Autoinjector
An autoinjector (or auto-injector) is a medical device designed to deliver a dose of a particular drug. The injectors were initially designed to overcome the hesitation associated with self-administration of the needle-based drug delivery device. Most autoinjectors are one-use, disposable, spring-loaded syringes. By design, autoinjectors are easy to use and are intended for self-administration by patients, or administration by untrained personnel. The site of injection depends on the drug loaded, but it typically is administered into the thigh or the buttocks. Autoinjectors are sharps waste. History. Automatic syringes are known since 1910s, and  many spring-loaded devices with needle protectors were patented in the first half of the 20th century, but it was not until 1970s when they became economically feasible to mass-produce (simple syrettes were used instead before). Design. Designs exist for both intramuscular and subcutaneous injection. Disposable autoinjectors commonly use a pre-loaded spring as a power source. This spring and the associated mechanical components form a one-shot linear actuator. When triggered the actuator drives a three-step sequence: Some injectors are triggered by simply pushing the nose ring against the injection site. In these designs, the protective cap  is the primary safety. Other designs use a safety mechanism similar to nail guns: The injection is triggered by pushing the nose ring against the injection site and simultaneously, while applying pressure, pushing a trigger button at the rear end of the device. Since spent autoinjectors contain a hypodermic needle they pose a potential biohazard to waste management workers. Hence the protective cap is designed not only to protect the drug and keep the needle sterile, but also to provide adequate sharps waste confinement after disposal. Injectors intended for application through layers of clothing may feature an adjustable injection depth . Other typical features include: A drug inspection window, color-coded spent indicator, and an audible click after the injection has finished. Uses. Military uses include: Variants. Another design has a shape and size of a smartphone which can be put into a pocket. This design also has a retractable needle and automated voice instructions to assist the users on how to correctly use the autoinjector. The "Auvi-Q" epinephrine autoinjector uses this design. A newer variant of the autoinjector is the gas jet autoinjector, which contains a cylinder of pressurized gas and propels a fine jet of liquid through the  skin without the use of a needle. This has the advantage that patients who fear needles are more accepting of using these devices. The autoinjector can be reloaded, and a variety of different doses or different drugs can be used, although the only widespread application to date has been for the administration of insulin in the treatment of diabetes.