Artificial organ

An artificial organ is a man-made device that is implanted into, or integrated onto, a human to replace a natural organ, for the purpose of restoring a specific function or a group of related functions so the patient may return to as normal a life as possible. The replaced function doesn't necessarily have to be related to life support, but often is.
Implied by this definition is the fact that the device must not be continuously tethered to a stationary power supply, or other stationary resources, such as filters or chemical processing units. (Periodic rapid recharging of batteries, refilling of chemicals, and/or cleaning/replacing of filters, would exclude a device from being called an artificial organ.) Thus a dialysis machine, while a very successful and critically important life support device that completely replaces the duties of a kidney, is not an artificial organ. At this time a successful portable self-contained artificial kidney has not become available.
Reasons to construct and install an artificial organ, an extremely expensive process initially, which may entail many years of ongoing maintenance services not needed by a natural organ, might include:
Life support to prevent imminent death while awaiting a transplant (e.g. artificial heart)
Dramatic improvement of the patient's ability for self care (e.g. artificial limb)
Improvement of the patient's ability to interact socially (e.g. cochlear implant)
Cosmetic restoration after cancer surgery or accident
The use of any artificial organ by humans is almost always preceded by extensive experiments with animals. Initial testing in humans is frequently limited to those either already facing death, or who have exhausted every other treatment possibility. (Rarely testing may be done on healthy volunteers who are scheduled for execution pertaining to violent crimes.)
Although not typically thought of as organs, one might also consider replacement bone, and joints thereof, such as hip replacements, in this context.