also known as classical "intradermal" injection), fat ("subcutaneous" injection), or muscle ("intramuscular" injection). The jet stream is usually generated by the pressure of a piston in an enclosed liquid-filled chamber. The piston is usually pushed by the release of a compressed metal spring, although investigational devices may use piezoelectric effects and other novel technologies to pressurize the liquid in the chamber. The springs of currently-marketed and historical devices may be compressed by operator muscle power, hydraulic fluid, built-in battery-operated motors, compressed air or gas, and other means. Gas-powered and hydraulically powered devices may involve hoses that carry compressed gas or hydraulic fluid from separate cylinders of gas, electric air pumps, foot-pedal pumps, or other components to reduce the size and weight of the hand-held part of the system and to allow faster and less-tiring methods to perform consecutive vaccinations. Jet injectors were used for mass vaccination, and as an alternative to needle syringes for diabetics to inject insulin. However, the World Health Organization no longer recommends jet injectors for vaccination due to risks of disease transmission. Similar devices are used in other industries to inject grease or other fluid. The term "hypospray", although better known within science fiction, originates from an actual jet injector known as the Hypospray and has been cited within several scientific articles. Types A jet injector, also known as a jet gun injector, air gun, or pneumatic injector, is a medical instrument that uses a high-pressure jet of liquid medication to penetrate the skin and deliver medication under the skin without a needle. Jet injectors can be single-dose or multi-dose jet injectors. Throughout the years jet injectors have been redesigned to overcome the risk of carrying contamination to subsequent subjects. To try to stop the risk, researchers placed a single-use protective cap over the reusable nozzle. The protective cap was intended to act as a shield between the reusable nozzle and the patient's skin. After each injection the cap would be discarded and replaced with a sterile one. These devices were known as protector cap needle-free injectors or PCNFI. A safety test by